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Re: [Amps] Safety grounding - was Mains Isolation Transformer

To: g3rzp@g3rzp.wanadoo.co.uk, r@somis.org
Subject: Re: [Amps] Safety grounding - was Mains Isolation Transformer
From: Gudguyham@aol.com
Date: Wed, 5 Jul 2006 10:09:09 EDT
List-post: <mailto:amps@contesting.com>
 
In a message dated 7/5/2006 7:44:38 AM Eastern Daylight Time,  
g3rzp@g3rzp.wanadoo.co.uk writes:

This is an interesting one. Is it assumed that you can't get a fault such  
that the full fault current flows down the ground (i.e. the protection) wire?  
Such fault current being greater than the normal load current? Intuitively,  
I'd sort of expect the protective ground conductor to be of at least the same  
gauge, if not bigger, than the  current carrying conductors. 
Also, do we assume that in the case of two lots of 110 volt appliances, the  
currents will be approximately in anti phase, and so cancel  in the  neutral? 
This would fall down with bad power factors of opposite sign. (OK -  opposite 
sine or rather, cosine, if you want a bad pun) 
73 
Peter G3RZP



Peter,  the way things work if all wiring is done to code is that the  bond 
wire could carry a fault current as great as the load current or more, but  
since the bond wire would not carry normal current it would never heat up.   A 
fault current would be and is considered to be when the chassis becomes  
energized by one of the regular current carrying conductors in which case it  
would 
be a dead short which will throw the breaker in a fraction of a  second.  The 
wire can easily handle the surge for the time it takes to  throw the breaker in 
a dead short.  SInce the neutral is suppossed to be  wired so that it is the 
only wire to carry current back to the source it will be  most capable.  In 
the case of a 240 v single phase the neutral  only needs to carry the 
unbalanced 
load.  If both phases A&B are  carrying X amps of current, there is no 
current flow on the neutral.   Lou
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