Floating ground question

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Floating ground question

Postby lfhoward » Mon Jan 11, 2021 5:46 pm

Hi all,

Since 2015 when I began building it, the trailer’s interior 12V DC and 120V AC from the inverter were floating. In other words, they were not grounded to the steel trailer frame at all and there was no connection to the vehicle wiring.

Now that I’ve installed a Propex heater, the Propex is necessarily grounded to the trailer frame through the copper propane line and propane tank bolted to the frame. (I checked and there is continuity.) That means the rest of the electrical system must be grounded to the frame through the Propex via its 16 gauge - negative wire.

Is it critical to install a ground strap now at the inverter, so that the electrical system is properly grounded to the frame? What are the pros and cons of not doing this?

Thanks!
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Floating ground question

Postby dmdc411 » Tue Jan 12, 2021 5:06 am

In my opinion, 120vac ground should be grounded to the frame. Any 12vdc, I would run "all" negatives directly using wire, to a common terminal strip. AC and dc are not the same, and should not be combined. Keep the two separate. This includes trailer light wiring.

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Re: Floating ground question

Postby lfhoward » Tue Jan 12, 2021 8:14 am

dmdc411 wrote:In my opinion, 120vac ground should be grounded to the frame. Any 12vdc, I would run "all" negatives directly using wire, to a common terminal strip. AC and dc are not the same, and should not be combined. Keep the two separate. This includes trailer light wiring.

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Hi dmdc411, thanks for responding. I want to clarify that everything does have its own ground wire running back to the fuse block on the DC side. The inverter of course also has its own cables to/from the battery. The AC and DC do not mix. Up until today when I installed the heater through the floor, the chassis was entirely out of the equation for the house electrical. Now there is a ground connection to the chassis because of the heater. I just want to make sure that there are no situations where there could be a danger of fire, since the only connection to the chassis from the rest of the electrical system is through the 16 gauge wires going to/from the heater (with a 5A fuse on the positive).
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Floating ground question

Postby tony.latham » Tue Jan 12, 2021 10:39 am

Now there is a ground connection to the chassis because of the heater.


You've got an interesting question going--and I don't have the answer.

I'm throwing my response in, just to read what the (real) wizards say about it. :thinking:

Tony
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Re: Floating ground question

Postby saltydawg » Tue Jan 12, 2021 12:43 pm

Okay what you have now will not burn the trailer down.

But what could happen if is something elsewhere to short to the frame the 16 gauge will try to take it back to the neg side of the battery. If its something like lights on a 15 amp fuse, the fuse should blow. Now if its something like the 50 amp fuse for the inverter, something is going to give and it probably wont be the 50 amp fuse ( i dont know how big an inverter, just an example ). Or of some how the battery shorted to the frame, something is going to give, and it will be the 16 gauge wire.

In the real electric world we ground via the ground wire the wire size needed to carry the short with out damage, to allow the over current protection to do its job. Example being a house, number 4 copper to the water pipe on a 200 am service. If you want to be safe run a 10 gauge to the frame, it should be able to handle any 12 volt short long enough to blow any fuse you have even a battery short it should allow the main fuse to blow, you do have a main fuse dont you?

Next this is an argument that will happen, yes your 120 volt ground should go to the frame as well.Its only a safety ground it should never carry any voltage. Why you want it, lets say you 120 v shorts to the frame,with out a path back to the over current protection your frame is now energized with 120 v, just waiting for path to ground. Now you decide you want to crank your jack up ( assuming its on a wood block or a plastic jockey wheel) just another inch to level it some more and your wearing damp shoes, guess what the path to ground now is? come on I will give you one guess.
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Re: Floating ground question

Postby lfhoward » Tue Jan 12, 2021 4:43 pm

Scott, thanks for your insights also. You’ll be glad to know I have a 125 amp fusible link going to the 1000 watt inverter and a 50 amp main breaker going to the Bluesea DC fuse panel. So the trailer is protected by those at least. Interestingly enough, the grounding screw on the inverter for the chassis ground is continuous with the negative cable to the battery AND the 120VAC ground. So a ground strap to the chassis is a good idea and I will get one installed ASAP. It should take care of grounding both systems. The inverter’s manual says an 8 AWG wire would be needed.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Floating ground question

Postby saltydawg » Tue Jan 12, 2021 8:40 pm

lfhoward wrote:Scott, thanks for your insights also. You’ll be glad to know I have a 125 amp fusible link going to the 1000 watt inverter and a 50 amp main breaker going to the Bluesea DC fuse panel. So the trailer is protected by those at least. Interestingly enough, the grounding screw on the inverter for the chassis ground is continuous with the negative cable to the battery AND the 120VAC ground. So a ground strap to the chassis is a good idea and I will get one installed ASAP. It should take care of grounding both systems. The inverter’s manual says an 8 AWG wire would be needed.


Sounds like you can just run the 8 wire and rake care of it in one shot.
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Re: Floating ground question

Postby lfhoward » Thu Jan 14, 2021 1:19 pm

Update: the inverter chassis ground screw is continuous with the 120VAC negative but NOT with the 12VDC negative. Does that change our thinking at all?
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Floating ground question

Postby lfhoward » Thu Jan 14, 2021 5:49 pm

I figure not. The only difference is two separate ground wires would be needed, one for the DC electrical system and one for the inverter.

Here is how things have looked for 5 years:
Image

Since I was working on the electrical today, I decided to install a main cutoff switch. I thought it would be quicker and safer than always disconnecting the battery terminals when I needed to work on something.
Image

I installed a ground screw through the metal floor. (After drilling holes for the heater, drilling this hole through the floor didn’t freak me out at all, lol.) The yellow 10 gauge ground is for the 12VDC system and the blue zero gauge ground is for the 1000 watt inverter.
Image

Here is the system put back together. I taped around the positive leads to prevent accidental short circuits.
Image

So in conclusion, no more floating grounds. The chassis is negative grounded, just like on an automobile. Interestingly enough, the aluminum skin has electrical continuity with the chassis and the negative battery terminal also.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Floating ground question

Postby saltydawg » Thu Jan 14, 2021 6:31 pm

lfhoward wrote:Update: the inverter chassis ground screw is continuous with the 120VAC negative but NOT with the 12VDC negative. Does that change our thinking at all?


We call the ground the grounding conductor and the neutral the grounded conductor in the real voltage world. My renogy inverter has a built in GFI in the circuit, it is self resetting, ie no button but if it senses any leakage to the ground it shuts off. And to sense that it measures the amps going out vs coming in, and the only way there can be a difference is a connection to ground outside the inverter. So it should have a connection to the ground prong and the neutral for that power to have a path back.

I wonder if they assume that you will use a neg ground on the 12 volt and ground the chassis to the chassis screw on the inverter and thats how they will "force" you to tie them together.
Scott
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Re: Floating ground question

Postby bdosborn » Thu Jan 14, 2021 7:03 pm

ARTICLE 551 of the NEC (Recreational Vehicles and Recreational Vehicle Parks) requires bonding to the frame for three things: the battery charger case, the inverter case and the ground bar of the AC panelboard. It would be the ground wire of your incoming power line if you don't have a panel. "The bonding conductor shall be solid or stranded, insulated or bare, and shall be 8 AWG copper minimum, or equal". I can't recall anyone actually installing a code compliant grounding scheme for their trailer, including myself. I'm pretty close but I have a #10 bond to the frame instead of a #8.

BTW, there are a lot of little known requirements in 551 like GFCI for all receptacles, panelboards listed for use in an RV, grounding of all exposed metal, requirements for where your shore power inlet is located, labeling of your shore power inlet etc.... Did you know that Autotransformers shall not be used? Who knew? :lol:

Bruce

P.S. I have one ground bar in my trailer and all the DC and AC grounds land there. The ground bar is bonded to the frame.
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