Contact Resistance Testing of Circuit Breakers — Micro-Ohm Method

The Contact That Quietly Degrades

Every time a circuit breaker interrupts a fault, its contacts erode a little. Over years of operations they pit, oxidise, and lose contact pressure, raising their resistance. A high-resistance contact runs hot, accelerates its own decline, and in the worst case fails to carry or interrupt current. Contact resistance testing with a micro-ohmmeter quantifies this wear at routine maintenance, long before it becomes dangerous.

1. Why a Micro-Ohmmeter

A healthy breaker contact measures tens to low hundreds of micro-ohms. Only a four-wire (Kelvin) micro-ohmmeter injecting a high test current can resolve that range and ignore lead resistance.

• Four-wire method eliminates lead and probe resistance.
• High test current (10 A or more) mimics service conditions.
• Resolution in single micro-ohms.

2. The Test Procedure

1. Isolate, lock out, and earth the breaker.
2. Close the breaker contacts.
3. Connect current leads outboard, potential leads inboard of the contact.
4. Inject current and read the resistance across the contact.
5. Repeat for each pole.

3. Interpreting Results

• Compare against the manufacturer's maximum contact resistance.
• Compare poles against each other — one pole much higher is suspect.
• Compare against the previous test — a rising trend signals wear.
• A reading 50% above the others or the limit warrants service.

4. Common Causes of High Readings

• Contact erosion from repeated fault interruption.
• Oxidation on contacts that rarely operate.
• Weak contact springs reducing pressure.
• Loose terminal connections (test those separately).

5. After Service

After cleaning or replacing contacts and re-torquing terminals, re-measure. Resistance should return to the expected range. Record before and after values in the maintenance log.

EK Recommendation

EK micro-ohm meters use the four-wire Kelvin method with selectable test current for breaker contacts, busbar joints, and weld quality. Combine with a thermal survey for a full switchgear health assessment. Talk to EK service teams.