Use of generator on 1 phase of 3ph supply

[D4dog]

Looking for some advice as to the art of the possible prior to making a numpty of myself in front of my electrician.
Current board is three phase which has been split so upstairs circuits on one, downstairs on another and outbuildings on the third ( house used to have all electric heating I think so it was probably an attempt to load balance).
I have two essential supplies one to the water pump ( on top of hill, no power=no water!) and the heating circulation pump which acts as a safety device to prevent solid fuel boiler boiling the heat store. This has a battery back up but would only last about 6 hours. Both these are on the same phase.

My question is can I safely supply one phase via a changeover switch from a 1ph generator and leave the other two phases connected to main supply? So I put a changeover switch on one of the tails only. What are earthing/neutral issues with this and what problems if supply was restored on two phases with generator still supplying the other? Is this even feasible?

If not or unsafe, wont embarrass myself by asking my local sparks!!

 

[ban-all-sheds]

The N is shared - you don't have to power up all 3 lines with a generator, but you must use a 4-pole changeover switch to isolate all of the incoming supply.

Do you get power cuts lasting more than 6 hours? Would increasing the battery backup capacity be a viable alternative? And storing some water in a tank?

Do make sure you find out what the starting current is for the pumps - they can be a lot higher than the running current and require a larger genny than you night otherwise think.

 

[D4dog]

Thanks b.a.s! I suspected I would have to isolate all incoming lines. Was trying to give myself a simple indicator when mains restored by leaving external outbuilding phase connected. Guess this would be unhealthy with a shared neutral though? I have considered auto change-over but cost is huge (self start genny etc)

Our water supply is from a 500l tank which fills VERY slowly from the mains as water supply only just gets up here. Will check the start up load of the pumps. Chances of them starting together tiny so will consider worst case.

Increasing battery backup possible...but by the time I have got the batteries and decent inverter it costs more than the genny. And longish power cuts possible in severe weather because power company cant get to us in bad weather. Would make sense if I had room for PV but dont face the right way and surrounded by trees.

Thanks for advice

D4.

 

[ban-all-sheds]

Was trying to give myself a simple indicator when mains restored by leaving external outbuilding phase connected. Guess this would be unhealthy with a shared neutral though?

You may need to rearrange CUs/DBs, and/or introduce a new one, or a new switchfuse etc, but I can't think of a single reason why you should not have the changeover switch after the supply to the outbuilding.

Conceptually:

 

[JohnW2]

You may need to rearrange CUs/DBs, and/or introduce a new one, or a new switchfuse etc, but I can't think of a single reason why you should not have the changeover switch after the supply to the outbuilding.

FWIW, I agree.

Kind Regards, John

 

[ericmark]

I saw such a simple method of using a generator for selected items.
A 32A blue socket was wired to existing consumer unit.
A new consumer unit fed all the important stuff and was fed by plugging a 32A blue plug into the socket.
When there was a power failure the plug was taken out of the socket fed from consumer unit and plugged into generator instead.
No way any part of the mains supply and generator supply could ever come together so simple yet so effective.

 

[ban-all-sheds]

 

[JohnW2]

Edited: <various things>

I think you may need to read, again, (more) carefully, what eric wrote!

Kind Regards, John

 

[ban-all-sheds]

Ahh...

Deleted now to avoid spurious topic diversions. You can leave your quote intact if you don't believe that reason.

In my defence I can claim that I was influenced by the fact that I personally would never feed one CU from another.

 

[JohnW2]

Ahh... Deleted now to avoid spurious topic diversions. You can leave your quote intact if you don't believe that reason.

Whatever the reason for your deletion, I have edited my post accordingly.

In my defence I can claim that I was influenced by the fact that I personally would never feed one CU from another.

I wouldn't normally, either, but in the sort of situation eric describes, it's probably not unreasonable. It's done often enough with the infamous (and often/usually unnecessary) 'garage/shed CUs'.

Kind Regards, John

 

[ban-all-sheds]

Tails -- switchfuse -- IEC 60309 socket <--- 60309 plug -- [priority CU]


And a 60309 "extension lead" to use with the genny.

 

[JohnW2]

Tails -- switchfuse -- IEC 60309 socket <--- 60309 plug -- [priority CU] ... And a 60309 "extension lead" to use with the genny.

Sure - but if, as I suspect would be the case, the total loads on the 'priority CU' would be very modest (given that they were sometimes going to be supplied by a domestic genny), then an MCB in the main CU would be essentially the same as your 'Tails - switchfuse' - as I said, something commonly done to supply a (probably unnecessary) garage/shed CU. It's not as if the total load of main + priority CUs would be any greater than would be the total load on the 'only CU' if it were not split into two.

Kind Regards, John

 

[Lectrician]

The N is shared - you don't have to power up all 3 lines with a generator, but you must use a 4-pole changeover switch to isolate all of the incoming supply.

Do you get power cuts lasting more than 6 hours? Would increasing the battery backup capacity be a viable alternative? And storing some water in a tank?

Do make sure you find out what the starting current is for the pumps - they can be a lot higher than the running current and require a larger genny than you night otherwise think.

I assumed by his description he had three boards, so likely three pairs of tails. It would be fine to install a DP changeover switch in one of those pairs of tails. Alternatively, fit a four pole changeover switch and parallel the lives of the generator side so all phases are supplied by the single phase generator. The drawback here is that your generator is unlikely to be powerful enough to power everything, so you would need to be wary what you used. I would only recommend that to attentive people with a clear understanding of current demand.

 

[ericmark]

The problem with any change over is to ensure break before make and if a refrigeration unit it used some time between break and make is allowed. The single plug with two sockets ensures some time between break and make and also ensures earth and neutral are also swapped. Be it a 16 or 32 amp socket and plug it also means one can simply hire a generator and plug it in should your own become faulty.

I believe in keeping things simple. You can get, in fact I have worked on, USP systems with will act as an auto synchronization units allowing single and three phase inputs plus battery normally around 48 DC voltage, but bitter experience was there were more power outs caused by the UPS than saved by the UPS, in theroy it should have been install and forget, in practice it was always going wrong. The idea was we required 24/7 coms Falklands to UK, this was in case the main generator failed, we had a problems with governor on generator throwing the fan belts of the RR Eagle engines, but by time the UPS was fitted we had solved the problem, so the UPS was a total waste of money.

When I came to fit ups systems on another job, found cheaper to have an UPS per device rather than a central one. With an UPS per device, where continual power is required, using the plug in generator system gives the time between supply fail, and return, to allow the UPS to synchronize each time. Change over switches are too fast and tend to upset the UPS auto synchronization, they fail safe, but really don't want them to fail.

 

[bernardgreen]

The altermative is to leave the supplies to consumer units as they are and put changeover switches in the supply leads to the water pump and the heating pump.

Switches or a simple plug and a choice of 2 sockets, one socket from the CU and the other from the generator.