Megger testing. What voltage?

[fastline]

I am curious why voltage ranges for megger typically exceed 1000V? It is my understanding to never test a motor with more voltage than it is rated? Am I missing something? I am looking to buy one and wanting to determine which features I need to shop for. I would mostly test motor coils but it would be nice if the thing did something else to justify its being.

 

[Mike odom]

When you test a motor, you are pumping current into the windings at a rated voltage. You do not test with a higher voltage.

When you do a megger test, you are testing the insulation between an electrical device and its case. You use a high voltage so that you get a small leakage current, measuring that current will tell you your insulation resistance from a circuit to the surrounding case.

This is from wikipedia:

• On the whole megger testing is non destructive. What happens is a DC voltage is applied to the device or winding under test. It is an insulation test to see is the insulation has been injured in any way to cause a short circuit when normal power is applied to it.
  
  On my megger there are 3 ranges 300V, 750V, and 1000 volts. Applying the proper voltage is essential to not damaging the device you are testing.
  
  Working voltages up to 240 volts should use the 300 range.
  
  Working voltages up to 600 volts use the 750 volt range and working voltages above 600 use the 1000 volt range.
  
  As you can see if you used the 1000 volt range on a device that had a working voltage of say 24 volts you could damage the insulation just by testing it. So meggering a device is non destructive if you use the tester as its instructions tell you to.

 

[Nigel Goodwin]

Interesting that your megger doesn't have a 500V range - that used to be the standard test voltage - particularly on the hand wound ones.

 

[Al Manzoli]

Interesting that your megger doesn't have a 500V range - that used to be the standard test voltage - particularly on the hand wound ones.

Megger is the name of the company that makes the device.

It is actually an insulation tester , but can be used for continuity tests.

AFAIK it is not to be used on electronic circuits .

The basic ones that I know are , 1000 volt and 500 volt. ( Crank Type)

I own a 500 volt Megger.

I have had it for over 30 years , and it still works like new .

 

[Nigel Goodwin]

Megger is the name of the company that makes the device.

I know, but it's become a general name for an insulation tester.

It is actually an insulation tester , but can be used for continuity tests.

AFAIK it is not to be used on electronic circuits .

The basic ones that I know are , 1000 volt and 500 volt. ( Crank Type)

I own a 500 volt Megger.

I have had it for over 30 years , and it still works like new . View attachment 81508

Mine has the WWII WD stamp on it

 

[dr pepper]

Meggers test at dc, if you had a 3 phase 415v motor, the peak voltage would be higher than the 415, so you need a higher dc voltage than the ac, and then a bit of headroom to make sure.

I use a megger on electronic stuff, I'm sure nige has too, handy for checking rectifier diodes on eht circuits, I repaired an old 'scope for someone recently using a megger.

 

[rumpfy]

The older designs used a hand cranked generator.
The test voltage depends on what is being tested.
The electricians type is a 'go /no go' sort of thing where a pass /fail is required.
Electric cable type testers more generally use a 500 volt DC test with a current amp which is calibrated up to 20 or even 200 million megohm. The need for a leakage measurement arises where insulation quality is to be assessed. Telecommunications cables almost universally are required to meet a 30 or 40, 000 megohm.km insulation resistance requirement at 500 volt dc even tho' the working voltage is 48 volt.
For test voltages above 1000 volt, it is most likely that a partial discharge type of test setup using an Ac test voltage is better. This is particularly the case with solid insulated coaxial cable.
My view about testing small motor windings is that a 500 volt dc megger is what is required; even if the motor is wound for 120 volt. In the USA, the industrial power supply voltage is 480 volt 3 phase I think, and motors for this supply might be better tested at say 750 volt.
For testing high voltage diodes, it's nice to use a variable high voltage supply so that the reverse voltage breakdown value can be identified.

 

[Nigel Goodwin]

I use a megger on electronic stuff, I'm sure nige has too, handy for checking rectifier diodes on eht circuits, I repaired an old 'scope for someone recently using a megger.

Rectifiers in microwave ovens!

 

[dr pepper]

Yes of course Nige, forgot that one, a megger would only produce a fraction of the working but better than a meter powered by a pp3.

Reminds me I have a robin megger that dunt work, something else I can play with using my new found component tester I've had 20 years.