In European, U.S. and Canadian legislation clear references are made to static electricity being a potential source of ignition for operations conducted in flammable and combustible atmospheres presenting a significant and credible risk to the health and safety of employees. Not only does an electrostatic ignition hazard present a health and safety risk, it can cause significant disruption to business operations, in some cases leading to site closures, and result in negative publicity for the company that has suffered from the consequences of a fire or explosion caused by static electricity.
Industry standard guidance that addresses the ignition hazards of static electricity can be followed so that static ignition hazards are identified and the appropriate precautionary measures are put into action. Two leading international guidance documents of specific relevance to the hazardous process industries are CENELEC’s CLC/TR 60079-32-1 “Explosive atmospheres – Part 32-1: Electrostatic hazards, guidance” (2015) and the National Fire Protection Association’s NFPA 77 “Recommended Practice on Static Electricity”.
Both of these documents identify the range of EX/HAZLOC processes that present electrostatic ignition risks and the practical measures can be adopted to mitigate such risks. The most practical method of avoiding the accumulation, and consequent incendive discharge, of static electricity is the effective grounding and bonding of equipment. Grounding and bonding ensures equipment cannot accumulate electrostatic charge when the equipment is in contact with electrostatically charged liquids, powders and gases or is situated in close proximity to other electrostatically charged objects.
In order to safely transfer electrical charges from electrostatically charged equipment to earth, the most critical factor in the performance of grounding and bonding circuits is to ensure the total electrical resistance present in the path from the equipment requiring static grounding protection to a verified true earth grounding point is known. Both CLC/TR 60079-32-1 and NFPA 77 promote a maximum resistance of 10 ohms in metal circuits, providing EX/HAZLOC industry participants with a clear benchmark to target as a safety function when grounding and bonding protection against the accumulation of static electricity is a critical fire and explosion prevention measure.
Target 10 ohms.
The Earth-Rite MULTIPOINT II is the kind of system equipment specifiers and end users alike can adapt for a wide range of EX/HAZLOC processes that require active static grounding of metal equipment. Whether the metal equipment requiring static grounding protection is a railcar, an IBC (tote), drum or potentially isolated parts of interconnected process equipment, the Earth-Rite MULTIPOINT II will only indicate a positive ground status if the electrical resistance in the grounding circuit for the equipment is 10 ohms or less.
One of the primary cost advantages of the Earth-Rite MULTIPOINT II is its ability to actively monitor the ground status of up to eight (8) discrete items of equipment. Whereas one standard grounding system comprising an interlock function is typically needed for a single item of equipment, the Earth-Rite MULTIPOINT II’s ability to monitor eight (8) items of equipment, simultaneously, means that economies of scale can be achieved when the total installed cost of a project is calculated.
The Earth-Rite MULTIPOINT II’s monitoring control unit and remote indicator stations can be installed in Zone 0 atmospheres. The power supply unit can be installed in Zone 2 atmospheres.
The Earth-Rite MULTIPOINT II consists of a monitoring control unit that features an array of red and green LED indicators that verify when the equipment in need of static grounding protection has a resistance to a verified true earth grounding point of 10 ohms or less. In addition to the LED indicators located in the monitoring control unit, equipment operators can refer to independent remote indicator stations that can be mounted closer to the process hazard. Each indicator station provides operators with a visual “GO / NO GO” reference that informs them when the resistance in the static grounding circuit is low enough (less than 10 ohms) to proceed with the operation.
Demonstrating its installation flexibility in the following application the system is specified to ground four mixing stations (1 to 4) and two filling stations (5 and 6). Each mixer is interlocked with an individual relay corresponding to the equivalent ground monitoring channel so that if the resistance between the grounding clamp’s connection to the drum and the verified earthing point exceeds 10 ohms the mixer will not operate. Channels 5 and 6 are grouped via the system’s group relay so that if either drum is not grounded the pump feeding the filling station is shut down immediately, thereby stopping the delivery of electrostatically charged liquids to the drums.
The Earth-Rite MULTIPOINT II’s monitoring control unit and remote indicator stations can
be installed in Zone 0 atmospheres. The power supply unit can be installed in Zone 2 atmospheres.
Economies of scale are realised when compared with the total purchasing cost of six individual generic static grounding systems and there are several reasons why installation costs are minimised when compared to generic grounding solutions. The remote indicator stations are powered with intrinsically safe circuits that are fed directly from the monitoring control unit. This is more cost effective than specifying expensive Ex(d)/XP approved indicator stations that would need to be powered with mains/line power in the 230 V to 110 V AC range. In addition to the reduced cabling and purchase cost of the Earth-Rite MULTIPOINT II indicator stations, they consume much less power than mains/line powered indicator stations.
Protecting workers and company assets from ignitions caused by static electricity can’t be left to chance. In facilities where flammable and combustible products are processed, there’s a very high probability that static electricity is generated by the movement of gases, liquids and solids. The risks of a fire or explosion caused by a discharge of static electricity in an EX/HAZLOC area are just too significant to ignore. To emphasise its significance static electricity is identified in North American and European legislation as a potential source of ignition in potentially flammable and combustible atmospheres.
Although static electricity is regarded as “witchcraft” by many people working in the hazardous process industries, static grounding and bonding protection methods are anything but complex. Industry guidelines like NFPA 77* and IEC 60079-32-1** identify specific processes susceptible to discharges of static electricity coupled with practices that can eliminate the threat of ignition.
The most effective and practical means of eliminating the threat of a fire or explosion caused by static electricity is to ensure static charges are not permitted to accumulate on equipment, vehicles and people. Grounding and bonding presents the most effective and reliable way of removing static electricity from an EX/HAZLOC atmosphere.
*NFPA 77: Recommended Practice on Static Electricity” (2014).
** IEC 60079-32, Part 1: “Explosive atmospheres – Part 32-1: Electrostatic hazards – Guidance” (2013).
NOTE: Cenelec CLC/TR 50404 “Electrostatics. Code of practice for the avoidance of hazards due to static electricity” (2003) has been superseded by CLC/TR: 60079-32-1″Explosive atmospheres – Part 32-1: Electrostatic hazards – Guidance” (2015).
Industry approved guidance on controlling static ignition hazards.
To ensure we are protected from ignitions caused by static electricity we need to follow some basic rules of engagement provided in publications like NFPA 77 and IEC 60079-32-1. The most important benchmark is to ensure we can achieve an electrical resistance of 10 ohms or less between electrically conductive plant equipment, including mobile plant, people and vehicles, and a “ground source” that is verified as a true earth grounding point. This point will have a low resistance connection to the general mass of the earth and will transmit electrostatic charge from equipment to the earth, regardless of how much static electricity is generated by the process. This, in turn, removes the static ignition risk from the processing location.
In order to achieve a safe level of protection from electrostatic ignitions we must provide an effective means of grounding the equipment. Historically, the grounding of portable objects like drums, vessels, and vehicles like trucks and railcars was achieved with basic clamps that were assumed to make a direct connection to the equipment.
Figure 1; Traditional plier type basic clamp
However, issues like product build-up and protective coatings that can impede the integrity of grounding clamp connections, combined with rusted and degraded cable connections, prompted the industry to develop ground status indicator systems, particularly for the larger hazards which are typically reflected in railcar, truck and IBC bulk handling operations. Ground status indicators (commonly referred to as static grounding systems) monitor the connection to the equipment at risk of charge accumulation and provide a visual reference to workers to indicate if they have a secure ground connection, which, if green, will indicate that they can proceed with the process. In addition to providing a monitored grounding circuit many of these systems contain output contacts that can be interlocked with the process. Normally the grounding system’s output contact will be interlocked with the equipment controlling the flow or processing of the product, thus ensuring the equipment is grounded before the process that generates static electricity can begin.
Figure 2: Traditional wall mounted ground status indicator system with internal output contacts. Note the green for ‘SAFE TO GO’ indication method.
Sourcing and specifying grounding solutions with the flexibility to meet your specific zoning, installation and operating requirements.
For somebody tasked with controlling static ignitions hazards, solutions tend to fall between basic clamps and cables and static grounding systems. Static grounding systems offer the most layers of control and protection over static ignitions risks, particularly as they can monitor the integrity of the connection to the process equipment, provide a visual indication to operators of a positive ground connection and shut down the process if the grounding connection is ever compromised.
Although the majority of solution specifiers would like to have multiple layers of protection over electrostatic ignition risks it can be difficult to source the budget for processes where many items require static grounding protection. In addition, the processes responsible for generating static electricity may be operated manually with no means of automating safe controls over the process. One example of this is facilities that carry out manual filling operations of large quantities of drums and smaller containers with flammable liquids.
Bond-Rites provide 2/3 the level of protection of static grounding system at half the cost.
The Bond-Rite® family of products enable product specifiers apply additional controls over electrostatic ignition hazards above and beyond basic clamps and cables, without the burden of justifying purchases associated with grounding systems with interlock capability. For 15 years Bond-Rites have enabled solution specifiers to move up the safety curve from basic clamps and cables to achieve enhanced levels of safety by providing workers with a visual means of verifying a solid electrical connection to equipment for the duration of the process.
Employing the well-recognised safety principles of GREEN for “SAFE TO GO”, Bond-Rites utilise a pulsing green LED to indicate when the equipment (e.g. drum) has a resistance of 10 ohms or less to the site’s verified ground network. All Bond-Rites continuously monitor the connection to the equipment until the clamp is removed. The green for “SAFE TO GO” concept is extremely easy for workers to engage with enabling them to take responsibility for their own safety and that of their colleagues.
In the hazardous process industries, more commonly referred to as the “EX” industries, static electricity is generated virtually all of the time. Various grades of crude oil, refined petroleum products like LPG, and a host of chemicals fall into a category of materials that are often referred to as “static accumulators”. Materials in this category are known to be powerful attractors of electrons from other materials and resist “letting go” of electrons they come into contact with. In other words they “accumulate” static charge.
In a typical rail tanker loading operation, the static accumulating product is transferred from a storage tank via a gantry loading system into a receiving rail tanker. We can refer to the equipment involved in the transfer of product collectively as the product “transfer system”. As the product makes its way through the transfer system to the rail tanker, the molecules in the product become electrostatically charged.
If the rail tanker does not have a direct connection to earth it will accumulate electrostatic charges on its surface, which will result in the voltage of the rail tanker rising dramatically in a very short space of time. Because the rail tanker is at a high voltage, it will seek to find ways of discharging this excess potential energy and the most efficient way of doing this is to discharge the excess electrons in the form of a spark.
Energy discharged in static sparks.
Grounded objects that are in close proximity to charged objects are good targets for electrostatic sparks and permitting the uncontrolled accumulation of static electricity in an EX atmosphere is no different to having an engine’s spark plug exposed to a flammable atmosphere.
If the rail tanker is not grounded, its electrostatic voltage can build up to hazardous levels in less than 20 seconds. Table 1 illustrates how much energy can be discharged by a spark from a rail tanker charged to 20,000 volts.
Table 1. Potential energy of sparks from various objects.
When the energy of sparks discharged by static electricity is compared with the minimum ignition energies of a wide range of petroleum products and flammable chemicals, it’s easy to see why the rail tanker and any equipment connected to it, like flexible hoses and piping, should be bonded and grounded.
As can be seen in Table 1, electrostatically charged rail tankers can discharge sparks with a huge amount of energy. At these energy levels the prevention of electrostatic shocks to workers is also an important safety consideration. Involuntary physiological reactions caused by electrostatic shocks could lead to trips and falls and could be particularly hazardous when personnel are working above ground level.
Of the several factors that contribute to static charging, the one variable that must definitely be controlled is the grounding of the rail tanker. Grounding the rail tanker ensures that the rail tanker’s resistance to the general mass of the earth is maintained at a level that does not impede the safe transfer of static charges from the rail tanker to ground.
In North America the grounding of rail tankers with dedicated static grounding systems is common practice. In Europe, the practice of rail tanker grounding is mixed. Some sites do it, some don’t. For sites that do not actively ground rail tankers there is an assumption made that the tank of the rail tanker is well bonded to the chassis and that static charges generated by the product transfer operation can pass from the chassis through the rail tanker’s wheels to earth or back to the loading gantry via bonding connections.
Fig. 1 The MIEs of common petroleum porducts. (Source: NFPA 497)
Hazop assessments, and the reports that follow on from them, are a great way of capturing and identifying processes and practices that could lead to the ignition of flammable atmospheres through discharges of static electricity. What Hazop reports are not so great at doing is identifying what the earthing solution to eliminate the risk should look like.
The task of identifying the right earthing solution falls to people like you and members of your team and it’s not likely to be something you deal with on a day to day basis. For most people, identifying and specifying the right static earthing solution is probably the kind of project they’ll handle once or twice in their career. But get it right first time and it quickly becomes an area where you can bring value to the table throughout your career. This guide is about helping you get started on the right path and can be best described as a door opener to the subject of hazardous area static control.
The guide is broken down into three distinct sections. The first section deals with industry guidelines that provide guidance on controlling static electricity in hazardous areas The second section helps you work out the “best-fit” for controlling electrostatic hazards at your site and the third section touches on hazardous area equipment approvals, specifically what you should be looking for when selecting an EX Certified approved static earthing solution.
1. Static Earthing Benchmarks.
Before embarking on this guide to specifying and sourcing static earthing solutions it should be asserted from the jump-off point that hazardous area certified equipment that carries the mark of a Notified Body like SIRA or BASEEFA is not a validation of a earthing system’s performance characteristics when it relates to providing static earthing protection. Although a lot of time and effort can be put into sourcing earthing solutions that match or exceed your EX zoning requirements, the first recommendation this buyer’s guide will make is to take account of hazardous process industry associations that provide guidance on preventing ignitions caused by static electricity. There are several documents published by highly authoritative and respected associations around the world that identify the industrial processes that can be the source of electrostatic ignitions.
The committees that are assigned the task of developing and updating these guidance documents in line with the latest state of the art techniques are employees of companies and consultancies active in the hazardous process industries.
Demonstrating compliance with the recommendations outlined in these guidance documents will virtually ensure all of the electrostatic hazards presented by your company’s operations are under your control. If you can specify earthing solutions that display compliance with the publications listed in Table 1, you will be ensuring your static earthing protection methods display the latest state of the art in preventing fires and explosions caused by static electricity.
Table 1: Hazardous process industry guidelines for preventing fires and explosion caused by static electricity.
The guidelines in Table 1 describe how and why certain operations, whether it involves liquids, gases or powders, can generate static electricity and result in the static electricity accumulating on the equipment being used in the process. The primary means of preventing ignitions caused by static electricity is to ensure all conductive and semi-conductive equipment, including people, are bonded and earthed to a verified “true earth” earthing point. This ensures electrostatic charges cannot accumulate on equipment and discharge a spark into an ignitable atmosphere.
Because the Earth has an infinite capacity to balance positive and negative charge, if equipment is connected to it, that equipment is at “earth potential” meaning it can’t charge up in response to static generated by the movement of material.
Fig. 1: To ensure equipment cannot accumulate electrostatic charge, the equipment should be connected to the general mass of the earth by means of a true earth grounding point. The resistance between the grounding point and true earth must be low enough to allow the electrostatic charge generated by the process flow to earth.
Just as many other safety related functions have benchmarks designed with factors of safety in mind, earthing and bonding circuits can, and should, work to benchmarks that exceed the minimum safety requirements. The minimum theoretical requirement for earthing electrostatic charges is usually described in academic circles as having an electrical resistance not exceeding 1 meg-ohm (1 million ohms) between the object at risk of charge accumulation and the general mass of earth.
However, it is well recognised that metal objects at risk of charge accumulation, e.g. road tankers, and the earthing and bonding circuits providing earthing protection, should never display an electrical resistance of more than 10 ohms if they are in good condition. This value of 10 ohms is the one value of resistance that is consistently recommended across all of the publications listed in Table 1. So wherever a earthing solution is being sourced for operations that involve metal objects like road tankers, railcars, totes, barrels and containers, earthing systems that display earth monitoring values of 10 ohms or less should be specified.
Another reason why the theoretical value of 1 meg-ohm does not have a role in real world applications is the requirements related to earthing Type C FIBCs. Although CLC/TR: 50404 (2003) states that the resistance through a Type C FIBC bag should not exceed 100 meg-ohm, the latest state of the art guidance published in IEC 60079-32-1 (2013) and NFPA 77 (2014) states that resistance through the bag should not exceed 10 meg-ohm. So clearly, a “theoretically acceptable” value of 1 meg-ohm is impractical when discussed in the context of metal objects that should display a benchmark resistance of 0 to 10 ohms or less, and Type C FIBCs that should display benchmarks of either 0 to 10 meg-ohm or 0 to 100 meg-ohms (depending on what standard the bag is manufactured to).
NOTE: If you are engaged in sourcing a earthing solution for Type C FIBC bags you must ensure you know what standard the bags are manufactured to. If you don’t know what standard your bags are manufactured to the bag supplier should be consulted. Once you know what standard your bag is manufactured to you should source a Type C FIBC earthing system that monitors the earthing circuit from 0 ohms up to 10 meg-ohms (NFPA 77 / IEC 60079-32 compliant) or from 0 ohms up to 100 meg-ohms (CLC/TR: 50404 compliant). Avoid earthing systems that do not monitor the full range of resistance as they are likely to fail bags that are designed to work up to 100 meg-ohms and pass bags that should only work up to 10 meg-ohms.
The International Electrotechnical Commission (IEC) has published a new Technical Specification called IEC 60079-32-1: “Explosive atmospheres – Part 32-1: Electrostatic hazards – Guidance”. This Technical Specification is a guidance document which is the latest addition to the IEC series of 60079 “Explosive Atmospheres” standards that are designed to limit fires and explosions caused by electrical malfunctions within hazardous locations.
The 168 page document is the first of two documents to be published by the IEC under the “60079-32” designation and is intended to aid the designers and users of process equipment minimise the risk of incendive electrostatic discharges within potentially explosive atmospheres. It covers a broad range of process scenarios that can lead to the generation of electrostatic charges, provides examples of what measures can be taken to reduce charge generation and accumulation and outlines how process equipment should be grounded and bonded.
The second part, IEC 60079-32-2, is entitled “Electrostatics hazards – Tests” and outlines test methods to determine factors like surface resistance, earth leakage resistance, powder resistivity, liquid conductivity, capacitance and the incendivity of electrostatic discharges.
The stated objective of IEC 60079-32-1 is to provide:
“the best available accepted state of the art guidance for the avoidance of hazards due to static electricity”.
To date guidance documents that address the ignition hazards of static electricity have either been published by national institutions like the NFPA or pan-European organisations like CENELEC. IEC 60079-32-1 has been collectively developed by a large number of technical committees from IEC member countries, making this document a truly global collaboration. It also builds on the work of national and regional guidance documents addressing electrostatic hazards, including CENELEC/TR: 50404, NFPA 77, BS 5958, TRBS 2153:2009 and JNIOSH TR42.
Although the Technical Specification can be purchased from the IEC’s webstore, it will be the responsibility of national standards institutes like the ANSI in the U.S., BSI in the U.K. and DIN in Germany to administer the circulation of the document in their respective national territories. The ANSI has the document available for purchase from its website. Cenelec has withdrawn CLC/TR: 50404 and replaced it with CLC/TR 60079-32-1.
Overview of the Technical Specification:
The Technical Specification is sub-divided into what are termed “clauses” that highlight the electrostatic hazards associated with various categories of materials, the hazards associated with people, including physiological shocks, and what grounding and bonding measures should be put in to practice. The clauses are presented as:
1) The handling of solids.
2) The storage and handling of liquids.
3) The handling of gases and vapours.
4) The storage and handling of powders.
5) The storage and handling of explosives.
6) Electrostatic problems caused by people.
7) Avoidance of electrostatic shock.
8) Earthing and bonding of plant and machinery.
There are also several Annexes that provide informative material, examples of which include a description of the various types of electrostatic discharges, the types of electrostatic discharges that can be expected from processes carried out within potentially flammable and combustible atmospheres and the provision of an illustrated flowchart for assessing electrostatic hazards.
Owing to the fact that the document is 168 pages long, it would be impossible to provide a comprehensive overview of the guidance contained in the document in just a few pages. However, it would be worth touching on guidance related to the grounding and bonding of specific processes that utilise portable equipment at risk of static charge accumulation.
The design and monitoring of grounding systems:
This section addresses the design and monitoring of systems dedicated to grounding permanent and portable plant equipment. Permanently installed plant equipment like reactors and pumps will most likely be grounded via the electrical grounding system for the plant. Electrical fault paths (and lightning protection paths) are more than adequate to dissipate electrostatic charge to ground.
For portable conductive equipment this section recommends that temporary connections using bolts or “pressure-type” clamps are capable of penetrating protective coatings, rust or product deposits that are typically present on the surface of such equipment, e.g. metal drums. It states that pressure-type clamps should be capable of establishing a connection resistance of less than 10 Ohms to the base metal of the conductive equipment.
Systems designed to monitor the resistance between equipment at risk of charge accumulation and earth (designated grounding points) should not only be capable of monitoring the resistance in the grounding circuit, but should also be capable of drawing attention to any changes in resistance. This is to ensure that malfunctions in the grounding circuit are detected as early as possible so that inspections and necessary repairs are made in good time.
Given that metal grounding circuits should not display a resistance above 10 ohms it would be prudent to specify grounding systems that are capable of identifying changes in resistance and alerting personnel as soon as 10 ohms in the ground path is exceeded.
Powder processing operations can generate vast quantities of electrostatic charge via the movement of powder. The standard method of charging on powder processing operations is due to tribo-electrification, which is basically the contact and separation of the powder with processing equipment, the powder itself or other factors that can cause charging, like surface contaminants. There are numerous types of equipment that can cause the charging of powders. Such equipment includes, but is not limited to:
Table 1. Equipment used in powder processing operations
The processes carried out by such equipment can lead to varying degrees of electrostatic charge generation. Typical charge quantities, from published literature, are tabulated below. The values are based on the amount of charge, in coulombs, carried per kilo-gram of powder.
Table 2. Charge generated on powders by different powder processing and handling operations (NFPA 77 / CLCTR: 60079-32-1)
A simple calculation will show that a metal drum with a capacitance of 100 pF being filled with 25 kg of charged powder, following a simple pouring operation, could be charged to a voltage of 25,000 V.
The potential energy that could be discharged from the drum in the form of a spark can be estimated to be:
By any standard, the voltage generated by an operation that is known to be at the lower end of charge generating capacity can still generate enough potential spark energies to ignite a broad range of combustible atmospheres. Table 2 lists the minimum ignition energy of a sample of powders when they are at a Minimum Explosive Concentration.
Table 3. MIE of various powders when suspended in a combustible concentration
If the powder is being discharged into a blender or mixer that contains a solvent, the MIE of the hybrid atmosphere could be much lower such that the initial ignition of the solvent vapour could propagate a combustible dust deflagration.
The safety factor that needs to be borne in mind with these calculations is the assumption that the equipment being “electrified” by the charged powder is not grounded. If the equipment is grounded, there is no risk of the equipment becoming electrified by static electricity.
Static Earthing protection in powder processing operations.
“Earthing”, in its truest form, is the method by which a low resistance electrical connection is made between equipment at risk of electrostatic charging and the general mass of the Earth. This connection is normally described as a “true earth ground”. The actual connection to earth is achieved via purpose designed earthing rods, or building structures, that are buried below ground level. These earthing systems are tested by engineers to measure their true earth ground resistances to ensure they are below values required in standards like NFPA 70 “National Electrical Code®” and EN 62305 “Protection Against Lightning”. Some static earthing systems on the market today will actually verify if the equipment they are providing static earthing protection for have a true earth ground capable of conducting static electricity.
In pharmaceutical operations, equipment like powder conveying systems, micronizers, blenders and sieve stacks all make up multiple component assemblies that can accumulate high levels of electrostatic charge should any of the components be isolated from a true earth ground. Connections made with items like bonding straps can provide an intentional bond between metal components or assembly mating surfaces may provide an inherent bonded connection.
Fig. 1. A blender getting charged with a powder. Note that the bucket discharging the powder should be bonded to the receiving vessel or grounded independently.
Regular disassembly for cleaning and maintenance can result in bonding connections being missed or not made correctly when the Regular disassembly for cleaning and maintenance can result in bonding connections being missed or not made correctly when the equipment is reassembled. Vibration and corrosion may also degrade assembly connections so it is imperative to ensure that no parts in the assembly become isolated from a true earth ground reference.
The most effective way of ensuring that equipment used in powder processing operations cannot accumulate static electricity is to provide a dedicated static earthing solution that will monitor the ground connection of components at risk of static charge accumulation and alert personnel to a potential hazard should a component lose its ground connection. This is especially important if the ground connection point to the equipment is not readily visible or easily accessible.
Discharges of static electricity from hoses are known to cause the ignition of combustible atmospheres during the transfer of material to or from vacuum tankers and road tankers.
There are normally three main reasons why discharges of static electricity from hoses can occur. One reason is that standard non-conductive hoses are incorrectly used to transfer material. Non-conductive hoses are used to transfer material. Non-conductive hoses are capable of accumulating and retaining high levels of static charge which can result in incendive brush discharges from the hose itself, or the charging of isolated conductive objects attached to the hose like a nozzle or coupling that can discharge a spark themselves. It is generally accepted practice within the hazardous process industries that non-conductive hoses should not be used to transfer potentially combustible liquids and powders and numerous standards and industry association publications repeat this recommendation.
Another common reason for static spark discharges from hoses results from connecting conductive hose, or interconnected conductive hose sections, to a vacuum tanker or road tanker that does not have a verified static ground connection. The third most common reason for static spark discharges from hoses is where the conductive components of the hose structure become isolated during normal activity.
Figure 1. Four hose sections joined together in a vacuum tanker operation with an OhmGuard® hose tester testing the first hose section.
Both the second and third modes of electrostatic discharge are the most relevant to the hazardous process industries, and are scenarios where improper use of conductive hoses can lead to the accumulation and discharging of static electricity within a combustible atmosphere.
1.1 Conductive hoses connected to ungrounded vacuum tankers and road tankers.
With no static earthing protection in place a tanker conducting a vacuuming or loading operation will become electrostatically charged as it has no means of preventing the accumulation of static electricity on its tank and chassis. Because the metal connections (couplings) of the hose should be electrically continuous with the tanker, the tanker will also transfer charges to the hose, thereby causing the accumulation of static electricity on the hose as well. The quantity of charge transferred to the hose will be high as unearthed tankers can build up very large electrostatic voltages in a short space of time.
Charge accumulation on the conductive metal components of the hose, like couplings or nozzles, are a particular concern as these are the parts most likely to be closest to any combustible vapours or dusts during operations and may seek to nullify their electrical imbalance by sparking onto objects like operators, tank walls or pipes. If a combustible atmosphere is present in the spark discharge gap ignition of the atmosphere is highly probable.
In one reported incident a vacuum tanker was sucking off-specification toluene from a below grade sump and although the hose was conductive, the tanker to which it was attached did not have a verified static ground connection. The hose itself consisted of a metal wire helix embedded in the hose tubing which bonded the hose couplings but given the high level of voltage induced on the hose via the ungrounded tanker, a static spark was discharged from the metal wire helix of the hose, across the hose tubing and onto the metal rim of the sump. The resulting spark ignited the toluene vapours leading to a fire .
1.2 Damaged conductive hoses connected to grounded vacuum tankers and road tankers.
A more insidious hazard is situations where the tanker has a static ground connection that is verified with either a tanker mounted or gantry mounted earthing system, but the hose(s) connected to the tanker has lost its electrical continuity resulting in the isolation of a metal component somewhere in its structure. A typical example of this would be when the metal wire helix of the hose becomes isolated from an end fitting like a hose coupling or a nozzle.
Metal wire helixes are commonly used to reinforce the hose structure against transfer pressures and bending kinks. Another common function of metal wire helixes is to bond end fittings to provide the necessary end-to-end electrical continuity that will prevent the accumulation of static electricity on the hose. If the metal helix, through normal industrial “wear and tear”, breaks or detaches from hose couplings or nozzles, these components now have the capacity to accumulate enough charge and enough energy to ignite a combustible atmosphere.If a hose section with an isolated coupling is fitted between other hose sections, the other sections are isolated from the grounded tanker also which could lead to multiple components becoming electrostatically charged near to, or within, the potentially combustible atmosphere. In this situation the isolated hose sections will become charged due to contact with the moving liquid or powder.
Figure 2: Examples of a loading gantry mounted (Earth-Rite® RTR) and tanker mounted (Earth-Rite® MGV) static ground verification systems.
Another important consideration is hoses fitted with two metal wire helixes, where one helix is present on the outer surface of the hose and a second helix is present on the inner surface of the hose. In some hose designs the inner helixes are not bonded to the hose end fittings and it is important to ensure that such helixes cannot discharge sparks onto the end fittings or operator, especially when the hose is removed at the end of a transfer operation when a combustible atmosphere may be present in the hose or the area surrounding the hose. A hose fitted with an internal metal wire helix caused a fire through a discharge of static electricity, and in addition to the wire helix being broken, both end couplings were not designed to be connected to the inner metal helix. Quoting from “Avoiding Static Ignition Hazards in Chemical Operations”, AIChE/CCPS, Britton L.G., 1999:
“A fire was reported during draining of toluene from a road tanker through such a hose and after the event it was found that the inner spiral was not only broken but was not designed to be bonded to the end connectors. Two post loading toluene fires occurred with a similar hose as the disconnected hoses were being handled by operators.”
2.0 Industry standards and recommended practice.
To ensure that the hoses used on vacuum tankers and road tankers are not an electrostatic ignition source in a hazardous area there are numerous standards and recommended practices that describe the required electrical continuity of hoses. However, owing to the various hose construction types and established industry sector “norms”, there are a range of electrical continuity values that preclude a “one size fits all” approach to ensuring a hose is safe to use in a potentially combustible atmosphere.
By far, the most common type of hose used on vacuum tankers and road tankers are hoses that contain metal wire helixes that may be sandwiched between layers of hose tubing or may be present on the inner or outer surface of the hose, or both.
The following table lists several standards and industry association publications that outline the conductivity requirements for hoses. The respective recommended values of hose resistance are derived for an equivalent 25 ft. length of hose.
Table 1: Standards and industry publications that address hazards related to electrostatic charging of hoses.
In reality, many companies specify their own internal inspection regime that requires periodic end-to-end electrical continuity testing of their hoses. Periodic testing is normally performed every 6 to 12 weeks by a trained technician who will use a multimeter to measure and record the test results. The normally accepted end-to-end resistance “PASS” benchmark for individual hose sections with metal helixes is 10 ohms or less. Depending on the test results the technician will either allow the hose back into service, schedule the hose for a repair or remove the hose from service altogether. Quoting from CLC/TR: 60079-32-1 (ref. Table 1):
“Due to broken bonding wires or faulty construction, it is possible for one or more of the conductive components of the hose (i.e. end couplings, reinforcing helices and sheaths) to become electrically insulated. If a low conductivity liquid is then passed through the hose these components could accumulate an electrostatic charge leading to incendive sparks. Therefore, the electrical continuity of the hose should be checked regularly. Care should be taken to ensure that all internal metal helices are bonded to the end coupling.”
Although periodic testing of hoses is important, from a static earthing protection viewpoint, it would be safer to test the hoses prior to every transfer operation. In the 6 to 12 week period that the hoses are in use, breaks in end-to-end continuity can, and will, occur. Normally the metal helix that bonds the couplings of the hoses together will either break or loosen from its connection to the coupling.
Figure 3 . An isolated coupling caused by a broken wire helix.
If hoses with breaks in continuity are kept in service there is a strong chance that they will be accumulating static electricity during loading or vacuuming operations thus increasing the probability of static spark discharges when the hose is being used in a hazardous atmosphere.
The ideal procedure for proving a secure static earthing path for all the primary components used in the transfer, i.e. the road tanker and the hose sections connected to the tanker, would be to verify a ground for the tanker via a tanker mounted earthing system (Earth-Rite® MGV), or a gantry mounted earthing system (Earth-Rite® RTR). When the ground path for the tanker is verified, the next operation would be to connect the hose(s) to the tanker and then perform an electrical continuity test through the hose sections back to the tanker. This would ensure that the hose will be capable of transferring static charges through its structure, onto the tanker and down to ground via the static earthing system.
Nowadays, the task of selecting an electrical system destined for use in a hazardous location can lead to a time consuming navigation through a myriad of approval certificates with different acronyms, product labelling details and an investigation into what the markings on the label(s) actually represent. This article will provide an overview of the various national and regional regulations governing hazardous area certified products, take a brief look at the standards against which they are assessed and examine if there is any scope for creating a “common language” for users of hazardous area equipment that may facilitate a future in which class leading hazardous area equipment, regardless of origin, is acceptable to the relevant regulatory body of the country in which the hazardous area operator is located.
(Examples of all product approval markings referred to in this article can be viewed at the end of this article).
Hazardous location Product certification requirements in the United States.
In the U.S. the Occupational Safety & Health Administration (OSHA) is responsible for ensuring safe working conditions for employees in the workplace through demonstrable compliance with its Code of Federal Regulations (CFRs) which are U.S. law. Under 29 CFR 1910.307, OSHA regulates worker safety in hazardous locations through a requirement that companies procure and install equipment that will demonstrate compliance the National Electrical Code, NFPA 70, via a list of safety standards deemed “appropriate” by OSHA. The primary safety standards recognised by OSHA, at minimum, demonstrate compliance with Articles 500, 505 and 506 of the National Electrical Code. These articles describe the classification of hazardous locations, what methods of electrical protection (protection techniques) are acceptable in these locations and how equipment operating in such locations shall be marked.
Article 500 describes the Class and Division system of hazardous location classification, the relevant protection techniques and product markings required. Article 505, added in 1996, relating to gas and vapour atmospheres and Article 506, added in 2005, relating to dust and fibre atmospheres, describe the Class and Zoning system.
Under regulation 1910.3079(g)(1), OSHA permits industry to work to the Class and Zoning system described in NEC 505. NEC 505.9(C) outlines the marking required on the equipment. Although OSHA guidelines do not address Class and Zoning systems for dust and fibre atmospheres, under NEC 506.20, equipment listed for Class II, Div. 1 and Div. 2 locations can be installed in the respective Zone 20, 21 and 22 areas provided the temperature classification of the equipment meets the requirements for the relevant dust group. Under 506.9(C)(1), Class II listed equipment with Division 1 and Division 2 approvals can be marked with additional Zone identification and dust group temperature classification markings. NEC 506.9(C)(2) describes the Class and Zone method of marking equipment that will be operated in dust and fibre atmospheres.
Under OSHA regulations, electrical equipment destined for installation and use in hazardous locations must be certified by a Nationally Recognised Testing Laboratory (NRTL). NRTLs test and certify equipment to standards, produced by recognised standards developing organisations, which OSHA deems “appropriate”. Examples of standards producing organisations include ASTM, ANSI, ISA, IEEE, Underwriter Laboratories and Factory Mutual. OSHA recognises and monitors organisations that apply for NRTL status in accordance with the requirements of 29 CFR 1910.7. OSHA will also recognise NRTLs based outside of the U.S., one example being CSA of Canada.
When OSHA recognises an organisation as having NRTL status they are issued a formal notification by the head of OSHA, who is the Assistant Secretary of Labor for Occupational Safety and Health. This notification sets forth the specific scope and other terms of the recognition. This recognition is reviewed every five years and will be revoked if the NRTL does not comply with the requirements of 29 CFR Part 1910.
It is the responsibility of the NRTL to submit the safety standards it intends to certify product against, test, certify and list the product to the relevant standards, monitor the use of the listed product in the market place and notify manufacturers if changes to standards are likely to impact on the certification of their listed product.
OSHA compliance officers carry out site inspections to ensure electrical equipment used in hazardous locations display the unique certification mark of an NRTL and have the authority to issue fines if the equipment is not installed in accordance with the equipment’s approvals or does not display the mark of a recognised NRTL.
Hazardous area Product certification requirements in Europe.
In the European Union, the “ATEX 95” Directive, 2014/34/EU, is a legally binding requirement on manufacturers and users of equipment intended for use in potentially explosive atmospheres. The scope and intent of the Directive is to enable the free movement of ATEX certified equipment throughout the European Economic Area, which is made up of all European Union and European Free Trade Association member countries. It was adopted on 23rd March 1994 and entered into force on 1st March 1996. It replaced all EU member state’s similar national regulations on the 1st July 2003.
The ATEX 95 Directive lays down the Essential Health and Safety Requirements (EHSRs) that specify the levels of explosion protection required for equipment destined for operation in potentially explosive atmospheres (hazardous areas). The primary source of technical adherence for the assessment and certification of products designed for use in explosive atmospheres is harmonised “EN” standards that provide “technical expression” of the requirements of the EHSRs. The full list of harmonised standards is contained in the Official Journal of the European Commission, the executive body of the EU.
In order to place an ATEX certified product on the market, the manufacturer must sign a Declaration of Conformity with the appropriate Directive(s). Declarations of Conformity with other Directives, particularly electrical equipment, may apply before a CE mark can be placed on the equipment (e.g. electromagnetic compatibility under EMC 2014/30/EU). In addition, for products that are certified for installation in hazardous areas, “Ex” marking on the equipment is required.
To support the Declaration of Conformity for ATEX, the manufacturer will have their product assessed and tested by a “Notified Body” who will issue an EC Type Examination certificate that states the product meets the requirements of the ATEX Directive’s EHSRs. The Notified Body tests the product in accordance with the European Commission’s list of harmonised “EN” standards that reflect the “latest state of the art” with respect to the protection methods that comply with the EHSRs of the Directive. The Notified Body must also asses the manufacturer’s quality assurance system relevant to the manufacturing of the certified product, ensuring product quality is assured under the requirements of the Directive.
Under the Directive, notified bodies are required to meet as part of the “Group of Notified Bodies” to ensure the technical requirements of the EHSRs are fulfilled via the latest standards and to ensure the standards are applied in a coherent manner throughout member states. CENELEC normally requests the International Electrotechnical Commission (IEC) to produce standards that will meet the requirements of the EU’s Directives. The most applicable standards relevant to hazardous area product certification is the IEC 60079 series of standards published by the International Electrotechnical Commission which, since 2006, have been adopted by the European Union, via CENELEC, as harmonised “EN” standards that support the EHSRs required by the ATEX Directive. The “zoning” concept used under ATEX has been adopted from the IEC system of hazardous area classification.
Internationally recognised hazardous area product certification.
The International Electrotechnical Commission (IEC) itself runs its own certification system for hazardous area equipment and it is called the IECEx scheme of hazardous area equipment certification. This scheme has the stated goal of becoming the global benchmark for hazardous area certified equipment so that any product carrying the IECEx mark will be acceptable to all national bodies regulating industries engaged in hazardous area activities. Under the IECEx “Ex” product certification scheme, manufacturers submit their product to a recognised IEC certification body, an ExCB, who controls the certification process for the product. Samples of the product are tested in accordance with the relevant IEC standards by a testing laboratory, an ExTL, under the coordination of the ExCB. The product must conform to the most applicable standards related to hazardous area equipment, most notably the IEC 60079 series of standards. The ExCB is also responsible for auditing the manufacturer’s production facility in accordance with the ISO 9001 standard. The manufacturer is only issued with a Certificate of Conformity if the test report (ExTR) and the quality assessment report (QAR) provide evidence of compliance with the relevant standards. The ExCB is then responsible for annual audits of the manufacturer with respect to the product that has been granted the Certificate of Conformity (CoC). The CoC and test report are held and controlled by the IEC and the most current version of the CoC, along with its revision history, is visible on the IECEx website.
Under IECEx rules an ExCB can only certify products against the IEC standards for which it has been assessed via the “scope of acceptance”, a process carried out by an Assessment Team made up of existing IECEx members. The ExCB and ExTL must demonstrate the capability (e.g. technical competence, laboratory equipment) to assess and test products to the standards covered by the scope of acceptance. Annual surveillance of ExCBs and ExTLs is required under IECEx rules and all ExCBs and ExTLs are re-assessed every five years.
The IECEx is made up of national technical committees that each contributes to the creation and continuous updating of standards. Updates to standards are frequent and are designed to reflect the latest state of the art with respect to protection concepts and test methods for equipment to be operated in hazardous areas. To illustrate, the latest standard for intrinsic safety, edition 6 of IEC 60079-11, published in 2011, was previously published in 2006. One example of the 2011 update was the addition of new test requirements for opto-isolators.
Convergence of hazardous area product certification systems.
If there is any likely route to the convergence of an internationally recognised hazardous area product certification “kite-mark” this is likely to be via the IECEx scheme. To enable harmonisation on a global scale the IEC requires the identification of “national differences” between national standards and regulations and IEC standards. Additionally, a transitional period must be defined in order to normalise such differences so that the IECEx standards have a broad consensus of agreement and are acceptable to all participating countries. National differences may be reflected in requirements like fire and electrical shock testing for the U.S. and any Directives applicable to electrical products for the EU and EFTA. However, it is not the goal of the IECEx scheme to replace or remove these additional requirements as the scheme is focussed solely on explosion protection certification.
The U.S. representative on the IECEx Scheme is the United States National Committee (USNC) which is administered by the ANSI. Under NEC Articles 505 and 506, hazardous areas are divided into Zones and the majority of ANSI, ISA or UL standards referenced in NEC Articles 505 and 506 are derivative forms of the IEC series of 60079 standards. Under ANSI guidelines, an IEC standard is either adopted in its identical form or modified to add to, or reduce, the requirements of the standard. The standard will normally be adopted or modified by one of the standards developing organisations like the ISA or UL and obtain national standard status via the ANSI. In relation to explosion protection, the majority of differences applied to IEC standards are designed to ensure compliance with the NEC. Such differences could relate from wiring methods through to the addition of sections such as information on protection concepts like zener diodes.
Although the technical and legal harmonisation process may take anywhere between 10 and 15 years, from a U.S. perspective, the real challenge to adoption may happen “on the ground”. One example is where the Authority Having Jurisdiction (AHJ) will need the technical capability to sign off on equipment marked and certified to the Class and Zoning requirements of the NEC. In reality, if the product has the mark of an NRTL, with IEC “Ex” Certified Body status, and the appropriate level of knowledge and communication is present between OSHA compliance officers and other AHJs, then the case for sole acceptance of IECEx marked products may strengthen in the medium to long term.
A recent development has seen the United Nations, through the work of UNECE, adopt the IECEx model of certification as the basis for establishing a regulatory framework for hazardous areas at a national level so that any U.N member country can use the IECEx model as the basis for national legislation. This will be especially beneficial for countries that require access to class leading hazardous area products but do not have legislation or regulations that mirror those of the U.S. or Europe, which can make the selection of equipment an onerous, or even impossible, task. Member countries with developed legislative frameworks may choose to align themselves with this development so that all countries recognise the same certificates.
This article will explore the current methods used to provide static earthing protection for vehicles operating in locations that do not have installed, or correctly specified, static earth monitoring systems. Although primarily designed to provide all tankers with mobile ground (earth) verification capability, the Earth-Rite® MGV has proven to be a success for vacuum tankers used by contractors providing cleaning, spill and material recovery services to companies with classified hazardous areas. The Earth-Rite MGV is also utilised on tankers that must collect from, or deliver product to, locations that do not have satisfactory static earthing protection for road tankers in place.
Vacuum tankers provide a wide range of services to the hazardous process industries ranging from storage tank cleaning to the recovery of combustible materials resulting from leaks and spills. A key feature of this type of service is the recovery of materials in locations with potentially combustible atmospheres.
Static electricity is a well known ignition source within the hazardous process industries and because the generation and accumulation of static electricity is not visible to the naked eye, this “below the radar” characteristic, makes it an exceptionally precarious and dangerous hazard. Normally, the only evidence of static electricity being present during a transfer operation is when somebody sees or hears a static spark discharge. By then it may be too late to prevent the ignition of the surrounding atmosphere if it is in its combustible range.
Earthing vacuum tankers operating in hazardous areas eliminates the threat posed by static electricity and is an action that effectively connects the tanker to the general mass of the Earth, which is sometimes called “True earth”. The voltage induced on the tanker by the charged material is the key factor in a static spark discharge. Earthing ensures that no voltages are generated and permitted to accumulate on the tanker.
A solution that is appropriate to the potential hazard.
For over twenty years dedicated static earth monitoring systems have replaced basic earthing reels on the road tanker loading gantries of petrochemical and chemical sites, pharmaceutical sites, tank farms and food and beverage manufacturing sites. Due to the combination of the large quantities of combustible material being processed, the amount of charge that can be induced on tankers and the potential outcome of the ignition of the atmosphere, bonding reels were replaced with earth monitoring systems that were designed to monitor the integrity of the road tanker’s connection to earth so that electrostatic charge could not accumulate on the tank or chassis of the road tanker while product was being transferred. To enhance the safety of transfers at these locations, gantry mounted earth monitoring systems normally have an interlock function that stops the movement of product if the earthing system is disconnected from the road tanker.
Even though the potential and consequences of fires is, at the very least, the same for road tankers at dedicated loading gantries, vacuum tanker service providers have not been in a position to provide this level of safety and protection of their personnel and tankers, or for their customer’s personnel and property.
Until now, vacuum tanker service providers have had to rely on very basic devices to earth their vehicles. This is simply because technology that is capable of verifying the quality of static earthing points in a mobile, quick and user-friendly way has not been available to drivers and operators. The method currently used consists of a simple earthing clamp attached to single core braided cable wound onto a reel.
Very often, vacuuming operations will be carried out on facilities and remote locations where “designated” earthing points may not be tested on a regular basis, are not accessible or do not exist. (More detail on earthing points is provided at the end of this article). Bulk transportation companies can also have the same difficulties when they deliver product to customer sites where earthing systems are not up to current specifications, or worse still, are not installed.
When compared to the performance and safety of static earth monitoring systems, single core bonding reels have several major drawbacks.
- Bonding reels cannot monitor the tanker’s connection to the earthing point for the duration of the transfer process. If the clamp’s connection to the earthing point is compromised, the drivers and operators will have no way of knowing this as they will be concerned with the safe and secure transfer of material.
- When the driver needs to connect the reel to secondary earthing points (e.g. pipe or structural support beam), the bonding reel cannot verify that the earthing point actually has a verifiable connection to a True earth ground.
- On many customer sites electricians are required to perform resistance readings with multi-meters to verify that the tanker has a 10 ohm or less bonded connection to a designated earthing point, via the bonding reel. This method has several major drawbacks.
- The electrician needs to be taken off maintenance, repair and installation work to perform this test and may be delayed, even up to a few hours, in performing the resistance check. This has the knock on effect of delaying the vacuum tanker team in proceeding with the cleaning, spill recovery or tanker offloading operation.
- In an emergency situation, like a spill or leak, the vacuum tanker team may not have time to wait for an electrician to conduct a bond resistance test and will have to bond the tanker to points that have not been designated as verified earthing points. In that situation, they will be hoping that the object they have bonded to will have a connection to True earth.
- The resistance check is a one-time bond resistance check between the points the tanker is connected to. It does not verify if the structure the reel is connected to has a connection to a True earth ground.
- Because the resistance check is a one-time check, the drivers will not know if the clamp’s connection is compromised during the transfer.
Unlike the security provided to road tanker drivers and loading gantry operators by gantry mounted earth monitoring systems, the vacuum tanker team running the recovery or transfer operation has no way of knowing if their tanker is connected to a good earth.
Contract service providers, and customers, have concerns due to such limitations because the teams are connecting reels to earthing points that have neither been tested nor verified as being connected to a True earth ground.
In order to remove this uncertainty and provide vacuum tanker service providers with the same level of protection that gantry mounted static earth monitoring systems provide, Newson Gale developed the Earth Rite MGV, which is a vehicle mounted static earthing verification system. MGV stands for Mobile Ground Verification.
Newson Gale developed the Earth-Rite MGV to give the providers and customers of vacuum tanker services the same level of safety and performance that a gantry mounted static earthing system can provide. The only difference between a gantry mounted system and the MGV is that the MGV is a permanent component of the tanker for which it is providing static earthing protection.
In recent years there has been a proliferation of new and low cost plastic portable containers. Containers ranging in size from 1 litre bottles, to 205 litre drums and 1000 litre IBCs have provided the supply chains of the hazardous process industries with a diverse range of material packaging options. While some packaging options will require plastics that demonstrate specific levels of material compatibly with different products, one of the major drivers of plastic packaging is their relative low cost in comparison to metal containers including metal drums and metal IBCs. The increasing use of plastic containers within the hazardous process industries is coming under increasing scrutiny due to the hazards associated with static electricity. This brief article will address the issues associated with static electricity on plastic packaging, draw on reports and expertise of industry and safety bodies and provide solutions to earthing non-metallic containers, with a particular focus on composite drums and IBCs.
Defining the meaning of the terms “static dissipative”, “conductive” and “insulating”.
It is important to define the terms “conductive”, “insulating” and “static dissipative” (anti-static) in order to fully appreciate the capability of materials to safely dissipate electrostatic charges from objects that are correctly earthed (grounded). Conductive materials permit the transfer of electrostatic charges instantaneously. In static dissipative materials, electrostatic charges are adequately dissipated, albeit at a slower rate than conductive materials. In insulating materials, or to be more precise, poorly conducting materials, electrostatic charges tend to be retained on the material and not readily transferred, even when the material is connected to earth.
Understanding the difference between volume resistance and surface resistance is also important. Resistivity is determined by the intrinsic properties of a material that resist the flow of electrical currents. Volume resistivity, ρ, represents the total resistivity value of a section of material through its entire volume. The overall resistance to charge transfer is calculated by multiplying the resistivity value for the material by its length and dividing by the cross sectional area through which the charge is flowing:
R = ρl/A
For example, the resistance through a large volume of 1 m length by 1m2 cross sectional area of PTFE with a resistivity (ρ) value of 1019 Ω.m, is equal to 1 x 1019 ohms(1). For a similar volume of copper with a resistivity value of 1 x 10-8 Ω.m, the resistance through the copper will be 1 x 10-8 ohms. So even if the PTFE is correctly earthed, charges will experience a very high degree of resistance to their movement to earth, whereas as for metals, charges will experience little or no resistance and be transferred to earth immediately.
Resistance experienced by current flowing through material is influenced by the resistivity value, ρ, for the material and the length and cross-sectional area of the material.
Surface resistivity, λ, represents the total resistivity across the surface of a material. In essence, a material with a high volume resistivity could be engineered to have a low surface resistivity value, meaning charges that would otherwise not transfer easily through the material, are allowed to transfer across its surface.
Overall surface resistance is calculated in a similar way, where the resistance is calculated from R = λ L1/L2.
Resistance experienced by current flowing across surface is influenced by the surface resistivity value, λ, of the material and the length and breadth of the material.
In general materials can be segmented into three categories, depending on their volume and surface resistivity values.
Table 1: range of resistivity values for conductive, static dissipative and insulating materials(1).
In regard to electrostatic ignition hazards within hazardous areas the correct use and specification of containers made from conductive, static dissipative and insulating materials is critical to the safety of workers and the processes in which these containers are used.
Testing of composite IBCs and industry guidance.
A report prepared for the Health & Safety Executive in the UK highlights key selection criteria hazardous area operators should take into consideration when using portable containers within hazardous areas(2). The report tested and quantified the levels of electrostatic discharge on containers ranging in size from small 1 litre plastic bottles to 1000 litre rigid IBCs. Rigid IBCs are supplied in a wide range of different materials of construction and can be made of insulating plastic, static dissipative plastic, and insulating plastics surrounded by metal sheet cladding or steel frames. 205 litre plastic drums were not included in these tests.
The generation and measurement of electrostatic discharge was conducted in accordance with EN 13463-1:2001, “Non-electrical equipment for use in potentially explosive atmospheres. Basic method and requirements”.
Controlled laboratory testing highlighted that levels of electrostatic discharge capable of igniting commonly used gases and vapours is possible from all container types. A plastic composite IBC, manufactured with a static dissipative outer layer was tested and this demonstrated safe electrostatic discharge levels, however, the report does indicate that a representative sample would need to be tested to determine if these characteristics are consistent.
Just some, of a number of the report’s conclusions and recommendations, are listed below:
- “It is very important with all designs that the frame and any other conducting parts are electrically bonded to earth during any operation where electrostatic charging may occur and that they should not be stored on a highly insulating surface unless separately earthed”.
- The earth connection between the frame and conducting parts of taps should be checked at regular intervals.
- Exposed plastic components (e.g. taps and filling caps) should be made of static dissipative materials.
- Metal frames and conducting objects located on IBCs should be “electrically bonded to earth” with a sufficient charge relaxation time permitted.
- A thorough risk assessment should be carried out to determine the most appropriate type of container with a particular focus on electrostatic charging potentials and the presence of flammable gases and vapours in the IIA, IIB and IIC categories.
The loading and unloading of road tankers with flammable and combustible products, presents one of the most serious fire and explosion risks for site operations within the hazardous process industries. A study conducted by the API in 1967 identified static discharges as being responsible for over 60 incidents in road tanker loading operations demonstrating just how long this potential threat has been acknowledged. The natural presence of static electricity in product transfer operations, combined with its associated ignition hazards, ensures that regulators take static control precautions for road tankers very seriously.
Static electricity and road tanker product transfer operations.
Powders and liquids with low electrical conductivities are the prime sources of static charge generation because their electrical properties do not easily permit the transfer of excess charges. Instead, non-conductive and semi-conductive liquids and powders retain and accumulate charges after they make contact with conductive objects. The most common interface for charging of non-conductive and semi-conductive product is contact with metal plant equipment including pipes, filters, pumps, valves, barrels, IBCs, mixers, blenders and agitators.
When the electrostatically charged liquid (or powder) is deposited into a container like a barrel, IBC, or road tanker charging of the container will occur if there is nowhere else for the charges to go. In this situation the charges are “static”, accumulate on the surface of the container and set up a potential difference with respect to earth.
Fig. 1 Levels of voltage generated on a road tanker, by an electrostatically charged liquid at approved flow rates.
Over a short time period (less than 20 seconds) potentials in excess of 50,000 volts can be induced on a road tanker’s container when it is being filled at normal flow rates with a product that is electrostatically charged. The magnitude of the voltage induced is directly proportional to the quantity of charges making contact with the container.
This voltage represents the ignition source and the potential energy available for discharge via a static spark at voltage levels of 50 kV can, for a typical road tanker, be in excess of 1250 mJ. The vast majority of flammable vapours and combustible dusts can be ignited at these energy levels.
For sparking to occur in road tanker product transfer operations, other conductive objects must come into close proximity with the charged container of the road tanker. Examples of conductive “objects” include the fill pipe entering the opening on the top of the container, fall prevention systems like folding stairs, and drivers or operators working around the road tanker.
The charges on the road tanker’s container attract opposite charges to the surface of the object and rapidly create an electric field between their respective surfaces. It is the strength of this electric field that causes the “breakdown” of the air between the container and the object. When the air is “broken down” a conductive path for the excess charges to rapidly discharge themselves is created, leading to a static spark discharge. If a combustible atmosphere is present in this space, ignition of the atmosphere is very probable. Under ambient conditions an average field strength of 30 kilo-volts is capable of causing the electrical breakdown of air over a spark gap of 2 cm.
Fig 2. Potential minimum ignition energies present on road tankers based on the time period of road tanker filling operations.
Standards and recommended practice governing the static control of road tanker product transfers.
As outlined earlier, regulators are extremely cautious about the ignition hazards presented by static electricity in road tanker product transfer operations. Three standards, in particular, provide clear guidance on what precautions should be taken. NFPA 77, API RP 2003 and IEC 60079-32 state that earthing (grounding) of the road tanker should be the first procedure carried out in the transfer process. Grounding effectively creates an electrical circuit that connects the road tanker to the Earth and it is this connection to earth which prevents static charges accumulating on the road tanker’s container. The reason the charges can transfer from the road tanker to earth is because the Earth has an infinite capacity to absorb and redistribute static charges, with the positive effect of removing the ignition source from a potentially combustible atmosphere.
The electrical resistance of this circuit from the road tanker to the “earth source” (or “grounding point”) which is in contact with the earth, is a key performance indicator of the entire grounding circuit’s capacity to provide a secure and safe product transfer operation. NFPA 77 and API RP 2003 state the resistance in a healthy metal circuit should never exceed 10 ohms, therefore the entire circuit between the truck and grounding point should be measured and be equal to, or less than, 10 ohms. If a resistance above 10 ohms is measured this will indicate problems with parts of the grounding circuit including the road tanker connection, the earth source connection or the condition of the conductor cable.
You don’t need to be a rocket scientist to safeguard against the hazards of static electricity.
For any person responsible for the safety of employees, colleagues, plant equipment and plant property, one of the most potentially confusing aspects of providing a safe operating environment is trying to determine if that site’s manufacturing or handling processes have the potential to discharge static sparks into flammable or combustible atmospheres.
Static sparks contain enough energy to ignite flammable vapours and dusts.
Electrostatics is a detailed subject area that, for most of us, appears to be a black art accessible only to academics and experienced process safety consultants. Because static ignition hazards occur at the “nuclear level”, it is naturally difficult to visualise how and why static electricity is a hazard in industries where flammable and combustible products are regularly processed. There are so many variables that have a role to play in electrostatics, it is almost impossible to predict the net effects of these parameters, in a hazardous prevention context, without feeling the need to conduct controlled laboratory testing to determine if a specific process could produce incendive electrostatic discharges.
If you consider that a walking across a carpet can generate 35,000 volts (35 KV) on a person, it is easy to see how normal everyday processes can generate potentials well in excess of 10,000 volts (10 KV). For a small object like a metal bucket, which has a typical capacitance of 20 pico-farads, the total energy available for discharge at 10 KV is 1mJ. This is higher than most flammable vapour minimum ignition energies (MIE’s). Scaling up, the ignition energy available on a human, at 10 KV, would be around 10mJ. In powder conveying operations voltages of the order of 1000 KV can easily be generated on parts of the conveying system. Road tankers undergoing loading can carry as much as 2000 mJ of ignition energy.
It can be time-consuming, and expensive, to investigate and determine the level of voltage that can arise as a result of these charging mechanisms. Complicating matters further, ignitable electrostatic discharges can occur in many forms ranging from spark discharges, propagating brush discharges, bulking brush discharges, to corona discharges. The effort required to assess, determine and combine these variables into a cohesive audit of a potential hazard is, by no means, easy.
Which standards should I follow to control static electricity in ignitable atmospheres?
Fortunately, there are several internationally recognised standards that provide guidance on ways to limit electrostatic hazards enabling those responsible for worker health and safety minimise the risk of incendive static discharges. Hazardous area operators who can demonstrate compliance with these standards will go a long way to providing a safe working environment and preventing the ignition of ignitable atmospheres. The most comprehensive standards are:
NFPA 77: Recommended Practice on Static Electricity (2007).
Cenelec CLC/TR60079-32-1: Explosive atmospheres – Part 32-1: Electrostatic hazards, guidance (2015).
API RP 2003: Protection against Ignitions Arising out of Static, Lightning and Stray Currents (2008).
API RP 2219: Safe Operation of Vacuum Trucks in Petroleum Service (2005).
The standards, particularly NFPA 77 and CLC/TR: 60079-32-1, describe a range of processes where static charges can be generated including flow in pipes and hoses; loading & unloading of road tankers; railcar loading & unloading; filling and dispensing portable tanks, drums and containers; storage tank filling and cleaning; mixing, blending and agitation operations; the conveying of powders and other operations. The API RP 2003 standard focuses on road tanker loading and railcar filling operations, storage tank filling and general operations involving petroleum products. API RP 2219 provides detailed guidance on protecting vacuum trucks from electrostatic hazards.
These standards outline what factors can be identified and controlled to limit electrostatic hazards and these controls typically depend on:
- Preventing the accumulation of electrostatic charges on plant equipment, people and the material transferred.
- Controlling the process to minimise the generation of electrostatic charges.
NFPA 77 (5.1.10) states that the transfer of just one electron in 500,000 atoms is required to generate voltages with enough energy to ignite flammable atmospheres.
Effective earthing and bonding is presented in the standards as the primary means of protection from electrostatic hazards and is the most straight forward, secure and cost-effective means of ensuring static hazards are managed and controlled correctly. Eliminating the accumulation of static charges will eliminate the static hazard.
Vacuum trucks provide an important contribution to the transportation and recovery of flammable and combustible products within the hazardous process industries. Their efficiency and versatility means they can fulfill a broad array of duties ranging from the transfer of chemicals in manufacturing production, to removing waste deposits from storage tanks or performing hazardous material recovery at the site of road & rail traffic incidents.
Equally, truck deliveries within retail gas & petroleum distribution and the food & beverage industry require transportation to locations where earthing systems may not be installed or verified earthing points may not be present to ground the tanker while it is transferring material.
Vacuum truck used for site chemical transport and recovery
In the recovery and transportation of flammable & combustible products the generation and build of electrostatic charges can pose a significant hazard to personnel and equipment if correct static earthing precautions are not put into action. As you may have read in previous technical articles, the relative motion and interaction of different materials leads to the instantaneous combination and separation of positive and negative charges. If these charges do not have a means to dissipate from the objects or materials they come into contact with, i.e. flow to true earth (ground) or share charge with available opposite charges, they become “static” and raise the electrical potential difference of the object or material on which they are accumulating.
In essence, this potential difference is equivalent to a stored source of energy which is immediately seeking to discharge itself in order to return the object to a natural state of electrical equilibrium (0V). If the energy is allowed to discharge in an uncontrolled manner it will do so, in the majority of cases, in the form of an incendive electrostatic spark. Should such an event occur in the presence of a vapour or dust, when they are within their respective ignitable flammable and combustible thresholds, there is a high probability that ignition of the material will occur.
The potential energy stored on an object that can be released in the form of an electrostatic spark is equivalent to:
W=1/2 C ∙V2
The total energy available for discharge, (W), is equal to the product of the object’s capacity to store charge (capacitance, C) and the square of the voltage, (V), of the body. The voltage of the object is increased by the generation and accumulation of electrostatic charges. To illustrate, a small object like a metal bucket has a capacitance of around 20 pico-farads. If electrostatic charges are permitted to accumulate on the bucket, raising its voltage by just 10 kilo-volts, 1 mJ of spark energy can be discharged by the object.
1 mJ is capable of igniting the majority of flammable vapours and gases. In real world processes the larger charge storing capacity of equipment like tanks, hoses, lances and trucks (up to 5000 pico-farads), when combined with high potential differences caused by the rapid interaction of liquids and solids, can generate much more significant levels of stored energy ready for uncontrolled discharges.
Examples of recorded incidents caused by uncontrolled static ignitions:
(a) In 1998 an explosion, and one fatality, occurred when granular polypropylene was being vacuumed from a dust collector into a large vacuum truck. The cause of the explosion was a static spark that discharged from the lance to the dust collector. The cause was a non-conductive hose that was used to connect the lance to the vacuum truck.
Because the hose was non-conductive, instead of static charges flowing through the hose to the grounded / bonded truck, static charges accumulated on the metal lance, raising its potential difference relative to the duct collector. In order to equalize the potential difference of the lance, the static spark discharged to the dust collector, igniting the combustible atmosphere in the process.
(b) A fire in a toluene sump was caused when a static spark discharged from the conductive metal windings of a rubber hose to the metal rim of the sump. Although the conductive windings of the hose were bonded to the truck, the truck itself was not grounded. This caused static charges to accumulate on the windings of the hose, raising its potential difference relative to the sump.
The common denominator:
The common denominator for these incidents is that the rate of electrostatic charge generation on the components of the system were permitted to exceed the rate of charge dissipation resulting in the accumulation of static charges on some part of the transfer system.
The transfer system includes the lance, hose, hose connections, truck collection chamber and the chassis of the truck itself. To remove the risk of an incendive static spark discharges causing a catastrophic accident these components must be correctly bonded and grounded.
In 2006, the U.S. Chemical Safety Board published the findings of a major study outlining the scale and devastating consequences of combustible dust cloud explosions that have occurred between 1980 and 2005 in US chemical processing operations.
In that period 281 explosions were caused by ignitable combustible dust atmospheres, resulting in 199 fatalities and the injury of 718 workers. In the UK the Health and Safety Executive recorded 303 dust explosions over a nine-year period and German records demonstrate 426 similar incidents over a 20-year period.
During one 10-year period a single insurer listed a total of 450 incidents across their client base that were attributed to dust fires and explosions. The total cost of damages amounted to $580 million, with the average gross loss for dust explosions costing $1.9 million and dust fires costing $1.2 million.
Since their report was published the CSB has repeatedly requested that OSHA take more action to regulate the safety of operations processing combustible and flammable powders. The 2008 sugar refinery explosion at the Port Wentworth plant of Imperial Sugar should be a warning to a broad range of industries just how risky and relevant dust explosions are. Approximately 70% of all chemical processing industry operations handle powders in a combustible form at some point in their manufacturing process.
CSB study 1980 to 2005: sectors with recorded incidents of combustible dust fires and explosions.
Several contributing factors need to be present to support the ignition of a combustible dust cloud in a static hazardous area, comprising:
- A dispersed dust cloud-oxygen mixture that is above its Minimum Explosion Concentration (MEC).
- Physical containment of the dust cloud that will lead to rapid pressure build-up causing deflagrations out of process equipment and into open workspaces.
- A heat source with enough energy to ignite the combustible atmosphere.
The locations of primary deflagrations normally occur within process equipment such as dust collectors and blending machines. Secondary explosions result from a containment breach, with the primary deflagration propagating through conveying systems or through mechanical breaches in the processing machinery. Secondary explosions cause the bulk of devastating damage to workers, buildings and equipment by unsettling and igniting layers of dust that have collected on surfaces. A 1.6 mm layer of dust that gets dispersed from primary explosions is all that is required to initiate secondary deflagrations.
Three separate studies with collective data totalling 1100 dust explosions gathered in the US, UK and Germany highlight process equipment that have proven to be known sources of primary dust explosions. The main processes that suffer from explosions are dust collection, powder grinding and pulverising, powder conveying operations, silo and container filling and powder mixing and blending.
German study: recorded sources of ignition in combustible dust explosion incidents.
The German data, which totalled 426 incidents, provides a percentage breakdown of known primary sources of ignition. Electrostatic discharges make up 10% of known primary ignition sources. The “unaccounted” category accounts for incidents where no physical evidence (electrical or mechanical causes) has been detected. The prime suspect in the “unaccounted” category is very often electrostatic discharges, but as no witnesses can provide evidence of seeing or hearing a spark, ignition sources of this type go unreported and unaccounted for.
Even though the majority of combustible dusts have higher MIE’s than flammable vapours the amount of energy available from electrostatic discharges within contained environments will ignite the vast majority of combustible dusts. This is because the rate of electrostatic charge generation and accumulation in powder processing operations is extremely high.
Minimum Ignition Energy of explosive / flammable materials (Source IChemE)
The UK’s Health & Safety Executive, in collaboration with the Chemical Business Association (CBA) and Solvent Industry Association (SIA) has issued general guidance outlining the type of assessments that should be carried out to manage the risks associated with IBCs storing flammable and combustible material.
Of particular interest is the assessment for managing the risk of electrostatic ignitions. The HSE refers to the SIA’s notice No.51a which provides guidance on minimising the risk of incendive electrostatic spark discharges when storing solvents in IBCs.
Electrostatic hazards and IBCs
The risk of electrostatic discharges in potentially flammable or combustible atmospheres is well documented in best practice standards like Cenelec’s CLC/TR:50404 and NFPA 77. Although identifying static as a hazard is difficult to visualise, as it is not readily tangible or easily detectable, the underlying theory and safe practices that can be put into place are relatively straightforward.
The flow of any material in pipes, filters and fittings, whether the material is conductive or non-conductive, results in the separation of charges. The separation of 1 electron in half a million is all that is required to provide the right conditions for an incendive spark discharge to occur. Much the same way a spark plug works in the engine of a car, electrostatic discharges result from the existence of a spark gap. The spark gap only needs to be momentary and if a flammable or combustible atmosphere is present in the spark gap, the energy released can exceed the minimum ignition energy of the surrounding atmosphere. Uncontrolled spark discharges have enough energy to ignite the majority of flammable atmospheres.
When liquid entering an IBC has surplus charges attached to it, it creates an electric field which induces opposite charges on the inner wall of the IBC. If the IBC is not properly grounded, it will act like a capacitor plate in an electric circuit, accumulating charges on the outer surface of the IBC.
The accumulation of charges is now a potential ignition hazard as surplus charges are available to discharge to objects in the vicinity of the IBC in an uncontrolled manner. The commonest form of object charged IBCs will discharge to are grounded conductors like surrounding plant equipment, dip tubes, forklift trucks and, most commonly, the operator handling the IBC. What is of critical importance is that the IBC is conductive and has a low resistance static dissipative connection to earth. This will enable any surplus charges to flow immediately to ground from the hazardous area in a controlled manner. The standards, including the guidance issued by the SIA, categorically state this resistance must be less than 10 ohms and regularly checked to ensure the IBC is always capable of dissipating charges.
A connection resistance of 10 ohms or less ensures there is no doubt that the rate of charge dissipation exceeds the rate of charge generation and charge accumulation, allowing the static charges to be dissipated safely from the IBC.
It follows that the first thing an operator must do before filling or dispensing from an IBC is to ensure the IBC has a positive static dissipative ground connection.
There are also a number of additional factors that must be borne in mind when using IBCs. Filling flow rates and the conductivity of the liquid are especially important factors to consider. When the IBC is filled initially, a potential spark gap will be present between the end of the filling pipe and the surface of the liquid. The SIA guidance recommends 1 m/s until the fill pipe is covered by the liquid and a limit of 2 m/s thereafter. Splash filling must be completely avoided as this will encourage the separation of charges.
If the liquid is conductive charges can dissipate through the conductive wall of the ground connected IBC. If the liquid is low conductivity (<50 pS), the appropriate charge relaxation times should be incorporated into the handling process. NFPA 77 provides a comprehensive list of flammable liquid conductivities and their corresponding charge relaxation time periods.
How to avoid the real cost of non-compliance in Static Control
The overall cost of non-compliance can stretch far beyond potential “savings” achieved by ignoring the risk of electrostatic ignitions or by using non-compliant static control methods.
More often than not, static caused fires and explosions result in expensive production downtime, legal and insurance costs related to personnel injuries & fatalities and damage to company property. In numerous cases static caused fires have led to the pollution of the local environment resulting in the loss of public goodwill and the payment of heavy fines imposed by local government. Under ATEX, the European Union’s legislation which governs the safety of personnel working in hazardous atmospheres, everybody from suppliers to company directors are open to criminal prosecution if a court determines that adequate Best Practice procedures and equipment have not been used to protect workers.
Fortunately, there are three industry produced Best Practice standards that provide the background to the nature of static electricity, the processes that are susceptible to electrostatic ignitions and the preventative measures that should be put in place to eliminate static electricity as a health and safety risk.
The best practice standards are:
- Cenelec CLC/TR:60079-32-1 (2015): Explosive atmospheres – Part 32-1: Electrostatic Hazards – Guidance.
- NFPA 77 (2014): Recommended Practice on Static Electricity,(National Fire Protection Association).
- API RP 2003 Eight Edition (2015): Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents, (2003), (American Petroleum Institute).
The standards are produced by committees made up of industry experts in hazardous process safety and show remarkable consistency in the precautionary measures identified for controlling the generation of static electricity.
For example, in tank truck* transfers both CLC/TR: 60079-32-1 and API standards (NFPA references API for tank truck transfers) recommend:
1. The use of interlocks to stop flow of product preventing the generation of static if the truck loses its earth connection.
2. Monitoring the bonding/earthing circuit to less than 10 ohms and providing positive indication to operators that a positive bond/ground connection is established.
3. State the first operation in road tanker transfers is to apply a full earth connection to the vehicle.
The API standard goes a step further stating the earthing clamp should not be removed until the tank truck body is sealed, i.e. removal of the earthing clamp should be the final operation in the product transfer process.
As recommended in each of the standards the most effective method of eliminating spark gaps is to ensure all conductive and semi-conductive objects are bonded and grounded with fit for purpose static control equipment. The static control equipment should be capable of making low electrical resistance contact with charged equipment, combined with maintaining secure and reliable low resistance static dissipative circuits.
A good margin of safety can be assured by ensuring that static dissipative circuits and their connections are regularly checked for resistances greater than 10 ohms. The NFPA 77 and API standards state electrical resistances higher than 10 ohms in metal circuits are indicative of a break in the continuity of the circuit, resulting in the potential and undesirable accumulation of static electricity.
Earthing clamps connected via cables to identified earthing points are the established and proven method of preventing electrostatic discharges from movable or fixed items of plant in flammable atmospheres.
With some operations requiring hundreds of connections to be made and broken every day it is essential that a good electrical connection is made each and every time. The effectiveness, reliability and durability of any static earthing clamp and associated cabling is fundamental to keeping process operations safe from the dangers of an incendive static spark discharge.
It is common for process plant, associated containers, drums and IBC’s to build up layers of product or rust, or have surface coatings present. These layers can form an unpredictable insulating barrier that can easily defeat certain designs of clamps like alligator clips and other “in-house” methods of static earthing protection.
Hazardous area certified (ATEX / Factory Mutual) earthing clamps provide extra guarantees over alligator clips and welding clamps.
The importance of effective clamp design and its suitability for use in flammable atmospheres has not gone unnoticed by regulatory and approval bodies around the globe. Approved ATEX, static earthing clamps must meet specific criteria to be certified as suitable for use in hazardous areas. For example, earthing clamps made from aluminium must be anodized to prevent mechanical sparking under normal operating conditions if they are to be used in a Zone 0 or Zone 20 atmosphere. There are also limitations placed on the amount of plastic that may be used in the clamp body as this may enable the accumulation of static charge. Under an ATEX assessment earthing clamps are also assessed for sources of stored energy and their ability to cause a spark if the energy is released in the hazardous area. One potential source of energy in earthing clamps is the spring which has the potential to generate a mechanical spark through contact with other objects if its escapes the body of the clamp. Therefore clamps are assessed for their structural robustness to ensure any stored energy is reliably contained within the clamp.
US approval bodies such as FM Global assess several other design criteria regarded as critical for static earthing clamp performance.
For use in hazardous locations, the electrical resistance through the clamp, including through the contacts and the clamp body must not exceed 1 Ohm when attached to plant equipment. Additional tests ensure the clamp must be suitable for use in normal industrial conditions. To achieve FM approval earthing clamps must undergo, and pass, the criteria set by the following tests:
- Separation force testing: to ensure clamps are not easily or accidentally displaced during operations.
- Clamp pressure testing: to ensure the clamp contacts can penetrate connection inhibitors like rust, coatings and product deposits and make a positive connection to equipment requiring static earthing protection.
- Vibration testing: at varying frequencies to ensure that approved clamps guarantee positive and stable contact with vibrating and portable plant equipment.
- Have a maximum resistance through the clamp body of 1 ohm.
Typical markings to be found on an ATEX and/or FM approved clamps
Newson Gale Studies
Lab tests, designed to reflect real world operating conditions, were conducted to investigate the impact layers of protective coatings and adhesives can have on the ability of clamps to establish positive contact with strips of metal. Based on Factory Mutual static earthing clamp approval requirements, the benchmark clamp resistance test was set at 1 Ohm.
The tests showed some surprising results. Most notably, in the ‘Coatings Test’ even the thinnest layers (400 μm) provided a wide range of clamp resistance readings that varied based on clamp design.
The test indicated the highest levels of clamp resistance (upwards of 100 meg-ohm) were exhibited in clamps with varying combinations of high surface area contact and poor to good spring pressure – e.g. alligator clips.
The clamps that exhibited consistent positive values (less than 1 Ohm) combined low surface area contact with good spring pressure. Low surface area contact, achieved via sharpened teeth (typically manufactured from tungsten carbide or stainless steel) supported by good spring pressure, enabled penetration of the entire range of test coatings.