Chassis Ground Question 120v and 12v

[brianjonesphoto]

I've got my wire run and im about to start hooking things up this weekend. I want to confirm my assumption that the chassis should be bonded to the shore power ground and the 12v and 120v system grounds would then be bonded together when plugged into shore power.

Thanks.

[troubleScottie]

Yes and no and maybe.

First, the 12V system needs to tie back to the negative pole of the battery. The most reliable method is to use a return wire. Each DC appliance must have a connection from the battery positive and to the battery negative. Obviously, fuses, circuit breakers, distribution blocks/bar along with wires are part of the connection.

Second, using the frame instead of a separate wire depends on the trailer frame being a continuous electrical connection. The model system is the automobile with its metal frame which has many of the grounds are connected to the frame as is the battery negative. Trailers which are bolted together may not be electrically connected. Paint, dirt, water, shifting can create separate zones. A welded frame should behave better.

Third, there is no requirement that the battery or return DC wires or AC system be grounded to the frame. It is not even a nice to have. Image the original TD with wooden frames. Also, if there is a short, the trailer may become charged. You contacting it would be a bit shocking.

Fourth, the 120VAC system has 3 wires. Just to make sure, you are talking about the ground wire, typically a bare or green insulated wire. The black or hot wire and the white or neutral wire should never be connected to the DC system. You can connect the ground to the battery ground. However there is no requirement that the two systems be interconnected. A better practice is to ground (have all the ground wire for each AC cable) connection to the power distribution box ground bar/block and the power distribution box to the shore power connection.

[John61CT]

There is no real Earth Ground until you plug into shore power, and that should be the only point for the AC side. Best to get a pro to check it and sign off.

The DC side should not rely on the chassis for Negative Return for anything high current, expensive or important.

But can/should tie those returns into "Chassis Common" at multiple locations, as appropriate, you do not want different potentials.

[Pmullen503]

The correct answer is neither should be grounded to the chassis. AC and DC don't mix and there is no guarantee that the shore power you hook up to is correctly wired.

I run separate ground/neutral wires for everything, keep each system completely separate (trailer lights, 12v interior lighting, 120v outlets). That way a problem in one system is isolated from the others.

The only thing that should be grounded to the trailer frame are the trailer lights and that is so it's common with the vehicle's ground.

[bdosborn]

Hmm, I wonder why the NEC and RVIA require the battery and shore power to be grounded to the frame?
Bruce

[brianjonesphoto]

Hmm, I wonder why the NEC and RVIA require the battery and shore power to be grounded to the frame?
Bruce

Thank you that was a simple answer I was looking for. I guess I didn't give enough background. I have wired a house and I have wired many 12v systems, but I have never combined the 2.

Shore power outlet will get bonded to chassis ground at entrance.

Thank you.

[brianjonesphoto]

Yes and no and maybe.

Third, there is no requirement that the battery or return DC wires or AC system be grounded to the frame. It is not even a nice to have. Image the original TD with wooden frames. Also, if there is a short, the trailer may become charged. You contacting it would be a bit shocking.

ummm that why you would want to ground the 120v to chassis. If then there is much less potential of you becoming the ground path by touching something. 99% of the time the shore power will be my honda eu2000 or my house. This is a very simple 120 system. total of 1 breaker and 3 outlets with a GFCI on the shore cable.

[featherliteCT1]

Mike Sokol says to bond AC ground wire to trailer frame. Same with ground wire for battery.

Here is a testing technique to test the adequacy of the AC ground connection.

Picture1.png (328.34 KiB) Viewed 1990 times

https://www.rvtravel.com/diy-safe-way-t ... -rv-frame/

He says the above test "is far superior to just using an ohmmeter since it drives an ampere or so of current through the ground-bond connection
Elsewhere he also says no need to bond to an "earth" rod, for both a generator and an AC system for a trailer."

I wired my trailer in accordance with his advice. He does an excellent job of explaining the theory to support his advice.

[Ottsville]

You contacting it would be a bit shocking.

I'm having trouble wrapping my head around this statement. Electricity is the system I understand the least. If there is a short, wouldn't the frame be just as likely to give a shock when you touch it whether the frame is used as a ground or not?

[brianjonesphoto]

You contacting it would be a bit shocking.

I'm having trouble wrapping my head around this statement. Electricity is the system I understand the least. If there is a short, wouldn't the frame be just as likely to give a shock when you touch it whether the frame is used as a ground or not?

Being shocked by 12v system is extremely unlikely as it the low voltage usually doesn’t have the voltage to overcome the resistance of dry skin.

The 120v felt more complicated. I won’t touch on inverters as I don’t have much experience with them. With power coming from “the grid” there should be a earthed ground somewhere in the system. By connecting the trailer chassis to that ground via the ground pin of the power lead any short (Loose wire, chafed wire insulation, faulty device) that makes contact with another conductive material attached to the chassis will have a direct path the ground through the frame causing a fuse or breaker to disconnect the circuit. Without the chassis ground you could become the conductor to complete the circuit to ground the fuse or beaker should still trip but it could hurt before that happens.