Thanks Bill, this is one of those concepts that is not apparent to the most
casual observer. I assume this is also the reason that 240v three phase
systems have 208vac between legs?
It is a great day when you learn something new!
73,
Gerald K5GW
In a message dated 10/15/2013 8:36:06 A.M. Pacific Daylight Time,
wlfuqu00@uky.edu writes:
Actually, if you have only 2 connections to a 3 phase system they are 180
degrees relative to each other.
The 120 degree phase relationship can only be observed with 3 connections.
In that case one becomes a reference
that the others are compared with.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of TexasRF@aol.com
[TexasRF@aol.com]
Sent: Tuesday, October 15, 2013 11:14 AM
To: w9ac@arrl.net; amps@contesting.com
Subject: Re: [Amps] Electrical Distribution Methods
Hi Paul, probably showing my ignorance here but isn't there a 120 degree
phase difference between any two legs of a three phase power distribution
system?
Seems that might pose a problem connecting the single primary of a
distribution transformer.
73,
Gerald K5GW
In a message dated 10/15/2013 7:40:04 A.M. Pacific Daylight Time,
w9ac@arrl.net writes:
Peter,
That's a good explanation.
> With Protective Multiple Earthing, provided there is enough distributed
> grounding of the neutral, that isn't a problem.
That's an area of concern of mine in one application of the U.S.
electrical
distribution system. Many of us have likely noticed that one terminal of
a
pole-mounted utility company residential split-phase transformer is
connected to one of three hot phases in a multi-phase distribution system
--
or single-phase distribution where the one hot phase is split from an
upstream three-phase source. In both cases, the primary return is
connected
to the Multi-Ground Neutral (MGN) messenger cable (AKA "strand"). That
cable is bonded and earthed down each and every utility pole. This cable
may also serve double-duty for attachment of either CATV or telephony
cables -- or where telephony and CATV is separated on different strands,
they are periodically bonded together.
It's unnerving to think that the bare copper bus wire that runs down each
pole is really connected to many, many KV and the only thing keeping a
barefooted kid safe when s/he touches an accidentally lifted/unearthed
ground wire is that (hopefully) the MGN cable strand has excellent
connectivity to earth on many poles up and downstream from the pole in
question. I understand as a matter of economics it must be this way for
single-phase distribution as is common in rural areas. But when a
240V/120V
split-phase transformer is hung on a three-phase pole, why can't the
primary
connect between two phases instead of one phase and the MGN? I can't see
a
direct economic impact in this case apart from perhaps better lightning
mitigation when one side of the utility transformer primary is grounded.
Sorry for drifting the subject off even further (notice the new subject
line) but it's been a nagging question of mine. In a past life, I was
Director of Engineering for several telecom and broadband firms and this
issue was raised from time-to-time (usually when a cable tech was
injured)
with no clear answer ever obtained from the utility companies.
Paul, W9AC
----- Original Message -----
From: "peter chadwick" <g8on@fsmail.net>
To: <amps@contesting.com>
Sent: Tuesday, October 15, 2013 9:49 AM
Subject: Re: [Amps] More on two pin 220vac
> The neutral is probably grounded at the distribution transformer.
That's
> likely to be a Y secondary and the centre point is 'grounded' and forms
> the neutral. Depending on the load and how much resistance there is in
the
> neutral, at any point along the distribution, the neutral voltage will
> rise above ground to some extent, so quite often, additional grounds are
> provided for the neutral. The problem comes if the neutral is ruptured:
> then the 'load' side of the break is connected through the stoves, A/C,
> radio gear etc to the live side of the line and the neutral, under these
> fault conditions is up at line voltage with respect to ground.
>
> With Protective Multiple Earthing, provided there is enough distributed
> grounding of the neutral, that isn't a problem. Where the neutral is
> connected to the house ground wiring, then you can get a problem,
because
> the stove, A/C etc would love that ruptured neutral to have a good
ground
> connected to it. If that ground is the external radio ground, all the
> fault current wants to go there through the cable in the wall and the
> transceiver power cord which can't take 50 or 100 amps....for very long,
> anyway.
>
> So here, it is allowed for the property earth to be separate from the
> supply 'earthed' neutral - and that's much safer in the ruptured
neutral
> situation.
>
> But different countries have different methods. At one time (and maybe
> even now) some outback properties in Australia used a ground return
> instead of a neutral.....
>
> As they say, 'there's more ways than one of skinning a cat' - although
my
> cats don't believe in any of them!
>
> I think Charles should firstly check the schematic to make sure that
there
> is nothing getting power from one side of the line and chassis -
although
> that's not been allowed for years and the amplifier probably wouldn't
work
> anyway. The use a meter to check that BOTH mains leads are a very high
> resistance to chassis - many megohms.
>
> Secondly, check the wiring to make sure the interlock wiring is as it
> should be - and Lou has given some pretty definite directions on that.
>
> 73
>
> Peter G3RZP
>
>
>
>
>
> 73
>
> Peter G3RZP
>
>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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