Wiring a generator for backup power

[Spark123]

I'll probably end up killing meself falling over the 10001 extension leads I have for mine!

 

[wotan]

Running a modern TV off a generator is not a good idea, the output is not smooth enough, you could end up damaging your TV.

Wotan

 

[ban-all-sheds]

you would need a change over switch ( 63A shown in link, you would need a 100A one )

Why a 100A one if the load through it will be limited by the breakers downstream of it?

 

[ColJack]

in my original reply, I said to put it between the meter and the CU.. ( or at least I thought I did.. )

so that would require a 100A changeover switch for the full load of the installation.. ( no seperate CU reqired for "essential circuits".. )

 

[ericmark]

Two points.
Yes when abroad I have seen generators outside shops with a lead having a plug on both ends. I have also seen many other very dangerous practices while abroad and I would say this is very near top of list for dangerous practices and is not the way to go.

As to earths again one should not rely on the suppliers earth when he is not supplying as he could have completely cut the cable to repair and not only may you not have an earth but you could also injure the guy doing the repairs.

Using the plug and socket arrangement the suppliers earth would be disconnected and using an earth rod would be best option. Using the suppliers power to test the earth rod I see no problem with so long as the safety items in Book 3 are followed.

This will mean somewhere you will need an RCD in the supply.

Note:- Some generators are not suitable to supply multi items because of the way the earths are connected. The duel voltage types often have 57-0-183 volt outlet when switched to 230 volt output. Do be careful selecting generator.

 

[ban-all-sheds]

in my original reply, I said to put it between the meter and the CU.. ( or at least I thought I did.. )

Umm... yes, you did - sorry.

It's just that I'd been thinking about a similar idea, had already decided it would be based on a separate set of priority circuits, I'd read the whole topic where this suggestion had arisen, and, well....

 

[ban-all-sheds]

My genny is connected with a widow maker

 

[SparkyStu]

My genny is connected with a widow maker

I used to connect my 110v tranny to a 110v supply using a lead with two 110v plugs on it. Then I plugged the 240v plug into a double socket and used the sedcond outlet for powering my cordless drill battery charger and radio. Works very well on a 110v only site - just smells a bit "fishy" when the tranny warms up!!

 

[ban-all-sheds]

You would fully have deserved being instantly thrown off site and never allowed back.

 

[ericmark]

Odd how such an old post has been re-started. But the transformer problem for isolation is something I have been considering. As to running a transformer in reverse it will depend on how wound where windings are alongside each other not a problem but one on top of other causes it to over heat.

But buying a 230v to 110v and a 110v to 230v pair of transformers is cheaper than one 230v to 230v and although there would be extra losses it is tempting to use this method to isolate which would then also allow 110 volt to be used.

Isolation is required so the shore earth can be dispensed with so reducing the erosion of the sacrificial anodes.

Having a 110v 60hz uninterruptable power supply it would be tempting to use that to power PC since the switch mode unit in PC does not ready care if supplied with 50 or 60Hz or anywhere between 100 and 240v ac/dc.

Something goes against the grain to transform down and up again though but can't see why. I know in Hong Kong the supply was 11Kv which we transformed down to 10Kv on the surface flexible cables then connected this to the tunnel boring train where it was transformed to 660v this was then transformed to 220 volt with a delta wound transformer forming an IT supply the only place I have ever worked on an IT supply.

I am sure the step down transformer 660 to 220 was really designed as a step up transformer as 660 side was star wound and this was installed by Robin who made the boring machine. Not sure of KVA rating but from size I would expect around the 100KVA mark it was not a small unit. So it does seem running transformers either way is permitted.

However never had so many shocks in my life as when working on the TBM so not in favour of IT system and I fully understand why not allowed in the UK.

 

[mike42]

To keep things simple I would wire those items wanted during a power cut to a separate consumer unit. 2KVA is less than 13A and either you could used a 13A or 16A plug to feed the consumer unit and simply unplug from mains and plug into generator during the power cut.

Of course one can use change over relays and switches but really not worth it for 2KVA.

Much depends on length of power cuts but if you want uninterrupted supply then inverting to DC and back to AC is the way forward you can charge 12 or 24 volt batteries both from mains and generator and then use an inverter to supply the 230Vac supply. However this would only really be done where power loss was critical.

I have not had a suppliers power cut in years which would have damaged food in fridge or freezer and more likely to be caused by a RCD tripping if the food is that expensive then auto resetting RCD are on the market but at £300 each not cheap.

I have emergency lights and that is my limit can't see the time without power is enough of a problem to do anything else.

My son has different outlook and has an inverter and two large 70A 12V alternators on his engine. He has considered changing one for a 230Vac combined alternator inverter set-up where engine speed is not important but decided not worth the expense. He relies on the 300AH bank of batteries in the main the 4th 100AH battery being reserved to start engine. He runs everything from a 16A supply and intends to upgrade inverter to 3KVA but he expects to be away from shore power for days on end. But I also was able to run everything from 16A supply in a caravan and unless you use electric for cooking and hot water 16A should be ample.

The 8A available from a 2KVA generator will not run washing machine etc. But as temporary supply should be ample.

Firstly, yes I know this post was created before Johnson's wife was born but its still valid technically I think.

The question I have is why use only 16A to feed the new CU? Why not use say 40A ? That way, you can run more stuff through your emergency CU which means when the time comes to switch in the generator you have more flexibility in what can be lit up. Of course if you did it the way I propose, then during the few hours of using the generator, you'd need to manually account for everything and ensure the wife doesn't put on the washing machine at the same time as plugging in a kettle etc since the generator couldn't satisfy anywhere near enough for a 40A load.

I'm no spark, but is the above correct or have I got it completely wrong?

 

[skenk]

The 2nd DB could have more circuits/loads (but is there any point in using a 2nd one if you still need to manually regulate the load/switch things off when running on the generator). There still wouldn't be any need to use a plug with greater capacity than the output of the generator to connect it to the changeover switch. A 2kva generator outputs less than 9 amps (and probably has loads of other issues with connecting to a house for backup purposes, such as no earth...). The connection between the mains supply and the 2nd DB (and the rating of the changeover switch) would still need to be suitable for whatever the sub-main circuit/supply arrangement to the 2nd DB is.

 

[ericmark]

Before TN-C-S there was not really a problem, but now the DNO can use same conductors for neutral and earth so there is current flowing through the conductor the voltage of the bonding may not be true earth. And this is more of a problem when the supply is lost, as we have no idea of the voltage of the PEN to true earth.

However within the equipotential area, we don't really have a problem, only when we move to an area not bonded to the PEN, this area is in general outside in the garden for example, and when we use a generator this is also likely to be outside, so out consideration has to be how we are going to bond the area where the generator is to the property bonding in a safe way?

The other option is not to connect the two together, like the shaver socket if supplying one item, then using IT is likely not going to cause a problem, however this assumes the generator has not got some internal bonding and likely it has.

So we need to do a risk assessment, and likely having a few items on a dedicated consumer unit which can either be plugged into the main consumer unit, or a generator is reasonably safe, there can still be a bonding problem, but the risk is low.

However be the emergency consumer unit be supplied with a 5, 10, 13, 16, 20, 32 amp plug, that limits the supply not only on the generator but also when plugged into the mains, so the main idea is to only have essential items on the supply, so a 13 or 16 amp plug is used so easy to swap generator to main supply.

This does cause problems as we look at freezers, boilers and lights all being on different circuits, if we were wiring from scratch it would be easy, select a colour socket for emergency supplies, and only have freezers and boiler on that radial, but we don't design homes that way, so we need to look at an alternative.

As long as the boiler is on a plug, extension leads would likely work for the odd time we lose power, the lights are the only items not on plugs, so likely we could use table and standard lamps during the outage.

So for me, a small generator, and a cable through the cat flap, and we move into using just 4 rooms of the house. The only change I need to make to use a small generator is to move the boiler from the FCU to a plug and socket, some times keep it simple is the way to go.

If I lived in a farm house with power cuts 10 times a year then would look for some thing better, but I have lived here for 3 years and never had a power cut, so preparing for a power cut is low on my to do list, a small generator so I can use garden power tools without long leads would be an advantage, better than having petrol tools, and swapping the boiler from FCU to plug and socket is easy enough.

Wish I had got an electric chain saw now, and a generator, that's the only petrol powered garden tool.

 

[mike42]

So my Gen arrived today - its 3KW/. I've been thinking how to protect it and the cabling. The "normal" way would be putting in a 13a FCU so that if the spare consumer unit tried to suck more than that, the fuse would blow.

The problem with that is it might happen rather often and I don't want to keep a supply of 200 13a fuses! Do FCUs not exist as fuse-less? Alternatively can consumer unit main switches come in very small sizes like 13A ?

How do others solve this issue? Maybe a big box of fuses is ok.....?

 

[JohnW2]

So my Gen arrived today - its 3KW/. I've been thinking how to protect it and the cabling. The "normal" way would be putting in a 13a FCU so that if the spare consumer unit tried to suck more than that, the fuse would blow. .... The problem with that is it might happen rather often and I don't want to keep a supply of 200 13a fuses!

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.

How do others solve this issue? Maybe a big box of fuses is ok.....?

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.

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).

Kind Regards, John