stainless steel fasteners for high current terminations 2

[quattro]

There is a theory that stainless steel fasteners should not be used in high current (15A+) wire terminations, for example attaching a crimped ring terminal to the stainless steel PEM nut installed into PCB or using a stainless steel screw. True or false?
Thanks.

 

[GRPatel]

As one goes on increasing the current (very high, order of a few kA), the magnetic material should be avoided in the vicinity of the current. This is more for the joints, as this will lead to heating and increase in temperature of the joint. Hence, one uses non-magnetic stainless steel for fasteners for clamping electrical joint for higher currents.
Considering this answer to your question in FALSE

 

[gordonl]

I suspect the point may be here the conductivity of stainless steel considering his example of a PCB board. I'm not involved in the area of PCB design, but I suspect there would be a current limit where you would want to ensure you had a good copper to copper connection. As long as the stainless is only the fastner and not the connection I would agree with GRPatel.

 

[quattro]

Thank you for your help. Maybe I should have stated the question more clearly. We had a SS PEM nut pressed into the plated through hole in PCB and then wire terminal carrying 15A was placed on the top of the PEM nut thus current was flowing through the fastener into the trace on the board.
The actual junction was: brass (ring terminal) - stainless PEM - copper trace.

 

[GRPatel]

How was the nut pressed into the PCB? How was the force applied between the nut and PCB? Was the lug fixed by using a screw? I am unable to visualise current transfer path. For information, I give resitivity of SS, MS and brass.
SS --> 0.6 to 0.7 micro ohm.meter
Structural steel---> 0.15 to 0.3 micro ohm.meter
Brass--> 0.05 to 0.07 micro ohm.meter
You can see the difference in the resistivity and evaluate the effect on heating of the current carrying path. In any case the resistance of the joint will contribute maximum to heating. The resistance of the meatl will be comparatively insignificant. Hence you may have to concentrate on the quality of the contact joint at current transfer point.

 

[prc]

I am not a PCB expert.But I think quattaro's original statement may be correct.I understand PCB's are heat sensitive and designers try to reduce the heat generated in it. For the same current(of the order of a few amperes) carrying condition heat generated in a stainless steel bolt or screw will be four times more than in a mild steel one and hence the recommendation as seen by quattaro.
In high current joints non magnetic (AISI 304 ,314)stainless steel fasteners are used to avoid overheating of the same .If magnetic bolts are used in such location the same will get heated due to stray losses generated in it from the magnetic field generated from the bus bar.In this case fasteners are not carrying any current but get heated up from the magnetic field.

 

[quattro]

Thanks all those who answered my question for your help.
Efraim