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#60272 - 12/27/05 06:49 AM Another Megger Topic
Old Town Offline
Member

Registered: 11/24/05
Posts: 21
Loc: Dallas,TX USA
The other Megger thread has raised some questions in my mind regarding isulation damage using a Megger. On THHN insulated copper for instance, would it be safe to crank 1000V through it testing for ground faults? Is insulation damage likley to occur? As you can tell I am new to Megger testing. Thanks in advance.

Regards,
Ty

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#60273 - 12/27/05 07:11 AM Re: Another Megger Topic
Zog Offline
Member

Registered: 09/13/05
Posts: 116
Loc: Charlotte, NC
Thats the point, would you rather the insulation failed during your test or when you turn it on? Now, you can damage insulation with a hipot test if done improperly.

AVO publishes a book called "A stich in time" which is a pretty good book for the basics.
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#60274 - 12/27/05 07:31 AM Re: Another Megger Topic
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Folks seem to be a little confused as to just what is tested during a megger test.

First of all, there is no such thing as a "megger test." There are, however, a number of different tests you may perform with a megger.

To begin with, a megger does not apply an extremely high voltage to the wire. Even 2000V is not that 'high,' when compared to what the wire is exposed to in the testing labs. That said, most megger testing is done at 500v, which is well withing the rating of usual building wire.
The real effectiveness of a megger lies in its' ability to measure extremely small changes in resistance. Think of it as a "micro" tester, if that helps.

The first test, the simplest test, is nothing more than a more sensitive continuity test. A megger is capable of detecting leakages at a much lower level than a continuity tester.

Another useful test is called a "polarity index" test, which relys upon differing voltages to be applied, a graph made- and compared to a graph made at an earlier time. This test reveals a breakdown in the resistance of the insulation to higher voltages, over time.

The third major test is calle the "saturation" test. Simply put, the wire is 'filled' with electricity, and, again over time, the amount that leaks out is measured. Once again, interpreting this test relys upon having previous test results available for comparison.

Not every "megger" is able to do all these tests. The meter maker ought to be able to provide you with complete instructions. It does matter which meter is used, over the course of testing.

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#60275 - 12/27/05 07:31 AM Re: Another Megger Topic
Old Town Offline
Member

Registered: 11/24/05
Posts: 21
Loc: Dallas,TX USA
Thanks Zog.

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#60276 - 12/27/05 08:53 AM Re: Another Megger Topic
Zog Offline
Member

Registered: 09/13/05
Posts: 116
Loc: Charlotte, NC
First off a PI test does not involve different voltages, it is a constant voltage test comparing the results at different time intervals (1 minute and 10 minutes)

Second, while we are on the topic, a megger is a brand name, like kleenex, that most people use to describe a Insulation Resistance test.

Thirdly, You can’t measure resistance

You are probably saying to yourself, “Are you nuts? I have an ohmmeter and a megometer in my truck that I use to measure resistance every day!” Well let’s get back to basics and review how an “Ohmmeter” works.

A megohmeter is actually measuring current. The meter applies a test voltage and measures the current flow through the circuit and uses Ohms law to calculate resistance which is displayed on the meter in Ohms. If the applied voltage is not high enough, the amount of current passed through the resistance is below the sensitivity range of the meter. I think that is what reno was trying to describe in his "micro tester" theory, but that sounds more like a microhmeter description to me, which is worthy of its own thread IMHO.
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#60277 - 12/27/05 09:48 AM Re: Another Megger Topic
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Back in the olden days we used a megger to blow open high resistance shorts. Same thing?
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Greg Fretwell

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