Why Measure It
Every closed contact pair in a breaker has a small electrical resistance. If contacts wear, oxidise, or loosen, that resistance rises, and under load it turns into heat. A micro-ohm measurement puts a number on it.
How the Test Works
A micro-ohmmeter passes a DC test current, commonly 100 A or more, through the closed main circuit of each pole and measures the voltage drop, then calculates resistance in micro-ohms. Testing is done with the breaker closed, isolated, and earthed per site safety rules.
Reading the Results
- Compare each pole with the manufacturer limit or the commissioning baseline.
- Compare the three poles with each other - a pole noticeably higher than the others deserves investigation.
- Track readings over time, since a rising trend matters even if the value is still within limits.
What a High Reading Suggests
Common causes are worn or contaminated contacts, loose primary connections, or insufficient contact pressure. Cleaning, tightening, or replacing the affected parts is the usual fix. Pair the result with a thermal imaging survey for extra confidence.
See the full test checklist. For contacts and pole parts, browse VCB spares or send an RFQ.