TROUBLESHOOTING THREE PHASE CABLE 4

[THooper]

Hello experts,

I am a past electric motor winder and builder. I just took on a new job in the coal mining industry. The testing equipment supplied is limited. They do not have meggers, hypots or surge testing equipment that I am more comfortable using for troubleshooting at my past job.

I am looking for information in general on troubleshooting
three phase cables with a ohm meter. Readings that would indicate such as phase to phase short, phase to gound short, moisture in cable, open leg etc..

Thank's in advance
kind regards,
THarper

 

[jraef]

Good luck with that.
What works for a 9VDC circuit from a VOM has little relation to what problems you will find at industrial volages. Continuity maybe, but cable capacitance can fool you on occasion. The only suggestion I can offer is that when dealing with 3 phase cable systems, you can sometimes spot a problem when there is a big difference in one cable compared to the other two. What you would be reading in terms of number values would otherwise be relatively meaningless however. For you own sanity I would suggest as a minimum, buy your own hand crank megger on ebay and write it off as a tool expense on your taxes next year if you can. I've seen them go for around $50 not too long ago, and they don't require batteries.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Try real hard for a megger.
A rule of thumb for #10 AWG is 1 ohm per 1000 feet.
You are not going to localize a fault in a cable by resistance unless you use a wheatstone bridge and use the multimeter in place of the galvanometer, and then the Wheatstone is best on hard grounds or shorts. Because of the low voltage used it is not particularly dependable on high resistance faults.
We have all encountered faults that were voltage dependent. The multi meter reads good, but the equipment fails the energisation test.
What can you do with what you got.
I would look for a small dry type transformer to get full voltage for testing. Better yet twice normal voltage. Even better, 1000 Volts.
Use light bulbs or better yet, resistors and one small lamp to limit the current. You will get a good indication of cable condition by whether the lights are bright, dim or off. Make your test connections and then step away before energising the transformer..
You can use your current range on the multi meter to get a good indication of the resistance of a high resistance fault. Again make the connections and then step aside before energizing.
CAUTION WARNING THIS METHOD IS DANGEROUS
THE ONLY THING MORE DANGEROUS MAY BE USING POWER CABLES WITH NOTHING BUT A MULTIMETER TO CHECK FOR FAULTS.
BETTER YET, Buy your own megger. It's your life at stake.

 

[THooper]

Thanks for the ebay tip, I just bought a brand new crank 500v megger for $70.00 off ebay using the buy it NOW!

Thanks again guys

 

[BJC]

Testing cables in a coal mine or coal handling area could be dangerous. 500 volts will make enough spark to make a a hell of a methane explosion. When I have been around mines and mine eletricans they did things a certain way for good reason. I don't recall how they tested cables but I would make sure I found out.

 

[waross]

Hi THarper
Glad to hear you have a megger. Please now disregard my suggestions and use the megger instead.
Yours

 

[jraef]

I Think BJC brought up a very important point (LPS vote from me). Make sure to check with the mine safety people before using in in potentially hazardous environments. It is still a good tool to have available however.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[OperaHouse]

What, nobody likes a TDR? I would think if you are talking about long runs that could get pinched and shorted, that would be the thing to have if time down is money. You can compare one phase with another and you can easily get a faults location (open ot short) to within 5 feet. That would save a lot of wire pulling. I bough a nice portable unit for cheap (it had a couple of problems I had to fix), but I've seen working ones on eBay for about $200.

 

[Skogsgurra]

Yes, TDR. Good thing. I cannot resist telling you about a simple TDR that I published in Electronic Design a long time ago. It still lives its own life on the web and I see it cited now and then. Google: +"cable radar" +gunnar and you will get a few links to it. It costs a few dollars and I have used it to solve problems in a lot of installations. The more interesting one was the Stockholm underground communication system.

Gunnar Englund

http://www.gke.org

 

[THooper]

BJC Did bring up a very important point, we are supplied with methane detectors and by law we have to take CH4 readings every twenty minutes while working in the face of the mine where methane levels are common. It is also required to perform CH4 tests before any welding or torches can be used or started in any part of the mine along with having five bags of rock dust and a fire extinguisher at hand. Methane is at the explosive range if it reaches levels of 5--15%

 

[jraef]

I always thought TDRs were very expensive, I'll have to look into that. I don't need one often enough to justify the $2000 I saw one quoted for, but if I can make my own or buy one on Ebay, I might go ahead. Thanks Gunnar and Operahouse.

PS. I visited your article site Gunnar, and I think it is humorous that so many people originally thought it was intructions to build your own RADAR!

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[BJC]

Google up "Online Partial Discharge Testing" Electrotest or whoever they are now have a good system. If I had a critical system I would use it.

 

[davidbeach]

Electrotest became Electrical Reliability Services.