True or False: Protective grounds must be large enough to carry fault current long enough to trip out the circuit.

• A. True
• B. False
• C. Not applicable
• D. Sometimes

Answer: (D)

The protective grounding conductor is there to give fault current a low-impedance path back to the source so the protective device can trip and clear the fault. Because of that, we size the ground path to be able to carry the expected fault current for the time it takes the protection to operate, so the insulation and equipment aren’t damaged while the fault is being cleared. But this isn’t an absolute rule in every situation. In some protection schemes the device trips so quickly, or other protective measures like ground-fault protection trip on leakage rather than relying on large fault current through the ground, that the ground conductor isn’t carrying substantial fault current for long. In those cases, the requirement to carry fault current long enough to trip isn’t the controlling factor, so the statement isn’t universally true; it’s sometimes true depending on the protection method and the fault current available.