Changeover for backup generator

[Mat Burchett]

Hi


I know this is an old post however I am wanting to do something similar


I am going to buy a 5.2KW generator and I would like it to power my house. I know I couldn’t use the oven at the same time as the shower. Im very mindful that when i'm on generator I am careful with what I use.


I will also have a meter so I can see what I am drawing at any one time.


I spoke to a friend who’s an electrician and he knows what he’s doing, However he hasn’t done generators before.


What he’s telling me is I can’t have my 32amp from generator to change over switch to consumer unit as the generator can’t power all of my appliances. I have said I won’t be using them all at once but he has said it’s not about what your using its about what can be pulled.


I thought it would just go from generator to change over to consumer unit and I switch on what I need.


Any help would be appreciated as found a good price on generator but don’t want to spend money if I can’t get this signed off.

 

[SimonH2]

I think your mate is talking bol...rubbish. It is true that you won't be able to power everything, and it's also true that you'll probably struggle for discrimination in that (for example) a fault in one circuit is more likely to trip the genny than trip the breaker. But there is no reason not to do exactly what you are talking about - and it's something I am planning to do when I replace my own CU.
Obviously the change-over switch has to be suitably rated for the mains supply - making it somewhat over-rated for the genny supply.

 

[JohnW2]

... and there are potential issues (about which opinions vary) regarding the earthing of the generator supply - has your electrician expressed a view about that?

Kind Regards, John

 

[bernardgreen]

making it somewhat over-rated for the genny supply

One option is to have two consumer units. Put all the circuits that the generator will supply on one consumer unit (Generator CU ) and all other circuits on the other ( Mains CU ) which is fed from the mains.

The change over switch can then connect the Generator CU to either the generator or to a feed from a suitably rated MCB in the Mains CU

 

[SimonH2]

I think two CUs is going to be overkill and not very flexible - as it means deciding in advance what you will and won't want to run off the genny, and running additional cabling. I think most people can learn the principle of "switch on cooker, lights go out" if the genny isn't big enough to run it.
A switch capable of switching the full mains needn't be too expensive. TLC have this 125A one, though I'm wondering where I'll find space for it

 

[Jackrae]

You'll never run a shower off that genny and it won't give you 32 amps either. Either you have typos or mis-information.
Unless you've a super-dooper cooker you might be able to run that and some house services.
Some makes of cheap gennies don't give good output regulation so some of your modern electronics (computer, tv, video, audio, boiler etc) might object by releasing the magic smoke if over-voltaged.
Having a genny to beat power cuts seems a good idea but you need to be careful what you run on it.
You might want to consider installing and inverter between the genny and the CU. The inverter will tolerate some input voltage swinging and will give a steady "clean" AC output, as well as incorporating the necessary transfer switching.

 

[ericmark]

One option is to have two consumer units. Put all the circuits that the generator will supply on one consumer unit (Generator CU ) and all other circuits on the other ( Mains CU ) which is fed from the mains.

The change over switch can then connect the Generator CU to either the generator or to a feed from a suitably rated MCB in the Mains CU

I think the two consumer unit is a really good option, the main consumer unit has a 32A socket wired to it, the secondary one has all the items you want to run off the generator on it, and it is fed from a 32A plug, so normally it is plugged into 32A socket from main consumer unit, with power failure you unplug from main consumer unit and plug into generator instead. It means total impossible for the two supplies to get connected together, and allows two independent earthing systems.

However that would not allow you to use all house items, even if not using them together. Not all generators are the same, even when they look very like each other, on the Falklands we had some single cylinder Lister and some twin cylinder Lister driven generators which worked very different with overload, the single cylinder on over load would lose the field excitation and auto shut down, remove the load it would start again. The twin cylinder one however would burn its self out with an over load.

This control is a real problem with generators, even the bigger ones can have unexpected results with an overload. We had some RR Eagle powered generators 4 running in parallel and you could get odd things happen when the load increased very quickly, a slow overload would cause the breakers to trip, however if one generator shut down on over heat for example when the fan belts broke, the load increase was so fast the engine revs dropped before the trip went out, with it running at 2000 revs instead of 3000 revs the engine did not have enough power to over load the trip, the exhaust would glow red hot, the volts would drop, the frequency also dropped and it would damage motors unless we manually removed the load.

So although in theory one may expect a MCB to protect the generator from overload, in practice that is not always the case. Also with smaller generators, (I have worked on up to 750MW) there is a problem with regulation, both cycles and volts may not stay within the tolerance required for even some domestic equipment, the fridge/freezer is easy to damage due to under sized generator selection. Except for inverter drive models the run amps may be only 0.250 but the start amps can be 10, if other items running stops it being able to get the power it needs the motors can stall. Stalling once or twice is not a problem, but repeated stalling sees off the overload built into the motor.

 

[JohnW2]

with power failure you unplug from main consumer unit and plug into generator instead. It means total impossible for the two supplies to get connected together, and allows two independent earthing systems.

That would presumably mean that both 'normally' and whilst using the genny, the earth connection to the 'secondary' CU would be via a plug/socket, and I'm not at all sure that would be compliant with regs, would it?

Kind Regards, John

 

[oldbutnotdead]

I've looked at the generator switchover sketch a few times and there's all sorts of issues. As others have said, unless you have an inverter generator then the output waveform is likely to be rubbish, fans, pumps and anything transformer fed won't run properly (IT kit running on switch mode PSUs run fine though but the PSU does get a bit warmer than usual). My solution (on a 3kva generator) was to stick a cheapo online UPS between gennie and load- the UPS corrects the sawtooth wave from the generator into a more or less sine wave which runs the heating fan and the microwave quite happily.

If you are looking at infrequent power outages in an occupied house it'll be much easier to invest in said cheapo UPS (or 2 or 3 for 5kva load) and some extension cables- if your (gas or oil) heating source requires electricity to operate then redo the wiring so there's a 13A plug and socket in the boiler circuit (so if mains goes off you can unplug boiler from mains and plug it into generator/UPS supplied extension cable), have a box of candles for light (or a stock of LED torches)

And forget using your (electric) shower - even the cheapest nastiest will draw about 8 kvA, which will cause your 5kvA gennie to have a bit of an episode.

 

[SimonH2]

It's been pointed out to me separately that the switch in the link I posted is just a 4 module wide DIN rail mounted switch in a big box* - so it could go in a smaller box, or in the CU itself. In principle it could replace the main switch in the CU - but then we get to the old chestnut of it not being part of a type approved assembly (not to mention the terminals almost certainly aren't in the right places).

* I assume the box is sized like that so that cabling for 125A circuits has plenty of room, especially if using inflexible cable like 6181Y.

 

[bernardgreen]

As others have said, unless you have an inverter generator then the output waveform is likely to be rubbish,

? ? ? A generator is more likely to produce sine waves than an invertor. Rotating a magnetic field between coils is going to produce a sine wave form. The output voltage is best regulated by varying the current fed to the rotating winding that creates the rotating magnetic field

That said there are generators that use an invertor type circuit to regulate the output voltage and these probably will create a pseudo sine wave form.

 

[bernardgreen]

Regarding the "earthing" of the generator. Make it replicate the arrangment of the mains supply. Domestic mains supplies are in almost all situations a substation transformer secondary with the Neutral side ( or star point ) connected to an effective and very low impedance Ground electrode.

So connect the neutral of the generator to a Ground electrode. Ensure the frame of the generator is not already connected to the output. Some site generators have the frame internally connected to the mid point of the 230 volt output to give 115 - 0v - 115 with the 0v grounded via the frame and maybe an additional ( optional ) earth electrode.

Then the generator supply can be dealt with in exactly the same way as the mains supply

 

[ericmark]

That would presumably mean that both 'normally' and whilst using the genny, the earth connection to the 'secondary' CU would be via a plug/socket, and I'm not at all sure that would be compliant with regs, would it?

Kind Regards, John

Why would using a plug and socket not comply? If the earth rod is used with both DNO TN-C-S and generator TN-S it changes name from extraneous-conductive-part to earth electrode again nothing stopping this, however it the earth is good then with a fault on the TN-C-S supply excessive current can run, if not good with a fault with generator supply a higher than 50 volt could be connected to the DNO supply. You are in a way correct some where you should bond the two earth systems together, but one does need to do a risk assessment before doing so.

 

[JohnW2]

Why would using a plug and socket not comply?

Maybe I have misunderstood. When you wrote ...

It means total impossible for the two supplies to get connected together, and allows two independent earthing systems.

... I presumed to meant that all of the 'incoming' connections (L, N and E) to the 'secondary' CU would come via a 32A plug, which would be plugged into either the DNO supply (via 'primary' CU) or the genny supply. If that were the case then (whether using DNO supply/earth or genny supply/earth), the only connection to earth from that 'secondary' CU would be via the 32A plug/socket.

I'm not certain that such would be non-compliant, but I strongly suspect that it would be. Any form of 'switch' in the earth connection is certainly explicitly 'forbidden', and I don't see a plug/socket as being very different from that.

How would you react if you were doing an EICR and found some sort of plug/socket in the path between the DNO's earth and the CU?

Kind Regards, John

 

[SimonH2]

Interesting point, but then every portable appliance we use has a plugged earth connection - more so if we use extension leads. There's an argument that it's different to having a switch in the earth supply for the simple reason that, assuming the plug/socket isn't broken, it's impossible to disconnect the earth without first having disconnected all line connections.
In a way, it's analogous to (say) a site cabin where there's a small CU inside for power and lights, and a blue inlet on the outside to plug the site supply into.