BTW, why does livestock make the difference ? Can't see how our sheep or
next farm's cows are going to reach any of the supply lines.

Can`t find anything more specific than this at present, and its not
particularly readable in my copied and pasted from page 20 of this pdf
file :-} :

http://www.dti.gov.uk/electricity-re...ns/gfesqcr.pdf

---
Distributors must not offer connections to earthing terminals from PME
networks for consumers=3F installations in caravans or boats. The
particular risk at these installations arises from the possibility of the
supply neutral conductor becoming disconnected from earth, possibly
causing the metalwork in the caravan or boat to rise to live potential
(assuming that the caravan or boat does not benefit from an independent
connection with earth).
Persons entering or exiting the caravan or boat would then be at risk of
electric shock.
There is a small inherent risk of danger to consumers (resulting from the
possible disconnection of the supply neutral conductor under fault
conditions) with all PME networks (see comments for regulation 7(1)),
however it is DTI=3Fs view that the risks are acceptable provided that:
(i) distributors comply with the requirements for multiple earthing
(regulation 9(2)) and take precautions to ensure continuity of the
supply neutral conductor (regulation 7(1)); and,
(ii) consumers comply with the equipotential bonding requirements of
BS7671 Requirements for Electrical Installations. Distributors
should note that at the time of connection they should be satisfied
that consumers=3F installations comply with BS7671 (see regulation 25
Connections to installations or to other networks).
Special consideration should be given to the earthing and protection
arrangements for certain installations where reliance on the connection
of the consumer=3Fs protective conductor with the distributor=3Fs combined
neutral and protective conductor could result in more significant risks.
For example, installations where it may prove difficult to attach and
maintain all the necessary equipotential bonding connections (e.g. farms
or building sites), installations at certain wet environments (e.g.
swimming pools and petrol filling stations) and certain installations
outside the equipotential zone of buildings (e.g. certain types of street
furniture). Further advice may be sought from the Health and Safety
Executive or the Institution of Electrical Engineers.
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