How To Meg Instrument Control Wiring.
- Thread starter goodoboy
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Disconnect cable at both ends... including the shield. Test insulation resistance between each conductor, each conductor and shield, each conductor and ground, and shield and ground...
Thank you Smart $
I am still a bit new to megging. But why test the insulation resistance between each conductor, each conductor and shield, each conductor and ground, and shield and ground?
Also, why test shield and ground?
Thanks
Test is for insulation degradation... so you test insulation resistance between every conductor combination including 'ground' as an additional conductor [n!-(n-1)! tests, where n is the total number of conductors plus ground]. In your case, it is desired for the shield to be grounded at only one point, to prevent circulating currents, especially those that cause signal interference. You must test insulation between shield and ground to verify the intentional connection to ground is the only connection to ground.
petersonra
Senior Member
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- Northern illinois
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- engineer
Be careful what voltage level you use for the test. a lot of instrument cable is not rated for much in the way of voltage.
ptonsparky
Tom
- Occupation
- EC - retired
To see if any fail the test. Otherwise give the megger away.
The insulation value gives an indication of the overall condition of the wire. A fault from shield to ground may not give you the grief of conductor to conductor but it will tell you something is amiss.
Thank you,
So we are looking for lots (megaohms) between the conductors to make sure no chance for shorting the conductors?
Which ground do I use during the test? Do I use the ground at the control system where the 2 conductors and shield will terminate at? And the ground at the instrument in the field?
Thank you
Thank you,
I will confirm the voltage rating for the conductors and set the test voltage to whatever it is rated for. So if the rating is 500V, I will set test voltage to 500V. But I wonder if other instrument cable in the tray will be effected by the 500V:??
Thanks
Yes... megohms preferred, but hundreds of kilohms may be acceptable.
Use ground at power-source end.
I think testing at 500V is your only option with a standard megger.
It won't affect other cables unless there is a serious problem somewhere... for which it would not matter because the problem is already there.
You can set up a lower voltage megger using a DC voltage supply and a micro-ammeter. Voltage leads to the conductors under test. Micro-ammeter in series with one of the voltage leads. Then it's just a matter of using ohm's law. NOTE: verify DC voltage supply is either ungrounded, or make sure you connect the grounded lead (typically black) to ground when testing to ground.
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gadfly56
Senior Member
- Location
- New Jersey
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- Professional Engineer, Fire & Life Safety
Is your cable TC-type or standard CMP? Regular cable for instrument use is usually listed to only 300V.
500V DC voltage test is typically acceptable for 300V-rated insulation.
The 300V rating is based on AC RMS nominal voltage which has [nominal] peaks of 424 volts. Nominal voltage tolerance is typically plus/minus 10%. That takes possible peaks up to 467 volts.
Thank you Smart $,
I will use the ground bus back at the control systems I/O card where the shield is normally connected.
Thanks
A cabinet ground bus is okay. Advise against using a ground terminal on an I/O card. You will likely be using a substantially higher voltage than the I/O cards are designed for.
Thank you Smart $,
The cabinet ground bus has other sheilds from other devices wires attached as well. Will the high voltages effect does devices while megging the cable?
Shouldn't as long as it is solidly connected to the power supply grounding conductor.
pjholguin
Senior Member
I was just at www.biddlemegger.com and came across TDR meters...from reading the specs on it seems to be prefect for your needs to check out the integrity of the cable.
Go check it out! JMHO.
Go check it out! JMHO.
Thank you very much
Thank you Smart $,
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