DC LV Battery Power Supply for Motorhome

12 volt appliances don't normally require an earth.

Some mains voltage appliances do of course need an earth, and this may be provided by the earth connection in a standard socket, that is connected via the wiring system to the vehicle bodywork and to an earth rod.

I had a look in a recently fitted out Housing Association maintenance team van with an inverter that supplies a double socket in the cargo area. Just inside the side door is a coiled main earth conductor and standard earth rod all tied up with cable ties, having never been used.


I passed comment to the electrician who drives the van that he had not used the rod, he said he had been told he could only use the inverter when the engine is running, so had only used it to charge cordless tool batteries when traveling between jobs and you cannot use an earth rod when you are doing 50 miles per hour on a dual carriageway.


Being selective about what gets plugged in and keeping that equipment and the inverter well maintained is the way to maintain safety.


Double insulation is a key protective measure, avoid equipment that needs earthing for safety.


Andy Betteridge

Contrary to a widely held belief, an RCD still gives some protection on systems with no CPC or on systems with neither of the supply wires earthed. (as with many inverters and small generators)

Most dangerous electric shocks are a result of current passing through the body from a live conductor to the general mass of earth or to articles connected thereto. Any current through the body of the victim is not returning to the source via the other circuit conductor, and would therefore operate an RCD regardless of what if any earthing arrangements exist.

Couldn't agree more about not being able to earth on the go etc. also let me assure you, there's no way anything I am looking at installing (with the possible exception of the extractor fan, which I haven't seen yet) will be able to give anyone a shock because it is in conduit and insulated with wood rubber and plastic at every point except from the obvious battery compartment which has an isolator switch. The electrode is intended to protect from failure in the PV panel on the roof. I have instructions to ground the frame. It's also probably the best I can do in the instance of any failure in the double insulation which the fuses will sort out anyway.


I've taken on board all the stuff about the fuses (too used to ac systems obviously) and everything you're saying. I suppose it's not really the electrics I have a problem with now. Think you've given me a good heads up on it to be honest.


It's the whole thing - gas, solar power, battery vapours, water, electrics and fire all in a relatively cramped space and being run by people who will absolutely park up under this most wonderful tree they found in the middle of nowhere despite knowing a thunderstorm is on the way, or changing fuses without isolating while touching the contacts with their bare hands,..  probably not great examples but making this easy for them to use is a real concern. I'm just going to have to put instructions on everything. Lol I'll end up spending more time on the instruction manual than the wiring.


Thanks everyone.


T

Touching the terminals whilst changing 12 volt fuses is fine.

Batteries should be kept away from sources of sparks, but are otherwise fairly low risk. I would not place anything that can spark in a battery compartment. The battery isolator and the main switch should be close to the battery, but NOT in the battery box or compartment.


I would normally fit a main fuse of from 30 amps to 60 amps near the battery, and run heavy cable from the main fuse to an automotive fuse box, with fuses of from one amp up to 15 amps according to circumstances. Relatively close fuse protection is prudent for small 12 volt motors, but less important for other loads like lighting.

I would normally wire 12 volt lighting and small power circuits in 2.5mm cable from a 10 amp fuse.

Various situations could arise, with different conclusions for "useless or useful". 

The fundamental point about the RCD is that it can only operate if there's an external current path between its two sides, other than the conductors that go through the RCD. (Let's call the sides the source and load side; cables could be on either side and are another source of faults from active conductors to external objects if getting damaged, trapped etc.)  Some current has to go through the RCD's active conductors then return by the external path such as earth or a protective conductor, in order to trip it. 


If everything on the source side is truly isolated from everything else, like a floating generator winding with negligible capacitive coupling, the RCD can never operate whatever happens downstream; just the same is true the opposite way round, if the load side is truly isolated. Taking a single-phase case, one of the two active and initially floating conductors at the load side of the RCD could accidentally come into contact with an earthed metal object: then the other active conductor becomes live, and the RCD does no good at all for an accidental contact between this one (also on the load side) and earth, as the whole fault loop is in the load side. Ok - this requires two faults together, but the first could happen long before the second and be unnoticed.


If instead several RCDs were used for different outgoing circuits, they'd at least protect against cases where faults happen on different circuits. If there is a fault to earth in the source or in possible cables between the source and RCD, and a further fault to earth on the other conductor on the load side of the RCD, then it also can operate.  So if you have an isolated source that "might go wrong" an RCD isn't quite useless.


An old guide HSE 482/2 suggests in para-12 using just an isolated system for a small setup (little chance of an accidental unnoticed fault).  It also confirms (para-39) my suspicion that cables are a likely culprit for the faults. Others e.g. who manufacture 'IMDs' (e.g. here) might suggest an isolated system with IMD; it's a neat way to avoid the shock currents that an RCD needs in order to start operating (and has other advantages too, except for cost).  I'm tempted to continue to other considerations of centre-tap and/or resistive earthing (and their pro and con), bonding of exposed parts, purpose of the earth rods that some like to use with every source, etc., but probably the above is enough to reconcile the views on usefulness of the RCD in this thread's case?


[And the above concerns a system with enough voltage to have need of an RCD. If the motorhome really doesn't have anything at 'dangerous' voltage, e.g. inverter-fed loads, the whole RCD thing seems a waste.]

ebee:

 

broadgage:

Contrary to a widely held belief, an RCD still gives some protection on systems with no CPC or on systems with neither of the supply wires earthed. (as with many inverters and small generators)

Most dangerous electric shocks are a result of current passing through the body from a live conductor to the general mass of earth or to articles connected thereto. Any current through the body of the victim is not returning to the source via the other circuit conductor, and would therefore operate an RCD regardless of what if any earthing arrangements exist. 

"systems with neither of the supply wires earthed" Have I misunderstood that bit? Yes without a cpc you`d still get an RCD to work for earth fault protection but if neither of the supply wires were earth referenced the RCD would not trip on earth fault, there would be no earth path therefore no danger , example bathroom shaver socket or isolating transformer

 

Small portable generators and some inverters, and some overseas mains supplies have neither wire connected to earth. An RCD still gives some protection in such circumstances for reasons already given.

Consider what happens if a person inadvertently touches one wire.

The installation might be so well insulated from earth that no significant current can pass through the victim, and no dangerous shock can be received. In that case the RCD has served no purpose, but done no harm.

No consider the alternative situation in which one side of the supply is inadvertently earthed, perhaps by cumulative leakage currents, or by a pre-existing earth fault. In this case a dangerous current could pass through the victim, perhaps with fatal results. A working RCD will trip and avoid this danger. A most wise precaution.

"one side of the supply is inadvertently earthed, perhaps by cumulative leakage currents, or by a pre-existing earth fault. In this case a dangerous current could pass through the victim, perhaps with fatal results. A working RCD will trip and avoid this danger. "


Yes indeed but you did say neither side was earthed, whereas it would in fact have an earth reference allbeit unintentional. Was my point