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Electrical safety when using 12 volts.
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adam2
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PostPosted: Sat Oct 27, 2007 1:42 pm    Post subject: Electrical safety when using 12 volts. Reply with quote

I felt that this deserves its own thread.

Many disscussions on these forums refer to useing 12 volt batteries to supply lighting etc. either directly or via an inverter.

Such installations are not entirely risk free and proper care should be taken.
For large or complex installations an experienced electrician should be consulted, however for small, simple or temporary installations the following points should be considered.

The risk of dangerous electric shock from 12 volts is negligable under any normal conditions, the real risk is fire from short circuits or overloads.

Remember that vehicle or leisure batteries can produce many hundreds of amps if short circuited, and that they are allways live.

Therefore anything connected to such a battery should have a fuse in the circuit as close as possible to the battery.
Large or complex installations should have a main fuse located close to the battery, and a proper fuse box to which the sub-circuits are connected.

For small or temporary installations an in-line vehicle type fuse holder should be located AS CLOSE TO THE BATTERY AS POSSIBLE.

Detailed advice regarding fuse and cable sizes may be obtained from the IEE wiring regs.
However for small, simple or temporary installations the following ratings would suffice.

0.5mm cable=fuse not exceeding 3 amps
0.75mm cable=fuse not exceeding 6 amps
1.0mm cable= fuse not exceeding 10 amps
1.5mm cable=fuse not exceeding 15 amps.

It would be unwise for the actuall load to exceed 50% of the ratings given above to avoid excessive voltage drop.

For example, although there is nothing unsafe in connecting a 12 volt 36 watt lamp using 0.5mm cable and a 3 amp fuse, the result would be dissapointing since the voltage drop in the cable (unless very short) and in the fuse, would result in a dim light.
Use of a cheap multimeter to check the voltage at the load can be useful, excessive voltage drop may be caused by loose connections, but usually is the result of cables being too small.

Great care should taken when a connecting two or more batteries together; A MISTAKE COULD BE FATAL due to burns or explosion.

Great care should also be taken to ensure that a 12 volt installation can not be inadvertantly connected to any other voltage.

Remember also that the output of an inverter is at 230/240 volts and should be treated with same respect as the mains.

Also remember that 12 volt flourescent light fittings generate lethal voltages internally, and should not be used with broken or missing covers etc.


Last edited by adam2 on Fri Jul 23, 2010 9:53 am; edited 2 times in total
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RenewableCandy



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PostPosted: Sat Oct 27, 2007 10:36 pm    Post subject: Reply with quote

Yes bear in mind that for a given power of device, the current going through the 12v version is 20 times that which would go through the 240v version (because power = voltage * current).

Current will not give you a shock in the usual sense but will heat up anything it flows through (hence the fire risk). So you won't get thrown across the room but you may very well get your hands burned if the wires aren't thick enough (low enough resistance) to carry said 20* current without heating up.

For some reason best known to itself the IEE is now called the IET. Not sure if the name of the regs has changed, though.
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mikepepler
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PostPosted: Sun Oct 28, 2007 9:35 am    Post subject: Reply with quote

I recently was given an almost-new car battery, so I'm using it on my solar DODGY instead of the 7Ah battery I had there (I know all about the deep-discharge issues...). I didn't have a separate fuse for the 7Ah battery, instead relying on the protection built into the charge controller. But with the bigger battery I've changed this, and used 50A cabling/connectors and a 100A fuse, all salvaged from an old UPS. The charge controller only works up to 30A anyway, so all this should be fine in normal use.
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MisterE



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PostPosted: Sun Oct 28, 2007 10:39 am    Post subject: Reply with quote

Very good thread, whilst I'm fine and up with the regs for 240, I know jack about 12v battery systems, and like most I naturally assumed they are even safer and more or less without risk. At least I know now that if I want some sort of back up system I'll have to hit the books and the newest IEE or T edition. Again thanks for the info guys Smile
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adam2
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PostPosted: Sun Oct 28, 2007 11:47 am    Post subject: Reply with quote

MisterE wrote:
Very good thread, whilst I'm fine and up with the regs for 240, I know jack about 12v battery systems, and like most I naturally assumed they are even safer and more or less without risk. At least I know now that if I want some sort of back up system I'll have to hit the books and the newest IEE or T edition. Again thanks for the info guys Smile


Strict compliance with regulations may be going a bit too far, especialy as some regs are impractical to follow when dealing with extra low voltages.

As an example, "all socket outlets rated at 16 amps or less, that may be expected to supply equipment used outdoors, shall be protected by an RCD" And where precisly do you buy an RCD for 12 volts? The fact that an RCD is not needed on 12 volts (or even 24) does not seem to have been considered.

The key point to remember is that 12 volts carries virtually no shock risk, but is at least as dangerous as the mains regarding fire risk.

A great many 12 volt appliances sold in car accessory shops etc. have no fuse protection. If such appliances are plugged into a car cigar lighter (which should be fused) then the risk is minimal. If however the appliance is connected direct to the battery there is a very real fire risk in the event of damage or failure. I would advise anyone using such an appliance direct from a battery to add an in line fuse holder to the wire.
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MisterE



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PostPosted: Sun Oct 28, 2007 4:05 pm    Post subject: Reply with quote

Again thats, because I'm putting leds in my lads ceiling (240v) and under his PC desk and in a square box running the length of the wall at ceiling height I am putting in green cathode rays 12v will be hard wiring them in via a 240 to 12v transformer, I'll now be paying closer attention to my cable runs I may even put them in conduit in the ceiling.

On another note, who hates black mortar!!!!!!!!! ME! Just took a ceiling down and now got panda eyes after a shower then a bath, I hated doing it as an apprentice but hey 30yrs on and I can at least now afford a mask and not a scarf around the head - the things they'd get away with when we were 16 shocking!
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mikepepler
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PostPosted: Sun Oct 28, 2007 4:43 pm    Post subject: Reply with quote

adam2 wrote:
A great many 12 volt appliances sold in car accessory shops etc. have no fuse protection. If such appliances are plugged into a car cigar lighter (which should be fused) then the risk is minimal. If however the appliance is connected direct to the battery there is a very real fire risk in the event of damage or failure. I would advise anyone using such an appliance direct from a battery to add an in line fuse holder to the wire.

I agree. Fuses are essential. People should be using a charge controller anyway, to prevent over-discharge as well as regulating charging from a solar panel. Charge controllers usually have at least a fuse built in - one of mine also protects against various wiring errors electronically.
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adam2
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PostPosted: Sun Oct 28, 2007 7:24 pm    Post subject: Reply with quote

MisterE wrote:
Again thats, because I'm putting leds in my lads ceiling (240v) and under his PC desk and in a square box running the length of the wall at ceiling height I am putting in green cathode rays 12v will be hard wiring them in via a 240 to 12v transformer, I'll now be paying closer attention to my cable runs I may even put them in conduit in the ceiling!


12 Volts from a transformer is generally less of a risk than 12 volts from a battery, the reason being that the transformer PROBABLY has overcurrent protection built in. For example a 12 volt halogen lamp connected to a transformer sold for the purpose does not normally require any additional protection. Whereas the same lamp connected to a battery certainly does require a fuse.

If your green lights and transformer are a DODGY sold for the purpose, then any required fuses or other protection should be built in, and all you have to do is follow the instructions.

If however the lights and the transformer where obtained sepaertly, and not intended to be used together, then installing a fuse would be prudent.
Size the fuse at about 150% of the load current, and ensure that the cable has a greater rating than the fuse.
If the load is greater than about 5 amps at 12 volts it may be better to divide the lights into groups, each with its own fuse.
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adam2
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PostPosted: Sun Oct 28, 2007 7:30 pm    Post subject: Reply with quote

mikepepler wrote:
I recently was given an almost-new car battery, so I'm using it on my solar DODGY instead of the 7Ah battery I had there (I know all about the deep-discharge issues...). I didn't have a separate fuse for the 7Ah battery, instead relying on the protection built into the charge controller. But with the bigger battery I've changed this, and used 50A cabling/connectors and a 100A fuse, all salvaged from an old UPS. The charge controller only works up to 30A anyway, so all this should be fine in normal use.


I am not sure that 50 amp cable and a 100 amp fuse is a very good idea, though an over sized fuse is better than none.
It might be better to reduce the fuse to 50 amps.
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mikepepler
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PostPosted: Sun Oct 28, 2007 8:42 pm    Post subject: Reply with quote

adam2 wrote:
mikepepler wrote:
I recently was given an almost-new car battery, so I'm using it on my solar DODGY instead of the 7Ah battery I had there (I know all about the deep-discharge issues...). I didn't have a separate fuse for the 7Ah battery, instead relying on the protection built into the charge controller. But with the bigger battery I've changed this, and used 50A cabling/connectors and a 100A fuse, all salvaged from an old UPS. The charge controller only works up to 30A anyway, so all this should be fine in normal use.


I am not sure that 50 amp cable and a 100 amp fuse is a very good idea, though an over sized fuse is better than none.
It might be better to reduce the fuse to 50 amps.

It's just what I had available, and given that the fuse is simply a piece of copper with a cross section of a couple of mm2, while the cable is many times that, I think the fuse would always blow first. Especially as the charge controller I use will limit itself to 30A - so any current above that should only arise from a short between the battery and the controller, and should therefore be high enough to blow the fuse quickly before the cables melt.

But as you say, a correctly sized fuse would be better still - I'm just building it all from salvaged parts where possible! For applications where I'm using my smaller SLA battery without a charge controller for short periods (e.g. camping light) I've bought some 10A fuses.
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adam2
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PostPosted: Sat Nov 03, 2007 11:20 am    Post subject: Reply with quote

Battery safety,
Ideally batteries should be placed such that the terminals can not be touched, or accidently short circuited.
For very large installations, a dedicated room or outbuilding should be considered.
For smaller installations using only a few batteries, plastic "rentacrates" are well worth considering. They are cheap, non-conductive, resistant to battery acid, and easily drilled to accomadate cables or flexible conduit.
The lids may be secured with cable ties.
Due to the great weight of batteries I would not stack crates containing batteries, though of course crates full of other, lightweight items may be placed on top of the ones containing batteries, thus saving space.

Low temperatures reduce the capacity of batteries, but prolong the storage life. High temperatures improve the capacity, but reduce the storage life and the life on standby/float charge.
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Andy Hunt



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PostPosted: Sat Nov 03, 2007 11:26 am    Post subject: Reply with quote

All good stuff - thanks for this thread Adam!

Smile
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MisterE



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PostPosted: Tue Nov 06, 2007 2:59 pm    Post subject: Reply with quote

I'm using these but in green http://www.ebuyer.com/product/51686 they are for inside pcs where the power supply is already bounced down due to the psu. I've never used them outside the pc but many other have. I'm either going to use and adaptor/transformer that you plug into the mains or this
http://www.airlinktransformers.com/transformer/et105-105v-dimmable-electronic-lighting-transformer.asp

Which seems a good deal and I'll be running 4 lights in total the wattage on those caths are very very low. They come with an invertor but the input is still 12v and not 240v

Here is an example of what some have posted whn using outside the pc

1) These are tubes of around 30cms long and about 1cm in diameter. they are a cold cathode tube, in other words a light! They don't give off any heat, are cheeeeeap to run, and last for around 15000 hours (normally!)
2) they are normally used inside computer cases (that have a window in the side panel) to illuminate the inner workings of a computer, but as you have probably read they can be used anywhere there is a 12v power source (i've got four illuminating my desk using a 12v 4amp power supply i got from Halfords.
3) The switch that comes with it controls the invertor (power supply) so it will only switch both tybes on and off at the same time.
4) They come in different colours, blue is the most popular! and a UV version is available - this has the effect of making flouresent and white colours glow brightly in the dark - funky effect.
5) they give off a steady light - not a pulse as someone has mentioned - he probably bought the bubble version!
6) Fitting the switch is a simple job - take out your spare 5.25 or 3.5" drive bay cover drill a hole to suit the diameter of the switch then push switch in (it has some locating lugs to secure it. NOTE the dual tube version does not come with a round adhesive 'washer' like the single tube version. remember to position it correctly if you want more than one switch!
Hope this helps somewhat.

Another comment I have put these all over my home cinema, behind speakers etc. All you need is a Doll's House transformer, which steps mains voltage down to 12v. (or us any other 12v DC transformer - I have one with a dimmer on!) Then cut off the wierd computer connections, and connect the two wires up either way round (lights have no polarity - work either way) to the 12v supply. They look amazing.

Well thats my project at the moment Smile
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adam2
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PostPosted: Tue Nov 06, 2007 3:22 pm    Post subject: Reply with quote

The cold cathode lamps refered too probably require a 12 volt DC supply, as would be found in a PC.
I dont think that they will work on 12 volts AC which is what a halogen lighting transformer produces.
I believe that you will require a transformer with a 12 volt DC output to power the lamps, these are readily available from MAPLIN and other suppliers.
To ensure reliability, the transformer should only be worked at about 50% of its rating. Fitting a small fuse, perhaps 1 amp for each light may be advisable, but perhaps not truly essiential unless you are using a very large transformer.

I have used 12 volt powered cold cathode lamps for lighting at home, I was not impressed with the life, the small/cheap/nasty inverter generally failed after at most a few hundred hours use.

To light a room in green, it may be worth be considering either green CFLs or green 1,200mm flourescent tubes.

(I have a surplus of green 1,200mm tubes, and dimmable fittings for them you can have them for free if you can collect, please PM if interested)
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MisterE



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PostPosted: Tue Nov 06, 2007 4:04 pm    Post subject: Reply with quote

Thanks Adam, I've only ever used them in PCs, other than snapping them when I've opened the case I've never had one go, lucky I guess. Now why didnt I think of using a flourescent tube for the pelmet doh Smile Thanks for the pm too will reply now Smile

http://www.mr-resistor.co.uk/item.aspx?&t=324&r=503&i=5146&a=1
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