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Robert M.O. Seidel avatar image
Robert M.O. Seidel asked

Earthing cable housing Multiplus and inverters

I have a question about the earthing connection or housing earthing of Multiplus and other inverter devices. Some operating instructions recommend that the housing/earthing connection should be at least 6mm2. Others state that it should be at least half the size of the DC supply cable. So with 95mm2 approx. 50mm2!!! That would be enormous. So what and why? So far I have assumed that it is primarily about the safety and function of the residual current circuit breaker, but if it is now written in such a way that it is related to the DC supply, I assume that it is about fault protection in this respect. If so, I doubt that the system is protected with it or I think that at such a moment, the whole thing will blow up in your face either way. Please only professional answers and no speculation. I am very grateful.

Translated

Groundingmultiple inverters
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Robert M.O. Seidel avatar image Robert M.O. Seidel commented ·

I should probably add that these are installations on boats that are also connected to the shore connection from time to time.

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cardy01 avatar image
cardy01 answered ·

Since your installation is on a boat the DC side will be bonded to 0V by the inverter negative cable to the battery and from the battery to the negative bonding point (hull if steel hulled) The inverter housing is then connected to the same bonding point with a cable one size bigger than the AC conductors ..... 4 or 6mm (12 or 10 AWG) usually.

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Robert M.O. Seidel avatar image Robert M.O. Seidel commented ·

Thank you! That is the perfect answer that explains my uncertainty! With a GFK hull, the mass comes to a metal through-hull which is below the waterline. top!

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gerard-van-seventer avatar image
gerard-van-seventer answered ·

If you want "only professional answers and no speculation" then you should contact your certified Victron installer/supplier. He or she will know the difference in earthing between mobile and fixed (housing) use.

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Alexandra avatar image Alexandra ♦ commented ·
@Robert M.O. Seidel

The answer is... What is the local regulation?

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sharpener avatar image
sharpener answered ·

UK wiring regulations mostly call for protective earth conductors to be one size smaller than the live/neutral conductors. If you buy sheathed twin-and-earth cable this is what you get.

I don't know of any reason why you would need especially large cable to protect the DC circuitry. DC fault protection for my Pylontech batteries is by a built-in circuit breaker controlled by the BMS. They have their own separate earth for the casing, but like the 5kVA Multi this is 6 sq mm cable. I don't think it is connected to the DC terminals, if the 48V -ve is connected to ground anywhere this is done inside the Multi.

As far as I know this setup complies with the UK Code of Practice for EESS which I studied before installing it.

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Kevin Windrem avatar image
Kevin Windrem answered ·

Connecting the case of the Multi to the safety ground and battery negative provides a path for fault current in order to trip a breaker or blow a fuse.Sizing the conductor for AC currents only risks a DC fault from burning the wire before the fuse in the DC positive lead can blow.

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sharpener avatar image
sharpener answered ·

Yes that is in Wiring Unlimited. However in that case surely the Multi should have an M8 ground stud which will take such a massive cable.

But it doesn't.

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Tony avatar image
Tony answered ·

The important thing is that the AC output has a good earth loop impedance. Good is a very low value. If in doubt get a qualified electrician to check there's sufficient earthing. Remember to install a RCCD, so that the power is disconnected in the event of a earth fault.

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sharpener avatar image
sharpener answered ·

The OP is asking about the cable for earthing the DC side.

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Robert M.O. Seidel avatar image Robert M.O. Seidel commented ·

In particular, the earthing of the inverter housing!

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Chris avatar image
Chris answered ·

screenshot-2024-06-01-21-53-30-899-comgoogleandroi.jpg

This section of text, or another version stating no smaller than one size lower than largest DC conductor appears in almost all victron wiring diagrams...


Also, the chassis ground on my multi 3K is an m8 stud...


No speculation here, just another hunter of the correct answer to this. Either the internet or victron or both seem to have something not in order.


screenshot-2024-06-01-21-53-30-899-comgoogleandroi.jpg (131.1 KiB)
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cardy01 avatar image
cardy01 answered ·

On my Multiplus 12/1200/50 16 the case connection point will accept about a 6mm2 conductor ... so I would suggest the case connection needs to be sized relative to the AC conductors.

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