Backup generators

[dmccormick]

Where we live we get a lot of power cuts (every 2-3 weeks). Mostly fairly short-lived (minutes), just enough to bugger up the PC and reset all the clocks. However, on occassion they have lasted nearly 24 hours. It's not like we live in the sticks either, just 5 miles from Manchester Airport, but anyway.

I was thinking about what would be involved in connecting a backup generator to provide power during cuts. It wouldn't have to power the whole house, just enough to make the oil fired heating boiler work and a few lights, the TV and kettle, probably around 5kw.

I am guessing it's maybe not a DIY job - can't just plug the generator into a socket as it might fry the electrician working up the road. However, I am interested in what kind of options we might have without going for a full blown industrial backup generator job.

 

[big-all]

generators seem quite expensive for 5kv £700
http://www.machinemart.co.uk/ranges.asp?g=113&r=2098
would have thought 2'8kv would run you computer tv video and a couple of lights but no kettle[camping stove ]for less than £400

 

[brown-nought]

A small portable generator is one thing but linking into your fixed wiring is another - its very complex and specialist work.

Buy a small UPS for the PC - it will allow the PC to shut down gracefully. (Most UPS do this automatically too). Torches, Wind up radios, candles, camping stoves and a Calor gas heaters are other ideas. TV off the generator?

You could consider an emergency maintained light near the stairs etc. (They can be useful near the CU too if you find it hard to remember where the torch was last used)

 

[dmccormick]

thanks for replies. I know I can manage with a camping gas stove, etc. and generator direct to TV; it's really the heating that's a problem as that's wired directly to a switched outlet. Was just wondering if there was an easy way to power up a few selected appliances, without having to run separate cables, etc. Also, was interested in general how backup generators work.

 

[brown-nought]

There are 4 main problems:-

1) You must ensure that the generator and mains can never supply a circuit simultaneously. Although generators can output 230v it would be out of phase with the mains.
2) Earthing. Generators are not tied into you houses earth and effectively operate as” IT” systems.
3) Ensuring the same level of fault, shock and overload protection regardless of supply source.
4) Noise. The more powerful the generator the noisier they get.

Most commercial systems work along side UPS’s for critical systems like Computers systems etc and rely on a short interruption to non critical systems like Air Conditioning etc as the generators fire up.

 

[andy]

There are 4 main problems:-

1) You must ensure that the generator and mains can never supply a circuit simultaneously. Although generators can output 230v it would be out of phase with the mains.

a 2no 2nc contactor can do this

 

[IanDB]

I've often pondered the same thing. The most critical item being the central heating. I wouldn't waste time getting a generator big enough to boil kettle. You can pick up really neat camping stoves for around £20 which would do the job far more cheaply.

You can get some small, quiet generators that will handle 1kVA quite happily. They're also reasonably priced. I was thinking that you could put a three pin plug onto the central heating supply and then either plug it into the mains supply or into a generator socket as appropriate.

The problem with backup generators is how they switch in and whether they introduce a glitch into the supply.

It gets expensive if you want self starting on power fail etc, etc. Brown-nought covered most of the problems.

 

[brown-nought]

In theory an arrangement would be similar to:-
[code:1]
Boiler ----- FCU
|
|
DP Break before make
Change Over
| |
MCB |
RCD 30ma Dp Switch
| |
| Original Mains Feed/Ring
|
DP Isolator
with lock out
|
Industrial Plug ---- Generator
[/code:1]

The generator connection and circuit is similar to a marina-boat /caravan connection.

I'm still uncomfortable with the earthing arrangement of relying on the pipes being protected via the boiler's earth path back to the generator.

If you are serious about doing something like this, I would strongly advise getting advice from a contractor experienced/insured to do this type of work. (I.e. Stand-by-power systems on the NICEIC contactor search page.) Just plugging fixed appliances into a generator isn't the way to go. Any solution would also need to ensure Gas/Oil/Building regs were adhered to.

 

[dw3101]

we have just this done at our house.. have a gas generator in the garage.. with jet wound right out.. just runs on house gas

Unfortunatley this is 110v.. so it goes though a transformer to step it up,

we then have a switch above the CU main switch, so when CU is of at main sw.. it operates a contactor and transfers the 'critical' loads over to a different CU which is fed via the Generator..

On mains returning this kills the generator automatically..

Dunno exact details as dad did it, but I could find out.. (he works for YE and does know what he is doing before anyone says anything)

He also has some nice 1mva + gas turbine generators, but I believe st james hospital probably have these..

David

 

[plugwash]

its not that hard but there are some issues you need to be aware of

generally when using a portable generator for anything more than a single tool you should link the neutral and earth in the plug used to connect to the generator (if you want a detailed explanation of why just ask)

then run from the inlet for the generator and from a 16A mcb in the non-rcd side of the CU to the inputs of your supply changeover switch. Automatic may be nicer but unless you have a generator that can auto start i don't see a huge amount of point in doing so

then after the changeover switch wire to a rcd and then radially to the sockets/fcus you want to cover (might be an idea to use a different style of socket so you can identify them easilly)

2.5mm should be ok throughout with this system (assuming a generator of 3KW or less)

 

[ban-all-sheds]

Buy one of these and flog power to your neighbours...

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=46412&item=4339374304&rd=1

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=1267&item=3855193169&rd=1

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=46412&item=4339753631&rd=1

But seriously...

I'm still uncomfortable with the earthing arrangement of relying on the pipes being protected via the boiler's earth path back to the generator.

Why would they be? You aren't going to disconnect the bonding...

 

[dmccormick]

thanks again all. the third one looks interesting especially as it says "will post to united kingdom". Wonder what the Royal Mail would make of that one.

Anyway, I'm obviously not going to try to rig any of this up myself, but I might mention to the proper spark I get to come to check over my foreigner job (previous post) to see if we can get some basic set of circuits working off a small gen.

Might become more of an issue for everyone if the doom and gloom stories about undercapacity of the national grid start to come true.

 

[dmccormick]

one last question on this post- why do generators refer to kVA rather than kW output ?

 

[andy]

apparant power and actual power. times KVA by 0.8 to get what roughly what KW it can power (0.8 is thepower factor, which varies)

 

[ban-all-sheds]

one last question on this post- why do generators refer to kVA rather than kW output ?

Because the makers of the generators have got no idea what the PF of your load will be, so what else can they quote?