Shed Electrics

message from N. Thornton on 22 May 2004

From: Andy Wade (spambucket@ajwade.clara.co.uk)

I think this is something of a misunderstanding, and a very common
one. An RCD doesnt limit fault voltage at all, just as it does not
limit fault current either. What it does is ensure that fault
voltage/current is short lived. With a TT system a leakage fault is
liable to give you more or less the full 240v on your hand drill, RCD
or no RCD. The RCD simply cuts the shock time down.

Given that appliance leakage faults are far more common than PME
earths going live, the advantage is clearly not with the TT option.

There is that too: RCDs do fail, and earths are normally not
maintained. I've seen whole house earths disconnected at the rod
before now.

In short TT should not be used unless there is no safer method
available. TT allows fault voltages to rise to 240v, and relies
entirely on an RCD to disconnect such faults. Neither of these
potential risks occur with supplier-earthed systems.

If this is going to go into an FAQ this should really be cleared up
first - unless I'm mistook somewhere along the line.

Regards, NT

N. Thornton replied to N. Thornton on 23 May 2004

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.

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

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

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.

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.

Regards, NT

Christian McArdle replied to N. Thornton on 24 May 2004

If it isn't zero, then the calculations for earth fault tripping via an MCB
are false, and the MCB may well take more than 10 seconds to trip, which may
be a fire hazard, depending on the nature of the fault. Remember that fire
protection is actually vastly more important than shock protection. Hardly
anyone gets killed by electrocution in the home, whilst many are killed by
electrical fires from appliance earth faults.

Christian.

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