Circuit Breaker and Switchgear Testing
Circuit Breaker and Switchgear Testing covers timing, contact resistance, motion, insulation, and interrupting-medium checks. The exam tests whether you can separate a contact problem from a mechanism or control problem.
Core rules
- Contact resistance is measured with a micro-ohmmeter or DLRO at a current high enough to break films; compare to nameplate or a previous baseline.
- Timing tests record open and close times, pole discrepancy, bounce, and travel. Slow operation can indicate linkage friction, weak springs, or trip-coil problems.
- Minimum trip voltage verifies that the coil operates at the specified percentage of rated control voltage.
- Vacuum bottles are checked for integrity; SF6 breakers require density, dew point, and leak checks.
- Insulation tests are performed on primary and secondary circuits with the breaker isolated.
Worked example
A 15 kV vacuum breaker has a nameplate contact resistance of 30 µΩ. Measured values are 32, 34, and 45 µΩ. The first two are within a typical 20% band; the 45 µΩ pole is 50% above nameplate. Reclean the contacts and retest. If resistance remains high, inspect the contact assembly and vacuum bottle. Do not adjust timing to compensate for a high-resistance contact; the root cause is mechanical or contact-related.
Traps
Using a low-current ohmmeter that cannot break oxide films; comparing contact resistance without temperature correction; testing timing without rated control voltage; ignoring pole discrepancy; assuming a vacuum bottle is good because the breaker is quiet.