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[OM] Re: [OT] 3 phase wiring

Subject: [OM] Re: [OT] 3 phase wiring
From: "Terry and Tracey" <foxcroft@xxxxxxxxxxx>
Date: Mon, 19 Apr 2004 20:07:35 +1000
I'm away from home at the moment hand have missed the start of this, so I
hope I'm answering the right question here.

Well, I've been in the power industry for over 25 years now.

The neutral in a three phase system exists only at the distibution level. It
is bonded to earth solidly at the sub station, and also to earth in your
house. This is called a multiple earthed neutral system.

In saying this there are always exceptions to the rule for various technical
considerations.

This keeps the neutral volts at zero volts. More people are killed by
neutral wires coming lose than from active wires in domestic situations.

The active is 240 volts to earth (UK, Australia). With AC, the voltage
varies continuously. In a sine wave. So each of these phases are 240 volts
to earth and neutral. But as they are different to each other in time they
have a difference in voltage between each other of 415 volts.

What does this mean? The neutral does carry current. Some of it is balanced
by the loading in the other phases. Ideally they should result in no neutral
current but this does not happen in reality on domestic systems. And anyway
There will always be neutral current until it gets to a correct place to
balance out stuff.

The high voltage lines do not have this problem. The distribution
transformers are connected such that neutral current becomes different
values of phase current. Enough of these out of balances should eventually
cancel each other out at the high voltage distribution level (11 kV, 33 kV
etc).

Back at the power station the neutral bears no resemblance to what you have
in the house. All the transformers between the generator and you change
things around.

>From a safety perspective, treat the neutral as alive. If you open circuit
it when it is carry current (or sometimes even if it is not) and touch it,
you will die. Or think that you are about to.

A balanced three phase load (such as a motor or a healthy three phase load
bank) will not result in neutral current and one is rarely run unless
required for control purposes.

This is different in the states where 60 Hz has affected the way they us
electricity. They use 110 volt for normalloads and 220 volts for high power
loads. For a certain load of power, less volts requires more current. More
current requires more copper. And this cost a lot. I'm a little grey on the
US distibution system, but I recall that they get three wires to their
house - two 110 volts to neutral and 220 volts between the two 110 volt
wires. Personally, I think they can't accept that they should have gone 240
volts 50 Hz!

Foxy

----- Original Message ----- 

> In UK homes, the neutral wire is at approximately the same voltage as
> earth
> (ie 0 Volts) if everything is working properly.  The voltage on the
> live
> wire varies on either side of this (plus or minus about 240V).  Hence
> only
> the live wire should light a neon in a test probe.   (A pseudo
> electrician
> trying to fit a wall light for me recently was baffled when he
> connected it
> up in such a way that both pins in the socket lit the neon in his
> probe BUT
> a light bulb did not illuminate.)
>


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