Road tanker earthing systems.

The standards state that an earthing (grounding) system, which can measure and monitor resistance in the grounding circuit, can be utilised. The system should verify if the ground connection to the road tanker is complete before loading or unloading is initiated.  The precautionary guidance regarding the grounding of road tankers in IEC 60079-32 reflects the IEC’s stated goal of providing the latest state of the art guidance.  It states that the “earth cable” earthing the road tanker should be part of a static grounding system that continuously monitors the resistance between the road tanker and the designated grounding point located on the loading gantry. It states that the grounding system should be interlocked with the transfer system to shut down the product transfer operation if this resistance exceeds 10 ohms. It also states that the grounding system should be capable of recognising when it is not connected to the chassis/tank of the road tanker. This ensures that situations where the tank of the road tanker is not connected to the grounding system, for example, where an operator could connect the clamp to an isolated metal mud-guard or wheel-nut, will not result in a permissive condition for the transfer operation, thereby eliminating the risk of electrostatic charging of the road tanker.

Road Tanker Truck

Dedicated tank truck grounding system which continuously monitors the  connection to the tank truck and the site’s verified grounding point.

An additional recommendation in NFPA 77 and API RP 2003 calls for interlocking the feed system (e.g. pump) with the grounding system so that if the grounding system is not connected to the road tanker, product cannot be transferred. This will ensure that electrostatically charged product cannot enter or leave the road tanker when the road tanker has no grounding protection in place. In general, interlocked grounding systems will complete the grounding circuit when the driver connects the clamp of the grounding system to the road tanker and the system sees a circuit resistance of 10 ohms or less.

Although the standards recommend a monitored resistance of 10 ohms, there are many grounding systems on the market today that monitor well in excess of this level. While it may be claimed that these systems are capable of dissipating static charges the capacity of a system to monitor at 10 ohms, not only provides an opportunity to demonstrate compliance with internationally recognised recommended practice, it also means that hazardous area operators know the system’s grounding clamp is making a secure and reliable connection to the road tanker, every time a product transfer is carried out. Grounding clamps should be designed to penetrate paint coatings, rust and general dirt build up as they are very effective at impeding secure electrical contact with the conductive metal of the road tanker.

Additionally, the grounding system must be capable of detecting minute changes in resistance when the transfer is underway and should not allow a high degree of change in resistance before shutting down the pump or alerting personnel. As soon as a resistance above 10 ohms is present in the grounding circuit, the grounding system should be capable of detecting this change and take control of the feed into the road tanker. Systems that permit resistances higher than 10 ohms have a greater degree of difficulty in detecting changes in the health and condition of the grounding circuit.

Road Tanker Recognition.

Because resistance monitoring systems operate when connected to conductive metal objects, additional features can enhance the protection of drivers, product and equipment.  A “road tanker recognition” feature can be utilised to ensure that drivers can only operate the feed system when the grounding system detects it is connected to a road tanker. A system like the Earth-Rite RTR will analyse the capacitance of the road tanker as part of the grounding circuit. If the capacitance presented is in the normal range for road tankers (1 nano farad or 1 x 10-9 farads), the grounding system will recognise that it has made a positive connection to a road tanker.

From the site operator’s perspective, this eliminates the risk of drivers unknowingly connecting the grounding clamp to parts of the truck chassis that are electrically isolated from the truck’s container. This isolation may be due to original design oversight like isolated mud guards or paint coatings insulating conductive parts like truck light enclosures from the chassis. In addition drivers have been known to attach the grounding system’s clamp to the loading rack in order to obtain a permissive state for the feed system to “speed up” the transfer. So while a permissive state for the feed system can be obtained with a standard resistance based monitoring system it does not necessarily mean the grounding clamp is electrically connected to the road tanker’s container. Specifying a grounding system with a road tanker recognition feature ensures the road tanker is safely grounded before drivers are in a position to begin filling it with product. Once the system has verified it is connected to a road tanker it should then monitor the road tanker’s connection to the grounding point to 10 ohms or less.

The Earth Source.

When a road tanker grounding system is installed it is assumed that the earth source (e.g. buried ground electrode) to which the system is connected has been independently verified as having a low resistance connection to earth. This connection is the foundation for secure and safe transfers and it is incumbent on the site operator to conduct seasonal “Fall of Potential” tests to ensure these ground connections do not deteriorate due to changes in soil composition, soil resistivity or corrosion of the ground electrode.

In winter, ground temperatures can reduce dramatically and cause an exponential increase in soil resistance levels. For the ground electrode these temperatures can have a significant impact on its contact resistance with the soil potentially impeding the transfer of static charging currents.

Standard grounding systems are not designed to verify this connection; however, the patented Earth-Rite RTR can remove this uncertainty. This system has a unique feature which verifies that it is connected to a grounding point which is capable of safely dissipating static charges to ground. In combination with road tanker recognition capability, this static “ground verification function” ensures that two vital connections in the grounding process are securely made before product is allowed to enter or leave the vehicle.

When both of these connections are confirmed, the system will continuously monitor the resistance of these connections at 10 ohms (or less) for the duration of the transfer process. Should either connection be opened during the transfer, the system will detect this and switch off power to the pump or valve actuators in order to stop the feed of charged liquids into, or out of, the road tanker.

Summary.

In accordance with the recommendations of industry groups and fire safety associations, the static earthing of road tankers is a key safety protocol in the loading or unloading of flammable and combustible products. Earthing ensures static charges are not permitted to accumulate on the road tanker eliminating the risk of the container becoming an ignition source. Additionally, national and international recommended practice advocates the adoption of static grounding parameters that will enhance the safety of the product transfer process including monitoring the grounding circuit to 10 ohms or less and interlocking the product feed system with a dedicated grounding system.

When selecting road tanker earthing systems, specifiers should also consider additional functions that can enhance the safety of the transfer process. Earthing systems which include road tanker recognition and static ground connection verification functions provide additional guarantees that a transfer process cannot take place unless the road tanker is connected to the grounding system and the grounding system itself is connected to a verified ground source. These features enhance the secure earthing of the road tanker and enable hazardous area operators demonstrate the highest levels of compliance with IEC 6009-32-1, NFPA 77 and API RP 2003.

Mike O'Brien

Author Details:
Mike O’Brien, Managing Director for Newson Gale

If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.


‹ Back to Knowledge Centre