As in joint resistance measurement? Usually with a high current milliohm meter, although even the big ones are working near the limit of their resolution.
If you want better precision then you will probably have to rig your own test using some sort of high current DC supply and a precision meter. I have used that technique on 18kA bars and it works well: I used a HP6681A power supply and a Keithley type 2000 bench meter. Beware of thermal EMFs in test leads, and make sure you fully understand how a Kelvin four-terminal resistance measurement connection works to avoid getting silly errors.
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Thanks, there is a commercial instrument available called a ducter not sure how well it will work. Also where did you apply the current this bus is a long run with many welds and joints.
On a 15kA bus a ducter will struggle to resolve anything worth knowing. Even the big 200A T&R type was basically useless because the variation in test results we recorded was larger than the expected value of the measurements. That was the reason for using a 500A DC supply and a precision meter. We could reliably resolve a few hundred n[Ω] once we sorted out a few teething troubles.
The HP supply has enough voltage output to drive current one end of the bus to the other so we used welding cable and bolted connections at the flexible links for the generator and transformer connections. Resistance measurements were performed at all bolted joints. The key things we found were to inject current well away from where you are placing the voltage probes either side of the joint, and to reverse the voltage probes and repeat the measurement then average the two values to compensate for any thermal EMF. Transformer bushings are awkward because it is difficult to inject current away from the voltage measurement point but there's usually a way of doing it.
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Thanks Scotty. What kind of values were you getting? Did you use any other method other than inspection to validate the welded aluminium joints. Did you paint the joints after welding?
No, no painting or anything after welding. The inside of the IPB shield was painted matt black to aid heat rejection to the outside world but the busbar was bright aluminium. It's a while ago but from memory we were getting joint resistances in the low [μ][Ω] range - maybe 4-5[μ][Ω]. The mechanical engineers had a look over the quality of the welding and weren't very complimentary as I recall. I think they were judging by the standard of welding normally produced by a coded pressure-vessel welder so it was perhaps a little unfair.
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Qalify your welder. Qualify your proceerure.Inspect your welds ( in process and after compete.
Idon't know of any inspecton othere than visual. X-Ray would work but I have heard of it being used.
If you get a good weld with good records shy test it?
I agree with your sentiments about testing welded joints, the joints I was testing were foil laminated, bolted joints from bushings to bus duct and the figures I quoted were for that type of joint.
Over here dye-penetrant is a common weld inspection method, as is ultrasonic. Both would require the weld to be dressed before the test, not something which the IPB manufacturers would normally do.
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Last week we faced a similar problem: 4000 A -rated busbar in LV distribution board. We have to measure contact resistance of bolted busbar joints. Unfortunately we have no microohmeter (we are mostly specialized in relay protection testing, this LV board came to us as a part of larger project).
We have good 750 A AC injection tester and I wonder whether it is acceptable to perform these measurements by A-V scheme using AC-source. It is clear that we will measure impedances instead of pure resistances, but acc. to me the difference will not be significant.
------------------------ It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
My opinion is that this would give acceptable results. I know in the past I've had a difference of opinion with Zogzog over use of high current AC tests in low resistance measurements, but I can't find the thread. I recall offering to do some comparative tests which I carried out but changed jobs right afterwards and never got around to posting the results. I will say that I didn't find any significant difference between an AC and DC test for breaker contact resistance measurements, but without the data to support that statement I understand if anyone wishes to dispute my claim.
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If we learn from our mistakes I'm getting a great education!
