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#83512 - 02/04/03 04:27 AM Megger
Reel-Break Offline
Member

Registered: 04/04/02
Posts: 179
Loc: nc
Lets say you`re megging some wiring from a disco out to 15 hp 600 volt motor.A&C read ok B reads 2.5 meg to ground. A&C are open I mean when I say ok. I was set to pull in some new wiring.Someone says its fine well I figured we`d show him since we`d only blow a few fuses but it worked.Now my question is wouldn`t the right thing to do be pull new wiring?And second when using a megger or meg ohm scale on a multi meter what readings indicate a true problem is there a range.Thanks in advance all the wires were diconnected on both ends completly by the way.

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#83513 - 02/04/03 06:02 AM Re: Megger
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
I don't believe the megohm scale or a DMM is will provide adequate voltage to test insulation values. Voltages from 100 to 1,000 are required to test insulation ratings up to 600 volts.
A typical rule of thumb for megging equipment below 1kV is one megohm. For equipment operating over 1kV, add one additional megohm per thousand volts of operating voltage.
Also, remember that megging helps establish a trend to determine the ongoig deterioration of insulation. Higher readings now, that are considerably lower in a year could help predict unexpected downtime. Conversely, somewhat lower readings that remain stable are probably a good indication.



[This message has been edited by Redsy (edited 02-04-2003).]

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#83514 - 02/04/03 06:34 AM Re: Megger
Reel-Break Offline
Member

Registered: 04/04/02
Posts: 179
Loc: nc
Redsy lets say you see the reading I described would you replace the wiring?This 2.5 meg was observed using a DMM.If I use a megger I`d think I`d see a grounded condition?I may go back and check it just for future reference.Thanks

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#83515 - 02/04/03 08:35 AM Re: Megger
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
I wouldn't do anything until I used a megger. Then you will probably get some reading (in megohms) from A & C as well. And your B reading may decrease. At that point you can compare the readings, and decide if any action should be taken.
Insulation quality testing can be complicated, as there are other tests that can be done(Insulation Spot Test, Dielectric Absorption Test, Step Voltage Test) which may provide more insight. A manual provided with the instrument will help determine the necessary tests.
BTW,
I am no expert on insulation testing but I have done it a dozen or more times. Each time I do it, I review the techniques & procedures.

[This message has been edited by Redsy (edited 02-04-2003).]

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#83516 - 02/06/03 01:04 PM Re: Megger
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Reel, read the manual for the tester. The web is an awful place to get technical education, but a terse article on insulation testing is http://www.branom.com/literature/irt.html There are a lot of “what ifs” in figuring hard-and-fast insulation-resistance numbers.

A useful booklet by Biddle is “A Stitch in Time”. Comparison to other tests for similar pieces of gear is often useful.

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#83517 - 02/06/03 01:37 PM Re: Megger
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Bjarney,

Good point. That is the manual I use.

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#83518 - 02/08/03 05:37 AM Re: Megger
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
A DMM will give an indication of a drastic fault, but the potential applied (typically 1.5 to 9V) isn't nearly high enough to give an accurate guide to the insulation's condition.

In the U.K., on our standard 1-ph and 3-ph (240/415V) systems we test at 500V and a result of 1 Meg or greater is required to satisfy "code."

As has been mentioned already though, there's more to it than just "over 1 meg good, under 1 meg bad."

If I tested at just over 1 meg on an old heating element, for example, then I'd be satisfied that I was reading normal leakage due to the age of the element. But if I tested even as low as 5 meg on newly installed cables I'd be wondering if I'd damaged them somewhere during installation.

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