"Christian McArdle" wrote in message
. net...

The armour of the SWA cable is earthed at the house end, but MUST be
isolated at the shed end.

It is the "MUST" part above that needs explaining.

Sorry for a rather delayed follow-up. If you don't break the earth then you
have effectively exported the house earth and your shed is a TN system, not
TT. And, unless you've sunk a massive earthing system, your local earth
electrode will be bugger-all use in preventing a rise in voltage above the
local ground potential if a supply network fault is trying to pull it up
toward 230V.

The need to get away from the supplier's TN earth is relevant where the
equipotential zone concept is difficult or impossible to apply - hence the
reference to greenhouses and Class I equipment used outdoors. Remember that
TN earths can rise to potentially dangerous voltages due to (for example)
cable faults on TN-S systems, or the classic broken service neutral on a PME
supply. OK, these things are pretty rare, but they do happen, which is one
reason why equipotential bonding is required. The big advantage of TT
earthing, properly implemented and maintained, is that your earth really is
at the local ground potential, and will stay there. Even if the earth
electrode has the maximum recommended resistance value of 200 ohms (see
OSG), then to trip a 100mA RCD shouldn't lift the earth by more than 20V.

The downside of TT is the reliance on RCDs, which don't always fail safe,
and the need to ensure that the earth system is maintained. Hence in dry
conditions it's probably best to stick with TN.

An alternative protective measure for PME installations now being
recommended by the IEE [1] is to provide your own earth electrode and
connect it to the main earth teminal in the _house_ installation - i.e. in
parallel with the supplier's multiple earthing. The recommended maximum
earth resistance in this case is, for domestic installations, 20 ohms - not
necessarily easy to achieve with DIY efforts in some soils. Then, in the
event of a break in the supply neutral, there is a path to earth which will
tend to reduce the touch voltage on your earthing. If you only have a few
lights switched on this will be a quite effective safety measure, but it
won't help a great deal if an electic shower is in use at the time of the
fault - that's when you really rely on the bathroom's supplementary bonding
to save your life.

We established in a previous thread that in a TN supply/TT generator
switchover system, that the TT earthing rod required for generator TT
operation may be connected to the TN supplier's earth. Why does the
same not apply here, provided that the earth rod is tested to below
200 ohms and the RCD requirements are met? Is it some sort of earth
loop current issue?

I hope that's clearer now from the above. A standby genny on your own
premises can always be operated as a TN system - why would you want use TT
for that? You do have to provide your own independent earthing of course -
and the parallel earth electrode approach referred to above would kill two
birds with one stone.

[1] See, for example, section 12.5.4 of the current /commentary/ on
BS 7671, ISBN 0852962371. (Recommended reading for all wiring
regs geeks.)

--
Andy