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Grounding and Bonding - what are the key benchmarks?
The earth has an infinite capacity to absorb charges and “earthing” (grounding) is the act of connecting a body to an electrode (or other buried structure) that has a verified contact resistance to the ground, typically less than 10 ohms. The U.S. National Electrical Code states that 25 ohms is the maximum acceptable resistance to ground. Grounding provides a path for static charges to rapidly flow to earth, reducing the voltage of the object to zero and thereby eliminating the presence of an ignition source. “Bonding” connects objects so that they are at the same electrical potential preventing discharges when they are positioned in close proximity to each other. If bonding is carried out, it is important to ensure that one of the bonded objects is connected to earth, thereby ensuring all parts of the bonded system are at zero potential.
Static Hazard = situation where the rate of charge accumulation exceeds the rate of charge dissipation |
Given that grounding is the primary source of static hazard prevention it is important to understand what parameters can be indentified as providing a satisfactory level of protection. The key to static hazard protection is ensuring that the path between the charged object and earth is of a sufficient quality to dissipate the static charges safely and rapidly.
The majority of plant equipment at risk of static charge accumulation is made of metal. Metals are excellent conductors and the natural resistive properties of metals ranging from copper through to steel means that electrical resistance to the transfer of charges from the body is low, provided that the body has good contact with earth. If the metal body is not earthed, this positive characteristic can quickly become a negative as isolated metal conductors are the primary source of static spark ignition hazards.
|
Material |
Typical Volume resistivity |
Resistance to charge transfer |
|
Copper |
1.7 x 10-8 Ω.m |
Low |
|
Steel |
4.52 x 10-7 Ω.m |
Low |
|
Carbon |
10 x 10-8 Ω.m |
Low |
|
Glass |
1 x 1010 Ω.m |
High |
|
Polymers |
1015 to 1022 Ω.m |
High |
To illustrate, a 10 m length (32 feet) of 2 mm diameter steel cable, in good condition, should have an overall resistance approximating to 1.44 ohms over its entire length (see table top right).
The maximum value of resistance present in metal circuits, which includes the body at risk of static charge accumulation, should be equal to or less than 10 ohms and is the benchmark value of resistance recommended by all four standards. If a resistance of 10 ohms or more is detected then there is a likelihood that the grounding circuit has been compromised and should be checked for corrosion or breakages.
|
2mm diameter cable |
25 metres |
10 metres |
5 metres |
|
Copper |
0.13 ohms |
0.05 ohms |
0.027 ohm |
|
Steel |
3.6 ohms |
1.44 ohms |
0.72 ohms |
Resistance values for a range of cable lengths
|
|
NFPA 77 |
API 2003 |
API 2219 |
CLCTR 50404 |
|
Metal Circuits |
10 ohms |
10 ohms |
10 ohms |
10 or 100 ohms |
|
Type 'C' FIBC |
must be grounded |
no reference |
no reference |
1 x 108 ohms |
Resistance values recommended by the standards for static earthing and bonding circuits
The table above outlines the maximum resistance levels for static dissipation circuits recommended by the standards for static control in potentially ignitable atmospheres. It is important to ensure that the static dissipative path, the path that channels the charging current to earth, is 10 ohms or less, and stays that way for the duration of the process.
How to Audit your processes for static hazards:
Figure 6.1.2 in NFPA 77 provides a decision tree flow chart which helps define a simple and effective way to help decide whether or not conductive objects should be earthed. It shows that the first step in an audit is to determine if there “is the potential to create an ignitable mixture”. If there is a potential for this to occur the next step states “bond and ground all conductive equipment”. There are further steps that query whether or not “electrostatic energy” can be generated and accumulate. As stated earlier, the process of determining these factors can be time consuming and require the expertise of process safety consultants. Very often, it is more cost-effective to earth the object, particularly if it is made of conductive metal, when it is known that materials with different properties come into contact. In order to provide a basic audit of processes NFPA 77 (Fig. 6.1.2) lists the following scenarios where charge can be generated:
Can charge generate?
If YES, can charge accumulate?
Does process include:
• Flow of material?
• Agitation or atomization?
• Powders or solids?
• Interaction with personnel?
• Filtration?
• Settling?
• Bubbles rising?
Does process include:
• Insulated equipment?
• Insulating materials?
• Isolated conductive equipment?
• Interaction with personnel?
• Nonconductive liquids?
• Mists or clouds?
When the answer to these questions is “YES”, it states that the potential MIE should be calculated to determine if it exceeds the MIE of the atmosphere present. This will probably be the hardest thing to calculate so the best advice is to earth the equipment as there may not be an opportunity to change the material being processed or the equipment, through which it is pumped, conveyed or handled.