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I am soon to be helping a friend with his electrics on his canal boat, he is looking at putting a small consumer unit, with a circuit for sockets and another for lights and upgrades to sockets wiring etc etc

All sounds easy enough but I am wondering how to select the correct short circuit protection, as the PSCC from the inverter is not very high, so even using a B16 MCB for the sockets the PSCC from invertor is well under 80A therefore in the event of a short circuit fault it will not meet required disconnection times etc. and if I got with a low-value MCB then it will cause nuisance tripping from over current.

Any ideas or is there something I am missing

Thanks

Marcus
 

Lucien Nunes

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Deleted post, comment later...
 

James

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If this is a small inverter I.e under 3kw,
You have probably not got to worry about protection for over current. The inverter will not provide enough current for long enough to damage the cable. Assuming it is at least 1.5mm
 

Lucien Nunes

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Exactly. Unlike the public mains, which can supply thousands of amps continuously until something disconnects the circuit, the inverter has very limited short-circuit output because it simply cannot convert any more from 12V without its transistors being destroyed. Therefore, even before its internal protection acts, the current is likely to be too low to damage the cabling. There will be some internal protection, possibly quite sophisticated, possibly just a fuse in the DC input. This will trip or blow in an attempt to save the inverter's transistors - it doesn't always! But as a numerical example, a 2.5mm² on a load circuit protected by a B20, might have to withstand 100A for 10 seconds before tripping, which it will. But I don't know of any inverter of a few kW rating that would do that without either shutting down or going up in a puff of smoke, so the MCB will probably never trip.

While considering protection, don't forget to check the earthing scheme of the inverter. Are N & E linked internally? Do you / can you make a link externally? Without it you have an IT system, single-fault tolerant but RCDs won't work.
 

PEG

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The inverter manufacturer,usually specifies any output fusing requirements....unless it's origin is coupled with an amusingly translated handbook...

"...special machine is wanting happy with load joined..." is one,i quite liked😍
 
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Exactly. Unlike the public mains, which can supply thousands of amps continuously until something disconnects the circuit, the inverter has very limited short-circuit output because it simply cannot convert any more from 12V without its transistors being destroyed. Therefore, even before its internal protection acts, the current is likely to be too low to damage the cabling. There will be some internal protection, possibly quite sophisticated, possibly just a fuse in the DC input. This will trip or blow in an attempt to save the inverter's transistors - it doesn't always! But as a numerical example, a 2.5mm² on a load circuit protected by a B20, might have to withstand 100A for 10 seconds before tripping, which it will. But I don't know of any inverter of a few kW rating that would do that without either shutting down or going up in a puff of smoke, so the MCB will probably never trip.

While considering protection, don't forget to check the earthing scheme of the inverter. Are N & E linked internally? Do you / can you make a link externally? Without it you have an IT system, single-fault tolerant but RCDs won't work.
Ok perfect I understand, so basically to the inverter cannot provide enough power to damage the cable, yes I was going to link the PE & N , and connect PE to the hull of the boat, therefor providing multiple earth fault protection and allowing an RCD to work

Thank you Marcus
Post automatically merged:

The inverter manufacturer,usually specifies any output fusing requirements....unless it's origin is coupled with an amusingly translated handbook...

"...special machine is wanting happy with load joined..." is one,i quite liked😍
Nice I will check out the manufacturer text, although I suspect it is in Chinglish as you say

"...special machine is wanting happy with load joined..." What product was that on
Post automatically merged:

If this is a small inverter I.e under 3kw,
You have probably not got to worry about protection for over current. The inverter will not provide enough current for long enough to damage the cable. Assuming it is at least 1.5mm
Cool thanks for that, yes over 1.5mm it will be 2.5mm flex
 

James

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be careful to not induce any current through metal parts of the boat.
galvanic corrosion can be set up and it can take Kg's of metal off a boat every day if left unchecked.

this is a specialised topic that I know only a little about, but it can be bad news for bearings, propellers etc as they just get eaten away.
 

PEG

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be careful to not induce any current through metal parts of the boat.
galvanic corrosion can be set up and it can take Kg's of metal off a boat every day if left unchecked.

this is a specialised topic that I know only a little about, but it can be bad news for bearings, propellers etc as they just get eaten away.
Ok perfect I understand, so basically to the inverter cannot provide enough power to damage the cable, yes I was going to link the PE & N , and connect PE to the hull of the boat, therefor providing multiple earth fault protection and allowing an RCD to work

Thank you Marcus
Post automatically merged:



Nice I will check out the manufacturer text, although I suspect it is in Chinglish as you say

"...special machine is wanting happy with load joined..." What product was that on
Post automatically merged:



Cool thanks for that, yes over 1.5mm it will be 2.5mm flex
This was a 3000W inverter,used off a pickup,to power show-jump timers,PA and associated pony carry-on.

I always read the paperwork...if not in Cantonese...one on a recent shower mixer,read "...undo not as pressure and wet quickly having..."

....I really see a new angle,for Jasper Carrot,with these instructions...:)
 

James

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worth a read!
 
I am soon to be helping a friend with his electrics on his canal boat, he is looking at putting a small consumer unit, with a circuit for sockets and another for lights and upgrades to sockets wiring etc etc

All sounds easy enough but I am wondering how to select the correct short circuit protection, as the PSCC from the inverter is not very high, so even using a B16 MCB for the sockets the PSCC from invertor is well under 80A therefore in the event of a short circuit fault it will not meet required disconnection times etc. and if I got with a low-value MCB then it will cause nuisance tripping from over current.

Any ideas or is there something I am missing

Thanks

Marcus
What about protection for the circuits if he hooks up to shore power?

Most shore supplies are 16A but some can be higher

Paul
 

Lucien Nunes

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True, the OCPD still wants to be there for shoreline use, although it probably doesn't matter if the inverter can't trip it.

On my regular mooring I am limited to 10A by a B10 in the wall cabinet. On board I have a B10 that I have recalibrated light on thermal but not magnetic, so that it always goes first on overload even when it's warmer indoors than out. I had to tame the inrush on the microwave though, to stop them both occasionally tripping magnetically.

Galvanic isolation is provided by a couple of exceedingly large diodes.
 

PEG

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True, the OCPD still wants to be there for shoreline use, although it probably doesn't matter if the inverter can't trip it.

On my regular mooring I am limited to 10A by a B10 in the wall cabinet. On board I have a B10 that I have recalibrated light on thermal but not magnetic, so that it always goes first on overload even when it's warmer indoors than out. I had to tame the inrush on the microwave though, to stop them both occasionally tripping magnetically.

Galvanic isolation is provided by a couple of exceedingly large diodes.
Are your diodes,so big,that they project both sides,like a scallop dredger? 🤗
 

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