On Sat, 08 Nov 2003 05:15:42 GMT, BigWallop wrote:

Some people don't understand the differences between the domestic
supply and a generator supply system, so they get stuck in this Live
and Neutral thing.

I fully understand where you are coming from and I believe we actually
agree, there seems to be a "separated by a common language" problem.
B-)

Local Earth - Properly installed connection to the local physical
ground via an earth spike etc.

Supply Earth - The "earth" connection provided by the public
electricity company.

Generator assumed to have no center tap to the alternator windings or
if it does the center tap is *NOT* connected to *ANYTHING*. A
generator with a center tap connected to local earth cannot make a
safe connection into a normal installation as both generator phases
are fixed at half the supply potential away from local earth.

For simplicity I have omited mentioning anything about how any
connections are made. Obviously the public supply L and N need to be
fully isolated from the CU before the generator phases are connected
and vice versa.

I agree that simply to connect the two "phases" from a generator to
the L and N terminals of a CU is unsafe. Both generator phases are
floating with respect to local earth. It is quite likely that the N
line will be at a significant voltage above local earth, ouch if you
come in contact with it. Single pole switches will not make the
connected equipment "safe" as they only isolate the L connection.

To over come this problem you pick one of the generator phases,
doesn't matter which, and bond that with correctly sized cables to
local earth, at the generator. This local earth bonded generator phase
you connect to the CUs N terminal. The unbonded generator phase to the
CUs L terminal. You now have a situation where the N wiring is at, or
very close to, local earth potential and all the L wiring is fixed to
be at the nominal output voltage of the generator. Single pole
switches now work "correctly".

Now the tricky bit. All the circuit protective wiring (aka the "earth"
wires in T&E etc) in the building are connected back to the main earth
termal next to the CU. The main earth terminal is connected to the
supply earth and equipotential bonding is also run from the main earth
terminal to the incoming service pipes etc

Trouble is that the supply earth could be at a different potential to
the local earth. Depending on the public supply type I think the
dangers of connecting the local earth to supply earth change. For TT
(local earth spike) and TN-C-S (PME) installations I don't *think*
there is much to worry about, as the supply earth is derived at or
very close to the premises.

I do have worries about making a local earth/supply earth connection
on a TN-S (supply earth provided from supply cable armour or metalic
sheath) system. This is the part that one needs to take advice on from
a qualified spark with experience of generator instalations.

Finally should the generator chassis be connected to the local earth?
Yes.

--
Cheers
Dave. pam is missing e-mail