North American Network Operators Group

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Re: Presumed RF Interference

  • From: Steven M. Bellovin
  • Date: Sun Mar 05 18:22:03 2006

On Sun, 5 Mar 2006 18:00:36 -0500 (EST)
David Lesher <[email protected]> wrote:

> > Cut the ground wire in your power cords but ground the equipment directly
> > to a metal frame.
> I would NEVER tell a client to do this. 
> That could easily kill someone.


The safety purpose of the ground cord is to cause a short circuit in
case line voltage energizes the case, in which case the breaker will
trip. If you cut that wire, the metal frame frame can become hot;
unless it's firmly grounded itself, there will be a potential between
it and ground.  Along comes the next well-grounded person to touch it
-- poof!

Even if the frame were grounded properly, that's a local ground, which
may differ in potential from the breaker box's ground.  The neutral
wire in the circuit is tied to ground at the breaker box, which means
there could be a potential difference between it and the frame.  That
also creates a potential shock hazard, though presumably not that great.

What might be useful -- ask an EE, not me -- is a circuit with an
isolated ground.  In that case, the ground wire from the power plug is
routed all the way back to the breaker panel, and isn't connected to,
say, the local electrical box that the cord is plugged into.  I've seen
computer equipment wired that way in the past.