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Old 08-24-2014, 04:43 PM   #1
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12 Volt grounding issue

I am working on a 1964 Bambi II. I have run all new 12 V and 120 V wiring. The 120 is not hooked up yet at all. I am working on getting the 12 V working using a toy train transformer to test. I understand low voltage wiring...I think.

The issue I am having is when one light (rear circuit) is working with the red (+) and black (-) wires (low voltage wiring for the 12 Volt), things work. When I touch just the black wire for the front light (hot wire) to the 12 volt post on the transformer, the front light will light without the neutral wire being connected.

This concerns me...so I took the front light down and checked to be sure I was consistent with the wiring connections front and back. Turns out all is fine. While the light was hanging down I tested again. Now the rear light works with the black and red wire attached, and the front light works only with both wires attached- as it should be. I pushed the front light back up into place and it now works on only the black wire.

The light base is metal. The inner skin is metal. Is the light unit grounding through the skin? And is this why it will light without the ground post being hooked up? Is this safe?
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Old 08-24-2014, 04:51 PM   #2
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12 Volt grounding issue

Your battery has a ground lug connected to the c channel of the trailer near where your battery is located. This ground will provide connectivity to your lights. I installed some running lights purchased from VTS which had one black wire and a white wire under the mounting screws, they work fine if my 12V ground is attached to the ribs of the trailer. Newer codes may frown on this, but these trailers were built way before the codes


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Old 08-24-2014, 04:53 PM   #3
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Yes, it's grounding through the skin.
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Old 08-24-2014, 05:15 PM   #4
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Quote:
Originally Posted by acstokes View Post
Yes, it's grounding through the skin.
No concern, right, Fred? I know that "hot skin" is a dramatic issue with high voltage, but not with 12V...am I correct?

Thanks!

Ben
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Old 08-24-2014, 05:30 PM   #5
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I'm no expert, but for testing I don't believe it's a concern. For permanent installation you want all your ground wires to run to a negative bus bar on a 12v panel near your battery bank. Your negative battery terminals are wired to that same bus bar and the bus bar grounded to the trailer frame. I'm sure someone will correct me if I'm wrong.
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Old 08-24-2014, 05:58 PM   #6
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12 Volt grounding issue

The frame/skin of your coach is half of the 12 volt electrical system. It is often referred to as ground. It is in fact the "common" side of the 12 volt DC. System.
The negative terminal of the battery and converter is connected to the "common" side of the circuits used in your coach. As is the negative side of each device in the coach.
If a light fixture (for example) has only one wire(black or red). That wire is the positive side of the circuit and should be connected to the wire going to the light switch on the wall. The base of the fixture if metal, must be attached in some way to the skin/frame of the coach. If plastic, there will be a wire (usually white) which should be connect to the skin in some fashion.
If the light fixture has a black and a white wire, the black wire goes to the positive side and the white goes to the negative ( (common) side (ie skin).
If the fixture has a black and a red wire (ie most pumps) the black wire is common and the red wire goes to the positive side thru a switch.
The discrepancy is due to the different standards between the auto industry and RV manufacturing industry.
In the auto industry:
Red = Positive
Black = Negative or Common (also referred to as ground)

In the RV industry:

Black = Positive
White = Negative or Common (also referred to as ground)

If the pump you have has a red and black wire. The red wire is the positive and the black wire is negative.
If the light fixture you have only has one wire. Usually black and sometimes red. That is the positive side of the electrical circuit. The fixture itself must be electrically connected to the skin with either a wire or thru a brass grommet in the fixture itself.
If the fixture or device has a black and white wire. The black wire is positive and the white wire is negative.
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