Merit plug wiring

Submitted: Sunday, Jul 28, 2013 at 11:48
ThreadID: 103441 Views:6236 Replies:2 FollowUps:3
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I am connecting a Merit plug to the exterior of a campervan. The centre pin of the plug is connected to the +ve terminal of the battery and the outside pin to the -ve terminal. I note, however, that the outside pin is part of the chassis of the Merit plug and is thus in contact with the metal shell of the campervan where I have made a hole to fit the plug. Is the connection of the outside pin both to the -ve terminal of the battery and the metal shell of the campervan creating a problem? OR do I need to isolate the plug in some way from the metal of the campervan?
My understanding is that only one connection, earthed to the metal shell of the campervan or connected to the battery terminal, is required. Thanks for any help.
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Reply By: Member - Frank P (NSW) - Sunday, Jul 28, 2013 at 12:12

Sunday, Jul 28, 2013 at 12:12
John,

If the shell of your van is properly earthed to chassis and negative of the battery you don't need to run a negative wire. But if you do there's no issue or problem.

The two-pin merrit socket is designed for mounting in non-conducting surfaces such as plastic interiors of cars. There is a single terminal version for mounting in metal surfaces. In your application it doesn't matter which one you use - just run the neg wire if you need to, or not if you don't :-)

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FrankP

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Follow Up By: JohnKT - Sunday, Jul 28, 2013 at 13:01

Sunday, Jul 28, 2013 at 13:01
Thanks for your reply. Excuse my ignorance, Frank, but would connection to both campervan shell and battery mean that the metal shell of the campervan would be 'live' if the campervan is not earthed.
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Follow Up By: Member - Frank P (NSW) - Sunday, Jul 28, 2013 at 19:15

Sunday, Jul 28, 2013 at 19:15
John,
The Bantam has pretty well answered your question below.

Apoplogies for using the common term "earthed". While in terms of 12V wiring it is probably incorrect, it is in wide use.

In your scenario, what I meant by "earthed" is that the shell of your van is electrically connected or bonded, to the negative of the battery. As Bantam has suggested, the construction quality of caravans is variable and this bonding cannot be guaranteed unless you test for it.

If it is, you don't NEED to run a negative wire from that outside terminal to the battery, but I agree with Bantam, it would be better if you did.

In terms of the 12V supply to the merritt socket, regardless of which way you do the negative, or if you do them both, the shell of your van will not be "live".

Just as the shell of your car is not "live".

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Reply By: The Bantam - Sunday, Jul 28, 2013 at 14:30

Sunday, Jul 28, 2013 at 14:30
lets clear up some matters.

These days almost without exception motor vehicles are "negative earth".
meaning the the negative terminal of the battery is bonded to the metal chasis of the vehicle.

HOWEVER, we can not rely on all metal parts of the body of either cars or caravans to be well bonded to negative.

Some ideas of being "LIVE" that pertain to 240V AC electrics are foreign to 12V DC electrics as found in vehicles.

Realy the concept of being "Earthed" as we know it in 240V AC systems does not exist in DC automotive electrics and many people refuse to use the term.

From the point of view of insualting the positive supply, all metal parts should be considered to be bonded to negative and a potential source of short circuit.

From the point of view of using the metal parts of the body and a negative return path, they should not be considered reliably bonded to negative unless proven otherwise.

As for caravans...well it is anybodies guess if the metal body parts or the chasis is properly bonded to negative at all.

Mostly it is best to run both a positive supply line and a negative return line all the way back to a distribution block or the battery and insulate both from the body.

I would not be particularly concerned if the socket you are using connects the metal body parts to the negative of the battery.

What is important is that the positive line is well insulated and properly fused...AND...there is a good quality return path to the negative terminal of the battery.

cheers
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Follow Up By: Ross M - Sunday, Jul 28, 2013 at 15:23

Sunday, Jul 28, 2013 at 15:23
Well stated Bantam!
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