In this country (RSA), the UK and many others, the "earth" wire is
connected to the neutral (centre) point at the step-down transformer. (The
transformer is usually a "star" type with 3 phases for the Live (Line) and
a common neutral point).
It isn't an "earth" as such, unlike the ground rod which is sometimes
provided for rf grounding.
The 3 phases are normally colour coded red, yellow and blue and in a
residential installation the 1st house on a street will be
connected to the red phase and neutral + earth, the 2nd house will be
connected to the yellow phase and neutral + earth and the 3rd house will be
connected to the blue phase and neutral + earth.
The 4th, 5th and 6th houses will be connected to red,
yellow and blue respectively (repeating the sequence for all the other
houses) to try and balance the total load on the transformer.
If a household needs a 3 phase supply the 3 phases are
brought into the property with the neutral, i.e. a 4 wire cable is used
with the outer sheath of the armoured cable being the earth connection.
In the single phase house the cable is a 2 core with live and neutral and
the outer armoured sheath is connected to the earth connection.
In modern installations the incorporation of an ELCB (earth leakage circuit
breaker) is fitted which, as Ian points out, measures the imbalance between
the live and neutral currents. Connecting either the live or neutral to
the earth connection causes the main 2 pole ELCB to trip out and disconnect
the utility supply from the entire installation. In this country the level
is set to 20mA, earlier installations used 30mA settings.
So you can see that connecting the neutral to the chassis would cause the
ELCB to trip. In any case connecting the live or neutral to exposed
metalwork is illegal under the wiring codes in RSA, the UK and
other countries following the European wiring codes.
It seems that the USA has a unique condition for house wiring which is
not necessarily used in other countries.
Constructors and amateurs should exercise extreme caution when utilising
information in the ARRL handbook or US manufactured equipment as the
connections contained are specifically for the USA conditions and may
cause electrical shock hazards or even death if incorrectly used in
other countries.
John ZS5JF
----------
> From: Ian White, G3SEK <G3SEK@ifwtech.demon.co.uk>
> To: amps@contesting.com
> Subject: Re: [AMPS] Line input wiring
To: <amps@contesting.com>
> Date: 26 December 1998 02:33
>
>
> Jim Smith wrote:
> > A chassis ground is not a ground at all. A chassis ground is a
neutral,
> >or common conductor. The chassis is used as this common conductor for
ease
> >of construction. It allows the manufacturer to do a lot less wiring. The
AC
> >line connection to this chassis ground will carry the imbalance between
the
> >2 hot conductors back to the panel. This conductor is defined by code as
a
> >neutral conductor, and is necessary for proper operation of the
amplifier.
> > An equipment ground is not necessary for the operation of the
amplifier,
> >but does make it safer. The NEC forbids using the equipment ground as a
> >conductor. The equipment ground should be bonded to the amplifier case,
> >which should be isolated from the chassis that the components are
mounted
> >to.
>
> In European wiring practice the neutral return is an insulated wire or
> busbar, and ALL the sheet metalwork is bonded together and connected to
> the separate "earth" conductor. Electrically there is no difference
> between "chassis", "case" and "equipment ground" - they all mean the
> same and are all connected to the safety "earth" (ie a rod in the local
> dirt). Even with the covers removed, there are no exposed areas of sheet
> metal at a different potential from the safety ground.
>
> Like the NEC, the European wiring codes forbid using the earth conductor
> to carry supply current. This means that the phase and neutral currents
> are exactly equal - unless there is an accidental leakage to earth. For
> safety purposes modern British house wiring uses Residual Current
> breakers that operate on any small difference between the two currents
> (some breakers will trip on as little as 30mA difference in 30A). Note
> that Residual Current breakers are not the same as Ground Fault
> Interruptors - they are two-wire devices that will operate without any
> ground connection.
>
> There are still amplifier designs in recent ARRL Handbooks that use a
> four-conductor plug and say "The AC neutral wire is connected to chassis
> ground, so the chassis must be isolated from the equipment cabinet,
> which must be connected to the AC ground conductor." But they don't tell
> you how! The antenna socket is connected to chassis, so as soon as you
> plug in a grounded antenna it cross-connects the the neutral and
> equipment ground. That in turn makes the equipment ground conductor
> carry a share of the supply current, which is contrary to your NEC.
> Isn't that exactly the *wrong* way to do it?
>
> Surely the right way is to connect all the 115V AC returns at the
> primary side of the mains transformer to a neutral bus which is
> insulated from chassis. (This would include neon indicators, the blower
> motor and all mains-operated relays.) Then everything else can be safely
> connected to the sheet metalwork, which is in turn connected to the
> equipment ground and finally to the rod in the dirt.
>
> I'm not an expert on the European safety regulations, but feel pretty
> confident that any equipment that doesn't keep all of its supply returns
> (neutral) completely separate from the chassis would be illegal to
> import into any of the European Union countries.
>
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.demon.co.uk/g3sek
>
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