Presumed RF Interference

Scott Morris swm at
Mon Mar 6 01:14:18 UTC 2006

The isolated grounds are definitely a recommended idea for telco/server
rooms...  Perhaps an array of them depending on the size power feed we're
talking about.  I'm assuming it's a sizeable UPS that runs your telco and
data equipment (or small server room).  The irritation, if you haven't done
this step already, is that adding a TRUE isolated ground after you've
already built your building and room is not exactly a cheap thing to do.  

Especially in nice metal framed buildings that like to have  a tendency of
becoming the nearest path ground themselves.  But I agree that it's
certainly something as a worthwhile "first path" to look into!


PS.  I agree it's not good business practice to kill your clients! 

-----Original Message-----
From: owner-nanog at [mailto:owner-nanog at] On Behalf Of
Steven M. Bellovin
Sent: Sunday, March 05, 2006 6:21 PM
To: wb8foz at
Cc: nanog at
Subject: Re: Presumed RF Interference

On Sun, 5 Mar 2006 18:00:36 -0500 (EST)
David Lesher <wb8foz at> 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.

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