Flexible Cables: Important Safety Issue [RE-wrenches]

Christopher Freitas --- OutBack Power cfreitas at outbackpower.com
Fri Aug 27 18:42:12 PDT 2004


 

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wallace stahle wrote:
> What are the dynamics of the current in a nice big 4/0 cable getting 
> from the CU and terminal flag, through the washer and the relative 
> bottleneck of the surface area of the nut on both sides of the terminal 
> flag?

When properly installed the current path should be directly from the 
crimped on cable lug to the stud terminal's base material - the stud, 
washers and nut are not in the direct current path.  Putting a washer 
between the lug and the base of the stud can result in lots of losses - 
I once saw a washer glowing red hot on the DC terminal of an SW series 
because of this very mistake.

>  Has that been looked at by those whose job it is to spec that method? Is 
>  there literature to that effect? Is it a mute point because the length 
>  of the reduction is so small?

Yes this has been considered and evaluated in the designs.  The 
technical term is "insertion loss" and is measured by putting a breaker 
in series on a cable - cut the cable, add two ring terminals and the 
breaker - then measure the voltage drop, current and calculate the 
wattage.  I usually poke my meter probes through the insulation of the 
cable to include the losses of the crimped lugs as well. The large 175 
and 250 amp was very low voltage drop - about the same as a DC current 
shunt if I remember correctly (10 watts or so at 250 amps I think...)  

> Some of the DC CBs  strike me as having very small studs relative to
> ampacity. Certain manus. I believe.

Yes - we order all of ours with at least 1/4 inch terminals (up to 70 
amp) and have special 8mm terminals on the 100 and 125 amp breakers.  

The higher the current - the more important the connections are.  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  

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