Bolted joint resistances. 1

[dbecker]

Hello,

I am trying to calculate roughly the value for bolted copper busbar resistance.

I have viewed this website

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec7.htm#Joint

But the information is less consistent than reality.

I am seeing more like 5-7 micro-ohms resistance across a bolted connection. About half inch thick copper on both ends overlapping about 2 inches, with belleville washers.

Does that sound reasonable?

Thanks

 

[desertfox]

A slight error on my part i should of squared the 25.4 to convert imperil to metric

 

[dbecker]

ahhh!!

I thought I was loosing my mind...

 

[dbecker]

I forgot to add, do you happen to have any suggestions for my post on crimped lugs? Looking for a typical value of resistance at the crimp for 646 crimped lug connections.

Thanks again you are great help

 

[desertfox]

Sorry, can't help with that one, however I would have thought the electrical resistance at the crimp would be very low if the crimp has been correctly made.

You said in your earlier post that this is related to the failed lugs so I have a question - where exactly did the failure take place - at the crimp or at the lug face?
If the failure did occur at the lug face then you need to look at the link I gave you earlier where it shows how to calculate stresses due to thermal expansion, because if the bolt was incorrectly tightened there is a very good chance that your failure occurred due to this fact.

If it occurred at the crimp then you need to go and have a look at the tooling and make sure that the tool is the correct one for the particular type of crimp. Also, there are standard pull tests for crimped joints, so you need to review these and ensure that your specimen crimp joints passed this test.

Regards
desertfox

 

[dbecker]

Hello desertfox,

The failure was a temperature value that was exceeded, not mechanical failure in that sense. I believe it was excess of 35C over a 55C ambient, which means that failure (according to code) occurs at 90C.

I do not know where exactly it occured (could have been under-torqued bolts, wrong washers used etc...), I am hundreds of miles away from the actual unit that was tested and they do not have very good data, just the fact that it hit 90C and failed.

In any case, the failure could also have been caused by poor ventilation of the system, inadequate cooling, too much current; we can speculate. My job at this point is to try and understand what the effects are of bolted joint design and temperature rise for a new design being implemented.

Would you be comfortable to say that the crimped lug connection is on the order of 1 micro-ohm or less? I would imagine so, they use a 12 ton hydraulic press to crimp the cables and its a permanent ordeal.

 

[desertfox]

Hi dbecker

No I wouldn't want to say I would be comfortable with guessing a figure of resistance of the crimp joint.
What I would say though is do a calculation of stress due to thermal expansion on the connection, you may have to assume a torque to get a clamping load but it might give you an idea of how tight your connections should or shouldn't be.
I also think you are more likely to encounter bigger problems with differential thermal expansion at the bolted joint than at the crimp joint, as the materials at the latter joint will be similar.

desertfox

 

[jghrist]

If the crimp joint is made properly, there will be a solid mass of copper or aluminum. The resistance should be lower than the resistance of an equivalent length of conductor.