During the test, I saw a large spark and the relay tripped. Was it a flashover or a breakdown - and how do I tell the difference?
Distinguishing flashover (external surface discharge) from breakdown (internal puncture) is critical because the corrective action is completely different.
Definitions (IEC 60060-1):
• Flashover: Discharge across the SURFACE of an insulator (external - insulation may be intact).
• Breakdown / Puncture: Discharge THROUGH solid or liquid insulation (internal - permanent damage).
|
Observation |
Flashover (External) |
Breakdown (Internal) |
|
Visible discharge path |
Along the bushing surface or air gap |
Through insulation - may not be visible |
|
Sound |
Loud, sharp "crack" (air ionization) |
Duller "thump" (through oil/paper) |
|
Arc location |
Visible between the live terminal and the ground |
Internal; no surface path visible |
|
Post-test IR |
Normal (insulation intact) |
Drops significantly (compromised) |
|
Post-test Tanδ |
Normal or slightly elevated |
Dramatically increased |
|
Post-test DGA |
No change |
C₂H₂, H₂ if puncture arcs through oil |
Decision tree:
1. Was the discharge path visible on the surface?
→ YES = flashover → clean, dry, re-test.
→ NO = could be internal. Measure IR → if low = breakdown.
2. Was discharge repetitive at the same voltage?
→ YES = internal defect (will repeat).
→ NO = random surface flashover (contamination).
Key practical rule:
• Flashover is a test setup failure, not equipment failure.
• Internal breakdown IS equipment failure - repair or replace.
• Always document: voltage at discharge, weather conditions, surface contamination.