Electrical system 12v / 240v confused.com

Mar 19, 2019
I am confused by my electrical system.

I have a 12v Master Switch, like this...


And a 240v consumer unit, like this...


Everything in the van that should work on 12v is working, but obviously requires the master switch to be on, so that's good.

If the master switch is off, but 240v is on, the only things that work are Sockets, Electric heater, water heater, but the two heaters are not labelled on the consumer unit.

The wiring diagram from the manufacturer shows the 12v and 240v systems being completely separate, and the 240v directly feeding Sockets, Electric heater, water heater, fridge and lights.

So the fridge and lights are the anomaly vs the wiring diagram, as they won't work on 240v alone, they need the 12v master switch to be on.

So, lights...... the wiring diagram shows 240v powered lights, the consumer unit has a sticker that claims to provide power to lights, but none of the lights work unless the 12v master switch is on. When you switch the 240v on they get brighter, but I'm guessing this is just the 1 volt increase in the system created by the battery charger, and the lights are still running off the 12v system. I don't seem to have any lights that work off 240v alone.

The fridge.....wiring diagram also shows 240v feed to the Fridge (Electrolux Dometic), the consumer unit has a sticker that claims to provide power the fridge, but the fridge doesn't work unless the 12v master switch is on, regardless of switch position on the fridge, which appears to make no difference whether it's on mains or battery. How am I supposed to know whether it even works on 240v? It could be running on 12v constantly for all I know.


Like I said, confused.com, wiring diagram is not consistent with what is happening in reality.
Mar 19, 2019
The other thing that makes little sense is why put the consumer unit in the bathroom wardrobe at the very rear of the van, when the 240v hookup is at the front? The battery charger and water heater are at the front, the sockets are front and centre, the heater and fridge are over the wheels. Why run cables all the way to the back of the van, just to mount a consumer unit, and run them all the way back to the front/centre. Why not just put the consumer unit at the front under the sofas where the rest of the wiring and the 12v fuse box is?
Jan 19, 2002
The fridge... I can't see why that should be influenced by the master 12v switch. The click selection on the fridge should select Off / Mains supply / 12V / Gas. Clearly there is facility to use one power source only (unlike the water heater than can quick heat using both gas and electric supply). The 12 V However is only operating from the car supply while the alternator is running - so should cool in transit, but not, for example, if sat on a ferry for a couple of hours. Many users will cool the fridge on mains a couple of days before a trip and then this is maintained by the 12v from the car during travel.
Lights - in my van all the lights are 12V so will be working off the caravan battery. These are then controlled by the master switch and then by switches on each individual unit. I have replaced with LED bulbs so are using very little power if I am off grid. You may have some additional lights that are mains supply that will only work when the electric hook-up is connected. Equally the pump is 12v and will only run when the master in on and the pump switch on the panel in the photo.
Incidentally if you have areas like that in the photo where the surface veneer is peeling you may be able to rectify this by gently heating with a hairdryer, which reactivates the glue, and then pressing down with a cloth in the affected area.
Nov 6, 2006
Mains lights were for many years only fitted to the front seating, and often, each light unit had its own switch at the fitting, but also, sometimes a push button dimmer type on its own panel was used.
The battery charger supplies 12v to that side when disconnected from the car, therefore the 12v master switch will have to be on to supply power to anything 12v related, whether the source is the van battery or the charger. The fridge relies on 12v power from the car when towing, but 12v power from the van battery on site, to run the fridge internal light, for which you should find a fuse in the 12v set
Mar 19, 2019
chrisn7 said:
but 12v power from the van battery on site, to run the fridge internal light, for which you should find a fuse in the 12v set

OK, that maybe explains things a bit. So the 12v master switch should only need to be on for the light in the fridge, but not for the fridge to run.

I shall try that, 12v master switch off, 240v consumer unit on, fridge switch on mains, and see if the fridge stays cold.

I assumed since the 12v master switch switched off the light, that it had switched off the whole fridge.
Oct 8, 2006
The power box doesn't show the fridge or heater but does show sockets. This is because the fridge, heater, and microwave if you have one will all be plugged into a 13A socket outlet in a cupboard or under a bunk adjacent to the appliance concerned. This is to allow for easy removal/exchange. Note the fan of the room heater runs off 12V so that it will still work on gas.

The easy way to find out if mains is on is to get a tester plug which will show if the incoming mains is reversed polarity (very common on the continent) and if an earth is present. It has three neon lamps in it, so if any of these light up you mains is working! Alternatively get this test kit - you hold the pen near anything live such as the front panel of a 13A outlet and if mains is present it will light up and beep.


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