UPS to the house

[Philip W]

Continuing from Vortex's thread on another thread I think a home UPS is the way forward. Andy H, has his inverter/charger/solarPV on one of his house circuits.

I am thinking of a similar setup (without the PV for the moment but with option for the future) but for my whole house.

I think I need:

1) Changeover switch (manual or automatic?)
2) Combined Inverter (pure sine) and charger (prob 2KW max say 3KW)
3) Bank of Batteries (12v or 24v?)

Probably would not want to spend more than ?2,000 on the whole setup.
I would have to pay an electrician to connect the switch to the mains (before the junction box I guess).

Questions:

1) Manual or Automatic changover?

If my lights continue blazing away uninterupted in rolling blackouts then I will draw attention to myself so manual could be better....
however with manual you have to keep checking outside to see if the power has come back on....
with automatic my better half will continue to use toasters, kettles etc...
whereas with manual you know you have to reduce consumption to the mininum...
With automatic, if we are away/at work fridges and freezers won't defrost and my solar panel will continue without overtemping....

Views?

Also any views on kit? It looks like the best bet are heavy duty marine versions, designed for gin palaces to connect and disconnect from mains AC at the dock.

Industrial UPS units look too expensive.

Anyone done any research?

12V versus 24V does it make a difference?

The main assumption here is not complete meltdown of civilisation but rolling powercuts 2 to 3 hours long across the country.

[adam2]

Secondhand UPS units can often be found very cheaply or even for free, ask anyone in the IT industry.
A secondhand unit will almost certainly need new batteries, and to achieve a decent run time these should be several times the capacity of the originalls.
Remember that the terminals of the external battery pack may not be isolated from the mains, they must be regarded as mains connections and protected against being touched.

If a UPS has been fitted with an external battery for extended run time, then it may be liable to overheating, and therefore should not be run at its full rating.

Operation is fully automatic, which as you point out means you might be unaware of the power cut.

Battery voltage is as determined by the manufacturer and can not be altered, it is typicaly 12 volts for units up to about 500VA, or 24 volts up to about 2,500VA.

If useing a standalone invertor instead of a UPS then operation is not normally* automatic, you would normally have to dissconnect the loads from the mains and connect them to the invertor, by means of a plug and socket arrangement or a suitable changeover switch.
Very great care should be taken that the invertor output can not ever, under any circumstances be connected to the mains, mistakes could be fatal. Suicide leads are so called for a reason!
*some more expensive models have an automatic changeover

Battery voltage is up the user, 24 volts should be slightly more efficient, but I would favour 12 volts, since you may wish to work some loads at battery voltage, and 12 volt equipment is more common than 24.
Some more expensive invertors incorporate a battery charger and will recharge the battery from the grid or a generator.
If a basic model is used then you will also need a suitable battery charger, preferably fully automatic.

It would be most unwise to connect the UPS or invertor to feed the whole house, only one or two selected circuits should be backed up.

It might be worth installing a dedicted circuit for essiential low power appliances.
Unless you have an improbably large UPS or invertor, a single 20 amp radial circuit should be sufficient, this could supply fridge, freezer, central heating pumps/controls, cellphone charger, cordless phone base unit, broadband router, computer, inkjet printer and a small tv.
Lighting could be by table lamps etc. plugged into the essiential circuit, or by connecting the lighting circuit to the same changeover arrangement as the essiential power circuit.

Things that you cant put on the essiential supply, electric shower, water heater, electric space heater, electric cooker, tumble dryer (unless gas)

Things you probably cant put on the essiential supply (but just about possible if its a very large one) kettle, toaster, microwave oven, boiling ring, washing machine, air conditioner, laser printer, vaccuum cleaner, power tools.

Remember that with automatic changeover, the system must be sized to supply the entire connected load , whereas with manual changeover, one might turn some loads off before selecting the backup power source.

[Mitch]

If you are looking at ex-I.T. stuff - just be sure to confirm the voltage. I had a couple of old 3kw units, but they had loads of small gell batteries in them in series. I can't quite remember, but I think the battery voltage was 160 or 180 volts. Our "big boy" for the server room at is 120v D.C., (10 x 12 volt batteries in series). The smaller UPS units that you get for individual P.C.'s are 12 or 24v - that I have seen anyway - but they usually only go up to about a kilowatt - (or 1,400VA or so).I think it's possibly easier to invert high D.C. voltages to 240 AC - much less D.C. current required - on bigger systems, hence the strange, high D.C. voltages in the design. I have a 3kw/12v unit on my boat, but it's not usually sold as an I.T. UPS, more as a "home" type inverter.

I did go 12v, but the thickness of the cables between the batteries and inverter is huge - and I still get a volt drop. If I was doing it again, I would definitely go for 48v. A 1 volt drop on a 12v system is huge - the inverter kicks off at 10.5, so even when I still have 11.5 available from the batt. bank, I lose the inverter. A 1 volt drop on a 48v system is minor and hardly makes any difference, and the cables can be quite a bit thinner.

Personally, for your percieved setup, I would go for a Victron Multi-Pus Inverter Charger, 48v. Probably be around ?1,500.00 for 2.5 or 3 KW. It may even have an automitic change-over switch built in. 4 x 12v 100 a/h batteries should give you about an hour at full clout - 2.5Kw. The Victrons can be coupled together - a very big plus in my opinion - so if you think you need more than the 3kw at a later date, you can just buy another Victron and common them up to double the capacity. I have never seen any other inverter that allows for this. The charger is probably about 25 amps at 48v, so 4 x batteries would be recharged in around 4 hours. I would use 35sq.mm cable between the inverter and batteries, for a length of up to 10 mtrs. Many Victrons can be coupled together, right up to about 26kw, I think, but the spec. should tell you more. Just google Victron Mult-Plus.

[Philip W]

Thanks guys useful tips there.

I like the idea of being able to couple the inverters together, then I can start small now whilst things are stable. Then scale up if necessary, also gives me some built in redundancy.

Good tip re the voltage.

I will have look at the spec for the Victron...

[Mitch]

The Victron series of inverters are real clever pieces of kit. Not only can they co-phase with each other, but they can co-phase with an external 240v input as well. An example would be if you had a 2.2kw genset as well as the inverter. You can set the inverter to draw no more than 2 kw off the genset max. If you are running a light load, say a couple of hundred watts, any excess from the genset goes to charge the batteries. If you then turn a load of things on - kettle, toaster and microwave at the same time, and need say 4kw, the inverter combines the 2kw from the genset with it's own output and will give you another 2kw from the batteries, protecting your genset and giving you the extra when you need it. (Wish I had bought one instead of the thing I have now - well one lives and learns......)

[adam2]

Although I have no experience of the Victron inverter chargers, they have a good reputation, and as Mitch points out can be added to.

If however you go for the model suggested, then that only leaves £500 for batteries and wiring.
Four 12 volt 110 A/H batteries would cost about £500, and would only give about an hours run time at 2KW or about 3 hours at 1KW.

Remember that the installation should be notified to the part pee police!

[Philip W]

It is ironic that with the new electricty regs people are more likely to do things themselves (and therefore potentially dangerously) than get in an electrician to sign it off. I bet I cannot find a leccy who will sign off what I have in mind. Not because it is dangerous but because they will not have come accross it before and won't be willing to sign it off....

[adam2]

It is ironic that with the new electricty regs people are more likely to do things themselves (and therefore potentially dangerously) than get in an electrician to sign it off. I bet I cannot find a leccy who will sign off what I have in mind. Not because it is dangerous but because they will not have come accross it before and won't be willing to sign it off....

Indeed, and apart from the hassle and expense, remember that the part pee police will have a list of who has backup power.
I doubt that TPTB are organised enough to requistion such equipment for "essiential" users, but I dont like the idea that they might.

Makes DIY a bit tempting does it not!

A U.P.S. or invertor/charger up to about 2.5 kw can be supplied from the mains via a standard 13 amp plug and socket, much over 2.5kw and the input current could exceed 13 amps when recharging the batteries.

The output from the invertor/UPS should be connected to a dedicated circuit feeding essiential appliances.
The UPS/invertor output will generally be by means of a plug and a length of flex, this flex should be jointed in a proper junction box, to the fixed cabling of the essiential circuit.
IT IS VERY IMPORTANT that there is no connection whatsoever between the essiential circuit and the remainder of the house wiring, apart from earthing, see below.

Almost all electrical circuits should be earthed, and the essiential circuit supplied from the invertor or UPS is no exception.
When the mains supply is connected, this earth connection is obtained from the normal mains wiring, just as with any other circuit.
However what if the UPS/invertor is unplugged from the mains? It would still be supplying 230 volts to the essiential circuit, which would now lack an earth. For this reason some authorites recomend that when a backup invertor is used to supply fixed wiring, that an additional fixed earth wire be used, independant of any plug and socket.
This additional earth should be connected between the earth wire of the essiential circuit, and either the instalation main earth terminal or an earth rod instaled for the purpose.(most dont bother with this however)

[Vortex]

If I was doing it again, I would definitely go for 48v.

Caution Will Robinson!

12v is safe(ish) - but 48V plus water can kill.

You live in a boat: your call.

[Mitch]

Adam2, I don't know if you - or anyone else - would be able to answer this, but earthing has baffled me for 35 years now. In electrical class at school we were taught all about how important earthing was to prevent people dieing. I was a beligerent little sod and kept asking why? Where was the circuit? I knew you DID get a shock from "live" to physical ground - experience had taught me that the hard way - I just didn't know WHY! Best the teacher could come up with was that one leg, (in a single-phase circuit - the common leg in a 3-phase), was physically "earthed" at the power station, hence the completed circuit from "live" to the ground you were standing on via your body. Therefore, all appliances, (they were all metal in those days), had to have a seperate "earth" wire bonded to the case. Should the "live" leg come into contact with the case, the fuse would blow. Well, what a completely daft idea, I thought. If the ruddy power company didn't "earth" one leg at the power station, for no good reason, there would never be any circuit between either leg and ground, and all this earthing hullabaloo could be done away with. It was six years, and after asking the question a thousand times, that a telecom's engineer explained that static build-up over the long distance transmission lines would arc through the insulators, permanently damaging them and causing huge losses and maintenance. To overcome this problem, power companies fitted auto-transformers at regular intervals and earthed one leg to drain the static - it suddenly all made sense.

Now I start messing with inverters - and off we go again. Boat safety reg's insist on full-on earthing, RCD's etc, etc, when istalling an inverter. WHY??? Sure, if you also have a shore-power hook-up, but why all the earthing paraphenalia if you don't? In fact, the inverter I bought was supplied with both legs floating!!! In order to make all the RCD gubbins on my boat work - so the Boat Safety chappie would pass my certificate - I had to "modify" my inverter and purposely wire one leg to boat chasis, calling this the "neutral". How daft, if I could have left it with both legs floating, I wouldn't have needed to go to all the expense of RCD's etc - there would have been no path, or circuit, from either leg to boat chassis. I could never have been shocked from either leg to "ground". I had to purposely create this lethal situation, just so I could spend loads of dosh and time suppressing a possible problem that couldn't have occurred, had I not created it in the first place!!!

What am I missing? Can anyone help me out here?

[adam2]

As you point out, one wire of most any public electricity supply is earthed at the transformer or generator, this is known as the neutral conductor and SHOULD be safe to touch, but please dont in case of a fault.
The other conductor, or the other three conductors if three phase, are known as live or phase and are dangerous to touch.

If such a conductor is touched the current will flow through your body, and return to the generator via the general mass of earth, perhaps killing you en-route.

If the supply is floating, with no connection to earth, it would appear that either conductor could be touched without danger, since there would be no return path for the current.
In the case a very small installation consisting of only one or two appliances, and a dedicated generator or invertor, this is true, it SHOULD be possible to touch either wire without danger, but again please dont try it, just in case.

However in the case of a larger instalation, such as a house connected to the grid, the postion is more complex.
There would be a real risk of an earth fault occuring and not being detected, the wire without the earth fault would now be at line voltage to earth, and dangerous to touch.
Since the earth fault could occur on either wire, rendering the other one dangerous, BOTH wires would have be regarded as potentialy dangerous, and the instalation equiped with two pole switches and MCBs at additional cost.
In event that two earth faults occured, on different wires, possibly in different buildings, fault finding would be a challenge.

In the case of a floating three phase supply, an earth fault on one phase would not only give 240 volts to earth on the "neutral" but would give 415 volts to earth on the two unfaulted phases, the insulation of wiring and appliances could be overstresed by this as it was designed for 240 volts.

In parts of Southern Europe, 3 phase, 220 volts delta connected supplies without a neutral were once common, but are now being changed to EU standard of 3 phase, 4 wire, 230/400 volt with an earthed neutral.

Specialist power supplies where reliability is crucial are often floating, in order to avoid power loss in case of an earth fault. It is vital that such instalations are monitored for earth faults, and that any found are rectified promptly, since any second fault would trip the supply.

In the UK it is a legal requirement to earth the neutral of public supplies, though not of supplies not made available to the public.

In the case of a medium size installation, such as a dozen appliances connected to a generator or invertor, the matter is less clear cut, and there are arguments both ways, though the balance of opinion appears to be in favour of earthing the neutral

When servicing or testing equipment that must remain live during the testing or adjusting (such as tv sets) it was good practice to supply the equipment under test via an isolating transformer.
Accidental contact with either wire should then not be dangerous.

[Mitch]

Fair point, Vortex - but I do try to keep the water outside - boats don't work very well if you keep the water inside!!

[Mitch]

In the case of a medium size installation, such as a dozen appliances connected to a generator or invertor, the matter is less clear cut, and there are arguments both ways, though the balance of opinion appears to be in favour of earthing the neutral.

Why is it less clear cut? What is the argument in favour of earthing?

The only thing I could see as a possibility, is that an earth fault one leg could develope on one appliance, and the fault occur with the other leg on a different appliance - you would need to touch both simultaneously to get shocked. I would imagine that the odd's of this scenario occuring would be extremely low though. Do you know of, or can think of, any others?

Oh, and thanks for all the answers Adam2 - much appreciated.

[adam2]

In the case of a medium size installation, such as a dozen appliances connected to a generator or invertor, the matter is less clear cut, and there are arguments both ways, though the balance of opinion appears to be in favour of earthing the neutral.

Why is it less clear cut? What is the argument in favour of earthing?

The only thing I could see as a possibility, is that an earth fault one leg could develope on one appliance, and the fault occur with the other leg on a different appliance - you would need to touch both simultaneously to get shocked. I would imagine that the odd's of this scenario occuring would be extremely low though. Do you know of, or can think of, any others?

Oh, and thanks for all the answers Adam2 - much appreciated.

It is generally accepted that almost any installation should have an earth wire, to which the metal cases of appliances should be connected, via the earth pin of plugs and sockets. So doing should ensure that no significant voltage can occur between any two appliances, even in the event of a fault, this earth wire should also be connected to any large touchable metal parts, such as boat hulls, water pipes, well caseings, structural metalwork etc. This should ensure that no significant voltage can exist between say the kitchen sink and an appliance near it, even in event of a fault.

What is less clear, on a small stand alone instalation is whether the neutral should be earthed, or left floating.
Earthing the neutral has the advantage that it closely resembles the public supply, that RCDs will work as normal, and that single pole switchs and MCBs can be used.

Leaving the neutral floating, may reduce the risk of getting a shock to earth, since there should be no return path for the current. In practice however there will probably be enough earth leakage from say a dozen appliances, that a shock could be received from either wire.
Since either wire is potentialy live, two pole switches and MCBs should be used, but seldom are.
With a floating supply, neon pilot lamps on appliances may glow even when the appliance is off, which is a lttle disconcerting.

[Mitch]

Ah, I think I get it - the problem is leakage. The leakage may well be infintismally small, (high resistance), so small as to have no effect on the installation, but yet could be large enough to kill, (only takes 20 mA to do you in at 240v). Yea, that makes sense - oh good, I didn't waste my time and money after all!!! Thanks Adam2