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On a MEGGER test 600V cables - what kind of ohmic values are we looking for ? 2

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bdn2004

bdn2004

Electrical
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We had various 600V cables recently Megger tested. Looking at the results - some of the ohmic values are in giga-ohms, some in mega-ohms and some in kilo-ohms. The cable was tested at 500V.

What kind of values are we looking for to say that the cable is good or bad?
 
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According to NETA ATS, 600 V cable should be meggered at 1000 V DC, not 500 V DC. The minimum acceptance value recommended by NETA is 100 MΩ.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Not to denigrate NETA (I held a Level 4) but the more common practice is to test in-service 600 volt cable in 480 volt service at 500 volts.

For older facilities, 100 megohms is sometimes optimistic. You can use it, but you're likely to find that you have a long list of circuits to look into. This might include disconnecting the cables from end devices to make sure the low reading is not due to a connected starter or other device.

there was an old 'rule of thumb' that said "A thousand ohms per volt", which would let you re-energize a 480-volt circuit with half a megohm resistance. Many compamies have their own specifications for minimum readings during maintenance testing. My company chose two megohms, probably by reading entrails of a pigeon.



old field guy
 
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vpc46w.gif


That's what I love to see, applied knowledge I can use - even if it's guided by pigeons.

Keith Cress
kcress -
 
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Testing IR on 480 V systems at 500 V DC ignores the fact that the AC peak voltage is close to 680 V. I'd much rather test at 1000 V DC for that reason. Also, for long cables from VFDs to motors where reflections might be a concern or for inverters, I might go even higher with the test voltage.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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xnuke - IR is not a test of dielectric strength of an insulation, it's more of a measure of dryness (or the wetness as the case may be) of an insulation. HV cables with low IR values have been working fine for decades.

Muthu
 
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Good point. That slipped my mind.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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oldfieldguy - we use the same rule of thumb... but we say 1megohm per kv, plus a meg for good measure
 
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I use what I have. And that is a megger that has 500 and 1000 V ranges. That one has done me good, so far. We had a DC excitation winding that meggered just below 1 Mohm a few years ago. The responsible guy said that should be OK but I wanted to replace the machine. "Too much work, we can't wait for that". So they started the mill. And next day the motor gave in completely. With steel in it. Which can be tricky to get out when squeezed between rolls and with half of it on the take-up reel. They had to wait for THAT to be cleared. Motor change didn't take long.

OK, that was not a cable. But marginal IR values should be examined before going live again. For a 500 V cable, I think that 1 kV is too little. I would rather go to 2500 V. Which "someone" usually can provide. Low IR is mostly something that happens at cable ends. Some alcohol and 100 PSI dry air can make wonders for it.

Meggering with cable connected is not a good idea. If there is a discharge somewhere, the steep discharge wave front does things to electronic devices. Even if there is no direct connection. Capacitance in small transformers is one way they can go. Had that on a paper machine once. I didn't know better. Then.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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I'd suggest downloading the booklet "A Stitch in Time" from the Megger website. It's a great introduction to insulation testing.
 
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You can get latest information on IR testing and values from the following excellent IEEE Standards.
1) IEEE 43 - Testing IR of rotating machinery
2) IEEE C57.152-2013 (earlier number 62-1995)-Clause 7.2.13 Diagnostic field tests of oil filled transformers.
 
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As I approach the last year of doing this stuff for a living, I can point to one of my former 'pets', a WW II vintage powerhouse. It had an overhead crane that could trolley from one end of the building to the other. The crane was 480 - 3-phase, fed by open trolley conductors. at each major structural member those conductors were supported by porcelain stand-off insulators that were installed in the early 1940's and never cleaned since then. I'd estimate we had a couple of dozen of those insulators on each phase.

During a shutdown maintenance period I decided to put a megohmmeter on those conductors.

Withe the shoes lifted on the trolley to take it out of the picture, each phase to ground read less than 90,000 ohms, the leakage being distributed over each of those poor old nasty insulators.

I talked with the boos and with the powerhouse engineer. We put the circuit back in service and as far as I know, it's STILL working.

old field guy
 
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When in doubt about a cable or piece of equipment because of a low megger reading, I look at the applied voltage and the measured resistance and calculate the Watts loss.
It's another perspective on the same condition which sometimes helps with the decision as to whether to energize or not.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Can the megger test be destructive? Seems I was on a job years ago, where they didn’t want to test the cable at the voltages specified on fear it would flunk the test and the cable would have to be replaced adding a big expense to the project.
 
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If a megger test is destructive, the insulation was bad. It's not the same as a hi-pot test. Do some reading maybe.
 
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If the cable/equipment got damaged by megger (which does not have enough 'puncturing power'), then the cable/equipment is unfit for service.

Muthu
 
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