battery breaker 'fuse' - never seen one in any of my boats

[simonfraser]

a fuse by the battery, so how are these rated to protect the wire / boat to the on off switch further up the chain ?
my battery switch in a 2021 boat is connected straight to the battery, no fuse, not for the house and not for the starter either
oh and not in the bow for the battery to the bow thurster either
and then do we need a fuse between the on off swich and the DC switch board too ?

 

[lustyd]

Or even protecting the switch. The BEP marine ones are rated to 275A which will be lower than the cable supplying it on a lot of boats so the old “fuse to protect the wire” may not be wise.

 

[pmagowan]

I once had the main cable from the battery to the engine break and come very close to shorting against the engine casing. Vibration over years had obviously caused it. I now have fuses at the battery terminal positive. They are sized to protect the wire. Other stuff has its own fuse if it needs protection. All smaller wires have their own fuse.

 

[Neeves]

Apart from parsimony is there any reason not to have, 'more', fuses?

Jonathan

 

[CapPugwash]

The fuse should always be as close to the source (battery bank) as possible as that is where the the risk of overheating is the greatest. It should be rated for the cable. I have a 100 Amp mega fuse connected between the battery bank and the battery Isolation switch which is wired to a busbar, from which there are fuses on the cables going to the switch panels etc.

 

[sarabande]

Apart from parsimony is there any reason not to have, 'more', fuses?

Jonathan

Each and every fuse is a potential SPOF, despite their designed MTBF. And that entails a number of additional spares to be carried, at additional expense and storage problem.

 

[PaulRainbow]

Each and every fuse is a potential SPOF, despite their designed MTBF. And that entails a number of additional spares to be carried, at additional expense and storage problem.

And every missing fuse is a fire risk.

Fit blade fuses for all circuits >40a, you need to carry about 4 fuses, costing 25p each. Plus a battery fuse and maybe a couple of midi fuses. Total cost about a fiver and all would fit in a fag packet with space to spare.

 

[simonfraser]

And every missing fuse is a fire risk.

yes, but a chafed battery wire touching the other polarity with a >100A fuse is just going to spark ?

 

[sarabande]

Paul, my point was that there's no need to go above the minimum designed number of fuses to protect the wires/cables. Adding redundant, extra, fuses , beyond professional "best practice", is a waste of money and introduces unnecessary complications and risks.

 

[PaulRainbow]

Paul, my point was that there's no need to go above the minimum designed number of fuses to protect the wires/cables. Adding redundant, extra, fuses , beyond professional "best practice", is a waste of money and introduces unnecessary complications and risks.

Technically you should have a fuse in the main cable from the battery, close to the battery. This should be rated at or below the cable rating, if the isolator switch is rated at a lower current than the cable the fuse should be rated at the switch max current (you might find that the switch is rated continuously at less than the cable, but with an intermittent rating greater than the cable, so you could perhaps still rate at the cable current, individual circumstances would dictate which apply).

If, for instance, the load cable/s from the switch split into lower rated cables, then they would need separately and additionally fusing. Say you have 35mm battery cables, fused at source with 240a fuses (which allows an emergency switch to be fitted between the engine and domestic isolators) then at the load side of the switch you have 3 x 10mm cables going to various panels/fuse boards, you would need to fuse each of these cables with a fuse no greater than 75a. Once those 10mm cables gets to the panel/fuse board individual fuses are fitted to protect each circuit.

Omit any of the above fuses and that cable has no over current protection. However, let's say one of those 10mm cables is 30cm long, going to a fuse board and there is nothing it can possibly short to, it could be argued that a fuse is not essential. Any possibility of a short and the fuse must be fitted.

 

[PaulRainbow]

yes, but a chafed battery wire touching the other polarity with a >100A fuse is just going to spark ?

Not entirely sure what you mean Simon.....

If you short a circuit with a fuse, the fuse blows. If it does not have a fuse the wire will get very hot, very quickly, possibly resulting in a fire. There is enough energy in a decent 12v battery to blow chunks out of battery posts and spanners, some 2.5mm wiring will melt instantly.

Here's the engine wiring loom from a 45ft motorboat, the stop solenoid shorted which left one of the wires below shorted to the negative on the engine block. Can you tell which wire shorted ? Luckily, the owner returned to the boat to find it filling with smoke and managed to get to the main isolator in the nick of time.