The only situation in which it's mandated by BS 7671 is where the circuit
earthing (CPC) is through metal conduit or trunking attached to the
back-box. Then you must fit a separate protective conductor between the
earth terminal in the back-box and the earth terminal of the wiring
accessory [Reg. 543-02-07].
True. But on most sockets the eyelet for the screw is usually part of the
sockets earth bar. To achieve your scenario would need the earth to fall out
of the socket terminals and "float" in the back box whilst at the same time
the live falls out of the socket terminals and touch the back box. In this
case the earth bonding lead is useless. Refer to LOZ34's earlier reply.
It does not matter if the socket is in contact with the wall or back box,
the socket is made from plastic!!!. A bit of corrosion, as you put it,
contradicts your first statement and would make the socket safer.
I'm sorry, but there we disagree. I've repaired curcuits that ceased
working after some time - years possibly - due to the screws for the live
or neutral not being correctly tightened in a socket or sockets when
installed. The conductor corrodes and ceases to make contact.
*I don't know what your problem is, but I'll bet it's hard to pronounce
Dave Plowman email@example.com London SW 12
There has to be -- aluminium oxide is a damn good insulator.
I was breadboarding a circuit which uses power MOSFETs for
directly switching mains. I temporarily used a bulldog clip
to clamp the MOSFET to a heatsink. Later, I clamped the second
MOSFET to the same heatsink.
It was only when I was disassembling the circuit to build it
properly that it dawned on me that I hadn't used any insulating
washers between the live MOSFET tabs and the heatsink (OK so far,
I was expecting the heatsink to be live whilst breadboarded),
and that the two MOSFETs had full mains voltage between their
heatsinks (oops!). The thing that saved them was that the
aluminium heatsink, like all aluminium, had coated itself in
a thin invisible hard layer of oxide, which was enough to
insulate it from mains even against the force of a strong
bulldog clip. The final build uses insulating washers obviously,
and although the heatsink is bolted to the base, it also has a
separate earth bonding connection using a sharp earth tag to
cut through the oxide layer.
This is also why aluminium mains wiring (common in US homes)
is rather a bad idea.
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