# Do leisure and vehicle earths need to be separate?



## Glandwr (Jun 12, 2006)

Are the two electrical 12volt systems discrete in a van, or can I use the positive from one system and the negative from the other?

I get the feeling that this is a really stupid question   . It concerns the fitting of an amplified radio antenna.

Dick


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## hogan (Oct 31, 2006)

Any chassis earth will be ok they are both common.


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## crusader (Jul 5, 2012)

As above , earth is earth , jim


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## listerdiesel (Aug 3, 2012)

Leisure batteries are not always connected to the same earth point as the vehicle battery, so I'd always use the same earth as the positive.

If you can establish that they are the same, then OK, go for it.

Peter


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## Glandwr (Jun 12, 2006)

Would it be sufficient to put a multi meter across the possible connections and if it says circa 12volts it's OK Peter? It would not give a reading if it were not a common earth?

Dick


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## listerdiesel (Aug 3, 2012)

Not really, a bad connection would still pass enough to show a good supply, while it would fail under load.

Peter


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## Glandwr (Jun 12, 2006)

There is less than a 10 ohm resistance between a known vehicle battery neg. and a known leisure battery neg. Peter. Would that confirm a shared earth or not?

Dick


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## raynipper (Aug 4, 2008)

As far as I have even found Dick, the chassis is negative or earth on all Euro vans.
But some American RVs have a centre tapped transformer and their white or mains negative can be 55 volts different to the chassis.

Ray.


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## listerdiesel (Aug 3, 2012)

Glandwr said:


> There is less than a 10 ohm resistance between a known vehicle battery neg. and a known leisure battery neg. Peter. Would that confirm a shared earth or not?
> 
> Dick


Good earths I'd expect to be less than 1 ohm.

Peter


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## rayc (Jun 3, 2008)

listerdiesel said:


> Glandwr said:
> 
> 
> > There is less than a 10 ohm resistance between a known vehicle battery neg. and a known leisure battery neg. Peter. Would that confirm a shared earth or not?
> ...


Testing the earth resistance / continuity with a meter is liable to lead to errors as the leads of the meter are also in circuit and it is being carried out at low voltage. On some meters you can zero them by shorting the leads together and zeroing so that the leads are not in the equation.
Earth continuity / resistance testing is normally done with something such as an earth continuity meter which does it at a higher current with an expected result of less than 1.0 ohm and in some types of installations a lot less.
In my work we did loads of earth continuity tests of equipment cabinet earth points such as the roof, sides and doors etc. They and the cabinet earth point also had the be less than 0.1ohm back to the equipment room earth bar. The earth continuity tester was set to deliver a current of 1.5 times the primary fuse rating e.g. on a 16A protected equipment it was 24amps.

Perhaps an easier way to determine with a simple meter if the battery negatives are connected is to do a voltage test. Put the meter positive on the Leisure battery positive and the negative on the cab battery negative - what voltage do you get? Compare it to what you get with the meter connected to the leisure battery terminals. It should be the same in both cases.


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## Penquin (Oct 15, 2007)

Being a numpty, it is important however to ensure that the leisure battery negative is connected to something as an earth since if it is not connected then no power can be taken from that battery.

AFAIK it does not make any difference of they are shared or common as long as the connections are really good, bad earth (negative) connections can cause some really odd effects on bulbs not lighting when expected - on an old car I found that the negative connection to the rear lights was poor and so when the indicators were activated the brake lights flashed.....

Just me ensuring that no-one thinks the negative connection is not important...... it is.....

Dave


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## listerdiesel (Aug 3, 2012)

rayc said:


> Testing the earth resistance / continuity with a meter is liable to lead to errors as the leads of the meter are also in circuit and it is being carried out at low voltage. On some meters you can zero them by shorting the leads together and zeroing so that the leads are not in the equation.


However you do it, anything over a fraction of one ohm is a problem.

Our Fluke DVM's read 0.6 ohms for the leads, we don't zero them as we are normally looking at far higher levels of resistance.

Peter


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## rayc (Jun 3, 2008)

Penquin said:


> Being a numpty, it is important however to ensure that the leisure battery negative is connected to something as an earth since if it is not connected then no power can be taken from that battery.
> 
> Dave


Perhaps half the problem is referring to battery negatives as earths. They are not, though they may be connected to the vehicle chassis which some people refer to a earth.
It is quite clear that the cab battery is bonded to the chassis as that is normal. It is not quite as clear as how the leisure battery negative is cross connected to the chassis /cab battery negative, though it must be if things such as recharge of both leisure and cab battery take place on EHU or from the alternator. In my Rapido the cross connection takes place in the CBE 12v distribution box.


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