Insulation Resistance & Surge Protection Devices

[Cing37]

This is my first time posting and the post is meant to be conversational, I don't necessarily need any answers or help so feel free to spitball, guess, or bring in ideas from left field.

I was using a Megohm meter at 1000VDC at the disconnect switch of a 75HP motor that drives a hydraulic pump. Two years ago, using the same method at the same place (load side of the disconnect switch to the motor) and I got 550Mohms (maxed out the meter if I remember correctly). This time around I got 0.8Mohms. This wouldn't necessarily alarm me, I mean it did at first, until I found a trend on the other 3 identical motors.

A little background: There are 4 identical motors with the exact same setup and I'm getting a similar trend on the rest of them. The readings I took two years ago are very high and this time around they are low... VERY low. The motors operate fine and their voltage/current recordings look nearly identical to the ones I recorded two years ago. This got me thinking and after I looked around some I noticed that there were new digital type surge arrestors on the disconnect switch enclosures that weren't there last time (as far as I can remember). I tend to think that the simplest answers are usually the right ones, but I figured I would have run into this at some point in the past right?

So the question is: Is it possible that these surge arrestors are messing with the IR meter?

I'm looking for photos of the surge device, If I find one I'll post it.

Thanks!

-Chad

 

[gar]

221102-1936 EDT

Cing37:

You have provided insufficient information, but I would very likely expect that your observed condition is a result of the surge limiters.

.

 

[junkhound]

Basic definition of an MOV spec:
Put 1000V across a 1000V rated MOV and you get 1 mA. 'resistance' reading of 800K is 1.25 mA.
The arrestors most likely contain 925V or so rated MOVs, typical for a 480 V line - then answer is obvious yes, your 800K reading caused by arrestors.

 

[Cing37]

Basic definition of an MOV spec:
Put 1000V across a 1000V rated MOV and you get 1 mA. 'resistance' reading of 800K is 1.25 mA.
The arrestors most likely contain 925V or so rated MOVs, typical for a 480 V line - then answer is obvious yes, your 800K reading caused by arrestors.

Thanks for the response. I guess my next question would be how I can get an accurate meg reading without removing any terminations (a stipulation in the scope of work). I suppose I could just remove the surge device and the meter would ignore it because it's an open circuit.

 

[junkhound]

Yes, a lot of specifications that mandate IR tests stipulate that arrestors can be disconnected for the test.