Why Commissioning Tests Matter
A breaker can look mechanically sound and still fail to interrupt correctly under fault conditions if contact resistance is too high, timing is off between poles, or insulation has been compromised during transport/installation. Commissioning tests catch these issues before the breaker is ever asked to clear a real fault.
Core Test Sequence
- Insulation resistance (IR) test — megohmmeter test between phases and to earth, both with the breaker open and closed, confirming insulation hasn't been damaged and there's no contamination-driven leakage path.
- Contact resistance (micro-ohm) test — measures resistance across each closed pole's primary contacts; a reading above the manufacturer's specified threshold points to worn, misaligned, or dirty contacts that will generate excess heat under load.
- Timing test — measures open and close times, and critically, the timing difference between poles (pole discrepancy). Poles that don't operate within the specified window of each other stress the mechanism and can affect protection coordination.
- Dynamic contact travel/velocity test — checks contact travel distance, overtravel, and speed during operation against manufacturer specification, since these directly affect interruption performance.
- Vacuum integrity test (VCBs specifically) — a hi-pot test across the open contacts of each vacuum interrupter confirms the vacuum hasn't degraded, since a compromised vacuum interrupter can't safely interrupt fault current even if it looks and operates normally mechanically.
- Protection relay functional test — secondary injection testing confirms the relay actually trips the breaker at the correct pickup and time settings, and that the trip signal path (relay → trip coil → mechanism) works end to end.
- SCADA/control wiring check — confirms remote open/close commands, position indication, and alarms all report correctly before the breaker is relied upon for remote operation.
Documentation
Every reading should be logged against the manufacturer's acceptance criteria and kept as the baseline for future maintenance comparisons — a contact resistance reading only tells you something useful when there's a prior reading to compare it against.
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