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Brandon avatar image
Brandon asked

Mobile trailer grounding

I am building a Tiny house on top of a steel trailer. My tiny house on wheels will be mobile and off grid with batteries charged by PV array. Will have the occasional plugin to a Generator for extra charging if need be.

My question is how does the ground circuit work in this scenario.

- Does the earth wire from the inverter, MPPT, PV panels grounds, battery casing, AC power outlets, lights, trailer frame, etc all join to a copper steak that I can drive into the ground when I park the trailer?

- Should the battery negative also be joined to the earth buss bar?

I have attached a rough diagram of how I think the earth circuit would go (earths in green). Could someone please let me know if this looks right. I quickly drew this diagram mainly for to show the earth circuit so I do apologise about the rough nature and any symbols that aren't quite right.


Thank you!


Multiplus-IIoffgridGrounding
earths.jpg (9.5 MiB)
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2 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

Your drawing looks good. You don't show a generator connection, but the safety ground for that would obviously also connect to your master ground bus.

There are two goals of a safety ground system. :

First, safety ground and neutral are connected at ONE place in the system. This provides a fault path that will trip a breaker. Without it, a fault in an AC hot leg could result in dangerous voltages on the the safety ground system including all metal in the system. This connection ("bond") typically occurs at the electrical service entrance. It's also handled in the Multi when it disconnects from incoming AC.

Second, the safety ground provides a path for static electricity to bleed off into the surrounding earth. This is where the ground rod comes into play. It is a fallacy that the the ground rod provides a path for AC fault currents. In fact, most ground rods may not trip a 20 amp breaker!

I would say it's a good idea to install a ground rod as you've shown if you are not connected to the grid. The ground rod may even be required by local/national codes even without a grid connection.

Note that many generators will not bond safety ground to neutral. If this is the case for your generator, you need to buy or make a plug that creates this connection when it's used as the AC source for your tiny house. You can check this for your generator with a multimeter: check for continuity between safety ground and the neutral at one of the output from the generator.


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Brandon avatar image Brandon commented ·

@Kevin Windrem

Thank you. I'll take all of this into account. So is it necessary for there to be a link between my negative battery cable and the master ground buss aswel?

Also, what is the purpose of the ground rod if it doesn't provide a path for AC fault currents?

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Kevin Windrem avatar image Kevin Windrem Brandon commented ·

Connecting the system negative to the frame provides a path for DC faults. Obviously this isn't a shock hazard but it prevents smaller conductors from carrying fault current.

The purpose of the ground rod (besides meeting code requirements) is to discharge static electricity from the frame of your trailer.

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Brandon avatar image Brandon Kevin Windrem commented ·

@Kevin Windrem thanks. Would I keep that earth stake in the ground when I am plugged into mains?

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Kevin Windrem avatar image Kevin Windrem Brandon commented ·

no

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Brandon avatar image
Brandon answered ·
@Kevin Windrem a further question on this topic I am hoping you can answer.

My system is now wired as per the above diagram. I have installed an RCD into it and it has the MEN link (which is taken care of by my Multi-plus).
If I am understanding correctly there could be one potential scenario where an earth rod could carry a fault current (albeit in a 'backwards' manner)- Out door power outlets. If a fault were to occur in an appliance that is outside the trailer on the ground and someone touches it, the fault current would travel down through the earth, back up the earth rod and back to 'the source' (Multi-plus), thus tripping the RCD. Is this correct?


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RCDs work by sensing the difference in current between the hot and neutral (or multiple hots and neutral in multi-phase systems). If there is current in one conductor and not another then the RCD trips. It does not matter where that current ends up going.


In your scenario, if the trailer were COMPLETELY insulated from earth, there would be a potential difference between the trailer and earth but no current would flow. The RCD would not trip but the person would not be harmed because there would be no current.

In practice, there is usually some leakage path from the trailer to earth (tongue jack, levelers/stablizers, stairs, etc.) and that would be enough to trip the RCD before the person was injured.

To take your scenario further, suppose that a second person is barefoot on wet ground touches the trailer. There would now be a fault current path through both people and the ground. The RCD would then trip.


At least that's my take on things.

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