"N. Thornton" wrote in message
m...

Good points, but 3 points tilt the table a bit:

1. Appliance leakage faults are many many times more frequent than a
supplier earth going live. This rather tilts what is more favourable.
The TT system performs worse on such faults.

Worse on touch voltage, but probably better on fault clearance time. But
does it matter, both systems comply with BS 7671 if properly designed and
built?

2. The fact that with most domestic TT installs, the earthing and RCD
functions are never tested after the installer leaves, not even once.

From posts here it seems to be coming quite common for a periodic inspection
to be done when a property changes hands.

It is these 2 primarily that lead me to conclude that the odds of a
nasty will be higher with a TT system.

Bear in mind the thread's subject. I'm certainly not advocating use of TT
earthing for house installations in preference to use of an available
distributor's earth terminal. That would be counter to the trend in the
supply industry, which is definitely towards universal PME (which principle
is ensconced in the ESQC regulations.) What I continue to maintain though
is the value of the TT system in outbuilding installations where the
equipotential zone principle is difficult or impossible to apply - e.g.
garages, sheds and workshops with damp floors, greenhouses or outdoor
equipment (Class I) with exposed-conductive-parts.

Note also the 170v scenario you present with TN only occurs when the
live to earth fault resistance introduced is zero, and that is
something rarely achieved by real world faults. So the situation with
TN is perhaps not as grim as it may have appeared.

See Christian's post. For design purposes you only need to consider s/c
faults. Any 'leakage' situation which produces enough voltage drop across
the CPC to give a dangerous touch voltage will generate an awful lot of
localised heat, and in practice will quickly turn into a
negligible-impedance fault as insulators track and carbonise, or whatever.
Certainly an RCD will give earlier disconnection in the event of this type
of failure (and might even prevent a fire) and in the TT installation you
get such protection 'for free' (the RCD has to be there anyway). But you
can fit RCDs in TN installations to give similar protection if you choose -
and you then get into the arguments about nuisance tripping (always a sign
of a definite problem, IME) and fire safety, premature loss of lighting from
exit route [see Peter Parry, /passim/].

Ultimately both systems have their imperfections, but with the death
rate from fixed wiring being at probably less than 1 per year,
considering all the other hazards in life it begins to become
academic in the end.

Tell that to the advocates of Part P. (Though I think it's a lot more than
one a year if you include deaths in fires started by wiring problems.)

--
Andy