600V cable megger testing stumper 1

[peterandersen]

1. Is IEEE 525 best source for insulation-resistance (megger) testing procedures & megohm ratings for 600V cables, or some other standard or book guide?
2. What is acceptable megger readings for 600V cables? 1MOhm?, 50MOhm? Does it depend on distance? Other factors?
3. Field question: megger tested 1000LF 480/277V UG feeder,THHN in EMT, got readings on 3 out of 4 conductors 600kcmil of 1GigaOhms, 4th conductor 1.6MOhms. Did again and got same result? Is 4th cable "bad"? Any other tests to prove 4th cable ok? Does superhigh Giga readings indicate questionable megger/procedure? Don't want to repull 1000LF cable, any suggestions?

 

[jahrtrix]

peterandersen, In working with motors and meggering, our
bottom line R was 1 megohm, anything less meant the
integrity of the insulation was lost. The cables are not
in such a harsh environment as the motor's windings.

The one cable with the low reading does indicate something
out of the ordinary.Years ago a long underground cable
was pulled on our work site with a similar issue. When they
dug it back up the tradesman checked out the intire length
and found a lump in the insulation. They trimmed back the
insulation and found a butt splice made by the manufacturer.
This P.O.ed the local contractor. The butt splice was not
crimped! good luck.

 

[Zogzog]

You can download "A stich in time" here, good into guide to megger testing.

http://www.electricalstoreonline.com/common_viewproduct.php?Refer=&Product=121

IEEE 400 is the main source for cable testing.

The internatioal electrical testing assn (NETA) hs procedures and standards for cable testing in the field.

I cant give you any advice based on the reading s you posted because there are too many varibles.

Are those readings phase to ohase, or ohsae to ground?
Were your phase to ground readings done with the other phases grounded?
Is the cable connected to anything or isolated in open air on ends?
What was your test voltage?
How long did you apply the test voltage?
What was the environmential conditions?

NETA requires a 600V rated cable to be at least 100M, regardless of length. There are other standards that vary on length, but none that I know of that would accept 1M.

i would recommend you have a NETA testing company come test this cable for you,should only be a few hundred bucks, much cheaper than replacing the cable. you can find one in your area at

http://www.netaworld.org

 

[rovineye]

Many variables as zogzog pointed out. But there is nothing suspicious about a reading above a gigaohm.

All insulation could have damage and you may not know it. A 600 volt megger may not show up if low humidity, clean and the jacket damage isn't against the conduit or another exposed conductor etc. Not suggesting you do, just pointing out why there can be differences at times.

Our IEEE insulation resistance requirements on ships is based on ampacity of the circuit. Don't know why.

 

[Zogzog]

Yep, but he had 1.6 Meg on one cable, that I would worry about but chances are something is connected to the cable.

 

[peterandersen]

Thanks for your posts! Contractor is going to cut "bad" cable in manhole and megger both ends to see which end is causing low readings then hopefully replace only half of cable and splice replaced cable half in manhole.

 

[Zogzog]

Wait a minute, you seem to be jumping the gun here, have a certified testing company test the cable before you go cutting it, if it is indeed a bad cable, then the testing company can locate the fault for you and you can just splice that part.

 

[peterandersen]

Thanks! We did get it tested a week or so later by local NETA certified firm. They got same results. This was their suggestion. Are you saying there is a method to exactly locate where in the line the problem is from either end of 1000LF UG ductbank? How would that help? We would still need to pull out cable and cut it at the MH so the splice would be accessible in MH, correct?

 

[waross]

It sounds as if you are on the correct path. Probably the cable on the bottom was abraded at a bend or curve in the duct. It can happen to the cable on the bottom of the pull. Possibly some feature of the duct scraped your lubricant from one cable.
1000 ft is a long pull and without adequate lubricant, cable damage is not uncommon.
All else being equal, I would suspect that the damage will be on the head end section of the cable. The most tension and the most probability of pulling damage is closest to the pulling line.
There may be other factors such as running out of lubricant part way through the pull and continuing to pull dry. This may cause tail end damage through increased friction, or head end damage through increased pulling tension.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Zogzog]

Peter, yes it can be located, however after the details you gave in your last post, you are on the right track. Locating does you no good if the cable is not accessable.

P.S. What NETA company did you use? Or what part of the country are you in?