Micro ohmmeter for cable crimp verification
Micro ohmmeter for cable crimp verification
(OP)
Our facility has recently had problems with failed cable crimps. Failure causes have ranged from workers forgetting to remove the varnish from the CTC (continuously transposed conductor) wire before crimping to improper fill of crimp joint, to incorrect crimp head selection.
The problem is that these crimps are inside oil filled power transformers and have led to failures in the testing, sometimes requiring the element to be untanked. I am trying to find the best way to verify that the crimp connection is good before the element is placed inside the tank. Currently our QA team goes out with a caliper to verify that the crimp haed compressed to the proper size, then attempts to pull the joint apart, which to me is not the way to verify a connection.
I am thinking about using a DLRO (digital low resistance ohmmeter) to test these joints, but have no experience to tell me if this is likely to produce any type of results. Before I ask my company to buy a $5000 piece of equipment that may not tell us anything, I wanted to ask other engineers that might have some experience out there. I am thinking about a 10Amp unit that states that it reads down to 1 micro ohm. Our cables range from #2/0 to 1000MCM.
Thanks in advance
The problem is that these crimps are inside oil filled power transformers and have led to failures in the testing, sometimes requiring the element to be untanked. I am trying to find the best way to verify that the crimp connection is good before the element is placed inside the tank. Currently our QA team goes out with a caliper to verify that the crimp haed compressed to the proper size, then attempts to pull the joint apart, which to me is not the way to verify a connection.
I am thinking about using a DLRO (digital low resistance ohmmeter) to test these joints, but have no experience to tell me if this is likely to produce any type of results. Before I ask my company to buy a $5000 piece of equipment that may not tell us anything, I wanted to ask other engineers that might have some experience out there. I am thinking about a 10Amp unit that states that it reads down to 1 micro ohm. Our cables range from #2/0 to 1000MCM.
Thanks in advance
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If it is broken, fix it. If it isn't broken, I'll soon fix that.






RE: Micro ohmmeter for cable crimp verification
For other applications many specs are now requiring a 100A DLRO, so you may be able to find some good deals on used 10A models from a used equipment dealer. Just be sure to have it calibrated annually, these tend to fall out of cal every once in a while.
I would still to the tug test, follwed by the DLRO.
RE: Micro ohmmeter for cable crimp verification
We compared measurements of crimped HV clamp resistance with microohmmeter MOM200 from Programma Electric, using test current of 100 A. Both results are in the range of 36 microohms, difference was about 2-4 microohms for several clamps.
According to me existing standards which require high current measurement (50 A, as I remember) simply don't take into account progress in electronics in the recent decades.
If your measurements are for your internal reference, acc. to me you can use some modern instrument. But if strict following of standards is required, you still have to check about minimum stahdard current and use high current instrument.
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RE: Micro ohmmeter for cable crimp verification
For busbars in switchboards we usually use a 100A micro-ohms meter or the SWBD nominal current (if lower than 100A) to check all connections.
I think that would be the best way to go about it.
I hope that helps.
Dan de Freitas
Field Services
RE: Micro ohmmeter for cable crimp verification
Thanks again!
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Micro ohmmeter for cable crimp verification
RE: Micro ohmmeter for cable crimp verification
In regard to your last question, use NETA Acceptance Testing Standard ANSI/NETA ATS-2009. Section 7.2.2.3.1.3 It states that for bolted electrical connections to do the following: "Compare bolted connection resistance values to values of similar connections. Inverstigate values which deviate from those of similar bolted connections by more than 50 percent of the lowerst value". These bolted connections implies "crimping" of the cable since the crimping is part of the electrical connection.
RE: Micro ohmmeter for cable crimp verification
RE: Micro ohmmeter for cable crimp verification
You will probably find that for a 300mm^2 cable lug connection your reading will be in the order of approx. 10 micro ohm if not less...
Hope that has helped.
Dan de Freitas
Field Services
RE: Micro ohmmeter for cable crimp verification
I guess I will give it a try.
Thanks for the advice
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Micro ohmmeter for cable crimp verification