[AMPS] Line input wiring

[Ian White, G3SEK]

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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