DC LV Battery Power Supply for Motorhome

This is correct Alan. This stuff Broadgauge seems to be putting out is simply incorrect. If he decides to draw himself a circuit he will see precisely why. RCDs are not a universal panacea for all known faults and possible dangers!

Yeah I'm a bit confused too.


On my side the issue is simple, I have a number of items requiring grounding to the van body itself, plus an unlikely but not impossible breakdown in insulation to the van from the 12V circuits. I expect that any problem from the 12V circuits will dissipate through the metal of the van and also any fault current will blow the fuses. But, the batteries themselves seem to require earthing to the metalwork and there's a solar panel which seems also to earth through contact with the roof. The regs for caravans and mobile transportable units say to earth/bond exposed conductive parts via the MET however I don't have a 240V system and therefore no MET. Providing an earth electrode seemed the best answer.


I also thought RCD's would probably be the best bet for detecting anything going wrong with the batteries and earth faults on the body of the van. It might also serve to protect the electrical SELV system from outside faults. So connecting these batteries to a small DB which had Positive, Negative and Earth might actually facilitate this. However I'm extremely sketchy on whether this will actually work the way I intend, or whether it will introduce problems to the SELV circuits.


Hence trying to work out the PSCC and PFC.


Trig

The regs for caravans and mobile transportable units say to earth/bond exposed conductive parts via the MET however I don't have a 240V system and therefore no MET. Providing an earth electrode seemed the best answer.

Think about a commercially available motorhome , how the wiring is configured and  how  the van can be used.


A fair bit of the time will be "off grid", that is, with no 230 volt A/C hook up.  You still will have a 12 volt system and no need for an earth rod. This seems to be identical to the proposed use of the van conversion in the OP.........12 volt system and no mains connection .

A solar panel system [with a charge regulator], with the panel on the roof should make no difference or present any problem. Everything is "earthed" through the vehicle chassis as is normal with a motor vehicle - "negative earth".

davezawadi:

This is correct Alan. This stuff Broadgauge seems to be putting out is simply incorrect. If he decides to draw himself a circuit he will see precisely why. RCDs are not a universal panacea for all known faults and possible dangers!

Not certain that I can agree with this.

Are you stating that if the brown wire through the RCD carries 1 amp, and the blue wire carries 0.95 amp, because 50ma is passing through a person who is touching the brown wire (or something connected thereto) that the RCD will not trip ?

Yep. In a nutshell RCD protection will only afford earth leakage protection if the earth return is to the supply side of the of any RCD rather than load side ie load thru RCD current coils imbalance. That`s how I check my RCD tester from an RCD protecteded supply L & N to supply the test socket connects L supply to L socket, N to Socket N and N also to socket E. The socket being a BS7288 trips and is timed by the RCD function of my MFT but leaves the supply (Domestic RCD protected)  untripped.

If the system is just 'safe' voltage, e.g. 12 V, then there's simply no point considering any earthing for safety reasons. In that case the only point in 'earthing' is in the sense of the word commonly used for vehicles, meaning connecting to the metal chassis (not the actual earth) in order to use that as a conductor in the circuit to save on wires. If you're happy to use separate wires for the whole circuit (not the chassis), you'd probably be better to leave the whole 12 V system isolated from the vehicle and earth.


It's only if there's a source of dangerous voltage involved (e.g. external hookup or internal inverter) that there's any reason to consider connections to  the chassis (other than the functional reason above) or to the actual earth.  In the case that the dangerous voltage is from an internal source and no part of the wiring will leave the vehicle (fixed internal load), there's no point in connections from the vehicle to the actual earth. 


Regarding the original question: the only protection you need for the 12 V system is to avoid danger from overcurrent, i.e. overheating during short-circuit or overload. You should be fine to follow vehicle conventions.  It should also be fine to follow mains installation conventions for overcurrent protection: most modern devices (e.g. domestic-type MCBs) are considered to provide full protection to wiring that's rated no more than the device's nominal current.  It was mentioned earlier that breaking DC is more difficult. Some modern MCBs do have a rating for DC systems, that would be more than adequate for your purpose. I'd happily assume any 230 V MCB to work fine with a 12 V battery (interested in any demonstrated case of this not being true), but if you feel a need to show that specifications are rigidly being followed than just look for one with a DC rating.