Updated May 2017
Twelve volt caravan systems may be twin wire or chassis return. Twin wire is usually best. This article explains why – and which to choose to avoid problems.
Twin wire has one conductor (usually red) for positive leads. The other (usually black) is for negative. The wires may be separate, or within one sheath. Experience long shows it to be more reliable for RV use.
Chassis return uses the vehicle’s metal chassis as a common negative. A heavy cable connects the chassis to battery negative. This usually works well initially. It also saves cable. In many cases, however connections are poorly made. Many are exposed to damp. They then corrode.
After a time earth return connections often look this! Pic: inspectapedia.com
Twin wire or chassis return – avoiding RV electrical problems
Faulty connections cause many RV problems. Auto electricians dislike them as faults may be intermittent. The need for a wire from appliance negative to chassis increases where problems lie.
Most (not all) vehicle makers use chassis return for on-road electrics. Those that do, however, make such connections thoroughly. Moreover, they protect them against dirt and damp.
Few caravan makers chassis return. Most professional installers recommend against it too. So do my articles and books.
Twin wire or chassis return – electrolysis
Chassis return also cause electrolysis. This is a form of corrosion particularly occurring in radiator and cooling systems. It also occur in engines and gearboxes.
With chassis return, electric current flows via every metallic path. It does this mostly via the metal chassis and cable to the engine. And also to battery negative.
Single core (usually black) is used for chassis earthing. Red is used for positive. A few companies however black (+ve) and white (-ve).
Problems occur if that main earthing strap loosens or corrodes. Or is inadequately sized. Return current then seeks paths of lesser resistance. This is particularly via the radiator and water pump. Both contain different types of metal. These are particularly attacked. They corrode as electric current flows through them.
In particular this occurs when earthing straps are not replaced. That often happens following repaired front end damage. Another, for example, is where spot lights’s earth return is via a radiator support bracket. Return current consequently flows through the radiator instead of a needed cable.
Twin-core cable is used for circuits that do not use chassis return.
Car radios, and a few fridges have internal negative/earth connection. Or from their metal enclosure. This reduces radio frequency interference. They nevertheless use twin cable. That cable must be connected accordingly. Not via chassis return.
Vehicle makers require routine service checks for electrolysis. It is done by measuring the voltage (to earth) of the radiator fluid. For a fuller explanation ‘Electrolysis Corrosion in Vehicles’ .
Twin wire or chassis return – further information
Caravan & Motorhome Electrics covers every aspect of designing, specifying and installing RV wiring. It also covers every type of lighting and appliances, plus solar and charging systems.
Solar that Really Works does likewise re solar for cabins and RVs. Solar Success is for homes and properties. Caravan & Motorhome Book covers every aspect of RV use. For author information: Click on Bio.