using a megohmmeter to test new, 15 kv shiled cable? 1

[HighPotter]

What REALLY would this prove? This is shieled, jacketed 15kv cable, 3 conductors per conduit. The "run" is approx. 1000' long, with pole top terminations at each end.

HP

 

[HighPotter]

The word is " shielded"..I am sorry. Long day.

HP

 

[wareagle]

"What REALLY would this prove? "

Probably nothing. You will put a 1000 volts on a 15 kv cable. What can you expect. Why are you testing the cable?
Is it a new installation.? With a HANDLE like you have
why not Hi Pot the cable?

 

[HighPotter]

Thats my point, wareagle.

I was told by an engineer to Megg a new installation of cable. I just dont understand what he was looking for from this sort of test.

I have always DC highpotted a new cable installation, to most of the time, ATLEAST twice the rating of the cable.

HP

 

[waross]

Hello HighPotter
There may be a misunderstanding of terms. There was a recent thread where some-one was refering to a megger test at voltages that I thought were Hi-pot voltages.
As you know, a Hi-pot test is a go-nogo test at an elevated voltage. A megger test is a test of the insulation value at some specified voltage. It may be a subtle difference in some cases.
How about asking the engineer what voltage he wants you to test at?
respectfully

 

[ScottyUK]

Good advice from waross: ask what the engineer means. I'd accept the use of the term 'Megger test' up to 5kV, which is the highest voltage I'm aware of being used in that class of instrument. On a 15kV cable, unless something is desperately wrong, it will tell you very little.

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I don't suffer from insanity. I enjoy it...

 

[waross]

Hello everyone;
Take a look at thread242-152737
This was the first I heard of the term "Megger" used for high voltage testing, but it looks like some folks use the term "meggering" where we would say high potting. I think that the difference is that high voltage meggering seems to be used to establish a base insulation value for future tests.
respectfully

 

[Zogzog]

First, there are meggers available to much higher voltages, we have several 15kV "Meggers" in our shop.

Second, Insulation resistance is the physical resistance from the conductor to ground. It is a measure of the actual insulation. Dielectric strength is the maximum voltage a dielectric can withstand without rupturing. To determine each value, tests need to be performed. To determine insulation resistance a test commonly referred to as a megger test is done. To determine dielectric strength a dielectric proof test (hipot) is performed.

Insulation resistance cannot and should not be taken as a measure of the potential required to cause a breakdown or puncture the insulation. However, the insulation resistance test preceding the high-voltage dielectric test determines to a large extent whether the insulation is likely to break down when high voltage is applied.

Insulation resistance tests should be made regularly on a monthly, semi-annual, or annual basis depending on the type, location, and importance of equipment. Records of test results are helpful in establishing trends in the insulation's condition. These test results should be recorded. In the absence of consensus standards dealing with insulation-resistance tests, NETA Standards test voltages have been developed for both ATS and MTS for various types of equipment at various voltage ratings.

Third, you dont seem to be following any guidelines for your test voltages. For example your hipot test voltages for 15 kV cable depends on your cable type, conductor size, and insulation level (100%, 133%). Is this a new cable or exsiting cable? The industry consensus is not to DC hipot exsisting cable, it is destructive. You would want an AC hipot, VLF test, tan delta, or Partial discharge test. (Which is better is highy debated)

I highly recommend you get some further training on IR testing before testing those cables (Or hire a qualifed testing company. There is a good book available from Megger called "A stich in time" that is required reading for all of my NETA techs.


Scott Peterson
Training Manager
Power Plus Engineering

http://www.epowerplus.com

 

[tinfoil]

I think one of the points of confusion here is that 'Meggar' is a trademarked name for a company that used to only make 'low-voltage' testers and now makes both low and high voltage test equipment. I believe that the company formally known as AVO has been bought by Meggar(tm), and its products will be marketed under the MEGGAR brand.

I'd ask exactly what is wanted.

 

[Zogzog]

You are correct, but Megger is the industry standard name for Insulation Resistance Tester. It is like Klennex,Superglue, and Q-tip.

 

[HighPotter]

On new installations of cable, our utility still requires a DC hi pot "proof" test, to what ever voltage the chart says. Our primary cable is EPR.

This engineer wanted us to " meg " the cable, using a our Biddle 15kv meggar. I disagreed.

I was looking for opinions on why he might want the cable tested in that manner.

It's kind of funny how some of you just assume that anyone who "ask's" a question on here, is an idiot.

HP

 

[Zogzog]

You are both right, you want to conduct an IR (Megger) test before the Overpotential (Hipot) test. What is the insulation level of the cable? I assume this is a new cable, is that right?