Peterson Coil

A Peterson coil is an adjustable iron-cored reactor connected between a power system's neutral and earth to neutralize capacitive earth-fault current during a single line-to-ground fault. Also called an arc suppression coil or resonant grounding device, it is tuned so its inductive current matches the system's natural capacitive fault current, reducing residual current to help the fault arc self-extinguish and improve service continuity.

In a manufacturing or shop floor setting, a Peterson coil is part of the facility's medium-voltage distribution grounding scheme, feeding large plant switchgear, substations, or process buildings. It is installed at the transformer neutral or star point and tuned to the network's capacitance so first ground faults produce minimal current. This allows the plant to remain energized after a first ground fault long enough to locate and isolate the fault without an immediate full outage. Plant electricians must verify tuning, tap settings, and protection coordination, as detuning reduces arc-suppression effectiveness. The coil does not eliminate hazard; it changes fault behavior and can delay traditional protective clearing if not properly coordinated.

• ⚠️Incorrect tuning or detuning: If the coil is not matched to the actual system capacitance, residual fault current rises and arc suppression becomes ineffective.
• ⚠️Misunderstanding the protection philosophy: A resonant-earthed system may not trip immediately on the first ground fault, so crews can incorrectly assume the circuit is safe when it is still energized and faulted.
• ⚠️Poor coordination with fault detection and bypass schemes: Failure to maintain and test relays, CTs/VTs, or bypass breakers can prevent the system from transitioning to conventional grounding when a fault persists.