vehicle earth paths

Hi Garth,
I seem to remember the reason being to do with the fact that each body panel will have a different resistance to earth due to the welds and other assembly methods.

The rear quarter panel earth potential will be different to the front mudguard as a connection point to earth. I found it in the instructions for either the installation of the Rotronics dual battery system or some other electrical toy i installed. Can't remember which.....

Garth
What you say is correct, the steel should be low resistance, if its welded, but a bolt on panel if its painted before asembly can be high. another reason for using two wires is combat magnetic fields, if the wires are together the fields from the two wires are equal and opposite. The times this is important are where there is sensitive things like radio receivers and cd players, phones etc. Eric

Use copper, it has a conductance six times better the 'pure' iron.

Electricity uses the easiest pathway and may actually have several parallel return paths back to the battery. It may even preferentially use a fuel line from the rear of the vehicle back to the engine bay.

Use copper if you can it is relatively cheap insurance to avoid an unexplained 'burst into flames' event.

When was the last time you heard of some one removing the main nbattery earth where it connects to the body to check for paint, corrosion or a insecure crimp job on the lug. No one I know ever checks till something goes wrong.

Garth

You are correct and have been so since all vehicle bodies have been welded together. Since the mid 50s all cars have been unitary construction and if the welds were a problem causing electrical resistance I don't think the cars would hold together for long. Cars with chassis often had earth return wires and the idea that you need a copper earth return has been an urban Myth since this construction disappeared.

Four wheel drive vehicles have their bodies welded together and this forms the basis of their earth return. When wiring up a trailer socket find a convenient earth point close by on the body and not the chassis as the body may be rubber mounted to the chassis.

Caravans are wired using the chassis as an earth return – the earth wire from the plug goes to the nearest practical point on the draw bar, the metal work of the van is in turn earthed to the chassis and the lights pick up their earth from the skin of the van. Their earthing problems are greater than cars.

Eric

Radiation from power or transmission lines is only a problem at high frequencies. At mains power frequencies the only people concerned with radiation are transmission line engineers (yes it is a concern when you have lines 100s of km long.) With DC the only interference problems you will experience is with things like computers or ignition systems injecting hash onto the DC lines – in this case you should suppress the hash at its source.

Kiwi Kia

Generally I don't think fuel lines will be a particularly good earth alternative. The resistance of the body will be a lot less than a thin pipe. This may be a problem if you have incorrectly wired the trailer plug to the chassis instead of the body and the fuel line is the best electrical connection between the body and the chassis.

I also do not think bad connections in the battery earthing will be much of a problem. If you have problems in this connection you will have problems in many other areas that will point you a corroded earth strap. Any problems here will be quickly dealt with promptly or you wont be going far.

PeterD

Hi Garth, running both a positive and negative cable for high current set ups IS a case of best practice, not “ just best practice “.

There is no way of knowing to the point of being 100% sure that the earth return via the body is free of any form of resistance points. Furthermore, there is no way of testing for potential high current resistive points so you only find out about them the hard way.

You may do 1000 set ups and 999 of are OK but there is no room for error. Unlike the vast majority of devices that use the body as the earth return which are low current, things like dual battery systems are high current and as such, the slightest resistance can have severe consequences, least of which is the failure to be able to charge the battery properly.

When you currents of 20, 30 ,40 even 50 amps trying to return through a resistive point, you have the potential for an extremely high temperature generating situation and your own imagination will give you some idea of where this can lead.

Best practice with any high current requirements is to run both pos and neg cables but if you want to get some additional earth return ability then run some earth straps at either end of the negative cable.

Here's a practical test - from the Rear Right of an NP current model Pajero - the resistance from the Third-seat mount, to the battery negative was the same as 4 Gauge wire. i.e. if I ran 8 Ga wire I would have 2.5 times the voltage drop !

Here's the theory - steel has resistivity ten times that of copper, so there must be ten times as much steel as a 4 Gauge wire - 21 sq mm.

Here's the industry practice - everything except Starter Motor and Alternator are earthed via the bodywork - even the Rear Window demister that draws 10 amps.

Reliability of connections to the body can be a problem - which most probably accounts for the "it doesn't work" stories. For any holes that go through to the outside and may get wet, cover the joints in paint or Lanolin Graese to prevent corrosion which will quickly turn a good joint into a nightmare.

Mike