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