Andy Wade wrote in message ...
N. Thornton wrote:

Sorry but it isn't. A fault (whether phase-earth or phase-neutral)
would blow the fuse quickly -- or if it didn't, due to gross
miscalculation of the circuit design, it would set fire to the cable
instead. Either way, the shower would be rendered inoperative.

I can be quite clear that that is not correct. Fuses require large
overloads for long times to blow. An L to E leak that took a
significant time to blow might be present. Fitting a new fuse and
getting under such a shower would likely kill.

You've moved your own goalposts. Now you're postulating a 'leak' that
would overload the circuit enough to cook the fuseholder, but not blow
the fuse quickly. Such a leak would be passing several amps and would
dissipate lots of heat, thus drawing attention to itself - unless the
leakage was purely reactive, but what could cause that? I suppose that
what could happen is an earth fault part-way along the shower's heating
element, turning the 8.5 kW shower into a 10 kW one - pretty unlikely
though.

I think youll find such a fault is normal and common. Its how millions
of immersion heaters fail, they split open but keep going for a while,
with current flowing through the water, and thus total load higher
than it should be. The supply wiring gets hot but keeps going. It can
take a very long time for a wire fuse to blow - in some cases like a
whole year.

Nothing is obviously wrong, it continues to function, and if earth
impedance is low, as is usually the case, no-one is any the wiser.

If the install is modern with rcd and bonding, this will not be a real
safety problem, but if it has neither of those, the OP might possibly
be in for a very nasty surprise.


They are, but that is not the only scenario with which the description
is consistent. It would be different with a portable appliance, but
we're talking an electric shower here. If it were mine I would
insulation test it first.

I would too, as a precaution, and also check the load current (easy if
you've got a clamp-meter). But, IMHO, checking the tightness of
connections, the earth fault loop impedance and the supplementary
bonding are all more important if you are worried about being
electrocuted under the shower.

I agree, either approach would ensure safety.


Regards, NT