Four-wire Kelvin low-resistance measurement

Micro-Ohm Contact Resistance Test Report Format, Free PDF

Ten test lines for joints, lugs and contacts, with correction to 20 °C, percentage change against a baseline and a phase comparison box.

  • Free
  • No sign-up
  • A4, 2 pages
  • Formulas and limits included
Preview of the Micro-Ohm Contact Resistance Test Report form
Download the PDF formA4, 2 pages

Print it at 100 % on A4. Page 1 is the form you fill in. The next page is the worksheet and reference.

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What is inside

  • Equipment details: panel, rated current, conductor material, temperature, instrument and test current used
  • Ten test lines with measured value, value corrected to 20 °C, baseline, change in percent, maximum allowed and result
  • A phase comparison block with deviation from the lowest phase
  • Formulas for copper and aluminium correction, change against baseline and joint heating (P = I²R), with worked examples
  • Guide bands, common causes of a high reading and method notes on four-terminal connection and test current

How it was checked

Temperature corrections use the usual coefficients of 0.00393 for copper and 0.00403 for aluminium near 20 °C, and the worked example was checked. The deviation bands are commonly quoted practice, not limits from a standard. For circuit breaker contacts, standards usually call for a DC test current of 100 A or more, so check that your instrument is suitable for the item.

Related

Common questions

What is a good contact resistance for a busbar joint?

There is no single value. The reading depends on size, material and design, so compare with the factory or last test, or with similar joints in the same run. A reading much above the others is the warning sign.

Why use four wires to measure micro-ohms?

A four-terminal (Kelvin) connection puts the current leads outside the voltage leads, so the resistance of the leads and clips is not included in the reading. That is essential when the value you want is in micro-ohms.

Using the form

  • Limits and constants come from the standards named above, as commonly quoted. Check them against the edition you work to, and use the stricter of those and your specification.
  • Where a value is a common guide rather than a limit from a standard, the form says so.
  • This is a calculation aid for qualified people. It does not replace the standard, the instrument manual or your site safety procedure.

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CIE supplies insulation testers, earth testers, oil BDV testers, micro-ohm meters and relay test sets, with test certificate available on request.

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