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#24100 - 04/04/03 11:55 AM MEGGER TEST  
seeks  Offline
Member
Joined: May 2002
Posts: 25
bufalo, ny 14228
Can anyone explain how to interpret a megger test result on a cable?
SITUATION: Chiller upgrade, power cables (480V, 3 phase)were scraped on sharp edge of bushing during pull. Contractor says only the pulling jacket was effected, not the insulation. Megger test was performed and all resistance tests "were in excess of 100 Megohms". What exactly does this mean? Does this mean, definitively, that the insulation has not been damaged?
thank you in advance for your response.


Bob

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#24101 - 04/04/03 12:18 PM Re: MEGGER TEST  
DBC1  Offline
Member
Joined: Mar 2003
Posts: 16
Tulsa, OK USA
If the megger indicated greater than 100M ohms your insulation is OK.

In a nutshell if a megger reads open like above, the test passed. If you get a reading the test failed.

A megger is simular to a ohm meter on a DVM, but it uses a AC voltage at 50 volts or higher. This gives it the ability to punch through pin holes or arc tracts. So what you are looking for is any type of a reading.

The old hand crank ones are great for driving worms out of the ground.


#24102 - 04/04/03 01:52 PM Re: MEGGER TEST  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
A megger tests with high-voltage DC.

If you tried measuring with AC you'd read the overall impedance, which includes the reactance formed by the capacitance in the cable.


#24103 - 04/04/03 02:48 PM Re: MEGGER TEST  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
Lorton, Va USA
FYI, it was about 1965 it was dropped, but the NEC used to reference "1 meg-ohm for 1 minute" as a passing score on insulation resistance. The many different manufacturers as well as styles of insulation, varying conditions, etc, made them drop the quote, but it is still a very good guideline. On several occasions manufacturers were out of business, and it was the one I've referenced.


#24104 - 04/04/03 02:56 PM Re: MEGGER TEST  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
For 480V-AC circuits, usually 1kV-DC and sometimes 500V-DC is used as a test voltage—versus around 3 volts for the typical digital multimeter set for a resistance function. "Megg-o-Testers" are available roughly in the 100V to 5kV-DC range.

You can visualize a "Megg-o-Tester" insulation-resistance test set as simply an ohmmeter with the equivalent of a high-voltage battery.

Producers of these instruments are AEMC, Megger, [formerly Biddle/AVO] Fluke and others. { Er, see www.aemc.com .. www.megger.com .. www.fluke.com }




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


#24105 - 04/04/03 05:06 PM Re: MEGGER TEST  
ThinkGood  Offline
Member
Joined: Aug 2002
Posts: 1,081
Milwaukee, WI
Quote
The old hand crank ones are great for driving worms out of the ground.


Don't let PETA find out!


#24106 - 04/04/03 05:32 PM Re: MEGGER TEST  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
The good megger reading doesn't necessarily mean that the insulation has not been damaged. It just means that under the current conditions that there are no phase to phase or phase to ground shorts. The insulation could be cut through to the copper and still get this reading if there are no conductive substances (wire pulling lube, water, ect.) in the area of the damage, and as long as the bare copper is not touching the conduit.
Don


Don(resqcapt19)

#24107 - 04/04/03 07:14 PM Re: MEGGER TEST  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
seeks,
Please be aware that an Insulation- Resistance test, only tests between the cores(and their insulation) of the cable.
It does not include the outer sheating, unless the cable has been severely damaged,
in which case, you would get a dead short. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#24108 - 04/05/03 03:55 AM Re: MEGGER TEST  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
ThinkGood,
When I first started my time as a Line Mechanic, I was sent with a guy to disconnect some old Lines, they were across a river but were still live, long story short, when the line hit the water, all the fish came to the surface!,lying down, stunned!.
Trout for tea, that night!. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#24109 - 04/05/03 04:45 AM Re: MEGGER TEST  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
For what it's worth, in England we test 240V 1-ph and 240/415V 3-ph systems with 500V DC, and 1 MEG is the lowest acceptable reading for insulation, either between conductors or from any conductor to ground.

If I obtained a reading that low on a new system, however, I'd be concerned that something had been damaged during installation.


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