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rjoustra avatar image
rjoustra asked

Grounding Multis, Charge Controllers and Solar panel frames

My question is concerning grounding in my rv. My intent is to ground the solar panel frames, 4 charge controllers chassis and the two multis chassis to a single point on the Motorhome chassis. The new lithium batteries are placed next to the inverters and all goes through appropriate fusing and disconnects for the size system. The cables for the old inverter (3/0) are in the compartment where I have installed all the new equipment and my intent is to use those cables to run back where the old batteries were and install buss bars to handle all the connections (chassis ground , house distribution and alternator charging) that went to the old lead acid batteries. All the aforementioned equipment have new pos and neg buss bars where everything is landed and the neg would be chassis grounded about 10 feet away on those old inverter cables on the same chassis rail but due to the location it would be a feet to ground it all in that location. Is this acceptable or could I have a potential for a ground loop somewhere even though all the equipment will only have one chassis ground but some in one place and some in the other. Also is it safe to daisy chain my chassis ground for the two multis with a cable that would handle 1 not 2 going into a ground faulted condition or would it be a better practice to run each one to the grounding point separately and I suppose the same question applies to the charge controllers as my intent was to daisy chain them as well using a cable sufficient to handle one faulted at a time.

Grounding
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Kevin Windrem answered ·

Grounding the negative system in two places would create a ground loop which you want to avoid. Safety grounds (chassis connections and AC green wire) should never carry current and topology is not all that important as long as there is just one connection to the chassis.

The old inverter cables WILL be carrying current so you want to choose a chassis bonding point carefully. I don't believe the negative side of the battery and the chassis are connected in the MPPTs and Victron inverter/chargers so you may be OK with a chassis ground bond at the old lead-acid battery location. Inverter/charger chassis IS connected internally to the AC input/output safety ground (green wire) and that will be bonded to the RV chassis somewhere. That might create ground loops if you also connect the inverter/charger chassis to the battery negative there.

I'm not sure I have a good solution for you

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rjoustra avatar image rjoustra commented ·

Okay so the mppts negatives will be bonded at some point to the motorhome chassis according to all victron schematics I have looked at. In all of them the negative bus bar is connected to earth which in my case would be chassis ground and as you said the ac side is bonded probably somewhere in the coach also including the generator. So from what I can tell I should bond the negative buss bar to the chassis and the multis cases to the motorhome chassis as well with a cable of the same cross section as the feeds in my case a 4/0. Does this all sound right? Don’t want to catch fire lol

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Kevin Windrem avatar image Kevin Windrem rjoustra commented ·

The safety ground is there to return fault currents to the source so they don't cause issues. In the AC world, this is done so that the chassis of a piece of equipment isn't raised to the voltage of the hot AC leg and hurt/kill someone. It also provides a safe return path for currents. Without it, fault current could flow through control connections not large enough to handle the current and pop the fuse/breaker. Worst case, that could result in fire.

Safety ground wiring, must be sized to handle the maximum fault current. The worst case DC fault would be a positive battery connection that faults to the inverter chassis or RV chassis. The safety ground path needs to of sufficient size to safely return the fault current to the battery negative and pop the fuse in the positive battery lead without overheating the safety ground wire.

The National Electrical Code specifies minimum wire sizes for given fault currents. These numbers are for AC line voltage circuits but I don't see any reason they couldn't apply to DC faults as well. 6 AWG copper is good for 200 amps; 2 AWG is good for 500 amps, 3/0 is rated for 1200 amps and 4/0 is rated for 1600 amps.

You need to insure that DC inverter fault currents won't flow through the AC safety grounds which are much too small. Remember that your inverter chassis could carry DC fault current.

In order to handle DC fault currents and prevent ground loops, your inverter chassis should be bonded to the RV frame with a separate wire than you are planning to use for the negative battery connection to your old battery location. This will handle any DC fault currents rather than relying on AC wiring safety grounds. Speaking more generally, any load-carrying conductor can not be in the safety ground path.

Good luck and hope this helps

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