Bonding and grounding is a critical safety function when transferring flammable liquids. But for the ground to function properly, how good must it be or in technical terms, how much resistance to ground is acceptable in the design of the safety system?
Various types of conductive grounding electrodes may be used, such as rods, plates, and wires which are sometimes used in combination to increase surface area contact with the earth. If the purpose of the grounding system is to dissipate static electricity, a total resistance of up to 1,000 Ohms in the ground path to earth is considered adequate. This may be measured using standard ground resistance testing instruments and is realistically and quickly achievable in most types of terrain and weather conditions. In different types of terrain and climatic conditions, it can be impossible to verify a proper ground according to the most commonly referenced standard (NEC Article 250) which is written to meet the requirements of electrical fault current grounding and lightning protection, both of which require a much lower “ground resistance” (<25 Ohms).
NFPA 77 (7.4.1.3.1) states that in order to dissipate static charges a “resistance of 1 Meg Ohm (10^6 Ohms) or less generally is considered adequate”, however with standard instruments typically used to verify a satisfactory ground, like Clamp-on Ground Resistance Meters, an elevated resistance at this level is impossible to measure – more specialized and costly equipment is required.
The proposed maximum of 1,000 Ohms is therefore seen as a realistic level which
1) can be achieved in most soil and weather conditions and
2) can quickly and easily be tested and verified
NFPA’s research in various types of terrain confirms that this level of ground resistance can be rapidly achieved and confirmed using commercially available test instruments.
7.4.1.3 To prevent the accumulation of static electricity in conductive equipment, the total resistance of the ground path to earth should be sufficient to dissipate charges that are otherwise likely to be present. A resistance of 1 megohm (106 ohms) or less generally is considered adequate.
7.4.1.3.1 Where the bonding/grounding system is all metal, resistance in continuous ground paths typically is less than 10 ohms. Such systems include those having multiple components. Greater resistance usually indicates that the metal path is not continuous, usually because of loose connections or corrosion. A permanent or fixed grounding system that is acceptable for power circuits or for lightning protection is more than adequate for a static electricity grounding system.
7.4.1.3.1.1 In field-based situations such as HAZMAT hazardous response operations or flammable/combustible materials spill control and transfer, it might be necessary to establish a temporary or emergency grounding system in a remote location in order to dissipate static charges. In such situations, various types of conductive grounding electrodes can be used, such as rods, plates, and wires, which are sometimes used in combination to increase surface area contact with the earth. If the purpose of the temporary grounding system is to dissipate static electricity, a total resistance of up to 1 kohm (1000 ohms) in the ground path to earth is considered adequate. This can be measured using standard ground resistance testing instruments and is realistically and quickly achievable in most types of terrain and weather conditions.