Ferro-resonance can generate overvoltages of up to 12 times line-to-ground source voltage upon opening of a single-phase device or a poorly synchronized three-phase device. Violent failure can occur, exposing personnel to the high-voltage failure and accompanying conditions. Ferro-resonant conditions can result in damage to lightning arresters, switching devices, buried cable, transformers, and associated equipment.

Ferro-resonance can be initiated when all of the following elements are present and the switching means at dip point or takeoff is either a single-phase device or an unsynchronized three-phase device that does not operate all phases within 1/2 cycle:

1. System grounded at the source but with no ground at the transformer bank, such as a transformer or transformer bank connected delta on a grounded-wye system.

2. Shielded cable or overhead conductor length sufficient to create the capacitance necessary.

3. Transformer size that permits saturation of the iron core at the operating voltage

4. Transformer unloaded or very lightly loaded.

Prevention or control of ferro-resonance may be accomplished by any of the following measures:

1. Using a wye-wye transformer connection with both neutrals grounded and tied to the system neutral.

2. Using only phase-to-neutral (not phase-to-phase) transformer connections for single-phase transformers.

3. Limiting length of underground cable between transformers and single-pole or poorly synchronized three-pole switching devices.

4. If single-pole or poorly synchronized switching devices must be used, ensuring that transformer and underground cable are loaded in excess of 2% resistive load of the transformer capacity.

5. If transformer primary is ungrounded-wye, temporarily grounding the neutrals of the transformers being switched.

6. Installing close-coupled, high-speed, three-pole switching devices to minimize the duration of the single-phase condition during opening and closing of the circuit.