Why is Insulation Test Voltage Set to 500V?

I would guess that its to do with the peak mains voltage plus a bit.....

For 240 Volts peak voltage is about 380 Volts (I think, no calculator here!!), so 400 Volts would be too close to the maximum supply voltage, 500 Volts would cater for most eventualities, anything more than that risks damaging the insulation that you're testing...

At least that's what I would think is the reason?

John.

Back in the mists of time I had to do some of this testing. I believe that the reason for the voltage was that the relevant standard stated to test at that particular voltage. My memory (admittedly poor) says that the test voltage was 600V. A check of the ASTM standards would probably shed some light on this.

Of course it was 10% + to peak value of 220/240 V supply. The very 1st Insulation Tester was from Megger operated manually.

Later low voltage Insulation Testers were introduced, 200V, 100V versions for testing components of lower wkg volts on rapid developement in Electrical & Electronics.

Latest are the fully electronic Insulation Testers having multy test volts