Micro ohmmeter for cable crimp verification

[TurbineGen]

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


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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Zogzog]

A 10A DLRO will be fine for this application. There is not really any "spec" for your readings besides comparing similar connections. A little testing and you should be able to derive your own acceptance criteria.

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.

 

[lz5pl]

This year we purchased microohmmeter manufactured by local company exactly for measuring of crimped connections, contact resistance of disconnector switches, etc. Resolution is 1 microohm, or less (I have no data sheet at hand). Measurements under 1 Ohm are performed by 2 A. By the way, price is well under 5000 USD.
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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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[defreitas]

At work we use a 10A Micro-ohms meter (CHK - LM05) to check this sort of connections.

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

 

[TurbineGen]

I guess I would need to establish a baseline for each type of crimp before any measurements could be useful. What kind of variation would I expect to see between a good crimp and a bad one? 15%? The problem is that these will be tested by less technically minded people, so I need to be able to determine a usable range for each type of crimp we do.

Thanks again!

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[mmt019]

A Megger Ductor Model BT51 should work for you (or similar model). The Ductor circulates a DC current between two probes through the component under test and measures the voltage drop (millivolts)to get resistance(usually in milliohms). I hope that helps!

 

[mmt019]

TurbineGen,

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.

 

[mmt019]

Correction: Investigate not "inverstigate" and lowest and not lowerst.

 

[defreitas]

An other way t look at it as well, is to calculate I^2*R so then you will have some idea of how much loss you are going to have on each connection...

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

 

[TurbineGen]

The low resistance issue is the whole reason for the question. I was thinking about using this for crimp acceptance testing. If a good crimp is 10-15 micro ohms and a bad crimp is 18-20, I don't know if the instruments available are accurate enough.

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.

 

[dpmac]

I think your best bet is to mess around a bit. Make a number of perfect crimps, then make a few bad ones and see what happens. Maybe rent a basic DLRO unit for week and see if it does what you want it to.