I am looking for advice on the easiest way to wire up a modified
electrical distribution panel here at home, to allow a generator to be
fired up and take over power supply to the house during one of our fairly
frequent mains failures. Ideally I would like a self powered contactor to
isolate the mains input to the board and make available a socket into
which the generator output is left plugged. Once power is re-established
from the mains this socket would be automatically isolated and the mains
takes over the running again. I need to (obviously...) avoid the scenario
where both mains and generator are connected at the same time! I would
like it automated so if I am not here the wife only has to check no
excess loads in the house are still on, albeit not working due to power
outage, go outside and start the gennie, with no switches to fiddle with.
I intend to have an LED on the inside fuse box to allow her to see when
the mains is back, so she knows when to turn off the generator. Possible?
Schematics? Web site showing this? I am sure it must be a fairly common
thing for people living with iffy mains supplies.

Thanks. I am in the UK. Mains is normal 240v single phase.
--
Best Regards,
Chris.

On Mon, 13 Oct 2003 12:54:54 +0100, Chris Wilson wrote:

I am looking for advice on the easiest way to wire up a modified
electrical distribution panel here at home, to allow a generator to be
fired up and take over power supply to the house during one of our fairly
frequent mains failures.

What you are wanting to do is not going to be a cheap option. If you
want a generator that can take over the whole house installation then
realistically you've got to be looking at something like 10kva capacity,
and that's exercising some caution on loading when it's running. As a
ball-park figure, cost is likely to run to perhaps £15-20k for a fully
automatic installation.

If you want to pursue a more or less automatic arrangement, but a little
less costly, I'd suggest splitting off some of the installation onto a
separate 'essential supplies' (ES) consumer unit. Typically this would
supply some lighting circuits, the central heating boiler and pump, and
possibly one or two sockets dotted around the place. You might then be
able to get away with a 2 or 3kw generator. You'd need to make sure that
SWMBO realises she can't run the washing machine or similar when running
from the generator!

You'd need to insert a two pole break-before-make changeover contactor,
in the feed to the ES consumer unit, with one side going to typically an
industrial style weatherproof male socket. The generator supply would be
connected to this using a female plug. You've also got to arrange for
battery starting for the generator, with mains-fail sensing. You need to
consider a small time-delay to allow for auto-reclosing on the mains
supply, say five minutes.

You've also got to think of generator earthing. Most leccy companies
will allow you to bond to their earth terminal *providing* the incoming
supply is PME. You should also allow for an RCD as well.

All things considered, I'd be surprised if you got away with a limited
capacity more-or-less automatic system for less than £4-5K. Your biggest
problem I would think would be sourcing a battery start generator at a
realistic price, and getting a suitable control system to operate the
generator. It's certainly possible to do what you want, but you should
have some idea about designing control circuits. It ain't really the
sort of thing that you should be hoping to do by asking questions here!

If you want a much simpler system, then use a break-before-make
changeover isolator instead of a contactor and automated control system.
If you have an ES consumer unit, then when mains power comes back to the
non ES consumer unit, your wife should be able to see that mains is back
and switch over the isolator to mains supply. You'll still need to make
sure that the genny is earthed properly.

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?

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?

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

Christian.

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.

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.

I suppose I was thinking of the RCD being after the double throw isolator
and its effect on discrimination when the system is on mains. If it is only
on the generator side, a whole house one isn't such a problem.

Christian.

"Wanderer" wrote in message
...

You've also got to think of generator earthing. Most leccy companies
will allow you to bond to their earth terminal *providing* the
incoming supply is PME.

But BS 7671 does not allow you to "rely upon the connection to the earthed
point of the public supply system when the generator is operating as a
switched alternative to a TN system" [Reg 551-04-03] so an independent earth
electrode is required - although it can be bonded to the PME terminal.

There is a whole section (551) of BS 7671 devoted to 'generating sets' -
which includes staic UPSs, BTW - that anyone seriously intending to install
permananent private generation should study. Liaison with the supplier
(network operator) is essential, as are interlocked switching (so you can't
back-feed the mains) and automatic load shedding if your genny isn't big
enough to supply the whole maximum demand.

--
Andy