Megger Testing of Instrument Cables 5

[sagarini]

Hi all,
It has been my experience and I have heard from others that it is not standard procedure to Megger instrument cables. In our specific case we are using a 300V cable for 24VDC service. What is the forums opinion of Meggering instrument cable and what megger setting would be used for our DC cable?
Thanks

 

[DTR2011]

you can megger just about anything. for 300V cable, I would select 500V.

 

[ScottyUK]

Is the cable rating stated as 300/500V or 150/300V, or something similar? Assuming the 300V rating is core-core I'd likely go with a megger test at 250V core-earth, 500V core-core.


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If we learn from our mistakes I'm getting a great education!

 

[racookpe1978]

Double check that all instrumentation is disconnected - at both ends (sensor and electronics)! - prior to letting the meggar go into the instrumentation room.

 

[sagarini]

thanks all for the help. to followup on your comment racookpe1978, we will be meggering individual twisted pair cables, just from the local junction box to the mounted instrument. should we disconnect both ends of the cable for the test?
sagarini

 

[hacksaw]

good call, in fact some firms prohibit megging instrument cables, requiring meg-ohm checks instead.

besure to discharge the cables after charging them up, if not done properly the charge will rebuild on its own, as the dielectric relaxes to its uncharged state.

 

[waross]

Relaxation re-charging is an issue when you are working at tens of thousands of volts and hundreds of thousands of volts. Not really an issue at 25o Volts or 500 Volts.
However I wouldn't let a megger anywhere near instrument cables. Check your cables with a multi-meter.
Yes, disconnect both ends before testing.


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

 

[sagarini]

waross, i'm curious as to why you don't think you should megger instrument cables. as i mentioned before, others share your belief but i don't know why. the "megger-a stitch in time" book from megger.com does have a procedure for megger testing instrument, control and communication cables. as i also mentioned before i am new to all of this and am gathering information. thanks for any explanations.

 

[dpc]

I think Bill's main concern would be the same as mine - the risk of inadvertently putting 500 V on something designed for 5 V. If you disconnect both ends of the cable prior to doing the megger test, it should not be a problem, but I've seen a lot of components accidentally meggered. On the power side, it's usually not a big deal. But on a instrument circuit, it could be costly.



David Castor

http://www.cvoes.com

 

[jonathn1]

In my experience it has been born out of, " We never did it like that before", syndrome.In this case my information has come from someone not doing the test correctly. They hooked up a megger to one end and forgot to unhook the other. Thus frying circuits. On a motor or transformer if you do this it is less likely to actually cause a problem. So the fear of hooking up to a communication cable is that you would fry something. And its a real threat, if , DONE INCORRECTLY. The Stich in Time from Megger is good but it is also highly lacking in details for some situations. ( sorry still looking for something better myself so no recomendation yet). Megger makes several testers that will check actual insulation resistance. if you are doing an initial installation and are getting a base reading I would note and log any information i could get. Including megger readings on communications cables. I dont like a resistance test only because it misses some issues. Megger has a good article that can say it better than i but here is a small quote from the article. "The field pair with the low resistance will also have a change in impedance caused by
moisture in the cable. The Ohmmeter is looking only at the resistance; the TDR is
looking at the total cable impedance. The total cable impedance includes the
resistance, the capacitance, and the inductance." By looking at the whole picture you can get a better look at what is really happening. check out the whole information here.

http://www.megger.com/common/documents/TDR Applications.pdf

Good luck

 

[OperaHouse]

Those are really good points. The truth is possibly that people don't want to know and only bad can come from a test. A lot of cables get damaged from kinks, scrapes, water, solvents and sill work for a normal design life. Stress it and they will fail.