Micro Ohm Meter for Circuit Breaker Contact Resistance Testing

Why Contact Resistance Matters

Circuit breaker contacts erode over time, accumulating oxide layers and pitting that increase electrical resistance. Even a few hundred microohms of additional resistance can cause severe heating at high currents, leading to nuisance tripping or catastrophic failure. Periodic contact resistance testing is mandated by IEEE 62 and many utility maintenance standards.

The Micro Ohm Meter (DLRO) Principle

A digital low-resistance ohmmeter (DLRO) injects a high DC test current - typically 10 A, 100 A, or 200 A - and measures the voltage drop across separate sense leads. This four-wire kelvin technique eliminates lead resistance, allowing measurements down to 0.1 microohm.

Step-by-Step Test Procedure

• Isolate and ground the breaker per safety procedures.
• Connect current leads to the breaker terminals; sense leads as close to the contacts as possible.
• Close the breaker fully.
• Inject the rated test current and record the voltage and resistance.
• Repeat on each pole of multi-phase breakers.

Acceptance Criteria

Manufacturer data sheets list typical resistance ranges - usually 30 to 200 microohms for medium-voltage vacuum breakers and 50 to 500 microohms for SF6 designs. A reading more than 50% above baseline justifies further investigation.

EK / ETCR Recommendation

Choose a DLRO with at least 100 A output, four-wire kelvin clamps, internal data storage, and PDF report generation for utility audit compliance.