On Mon, 13 Oct 2003 15:48:26 +0100, Christian McArdle wrote:

You'll still need to make sure that the genny is earthed properly.

Out of interest, what would you need to do if you have PME earth?

Is that what you meant to ask? That's exactly the situation I *did*
cover in my comments.

You would obviously need an earth rod and 100mA Type S RCD arrangement.
However, do you have to disconnect the supplier's earth upon power failure?
If so, presumably it is simpler to just use the TT system at all times, or
is there a way of dealing with this?

Things may have changed since I left the industry and individual leccy
companies will have their own rules for the connection of stand-by
generators. In general terms, it was always considered better to leave
the earthing bonded up solidly, and drive a couple of earth pins close
to where the generator is to operate, as part of the genny installation.
I would also recommend the use of an RCD as additional protection for
the genny supply, although I can't see your reasoning for using Type S.
If an earth fault occured when the installation was operating from the
genny, I'd want it to operate immediately.

Can you assume that there will be nothing unduly bad on the PME earth and
leave it connected through?

Yes, subject to the comments above. What people always tend to overlook
is the philosophy behind PME, which is that it relies on the Faraday
cage for protection, afforded by bonding everything! I also realise that
it's quite fashionable to knock the principle as old-fashioned, but it
works.