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How do I determine busbar junction resistance?

How do I determine busbar junction resistance?

(OP)
I am looking for advice on determining 'tyupical' busbar junction resistances.  My applications run up to 50kADC.  Usually minor losses aren't critical to me, but sometimes I need to be able to estimate what's going to happen every 12 feet when I bolt all that copper together.  Any advice?

Paul Matejcek

RE: How do I determine busbar junction resistance?

The copper development assosiation (UK) www.cda.org.uk publishes a publication "copper for busbars", which contains information on that topic.

For a good connection (suitable area, flat, free of oxide and bolted with hardware giving the right force in the whole temperature range) the loss is usually negliable.

RE: How do I determine busbar junction resistance?

(OP)
electricuwe,

Thanks for the lead.  I didn't find that document on the US CDA page.  I briefly browsed the article on joints at the UK site, and it looks as though it will be helpful.

Regards,

Paul Matejcek

RE: How do I determine busbar junction resistance?

Maybe I misunderstood the question, but measuring joint resistance requires a Kelvin connection.  First, a DC current is passed through the joint with one pair of leads.  [Microohmmeter are available with up to a 500-ampere test current, so the leads may need to be fat.]  Then, the voltage drop is measured with a second set of leads [with small AWG] connected to a (milli)voltmeter.  Then: R=V/I, making sure the decimal point is in the right place.  Simply, if precisely 1 ampere is passed, the each measured millivolt equates to 1 milliohm.

RE: How do I determine busbar junction resistance?

(OP)
busbar,

Thanks -- I understand Ohm's Law, and how to get the data empirically.  My questions was about predicting the resistance.  Based on what I have learned today, it seems not to matter, because a well-made overlap joint (the sort I'm using) has a lower resistance than a similar length of bus.

Thanks again,

Paul Matejcek

RE: How do I determine busbar junction resistance?

Very simple!

Run  100Amps DC from a current generator and read the voltage drop on the contact. The higher the temperature the higher the drop voltage.

That means  resistance is not constant!!!

Traian

RE: How do I determine busbar junction resistance?

Paul - I agree that measurement is not relevant since we now understand your interest is in predicting junction resistance.

But I wanted to mention that Busbar's point was not simply a statement of ohm's law. He was explaining the 4-wire resistance measurement principle which almost entirely eliminates errors from lead resistance and probe contact resistance. Simple resistance measurements with 2-lead multimeter/ohmmeter are not nearly as accurate when measuring low resistances. Also a good discussion by Traian.

RE: How do I determine busbar junction resistance?

(OP)
Thanks to all who have replied.  What I have subsequently learned probably should have been intuitively obvious: a properly made overlap joint has less resistance per unit length than the bus it joins.  A well-made joint is 'better' than a continuous run of bus.

Paul

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