Battery Post Fuses!

[Dougal]

MCA coding for a 15m charter yacht insists that battery cables are either fused or double insulated. This would appear to address chafing.

...... you cannot be sure some foreign object isnt eating your insulation.

Indeed.... I have a wife and two dogs aboard;-)

 

[lw395]

I'm inclined towards fusing everything that I reasonably can. which I think we all are so we're just debating what's reasonable.

I think I could blow the fuse, but I'm willing to be corrected. Good crimps are alleged to have a resistance equivalent to a similar length of cable, which with 25mm2 is going to be neglible. 6ft of AWG 4 has a resistance of approximately 0.001 amps, plus about 20 milliamps for the internal resistance of the battery... so 21 milliamps worst case if we have a good strong contact with whatever is compromising our cable.

I = V / R = 13 / 0.021 = 619 amps, which is below the CCA of my battery.

They're slow blow, if my starter does use what I expect (150amps) then I could manage no problem on a 200 amp fuse, which on a dead short, going by the Blue Sea graph (https://www.bluesea.com/products/5187/MRBF_Terminal_Fuse_-_200A) ought to blow in a few seconds, but not blow just on the surge as the starter engages.

Assume some of your amps are ohms...

CCA is specified with 1.2V per cell.
And that's for a new battery.
It's easy to find a few milli-ohms in anything that's been on a yacht a few years.

It all seems like a very small window between the surge into the starter and the short current.
I would be looking at robust insulation and protection rather than relying on the fuse.
Covers for the + terminals, an extra layer of heat shrink here and there. Conduit in any vulnerable areas? That sort of thing.

 

[yachtorion]

Assume some of your amps are ohms...

Aw feck. Fixed that, thanks.

CCA is specified with 1.2V per cell.
And that's for a new battery.
It's easy to find a few milli-ohms in anything that's been on a yacht a few years.

Can't argue with you there.

It all seems like a very small window between the surge into the starter and the short current.
I would be looking at robust insulation and protection rather than relying on the fuse.
Covers for the + terminals, an extra layer of heat shrink here and there. Conduit in any vulnerable areas? That sort of thing.

Believe me on my boat all of the above are a given, I'm definitely not seeing it as an either/or thing. But having seen the melted, singed mess that was the engine wiring harness on the boat when I bought her, and how occasionally even a well made crimp can fail or fracture... I hate having unfused links. No to the fuse may of course be the best answer in the end - but I value considering and discussing it!

Then again over thinking stuff like this is probably part of the reason it's taking so long to get this boat back in the water.

 

[gandy]

But are the cables to the main distribution board (or wherever the next fuses/breakers are) capable of standing 750A for a second to blow the fuse?

If you're interested, look up the "Adiabatic Equation" as used by electricians. Essentially this tells you whether a given cable, in a given overload situation, will rise to a damaging temperature before the over current protection operates.

 

[lw395]

If you're interested, look up the "Adiabatic Equation" as used by electricians. Essentially this tells you whether a given cable, in a given overload situation, will rise to a damaging temperature before the over current protection operates.

I'm aware of that.
It's often simpler with mains electricity because you can generally assume the supply has got a very low impedance compared to the worst overload you have to think about.
Likewise, you can usually rely on a 12V battery to blow fuses up to 30A or so when required.

 

[mjf107]

A lot of these fuses may never blow.
You need to consider the cable resistance and battery CCA curves.
A 300A (nominal) fuse will carry upwards of 450A for a very long time.
https://www.bluesea.com/products/5108/MEGA___AMG_Fuse_-_300_Amp
Your battery and cables will possibly struggle to provide 450A for more than a few seconds, particularly if there is much cable length involved. After a few seconds, the cable heats up and the resistance rises.
But a short will still be dumping enough amps into the cable to cause a fire.

You have to weigh up the actual protection you are getting vs unreliability you are adding.
I am inclined towards fusing large house banks but not starter circuits.
The main thing though is to avoid it being an issue by ensuring all cables etc. are in good nick and protected from damage.

What you describe is becoming more like an overload than a short circuit therefore a circuit breaker with thermal and magnetic magnetic trips would give better protection
Michael

 

[Colvic Watson]

It seems to me there are many boat "safety" devices being fitted these days by owners that for decades have not been a part of standard fitment on boats.

So are owners just looking to spend money and tinker or have owners become far too paranoid?

Maybe, but statistics for the US (I don't have UK figures) say that 55% of boat fires are electrical in origin, with 45% being down to DC shorts/wiring.

30 years ago the boats were smaller, the electrical loads smaller and far fewer devices. Then look at battery bank size, it's gone from one battery for both to domestic banks of 300ah being common. 300ah of dead short will make a very big fire. I can't see the issue here, a 70ah fuse is fit and forget, it's fail safe, requires no maintenance and is only needed in a catastrophic cable short. Times move in and thankfully we've learnt from other people's mistakes, that's how progress works. Our domestic fuse took half an hour to fit, 99% chance it'll never blow but by golly it will be half an hour well spent if it ever does.