Grounding lithium house and earthing inverter

DIY
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1

I’m currently upgrading my boat electric system and this is what I see to be the final layout.

I’m unsure whether to connect the DC grounding bus bar:
A) to the starter negative bus bar (should I then upgrade the negative cable from the starter battery to the starter engine from 35mm² to 120mm² to match the highest current potential?)
B) directly to the starter motor (120mm²)
C) directly to the bonding bus bar (120mm²)

Do the 4mm² bonding cables need enlarging?

About the AC system, does the PE in and out of the inverter/charger look correct?

Many thanks in advance

Drawing-6.sketchpad (1)
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2
  1. The wire from the DC grounding bar to the lynx distributor can be 95mm^2 or possibly as light as 50mm^2. Ground fault cables are only expected to carry current for as long as it takes a fuse to blow, not continuously rated, and the cable only needs to carry one fault current at a time, not the total current.
  2. The DC grounding should also have a wire to the Anode bonding bar, this can be 6 - 16mm^2.
  3. AC side, you should have an isolating transformer for the shore power - the multiplus does not provide galvanic separation.
3

Thank you very much for your clear and quick reply - even after reading manuals and forums it’s difficult to understand this topic.

  1. it makes sense. Based on what you said, is it ok to use a bus bar rated 200A for the DC grounding considering the max amps for this system is 400A?
  2. Great, this sounds the most logical option to me. How do you calculate the cable size for this connection? (I’m trying to understand the theory)
  3. In the scheme, there is a galvanic isolator on the top-left that is between the shore power inlet and the Multiplus.

Thanks again Mike, I really appreciate your response!

4

The size of the ground bonding wires is a difficult topic to determine or understand.
Basically these wires should NOT carry any current - they are there as equi-potential bonding - if there is any significant current then this indicates a problem that needs resolving - or it will lead to serious corrosion somewhere.
So, as the wires don’t carry (much) current - how do you size them?
There are 4 factors determining the minimum wire size:

  1. Are they lightning conductors - if so, or if this may be so, then they need to be heavy copper tape / braid.
  2. If not lightning, are they likely to carry any DC / AC fault currents? - if so size according to the current.
  3. Are they signal grounding (noise prevention) wires? these can be small - ~1mm^2
  4. If none of the above, then they need to be sized such that they are resistant to corrosion - this would normally be 6 mm^2, though it also depends if they are copper or tinned copper wires. bare copper is a lot more prone to corrosion than tinned copper, so a larger section is needed.

Galvanic isolation:
It is not sufficient to use a galvanic isolator in just the ground / PE connector. You really should have an isolating transformer in the AC connection, this will give an increased level of protection.

5

Thanks again Mike, I think I’m starting to get my head around it.
Really appreciated!