Wiring a generator for backup power

[EFLImpudence]

Do FCUs not exist as fuse-less?

Yes, they are called Flex-outlets or ones with switches are called - Switches.

 

[mike42]

I'm not even sure that it would achieve what you think it would achieve. It takes about 22A flowing continuously to blow a 13A fuse, and appreciably higher currents than that for shorter times.

It would protect the generator.

No. As has been said, if you are going to have a separate CU for generator-supplied things, the sensible approach is to only have that CU feeding things that the generator can supply (simultaneously) - otherwise, as has been said, there's really no point in having a separate CU.

The problem with that is that would require re-jigging of the circuits which isn't financially feasible. Take an example. One ring main powers a bunch of sockets only one of which I would need in a power cut (the fridge). There is another freezer which I also need but its on a different ring main.

My existing board is 18 way and I would need probably 7 of those. 4 are lighting circuits, 1 is a 16A boiler+pump and then the two others are 32A socket circuits each with a fridge.

I am thinking to move those 7 and then protect my generator in case someone plugs in too much during the power cut.

Nor do I really understand what you think the hypothesised fuse would achieve - a generator cannot supply more current than it can supply. If you tried to draw more current than it could supply, either the voltage would fall or, more likely, the generator would stall (stop).

It would blow if too much current flows through it. I don't want to see how the generator would behave - I would rather have the load reduced to zero by a fuse blowing.

Kind Regards, John

 

[mike42]

Yes, they are called Flex-outlets or ones with switches are called - Switches.

I appreciate your time to reply. However as I understand it a switch is a manual device that an operator can turn on or off.

An FCU will automatically cut the circuit when the load flowing through it reaches a threshold somewhat higher than its rating.

A miniture circuit breaker (mcb) sits inside a consumer unit and does something similar but there is no fuse. I was wondering if there existed anything like that in a switch like form factor.

 

[JohnW2]

It would protect the generator.

As I said, the generator doesn't really need the sort of 'protection' you are talking about. Unlike the National Grid which can provide an almost unlimited amount of current if large loads are connected to it, as I said a generator can only supply however much current it can supply (in practice, pretty little).

The problem with that is that would require re-jigging of the circuits which isn't financially feasible. Take an example. One ring main powers a bunch of sockets only one of which I would need in a power cut (the fridge). There is another freezer which I also need but its on a different ring main.

I obviously don't know what you would regard as 'financially feasible', but it would not be a massive job to install new sockets for just the fridge and freezer (and, presumably boiler etc.) fed from the 'generator CU'

It would blow if too much current flows through it. I don't want to see how the generator would behave - I would rather have the load reduced to zero by a fuse blowing.

As I've said, you will not harm the generator by overloading it, and nor would any cable be at risk, since your 3kW generator could not provide enough current to harm any credible size of cable.

In essence, I would say that you are being unnecessarily cautious/concerned.

Kind Regards, John

 

[JohnW2]

An FCU will automatically cut the circuit when the load flowing through it reaches a threshold somewhat higher than its rating.

Yes, but if it's a 3kW generator, there is no way it is going to supply enough sustained current to blow a 13A fuse.

If there were a 'short circuit' (rather than sustained 'overload' due to items being plugged in/switched on) then the fuse would blow, but that would all happen in milliseconds, before the generator had a chance to stop.

A miniture circuit breaker (mcb) sits inside a consumer unit and does something similar but there is no fuse. I was wondering if there existed anything like that in a switch like form factor.

You can put an MCB (of any rating you wanted) in a small enclosure but, again, I'm not sure what you would be trying to achieve by using it.

Kind Regards, John

 

[mike42]

Yes, but if it's a 3kW generator, there is no way it is going to supply enough sustained current to blow a 13A fuse.

Thanks John for your advice and especially the line above which I hadn't thought properly about. Seems like I was overly worried about that.

However, regarding new sockets, it is obviously not an issue chopping in some sockets but the cabling to them would most certainly would be as would require plastering/tiling/painting and moreover would mean moving semi-built in fridges to access sockets behind during a power cut.

 

[JohnW2]

Yes, but if it's a 3kW generator, there is no way it is going to supply enough sustained current to blow a 13A fuse.
Thanks John for your advice and especially the line above which I hadn't thought properly about. Seems like I was overly worried about that.

Indeed - as I said, I think you are being unnecessarily concerned.

However, regarding new sockets, it is obviously not an issue chopping in some sockets but the cabling to them would most certainly would be as would require plastering/tiling/painting and moreover would mean moving semi-built in fridges to access sockets behind during a power cut.

Fair enough. As I said, I couldn't judge what you would regard as reasonably do-able (without too much hassle).

Kind Regards, John

 

[ericmark]

Many years ago when working in the Falklands we had a fleet of 3.5 kVA and 7 kVA generators, one single cylinder Lister the other twin cylinder and the generators looked near enough the same design but one larger than other.

However with on over load would cause the field to collapse, the other would burn out. The locals would fit the 7 kVA on a single cylinder, then over load would cause engine to slow down rather than the generator be damaged.

Today generators tend to how overloads built into the generator, but I think that is the least of the worried.

1) Is to ensure the DNO supply and the generator supply can never get connected together.
2) Is to consider how to earth.
3) Is how to stop an overload.

In my younger days I did make errors, with a 75 kVA generator and above, largest I have worked on was 750 MW at Sizewell, the generators are designed to have earth rods or mats and not really be moved too often, and have been designed with all the safety stuff in mind, at over 750 kVA often not low voltage, and with some method to synchronise.

It is the smaller versions which need a lot more care, they have some unusual features, which often don't spring to mind, like having a DC link to parallel up, the generator is actually producing DC and an inverter turns it into AC and this can allow the generator speed to fluctuate without the output frequency changing. Also the earth is not always bonded to neutral, but some tapping. And even current sensors so it can idol until required, often with a lower voltage and frequency. I know I have had mag mount drills fall off when a flood light bulb failed as the generator dropped to stand by mode.

How the generator is connected up depends as much on the generator as to the installation, some of the narrow boat systems are really clever with the way they synchronise to shore supplies.

Visiting Turkey I saw many widow makers, where the pins of a plug could become live, some really dangerous systems.

This country we have to be so careful, in the main due to problems associated with a broken PEN.

Things are moving on with battery packs and solar panels designed to auto disconnect from the DNO supply on a power failure, and run the home from batteries and solar panels, it is the inverter that does all the clever stuff, and to connect DC gear together we don't need to worry about synchronising.

Personally having an extension lead with maybe 8 sockets on the end and manually swapping plugs in the main is enough for a power cut, the idea of using some change over switch is just waiting for some thing to go wrong.

 

[SimonH2]

My take on it is ...
For a cheap and cheerful setup, just make sure your boiler/heating is using a plug and socket rather than hard-wired via an FCU (many heating "engineers" still think using an FCU is mandatory - I had to "strongly request" they refitted the socket when they replaced my mother's boiler a couple of years ago); and have some extension leads readily accessible. That's my current emergency plan.
Other than that, IMO the sensible way is a C/O switch between meter and CU, with a power inlet (wall mounted plug) on the other inlet to the C/O switch. That's what I plan to do when I get around to replacing the CU (part of the plan to get rid of the inconveniently located CU in the extension). But then you have to consider earthing and generator wiring ...
This thread over on the Electrician Forum is "interesting" - in that it hilights the problem that there isn't any way to do a generic setup since there are interdependencies between how the genny is wired and how you need to wire the rest of the setup. So in my opinion you can only provide the 3 pin power inlet with a local earth - and it has to be the responsibility of whoever hooks up the genny to ensure the genny and connecting cable are suitably wired to provide a "TN-C-S like" supply. If the genny is hard-wired then you can make sure it's wired correctly - but once a plug in connection is involved then you have no control over what someone plugs in later on.

 

[JohnW2]

My take on it is ... For a cheap and cheerful setup, just make sure your boiler/heating is using a plug and socket rather than hard-wired via an FCU ... That's my current emergency plan.

That's certainly what I do, but I don't know that I would necessarily classify it as "cheap and cheerful"

In conjunction with that, I have a number of sockets scattered throughput the house, and a number of strategically placed lights throughput the house which are only ever powered by the genny (i.e. only connected to the inlet into which the genny is plugged, when in use) is connected to when used - i.e. wiring totally separate from the grid-powered sockets and lights.

Other than that, IMO the sensible way is a C/O switch between meter and CU, with a power inlet (wall mounted plug) on the other inlet to the C/O switch. That's what I plan to do when I get around to replacing the CU (part of the plan to get rid of the inconveniently located CU in the extension). But then you have to consider earthing and generator wiring ...

That is the obvious thing to do, and how I originally did it. However, I eventually got rid of that in favour of what you call the "cheap and cheerful" approach above. For a start, I didn't want the hassle of having to switch off circuits (or remember not to use certain loads) derived from the CU when it was genny-powered. More to the point, I have multiple CUs scattered around the house, which made the 'sensible approach' less than convenient if I wanted genny-power and genny-powered lights available throughout the house.

Kind Regards, John

 

[SimonH2]

More to the point, I have multiple CUs scattered around the house, which made the 'sensible approach' less than convenient if I wanted genny-power and genny-powered lights available throughout the house.

Do you have multiple supplies ? Unless you do, it doesn't matter if you have multiple CUs, you just put the C/O switch between the meter and "all your installation" - i.e. between the meter and where the tails/submains split to the separate CUs.
I agree that managing demand could be an issue in some properties - particularly if there are multiple people, some of whom don't really understand how much power things use, or struggle with the concept of "don't switch on X". That's very much going to be a YMMV thing.

As an aside, at a previous job we had fairly frequent power cuts for a while - such that manglement asked about us having our own genny. Given that we had our own 1/2MVA substation, a "run everything" genny would be somewhat more than they envisaged spending. So we considered options ...
One was the "have dedicated circuits for critical stuff" - but we discounted that on the basis that it would be a never ending game of rewiring the place as things moved around. For example, one "important" bit of kit was considered to be the PC that was used for uploading the payroll information to the bank - and that section seemed to move around a bit. And of course, you can guarantee that if (for example) someone "needs" to do some copying/printing and see's that some equipment is working, then they'll simply start moving plugs ...
Another option was to have something in between and switch off large loads. That would keep the offices running, and some of the work in the factory (much of production involved electric heating, but there was usually plenty of finishing & packing work that only needed light). One idea I had was to have "keyed bars" which would have to be positioned in each DB to would lock off the MCB for anything not to be genny powered and interlock with the main switch for the DB. But as manglement didn't have the appetite for the cost of that size of genny either it was a very short discussion.
Once the power came back on, the question simply disappeared. It got to the point where the guy who had to deal with this would automatically grab the file from his cabinet if the power went off, and somewhat brusquely announce "this is what I told you last time" when the inevitable "how much would a generator cost ?" question came up yet again.
But since it was a candle factory, at least we weren't short of light when the power went off

 

[JohnW2]

Do you have multiple supplies ?

No, just a single 3-phase supply.

Unless you do, it doesn't matter if you have multiple CUs, you just put the C/O switch between the meter and "all your installation" - i.e. between the meter and where the tails/submains split to the separate CUs.

In theory, yes. However, although 4-pole changeover switches exist, they tend to be physically large and pretty expensive. When I used a c/o switch, I therefore put a DP one just in the feed to the CU serving most of the ground floor (including heating system), which obviously meant that the upper floors could not benefit from the genny supply, even for lighting. In any event ....

I agree that managing demand could be an issue in some properties - particularly if there are multiple people, some of whom don't really understand how much power things use, or struggle with the concept of "don't switch on X". That's very much going to be a YMMV thing.

That was probably the greatest issue for me. It's not just 'people' - on the upper floors there are things like immersions on time switches, which would have to be manually disabled if they were being fed from a CU that had been switched to a genny supply.

So, the 'modified cheap and cheerful approach' really suits me better. Now that everything is LED, all of the lights on the ';genny only' circuit sre usually left on, some light on every floor appears the moment the genny is started up. The heating (13A [plug) has to be manually transferred to a genny-only socket, and anything else I want to (and can!) run from the genny is then plugged into one of the other 'genny-only' sockets dotted around the house.

Having said all that, in the 25+ years I've had the system in place, I have hardly ever had to use it in anger. Whilst, being in the sticks, 'power cuts' are a very common phenomenon, they are usually extremely brief, and don't usually even give me enough time to go outside and fire up the generator!

Kind Regards, John

 

[ericmark]

This thread over on the Electrician Forum is "interesting" - in that it hilights the problem that there isn't any way to do a generic setup since there are interdependencies between how the genny is wired and how you need to wire the rest of the setup.

This is what I found, one generator had a tapping for earth bonding not neutral.

The problem is on this forum many are not electricians and try to DIY things beyond their skill set, so telling them to do things can easy cause errors.

So easy to end up with a widow maker.

We can talk about TN and TT, centre tap and earthed neutral, but what will the DIY person understand?

I remember on the Falklands finding a 110 - 0 - 110 volt supply, but the problem was the consumer unit did not have linked double pole MCB's in it, found same in UK with a system designed as 0 - 110 volt where the transformer had been renewed for one with an internal central tapped link to the frame, so again no over load on one leg, this resulted in whole machine needing a rewire after a neutral to earth fault.

As to earth rods, again open for error, a 200Ω rod will cause a RCD to trip with a faulty appliance outside, in doors everything bonded so even with no rod it would still trip, but a 1Ω rod could allow too much current to flow, and connecting neutral and earth together is clearly not permitted when connected to the DNO supply.

And with fault conditions no one knows how much current will flow if earth and neutral linked.

Too many times at work been asked how to I do this, and I tell them, then visit the home, and see what they have done, and realise they have not understood correctly what I had said, so DIY still feel unplug from DNO supply and plug into generator supply is the way to go, and any link neutral to earth should be inside the generator, and any earth rod should be connected to the generator earth lug, so if generator changed there is no possibility of a short circuit.

 

[mike42]

So my setup is nearly complete. I avoided the widow maker issue by making up an HO7 flex with 13a plug at one end to go into the generator and a commando _female socket_ at the other. That plugs into a male socket on the outside of the house which in turn goes to the changeover switch.

My plan would be to link E and N in the dedicated flex plug and also connect the rod to the earth terminal on the generator. I have already tested that E on the generator is linked to E on the socket (as well as the frame etc) and is not connected to N.

One thing I'm not sure of though is the quality of my earth rod installation. I mean as far as I can see the rod is held snugly and the connections are well made but I don't have a dedicated earth rod testing kit. Now I did measure the resistance between my rod and the main earth terminal of the house. That gave just under 100Ohms. I figured that should give me an upper bound (worst case) resistance of the rod but I admit my knowledge is lacking here.

The earth in the changeover switch is permanently connected to the suppliers earth (PME). On changeover, that connection is still there but then the earth rod will be plugged in and so connected to the supplier's earth. Since PME generally earths the neutral supply at multiple places along its route, I figure this is just like one more earthing. Other than seeing other's on the Electrician's forum doing this which gives me reassurance, I'd like to understand a bit more of the theory to feel more comfortable.

 

[SUNRAY]

We can talk about TN and TT, centre tap and earthed neutral, but what will the DIY person understand?

I remember on the Falklands finding a 110 - 0 - 110 volt supply, but the problem was the consumer unit did not have linked double pole MCB's in it, found same in UK with a system designed as 0 - 110 volt where the transformer had been renewed for one with an internal central tapped link to the frame, so again no over load on one leg, this resulted in whole machine needing a rewire after a neutral to earth fault.

On the subject of which, I've encountered dual voltage (220/110V) generators which are 110V centre tapped to ground terminal but 55-0-165V with 55V between neutral and earth terminal. On top of that they are often a single pole overload device.
Linking neutral and earth is very much a no-no on those. And it's not always the cheap ones.