[SI-LIST] Re: HIGH DC Current on GND Plane

  • From: Bill Owsley <wdowsley@xxxxxxxxx>
  • To: kgrhoads@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Tue, 19 Jun 2007 10:50:40 -0700 (PDT)

This should be fun...
  I've got this piece of telecom gear with a 48VDC supply at about 130 amps 
that has to pass the most severe earthquake test, zone 4? (memory skip there) 
and it's all cabled up to each of blades and out of test area (nevermind why, 
customer wants it, I think for the video) and it all has to function through 
the test.  The telecom passed, the rack failed, for those that wonder.  So for 
those of us standing on a stable reference plane - earth, the test bed endured 
some significant motional effects.  Someone could contribute the max G's and 
displacements if they like.
  There were no recorded errors from the electronics.  
  But next time I'm going to hook up a couple more sensors to look for this 
effect.
  
"Kevin G. Rhoads" <kgrhoads@xxxxxxxxxxxx> wrote:
  
>I have a midplane carrying 60Amps of current
> ...
>Is there anything else that can go wrong due to this DC , AC current 
>mixing in planes ??

Most of the issues have already been addressed. However, there is one
case which can crop up here that has not been raised, so I will comment
on that.

1) V = d/dt(LI) normally L is constant, so only dI/dt is considered.
But with 60 amps you have a lot of I and large H,B fields can result.

2) at this point MOTIONAL effects are no longer trivially ignorable
as negligible. THey may not matter, but you can't dismiss them out
of hand. That is because motion can change the effective L (or M for
mutual inductance) and with large I small dL/dt can still give large
voltages. V= d(LI)/dt = L dI/dt + I dL/dt You need to get a
handle on dL/dt, which leads to:

3) what size is the board? Will it experience vibration? Any moving
parts nearby? If yes to vibration or motion -- how near? how fast?
Are the things moving conductors? (like cable bundles flapping in
the breeze or vibrating or flex couplings or ...)

HTH
Kevin
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