Before personnel protective grounds are selected, the following criteria shall be met for their use, size, and application.

1. A grounding cable shall have a minimum conductance equal to #2 AWG copper.

2. Grounding cables shall be sized large enough to carry fault current long enough for the protective devices to sense and the circuit breaker to clear the fault without damage to cable insulation. An example would be a 4/0 Neoprene-insulated welding cable that will pass 30,000 A for 0.5 sec without melting its insulation.

Figure 4-9. Equipotential personnel protective grounds are used to protect electrical workers while they service, repair, or are close to circuits that can be accidentally reenergized.

3. The following are factors that contribute to adequate capacity:

a. Terminal strength depends on the ferrules installed on the cable ends

b. Cross-sectional area to carry maximum current without melting

c. Low resistance to keep voltage drop across the areas in which personnel are working at a safe level during any period to prevent reenergization. The voltage drop should not exceed 100 volts for 15 cycle or 75 volts for 30 cycle clearing times.

d. Verify that the grounding cable and clamp assembly is tested periodically by using the millivolt drop, micro-ohm meter, AC resistance, or DC resistance test methods. For example, if it is desired to maintain a maximum of 100 volts across a worker whose body resistance is 1000 ohms, during a fault of 1000 amperes, a personnel protective ground resistance of 10 millohms or less is required.

4. For further information on the construction of personnel protective grounds, refer to ASTM F855-90, IEEE 524A, IEEE 1048, and Section 7.5.

4.15.3 GROUNDING CLAMPS

Grounding clamps used in personnel protective grounds are manufactured specifically for this use. The size of grounding clamps shall match the size of conductor or switchgear bus being grounded.

The ground clamp also shall be rated to handle the full capacity of the available fault currents. Fault currents can typically range in magnitude up to over 200,000 A.

4.15.4 SCREW-TIGHTENING DEVICES

Approved screw-tightening devices designed for the purpose of pressure metal-to-metal contact are required for connections to an adequate system ground.

4.15.5 GROUNDING CABLE LENGTH

Grounding cables should be no longer than is necessary, both to minimize voltage drop and to prevent violent movement under fault conditions. For example, as a general rule, grounding cables should not exceed 30 ft for a transmission line and 40 ft for substation use.

4.15.6 GROUNDING CABLE CONNECTION

Grounding cables shall be connected between phases to the grounded structure and to the system neutral to minimize the voltage drop across the work area if the circuit should become inadvertently reenergized. Workers shall install the ground end clamp of a grounding cable first and remove it last.

4.15.7 CONNECTING GROUNDING CABLES IN SEQUENCE

Grounding cables shall be connected to the ground bus, structure, or conductor first, then to the individual phase conductors. The first connection of the grounding cables to the circuit phase conductors shall be to the closest phase of the system and then to each succeeding phase in the order of closeness.

4.15.8 REMOVING PROTECTIVE GROUNDS

When removing personnel protective grounds, reverse the order they were applied to the phases. The grounding cable conductors attached to the ground bus, structure, or conductors shall always be removed last.

4.15.9 PROTECTIVE APPAREL AND EQUIPMENT

Protective apparel shall be worn when applying or removing grounds. An insulating tool (hot stick) shall be used to install and remove grounding cables. (See 29 CFR 1910.269(n)(6) and (7), 1926.954(e) and NESC Rule 445).

Protective apparel (personnel protective equipment) should include at least the following:

1. Safety glasses and, if necessary, face shield appropriate for fault currents available.

2. Hardhat (Class B) (See 2.12.3)

3. Appropriate electrical gloves and protectors (See 2.12.3).

4. Appropriate clothing (See note in 2.11.3).