Question

[Clever]

Hi everyone,
My name is Gerson, and I reside in Namibia - Swakopmund. I’m a qualified installation electrician, and I’m currently working on obtaining my MIE. At present I’m employed as a training officer, Electrical at the Namibia Institute of Mining and Technology (NIMT), which is one of the leading FET colleges in Namibia. I’m also an Electrical Engineering student at UNISA, Stream – POWER.

Can someone please tell me the following:
When carrying out insulation resistance test on a three phase motor, what is the test voltage one should use?

We normally use a scale of 1000V, someone tells we've been doing it wrong and the test voltage should be 500V regardless whether the motor is a single phase 230V or three phase 380 V

I know what regulation 8.7.8.1 states, but now i'm a little confused.

[AndyD]

Depends on what the reason for doing the IR test in the first place ie is it for predictive planned maintenance or fault finding. Also could depend on the motor itself and on the method of supply ie whether there's a VFD or whether DOL. Obviously it would also depend on the motor supply voltage ie is it an LV or MV motor? To complicate matters further many facilities and/or companies also have their own testing requirements that are set in stone regardless.

Most insulation testers use a DC test voltage. If the motor is supplied with 400VAC DOL for example then the peak waveform voltage is around 566V so I'd say the IR test voltage needs to be as high or higher than that value to be meaningful. Depending on the instrument being used the next test voltage available would often be 1kV so therefore that's what I'd use. The regs require a test voltage of double the supply voltage so this would also suggest a test voltage of 1kV.