Wanderer wrote in message . ..
On 6 Sep 2003 05:31:44 -0700, N. Thornton wrote:
snip

Err, are you suggesting that the Faraday Cage doesn't work? So how come
most leccy companies now do so much 'hands on' live working?

A bathroom is not really like a faraday cage, since it has mains in
there that is not at the 'cage' potential. Its more like an earthed
cage with wired appliances in it, which occasionally become live.

Occasionally? I hope not.

Of course it does, faults happen, thats why aditional safety measures
are needed.


2 issues with equipotential earthed bonding a

a) a shock from light fitting to bonded metalwork is worse than a
shock from light to unbonded metalwork
and
b) should there be a fault with the earth feed lots of metal will
become live instead of a little.

Those points dont make it bad, but they do make it less great than it
first looks.

So you're suggesting that PME is not worth bothering with?

No, I haven't criticised PME.

May I
respectfully suggest that you need to go away and quietly reflect on
just what it is you're saying.

*If* an appliance or wiring installation develops an earth fault then
the various safety measures *should* act to isolate that fault.

Indeed, they shuold. Domestic wiring regs also cover situations where
safety measures dont resolve the problem, due to the protection
measures being faulty. Multiple protections is the approach.


If they don't, then the whole fabric of the building rises to some
voltage above zero with respect to earth,

No it doesnt, if an appliance develops an earth fault you may be
shocked by current passing from appliance case to concrete floor,
metal door frame on damp bricks, etc.


coz the leccy companies
connect the neutral of the distribution system to earth at the
transformer.

Right, but even a live 'earth' feed wont make the whole fabric of a
building live, it would introduce numerous potential electrocution
situations within the building.


Because you in your dripping wet state are safely contained
within a Faraday cage, formed by the equipotential crossbonding of the
installation, the risk of electric shock is minimised, because you are
at the same voltage as everything around you.

You seem to be forgetting that faulty light fitting above you as you
get out of the bath.


But I would look at equipotential bonding from another point of
view...

Please don't.

Pretty significant point.


Regards, NT