Question about the difference between an ohm-meter and a "megger"

[maryhillrat]

Quick question with a long story for background.

The question is, shouldn't a megger with it's higher voltage always read a lower resistance across insulation than a standard ohm meter due to the increased current leakage?

The background, we had a 480V submersible pump trip it's breaker within a second of starting it. I didn't have the megger handy yet, and while someone else was getting it, I put the multimeter on it to see if it had a low enough insulation resistance to show up on the meter. It read 30K, then when the megger showed up we tested it with that and got a reading of 500 Megs. This is completely backwards from my understanding of how these two types of meters work, and I am hoping someone can enlighten me as to how this could happen. Everyone with me was just as surprised.

The two meters were both Flukes and are reading accurately on other items so I'm pretty sure they are not broken, although I haven't had them calibrated since then to be certain.

Thanks!

 

[charlie b]

This is just a guess, but I shouldn?t think that either one will, as a matter of course, read lower than the other. I just think the megger is a more accurate instrument for this application. Put simply, if you divide a big number by a big number (i.e., applied voltage divided by measured current to give you calculated insulation resistance), you are more likely to get an accurate answer than you would by dividing a small number by a small number.

Welcome to the forum. I hope someone with more experience with instrumentation will have a more helpful answer.

 

[maryhillrat]

Ah, I figured it out when I read the fall 2008 tech bulletin from AVO. It's because of the current limit on the megger dropping the output voltage but still using the set voltage on the ohm's law calculation.

 

[growler]

The background, we had a 480V submersible pump trip it's breaker within a second of starting it. I didn't have the megger handy yet, and while someone else was getting it, I put the multimeter on it to see if it had a low enough insulation resistance to show up on the meter. It read 30K, then when the megger showed up we tested it with that and got a reading of 500 Megs.

I think time is the main factor here. You checked the circuit right after it tripped with the meter and there was a delay before useing the megger ( if I understand what you are saying correctly)

I had some tank heaters that would check good when cold but not after a minute or so of opertation. Then if you were to turn them off and let them cool down they would check good again.

Try your megger and meter on a known section of good cable or at least check this circuit under the same conditions ( either hot or cold).

 

[zog]

A ohmeter, is designed to measure ohms. actually it dosent measure ohms it calulates ohms based on an applied voltage and a measured current, the meter has a minimum current it can measure and if below that value will give an error on the display (E.G. - OL = Outside Limits), therfore an ohmeter has a limited maximum resistance it can measure, usually a few thousand ohms.

A Megohmeter (Megger is just a brand name), is designed to measure, .....Megaohms. Same basic principle but with the higher voltage driving more current and some differences in the design of the meter, it can measure millions (Meg), Billions (Gig), or for the good ones, trillions (Tetra) ohms.

And there is a microhmeter, guess what that measures? Yep, microhms, basically a kelin bridge circuit designed to measure microhms, your ohmeter cant measure microhms either for the same reason it cant measure microhms.

 

[maryhillrat]

A ohmeter, is designed to measure ohms. actually it dosent measure ohms it calulates ohms based on an applied voltage and a measured current, (snip)

Right, I understand that, if the megger reads 500 Meg, the ohmmeter should read an overload, but it didn't. It read 50K or so. that's the wierd part of it.

 

[zog]

Right, I understand that, if the megger reads 500 Meg, the ohmmeter should read an overload, but it didn't. It read 50K or so. that's the wierd part of it.

that is weird, same test setup?

 

[maryhillrat]

yeah, even taken within a minute of each other. I'm going out later today to take the readings again to see if anything changed, and I'm going to try to scavenge a meg-ohm resistor out of our scrap to put the meters on.

 

[dbuckley]

The two meters should read the same for a given resistance.

The reason to use the megger is that 500V or 1000v will persuade electricity to flow places where a volt or two wont. So if there is a difference in the readings, the megger should read lower.

 

[ohm]

The two meters should read the same for a given resistance.

The reason to use the megger is that 500V or 1000v will persuade electricity to flow places where a volt or two wont. So if there is a difference in the readings, the megger should read lower.

True, Megger refers to this as the Step-Voltage Method: Apply a 500V test for 60 sec then after discharging the sample apply say a 1000V for the same time period. If the higher voltage results in a lower reading further investigation is waranted.