Circuit Breaker and Switchgear Testing
Circuit Breaker and Switchgear Testing
This domain tests contact resistance, insulation, overpotential, timing, travel, trip/close coils, minimum pickup, and interlocks on breakers and switchgear.
Core rules
- Contact resistance is a micro-ohm measurement; compare phases and previous data. A rising value suggests pitted contacts or loose connections.
- Perform insulation resistance and overpotential tests at the specified voltage for the breaker class.
- Time close, open, and trip operations; compare to manufacturer curves and nameplate.
- Verify trip-coil minimum pickup, usually at or below 70% of rated control voltage, and check close coil operation.
- Vacuum bottles: use a vacuum integrity test where specified; SF6 breakers require gas density and moisture checks.
Worked example
A 125 V DC trip coil picks up at 82 V. Pickup percentage = 82/125 x 100 = 65.6%, which is below the 70% maximum and acceptable. The reasoning: the coil must trip the breaker at the lowest expected control voltage, so a pickup above 70% could fail to trip during a battery low-voltage condition.
Traps
- Testing only one close-open operation and missing a late trip.
- Using the wrong overpotential voltage for the class.
- Ignoring contact-resistance trends because the value is within range.
- Not checking both local and remote trip circuits.