Low-Voltage and High-Voltage Circuit Breaker Testing
Low-Voltage and High-Voltage Circuit Breaker Testing covers mechanical condition, contact integrity, insulation, and trip performance. LV molded-case and power breakers are often tested for contact resistance, insulation resistance, and trip characteristics. HV breakers add timing, travel, coil current, and sometimes vacuum-bottle or SF6 checks.
Core rules
- Isolate, lock out, and verify de-energized before removing covers or connecting test leads.
- Measure contact resistance with a micro-ohmmeter or DLRO at the manufacturer-specified test current. A common DMM cannot resolve micro-ohms reliably.
- Perform insulation resistance tests phase-to-phase and phase-to-ground, then discharge.
- Use a breaker analyzer for close, open, and trip times, contact bounce, stroke, and coil current. Test at rated control voltage unless the procedure says otherwise.
- Compare results with nameplate, manufacturer data, and previous tests; a pass/fail number alone is weaker than a trend.
Worked example
A 1200 A LV power breaker shows contact resistance of 45 µΩ, 48 µΩ, and 92 µΩ on phases A, B, and C. The C phase is about twice the others. The technician repeats the test with clean contacts and good lead placement, then inspects for pitting, misalignment, or loose connections. The reasoning is that phase balance matters: a single high phase points to a contact problem even if the absolute value is below a generic limit. Trip-unit pickup and timing are tested separately.
Traps
Using a DMM for contact resistance; testing timing at the wrong control voltage; not recording ambient temperature; ignoring one high phase because the average looks acceptable; and failing to discharge after insulation testing.