Q: We were at 90% voltage and heard a sharp "Snap" (flashover) inside the tank. The system tripped. Do we have to open the GIS and fix it now?

A: Not necessarily. This is called a Self-Clearing Flashover.

The "Three-Strike" Rule (IEEE/IEC):

1. Strike 1: If a flashover occurs, discharge the system and restart the test.

2. Observation: If the second flashover occurs at a Higher Voltage than the first, it means the electricity "burned off" a small, sharp burr or a particle. The GIS is getting cleaner.

3. Strike 2/3: If the flashover happens at the Same or Lower Voltage, you have a real problem (a cracked insulator or a major structural defect). STOP the test immediately.

Verdict: A single flashover followed by a successful 1-minute hold at full voltage is usually acceptable per IEC 62271-203.

How do we choose the test voltage for an "on-site" test?

Q: My GIS is rated for 220 kV. Should I test it at the same voltage the factory used during their Type Test?

A: NO. Field test voltages are lower to avoid "Over-stressing" equipment that has already been shipped and handled.

The "80% Rule":

1. Factory Test: The GIS is tested at the manufacturer at 100% of the standard withstand level (e.g., 460 kV for a 220 kV system).

2. Field Test: We typically apply 80% of the factory test voltage (e.g., 368 kV).

3. Duration: Usually 1 minute at the 80% level.

Why 80%? It is a balance. It is high enough to find any assembly errors or transport damage, but low enough that it won't cause permanent aging of the insulators if there's a minor imperfection.