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Busbar Joint Resistance Measurement — Preventing Hot-Spot Failures

2026年06月08日

A loose busbar joint is a fire waiting to happen. Micro-ohm measurement finds it before thermal imaging does.

Why Busbar Joints Fail

Busbars carry the largest currents in any switchboard, and every bolted joint is a potential weak point. A joint that loosens with thermal cycling, or corrodes at the contact face, develops a small extra resistance. At hundreds or thousands of amps, even a few extra micro-ohms generate enough heat to char insulation and eventually cause a flashover. Busbar joint resistance measurement with a micro-ohmmeter catches the problem while it is still measured in micro-ohms, not in smoke.

1. Why a Multimeter Cannot Do This

A good joint measures single-digit micro-ohms. A multimeter's lowest range is milli-ohms at best, and its test current is tiny — far too coarse to distinguish a good joint from a marginal one. You need a four-wire (Kelvin) micro-ohmmeter that injects a high test current.

2. The Four-Wire Method

  1. Two current leads inject a known high current (typically 10 A or more).
  2. Two separate potential leads measure the voltage drop across the joint only.
  3. This eliminates lead and contact resistance from the reading.
  4. The instrument computes resistance from V and I directly.

3. What to Measure and Compare

  • Measure each bolted joint individually.
  • Compare against the manufacturer's maximum joint resistance, or against a known-good joint of the same type.
  • A joint reading more than 50% above its neighbours is suspect.
  • Record values for trend comparison at the next outage.

4. Common Causes of High Readings

  • Under-torqued or loosened bolts.
  • Oxidised aluminium contact faces without joint compound.
  • Missing or degraded Belleville washers.
  • Dissimilar-metal corrosion at copper-aluminium interfaces.

5. Fix and Re-Test

After re-torquing to specification and cleaning the contact faces, re-measure. The resistance should drop to the expected single-digit micro-ohm range. Document before and after values.

EK Recommendation

EK micro-ohm meters use the four-wire Kelvin method with selectable test currents for busbar, breaker contact, and weld-joint measurement. Pair the data with a thermal survey for a complete switchgear health check. Talk to EK service teams.