Hi,

BigWallop ) wrote:
:
: "Dave Liquorice" wrote in message
: . 1...
: 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-)
:
: And if the generator has any safety earth leakage device, then placing any
: of the phase to earth will activate the device and kill the supply from the
: generator.

The neutral connection must be made before any earth leakage detection
device. Dismemberment of the generator may be necessary to achieve this.

: 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.
:
: That's the situation, yes.
:
: 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.
:
: Exactly.
:
: 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.
:
: Right. Exactly.
:
: 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.
:
: Exactly.

No, it depends entirely on how good your insulation is. Assuming
good insulation all round, it means that you become the neutral
reference when you touch one of the conductors and you don't feel
a thing. This is fine unless there is anything else drawing a
current to earth at the same time...

: 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".
:
: Then the phase you've chosen to stick in the earth will need less than one
: ohm impedance to be of any in supplying a neutral at the consumer unit and
: this would need to be calculated correctly in all weather and ground effect
: situations. It may also trip any safety devices on the generator itself.
: So that needs further investigation.

Why is such a low earth impedance required? Fault currents will rarely
flow through the rod itelf and in the rare cases they do they should be
picked up by an RCD. The rod only provides a reference for the whole
system. Think of the arrangement as having the very low earth fault
loop impedance of a TN-C-S system for faults to bonded equipment
(generator permitting!) but the relatively high earth fault loop
impedance of a TT system for true faults to ground. This does mean that
an RCD on the live and neutral from the generator is essential to catch
true earth faults though. I would also be wary of earth fault loop
impedance figures when looking at the supply from a small generator -
I suspect there will be quite a lot of resistance in the generator and
an RCD would be required even if you have the best earth rod in the
world.

: 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
:
: Exactly.
:
: 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.
:
: But if the supply earth is receiving back fed half phase supply from the
: generator, which has a phase stuck in the ground at less than one ohm
: impedance, then the earth, no matter where it is coming from, will have
: picked up this charge and will supply it back to the connections that
: already exist.

??? But if it is a true earth then you have the luxury of attempting
to charge the whole planet and nobody will care. The problem occurs
when you have more than one earth and they are not quite the same.
The biggest problem is the effect you may inadvertantly have on your
electricity supplier.

For a TT supply, there is no problem whatsoever. TT is convenient in
that there is only one possible earth reference, the earth rod. This
would provide the earth reference and the installation protective
conductors. One side of the generator, all metalwork bonding, etc would
be connected to this. The external supply only gives you live and
neutral and there is no risk of earth loops.

For a TN-C-S supply, you will have two earths, the supply earth (derived
from the supply neutral) and your own earth rod. Your own earth rod is
required to provide an earth in the event that the supply cable is cut.
I would be tempted to connect the PME earth and the earth rod together
but there are two caveats: 1) There is a risk that you might experience
more rapid than expected corrosion of your earth rod due to any voltage
present on the supply neutral derived earth during normal operating
conditions (this shouldn't be an issue since otherwise any cross bonded
buried pipework would also suffer) and 2) under fault conditions during
generator operation the voltage on your earth rod will rise taking the
voltage on the supply neutral conductor with it. This shouldn't be a
problem assuming that everything locally is RCD protected, cross bonded
and the earth through the rod is reasonable but it may cause some
current to flow into the supply neutral and I don't know if the local
electricity supplier would be expecting this. I don't know the official
answer in this circumstance - it will either be specified in the relevant
IEE regs or you will need to consult your electricity supplier.

: 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.

The same applies to a TN-S system as for a TN-C-S. There is a risk
that under fault conditions you will cause an increase in the potential
of your bonding which can be seen at the end of your supply cable (on
the sheath rather than the neutral in this instance). Again, assuming
RCD protection, proper cross bonding and a reasonable earth rod there
shouldn't be an issue but I would check with the electricity supplier.

: The two earth potentials must be totally separate. No domestic supply from
: the grid can be connected to the system if you are going to change over to a
: generator supply. Any common connection between them will also become part
: of the supply from both. They must be totally separated in the phase
: connections and earth potential connections. Totally.

I don't think that is hugely practicable and in most cases there
shouldn't be a problem. In the case of TN-S and TN-C-S supplies I would
do a bit of research or consult the supplier to understand what their
concerns are with regard to their earth but keeping the earths separate
will mean that there are switching devices in the earth, a very bad idea
in my opinion. Cross bonding will generally mean that the two earths
are effectively connected anyway. If there is any significant voltage
difference between the two available earths, then I strongly recommend
that you do not use mains applicances outside the house!

Keeping the neutrals and phases separate is obviously a given unless you
have some relatively unusual arrangements with your supplier and a whole
load of protective equipment to ensure that you don't feed into a dead
public supply.

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

: But then you'll supply the earth connections with both local and supply
: earth potential. Not a great idea in my experience.

The generator chassis should be connected to earth and the local one
makes the most sense since that is the one guaranteed to be present all
the time. As I suggested above, connecting the local and supplier earths
together is probably the answer anyway.

Have fun,

Dale.