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Low-Resistance Measurement for Weld Quality — Verifying Bonds

2026年06月08日

A bad weld looks fine but reads high. Micro-ohm measurement proves the bond before it is buried.

The Weld That Looks Good But Isn't

Welded electrical bonds — exothermic (thermite) welds in earthing grids, rail bonds, and structural connections — must carry fault current reliably for decades, often buried where they cannot be inspected again. A weld can look perfect on the surface yet contain voids or poor fusion that raise its resistance. Low-resistance measurement with a micro-ohmmeter proves the bond electrically before it is covered.

1. Why Visual Inspection Is Not Enough

Porosity, incomplete fusion, and slag inclusions inside a weld are invisible from outside but add resistance. At fault-current levels, that extra resistance generates heat and can fail the joint when it is needed most.

2. The Four-Wire Method

  1. Inject a high test current through two outer leads.
  2. Measure the voltage drop across the weld with two inner leads.
  3. The instrument computes resistance, ignoring lead and contact effects.
  4. A good weld reads in single-digit micro-ohms.

3. Acceptance Criteria

  • Compare against the resistance of an equivalent length of the parent conductor.
  • A good weld adds little resistance over the conductor itself.
  • A weld reading significantly higher than the conductor indicates a poor bond.
  • Reject and re-make any joint exceeding the criterion.

4. Where This Matters Most

  • Earthing-grid exothermic welds in substations.
  • Rail traction return bonds.
  • Cathodic protection bonds.
  • Structural steel bonding in tall buildings.

5. Test Before You Bury

The whole point is to catch a bad weld while it is still accessible. Build the micro-ohm test into the work sequence immediately after welding and before backfill or concrete.

6. Documentation

  • Record each weld ID, measured resistance, and verdict.
  • Photograph the joint with the reading.
  • Attach the instrument calibration reference.

EK Recommendation

EK micro-ohm meters use the four-wire Kelvin method with selectable high test currents for weld, bond, and joint verification. Talk to EK service teams for an earthing-installation bundle.