9th March 2020 - What Voltage for IR Testing?

Double The Voltage Mate?

I often get students telling me (in their most serious voice) that for insulation testing that you must use at least double the voltage that the cable is normally supplied with.  Most insulation testers can put out 250V, 500V or 1000V so if a cable was supplied with 400V then a test voltage of 1000V would be used.  Or so they say.

What Do The Standards Say?

The wiring rules says “The integrity of the insulation is stressed by applying a direct current of 500V for low voltage circuits” (pg 422 AS/NZS 3000:2018).  In Australia and New Zealand nominal voltages used are 230V for single phase and 400V for three phase.  Therefore we just use 500V for insulation tests.  And that’s it!

But Why Not Double The Voltage?

In the 1990’s we adopted the International Electrotechnical Commission (IEC) standards which says that for extra low voltage the test voltage is 250V.  Up to 500V the test voltage is 500V.  Above 500V the test voltage is 1000V.  So as you can see that according to the IEC standard for 230V and 400V we use 500V.

Can’t I Use Double The Voltage Anyway?

Um, no, probably shouldn’t.  If you look at your standard TPS cable you will see 450V/750V stamped on it which means that the cables insulation resistance is good for 450V from the core to the outer sheath and 750V between core to core.  So if you use 1000V between cores then you are exceeding the rated voltage of the cable (not really good for the cable).  There are cables that have ratings of 0.6kV/1kV but you are required by the wiring rules to use 500V regardless if it is three phase or not.

Summary

  • Double the voltage should be relegated to folklore.
  • Whether single phase or three phase it is 500V.
  • Be nice to us older electricians when explaining this concept of following the wiring rules.

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