Core to earth and core to core, DC test

Cable Insulation Resistance Test Report Format, Free PDF

A one-page megger record for power and control cables, with a per-kilometre column so cables of different lengths can be compared, and a reference page for test voltage and minimum values.

  • Free
  • No sign-up
  • A4, 2 pages
  • Formulas and limits included
Preview of the Cable Insulation 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

  • Cable details: type and size, rated voltage, length, installation, conductor temperature and humidity
  • Nine ready test lines: each core to earth, core to core, neutral and armour or sheath, with 1 minute and optional 10 minute readings and PI
  • A per-kilometre column (IR × length in km) so you can compare a 100 m run with a 1 km run
  • Before-and-after checks, a result block and a sign-off
  • Page 2: test voltage by cable rating, BS 7671 LV minimums, what common patterns of readings usually mean, and method notes

How it was checked

The per-kilometre conversion and PI are simple ratios with a worked example. The BS 7671 minimums are 0.5 MΩ at 250 V and 1 MΩ at 500 V or 1000 V. The test voltage table and 1 MΩ per kV idea are common practice guides, so use your cable specification where it states a value.

Related

Common questions

What is the minimum insulation resistance for a cable?

For LV installation circuits BS 7671 asks for at least 1 MΩ at 500 V for circuits up to 500 V. That is a minimum for installations, and a healthy new power cable usually reads far higher, in the thousands of MΩ. Many sites also compare with the factory test and the previous test.

Why does insulation resistance fall with cable length?

A longer cable has more insulation in parallel, so more leakage paths. That is why the form includes a per-kilometre column, so different lengths can be compared fairly.

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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Need the instrument as well?

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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