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Re: [Amps] Transformer question

To: "Amps Amps" <>
Subject: Re: [Amps] Transformer question
From: "Jim Brown" <>
Date: Mon, 09 Jul 2007 15:56:29 -0700
List-post: <>
On Sun, 8 Jul 2007 21:25:22 -0400, Tom W8JI wrote:

>It appears there may be some confusion about what a neutral 
>and a ground are.

Tom's reading of NEC is the same as mine.  

However -- I have heard that there used to be some exception for 
certain heavy appliances. If that execption exists, perhaps 
someone could cite the specific paragraph. 

There is a VERY good reason for the requirement that the equipment 
ground not be a current carrying conductor, and that the neutral 
be bonded at one, and only one point. The principal reason is 
leakage flux and inductance of the return path for load current. 
If the neutral carries all of the load current, all of the flux is 
confined in a small area between the phase and neutral conductors.

If, however, ground is a return path, the return current divides 
between neutral and "the building" -- that is, all of the parallel 
ground paths -- according to ohms law. in that condition, most of 
the load current will NOT be in the neutral, so most of the flux 
will be leakage flux that spreads out over the entire cross 
sectional area of the path. This can establish a massive hum field 
(depending on the magnitude of the current following that spurious 
path), which, by ordinary magnetic induction, will couple into 
lots of wires we don't want it to couple into. Like the shields of 
interconnect audio wiring, and the returns of RS232 cables, and so 


Jim Brown K9YC

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