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A campervan or motorhome may have either, or sometimes a mixture of each, of two ways of how the ‘house’ wiring is arranged.
The first way employs two conductors (a conductor is part of the cable that carries the current – the rest is insulation) that run from the battery to each connected appliance. One (commonly red) cable runs from the positive terminal of the battery to one side of the load. The other (commonly black) cable runs from the negative terminal of the battery to the other side of the load. Two conductor wiring can be done by using two separate single-conductor cables – or by using ‘twin’ cable. This has two similar-sized appropriately colour-coded conductors in a single sheath. Either way, a fridge or a light, or whatever, that is five metres cable distance from the battery requires 10 metres of conductor.
Most campervans and motorhomes are cabled differently. Because they are built on a heavy-gauge steel chassis and commonly have sheet steel bodies, that metal may be used in place of (typically) the negative conductor. There is so much of that metal through which current can pass that its resistance can be ignored. Thus earth return effectively halves overall circuit resistance. As five metre distance requires only five metres of single conductor cable, this enables a lighter gauge to be used. Or if the two conductor size cable is used, voltage drop is usefully reduced. Earth return wiring (as it is usually called) is common in campervans and motorhomes, but less so with fifth wheelers and conventional caravans because less metal is used in their construction.
At first sight, earth return wiring would seem the obvious choice. It is simpler. It is lighter. It is cheaper. In practice, however, earth return tends to become a nest of troublesome metallic worms – and to such an extent that I strongly recommend against its use.
The first objection is that it is only too often poorly done (time after time one encounters an earth return lead attached by an ill fitting self-tapping screw). Auto electricians say that poor earth connections are responsible for a huge percentage of electrical problems. Even if done properly, earth return connections are often made to parts of the chassis exposed to water and road grime. Then, corrosion is only a matter of time.
There is a further and potentially serious problem that can cause extensive damage to a vehicle’s more costly bits like engine and gearbox, and especially the cooling system.
In an earth return system, current will return to the battery by every path it can find that leads directly or indirectly to that battery - because all such paths are in parallel. Normally there will be a heavy earthing strap from chassis to engine and also to battery negative that carries virtually all of the earth return current. And that which doesn’t is so low it can be ignored.
But if that earthing strap or its connections becomes loose, or more typically corroded, then the earth return current will follow various paths of lesser resistance. These may and usually do include the engine cooling system and parts of the engine and gearbox. All are likely to contain disparate metals and these may be attacked by so-called electrolytic action as current flows through them. Such areas in effect become miniature electroplating plants.
Vehicle manufacturers take this seriously. Some makers have stopped using chassis earth return for any part of the vehicle electrics. Others check to ensure it is not happening: Nissan, for example, includes voltage to earth measurement of the cooling system liquid as part of routine servicing.
When installing the negative cabling, and to avoid a multiplicity of cables to battery negative, consider taking one very heavy cable from battery negative to one of those connection boxes made for the obligatory earth/neutral link in domestic (but NOT within RV) mains electrical systems. There is a range of such boxes, made by companies such as Clipsal, that will accept large battery cables plus any number of lower current cables. They are not cheap but do the job superbly. In my view it is probably better not only to avoid earth returns but to omit earthing the negative side of 12/24-volt house systems altogether. Avoiding this however can sometimes be difficult as car radios, and a few fridges (including Engel) have an internal dc negative/earth connection. Such earthing can readily be removed from fridges but rarely from car radios.
It is however worthwhile earthing the metal cases of electrical devices as this reduces RFI (Radio Frequency Interference) that may cause a hum on radios etc, and sometimes white streaks across TV screens.
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