mitchd wrote:

would agree regs quoted are relevent, to every installation, just don't
see the relevance to an earth rod being required on an outbuilding

131-01-01 (i) requires you to /design/ an installation to be safe. A
PME earth connection is potentially unsafe if it's exported outside the
equipotential zone and there is risk of simultaneous contact between
exposed-conductive-parts of the electrical installation and the general
mass of earth. These risks will arise if the DNO's combined neutral and
earth (CNE) supply conductor is broken, or if a phase-neutral or
phase-earth fault occurs elsewhere on the distribution network,
temporarily raising the PME 'earth' potential above ground, due to the
voltage drop along the CNE conductor. Such faults can take more than
0.4 s, and even more than 5 s to clear.

There's usually no difficulty if the outbuilding is dry. Any incoming
service pipes that constitute extraneous-conductive-parts can be
main-bonded to the incoming submain's earth in the usual way, creating
an equipotential zone. The earth conductor in the submain (usually the
SWA's armour) then functions as both a CPC and a main bonding conductor.
Because of the latter function the min. size for this 'earth' is 10
mm^2 copper, or 22.6 mm^2 steel armouring. This is met by 2-core SWA
(BS 5467) of size 6 mm^2 or larger.

So far, so good. But if the outbuilding is a garage with a damp floor,
or a greenhouse, the only way you could create a dependable
equipotential zone would be with a buried earth mat in the floor
structure, or in the soil - not very practical for existing structures.
Worse still is when the user uses an extension lead to use Class 1
equipment outdoors. The problem could be avoided by using only Class 2
equipment in the outbuilding, but that would preclude installing any
3-pin socket outlets, using metalclad accessories, which are usually
desirable on grounds of robustness.

The problem can be avoided by using TT earthing for the outbuilding - a
much safer arrangement all round, provided that the earth system and the
RCD(s) can be maintained. This is fairly standard practice, and has
been since use of PME became well-established during the 1970s.

The ESQC regulations legally forbid suppliers from providing PME earth
connections on supplies to caravans or boats. (Regulation 9 (4) "The
distributor shall not connect his combined neutral and protective
conductor to any metalwork in a caravan or boat.")

--
Andy