Bill,
We start off with a 3 phase star (wye) connected supply at nominally 415 volts
per phase. Each house gets one phase and the neutral i.e. 240 volts give or
take. The neutral is grounded (earthed) at the substation. Along the
distribution, the neutral is grounded at various places. When distribution
tended to be lead sheathed or steel armoured wire cable, the sheath or cable
was the neutral. Clamped in cast iron junction boxes, in certain soils, the
neutral return resistance could be problematical. The argument was that to 'use
higher technology cabling' - i.e. cheaper! - where there were three conductors
and a metallic sheath, said sheath would be neutral and earth. To avoid
problems with neutral and earth being at different potentials and to make sure
the 'earth' was earth, earth and neutral are tied at the entrance to the
property. Now if neutral is ruptured, the neutrals connected to the various
equipment float up in voltage. So all earths are tied to the neutral in the h
ouse. The problem then comes if you bring in an external earth.....
Fortunately, you can insist on having a local earth and no connection to the
neutral. So the power pole feeding my house - right at the end of the line -
has a ground wire and stake grounding the neutral. I have all the ground pins
of my house taken to separate ground and as they are tied back to the station
RF ground, it gives under 10% of the approved ground stake resistance.
But if you have a grounded neutral and you go outside and touch something
connected to the internal earth pin when there's a ruptured neutral fault, you
can be in very deep sh*t! So certainly here, it's safer no to assume you can
ground the neutral.
Now in the US, if you get a ruptured neutral, the two sides of the line divide
the voltage between them. The side with the lowest resistance will float
towards 120 on both sides....not necessarily actually getting there, but maybe
well above 120 volts. But the in-balance if you have a 'grounded' neutral then
depends on how good the ground is and what the load is.
Does this explain it for you? I regret it isn't easy...Hell, standards people
are involved and making it easy mean they aren't needed. I know - until June, I
was one - although not in this field.
73
Peter G3rzp
========================================
Message Received: Sep 19 2013, 05:02 PM
From: "Bill Turner" <dezrat1242@yahoo.com>
To: "Amps" <amps@contesting.com>
Cc:
Subject: Re: [Amps] Re Direct rectification of AC mains to derive the amp,
VDD, supply
ORIGINAL MESSAGE: (may be snipped)
On Thu, 19 Sep 2013 12:44:45 +0200, Peter wrote:
>The 240 volt EU supply is a very different animal to the US 120-0-120.
REPLY:
Hello Peter: Could you elaborate on this a bit? I assume the difference is
how the neutral is used or not used. Is that correct?
In the US the typical wiring is two 120 "hot" wires, a neutral and an earth
ground. In the proposed power supply, the neutral would not be used at all.
Just the two 120 hots and the cabinet would be earth grounded.
Thank you,
73, Bill W6WRT
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