DC LV Battery Power Supply for Motorhome

If you are using 12V Earthing is pointless and unnecessary. Use standard vehicle fuses etc rather than MCBs, easier and cheaper and less subject to problems with vibration. The PSSC will depend mainly on the wiring. Big conductors from the battery 35mm2, will probably give you 2-3 kA PSSC, a few metres of 3mm vehicle cable will have this down to 1 or 200 A. If you fit a mains invertor and basically run as a 230V installation is covered in BS7671 section 717, far too much to cover here. It is useful to be able to plug into a standard caravan pitch, but this with an inverter supply needs a change over switch along with RCCB and suitable inlet socket. Note that CPDs (MCBs) and an inverter supply which has seriously limited current available will not be much use for overload or short circuit protection, but will be needed for mains supply operation.

With only a single 12 volt battery, I agree that vehicle fuses are the way to go, cheap and readily available.

For a larger or more complex system that uses 24 volts, or multiple parallel 12 volt batteries, then I am not convinced that vehicle fuses have sufficient breaking capacity and would prefer BS88 fuses.

But for a simple, single battery system, vehicle fuses should be fine.


PSSC can be estimated by measuring the voltage drop under heavy load at the battery. A voltage drop of 1% under a 50amp load suggests a 5000 amp short circuit current. This is only an approximation and may subsequently vary if a different type of battery is fitted.


It is common to "earth" the negative pole of the battery, but this usually means by connection to the vehicle chassis, and a not a true external earth connection.

Thank you. I did actually consider vehicle fuses, but then changed my mind when I heard "remote beauty spots" being mentioned, still, as long as they keep enough spares it doesn't really matter, just thought MCB's would be safer and easier to reset..


As far as I know they have a hook up unit for campsites and all I have to do there is attach it by earth nut to the vehicle body. When they're on a site without a supply I thought the earth electrode would be the best bet. Unfortunately they have already bought the batteries so that's what I have to work with.


Trig

There's a whole new potential career to consider as a Caravan and motor home technician.


Andy B. 

The SELV part does not  need an electrode at all. However the words 'hook-up' on a caravcan site usually mean a rather ratty 230V lead trailed in the mud joining the vehicle at a CEE  (commando) inlet.  That will need a bit more thought, even if all it supplies is one socket for the battery chargers. In a larger van, it is likely to develop mains leads for the fridge,  extra lights, telly etc.

Earth rods and mobile vehicles tend to get left off, whatever the instructions  say, as it works without it. so an RCD is required, and I'd add some sort of L-N reversal detector, if only a plug in socket tester.

just thought MCB's would be safer and easier to reset

Be very careful about using equipment designed for a.c. on d.c. systems. Anything that switches or breaks a fault current will have a much harder job to do on d.c. as there isn't the luxury of the arc being automatically extinguished for you 50 times a second as the supply current alternates. Generally d.c. switches/MCBs need larger contact gaps and or have a much lower breaking capacity than their a.c. equivalents.  Being 12V doesn't really help on that count either - if anything the currents tend to be larger rather than smaller.


Beware of voltage drop - it's surprising how little cable you can use at 12V before exceeding recommended limits (3% from memory).


If you ever have to work on (towed) caravans, rather than motor caravans, be aware that 'chassis return' is only used for the road lights (if at all) - all the habitation circuits (both Fridge, and from the leisure battery) are strictly isolated from the chassis (it's all to keep the 'neutrals' separate and so prevent overheating in the tow connectors especially should one contact have slightly increased resistance). Also there's a requirement that all leisure circuits other than the fridge are disconnected when the caravan is being towed.


   - Andy.

Ok, more info coming up...


The vehicle is some kind of sprinter van. It includes 2 x 3w usb chargers, 1 mains double socket (which I've already advised won't actually work properly or safely on 12v.), 2 LED strip lights, 1 waterpump to be installed in a tank under the chassis, 1 extractor fan (biggest headache of all), gas powered fridge, cooker and shower and a log burner (I've requested that flame proofing is applied to all the wood, might suggest installing a canary).


I've taken voltage drop instructions from A721.523 "not to exceed 0.8V" I take this to be the whole circuit including the battery. I've just managed to convince the client to get battery monitors and had a bit of a look at invertors and think that will definitely be the best thing to do regarding the mains socket, which you are absolutely right on, things will creep into this van I'm sure.


So the next question is this. I have in the design an RCD on the extractor fan circuit, it'll be zone 1, very close to a gas shower and it has a light on it, goes straight through the body of the van. Needless to say, not very happy about all of that would be an understatement, I had calculated 5.5m of 1mm stranded cable which is at the higher end for volt drop anyway. I'd much rather get rid of the gas altogether and get a big enough invertor to do the job.


I guess having a long conversation with the client is what I'm going to be doing today. Gas fittings + log burner = boom.


Once again, thank you all, I do appreciate it, bear in mind I have only had a week with the proposed system, still at the very beginning so there's still time to turn it around.

If the extractor fan is 12 volts, then no RCD is needed.

Mains voltage switches etc are generally fine on 12 or 24 volt nominal circuits. DC is indeed much more destructive of contacts than AC, but a general "rule of thumb" is that contacts in switches, relays, contactors and the like if intended for AC may be safely used on DC provided that the voltage is limited to about 10% of the AC rating.

Would I use a standard light switch on 12 or 24 volts DC ? yes I would. But on 110 volt DC, no way.


I would not normally use 13 amp sockets on 12 or 24 volt circuits, there is nothing directly electrically unsafe in so doing, but dangerous confusion might be spread. USA type mains plugs/sockets have become a de facto standard for 12 volts DC at up to about 15 amps.

For 24 volts, BS 543 types are popular.

Yeah it's the client plugging things into these sockets which require an earth but not really understanding that there isn't one that I'm most worried about there.