Battery fuse (again)

boatmike

boatmike

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Having considered all the answers on the previous thread in favour of main battery fuses I have one remaining question. Assuming that the main fuse (or fuses) on the battery are there to primarily protect the main cables from a dead short would it not be simpler, rather than separately fuse each bank on the positive cables, to fit a single fuse of say 220 Amp to the main earth lead???? My motivation is that I have limited space and no room close to the battery to fit fuses on the positive cables but could easily fit a single fuse holder to the earth cable close to the main cable exit. I recognise that a blown fuse here would knock out both batteries but from a safety point of view would do the same job would it not?
 
The run of the wire between the battery positive and the fuse is not protected against a short, so this is why it is recommended that the fuse is as near to the positive battery terminal as possible. The longer the distance from the battery positive to the fuse, the longer the run of unprotected wire.

If you install a decent main fuse holder eg
https://www.asap-supplies.com/elect...-holders/fuse-holders/mega-fuse-holder-729372

I am sure you can find a way of wiring it in.
 
You don't seem to have recognised my point. To have a battery "short" as you put it you need a complete circuit from positive to negative. I believe that any break in that circuit protects the cables so a single fuse in the negative line close to the battery would be just as effective as 2 fuses on the positive side. I am seeking arguments regarding such an arrangement. Your post does not address my question.
 
boatmike said:
Having considered all the answers on the previous thread in favour of main battery fuses I have one remaining question. Assuming that the main fuse (or fuses) on the battery are there to primarily protect the main cables from a dead short would it not be simpler, rather than separately fuse each bank on the positive cables, to fit a single fuse of say 220 Amp to the main earth lead???? My motivation is that I have limited space and no room close to the battery to fit fuses on the positive cables but could easily fit a single fuse holder to the earth cable close to the main cable exit. I recognise that a blown fuse here would knock out both batteries but from a safety point of view would do the same job would it not?
Click to expand...

Yes, you can fuse the negative lead. Naturally, for optimum protection, this should be as close to the batteries as possible. One possible downside is that, in the event that the fuse blows, you would (as you've acknowledged) disconnect both batteries. So, if for example you have a problem with the starter circuit, you'd also lose the ability to run the lighting or VHF on the domestic circuit, which might be inconvenient.
 
pvb said:
Yes, you can fuse the negative lead. Naturally, for optimum protection, this should be as close to the batteries as possible. One possible downside is that, in the event that the fuse blows, you would (as you've acknowledged) disconnect both batteries. So, if for example you have a problem with the starter circuit, you'd also lose the ability to run the lighting or VHF on the domestic circuit, which might be inconvenient.
Click to expand...

Well yes indeed pvb I recognise that both batteries would go in the unlikely event of a blown fuse but my batteries are hard up against a bulkhead and the positive leads are in trunking to the other side which means that without a complete remake of the battery box its either going to be a considerable length of unprotected lead before the fuses or a negative side fuse close to the battery. Not ideal I suppose but would protect against a complete meltdown and fire. You make a good point in that if it were to blow at night you would be plunged into darkness so a powerful torch is always a good standby and I have a handheld VHF as a backup also so I guess that's covering risk pretty well but thanks for reminding me!
 
yerffoeg said:
The run of the wire between the battery positive and the fuse is not protected against a short, so this is why it is recommended that the fuse is as near to the positive battery terminal as possible. The longer the distance from the battery positive to the fuse, the longer the run of unprotected wire.
Click to expand...

That argument works in a metal-bodied car, where a short can form between the positive supply and any number of things, but only between the earth and something positive (which is why we disconnect the earth first) but doesn't apply to boats. And, of course, cars don't have battery fuses anyway, because, erm, um, ah ...
 
JumbleDuck said:
That argument works in a metal-bodied car, where a short can form between the positive supply and any number of things, but only between the earth and something positive (which is why we disconnect the earth first) but doesn't apply to boats. And, of course, cars don't have battery fuses anyway, because, erm, um, ah ...
Click to expand...

Mates son designs fuse boxes for cars, one thing he does is semtex fuses, when the airbag goes off it blows a link in the main battery feed, faster and safer than a normal fuse. So they may have them, last car he worked on was the new Golf platform so that may have one.

How far away do you need for the positive cable ?

Brian
 
boatmike said:
Well yes indeed pvb I recognise that both batteries would go in the unlikely event of a blown fuse but my batteries are hard up against a bulkhead and the positive leads are in trunking to the other side which means that without a complete remake of the battery box its either going to be a considerable length of unprotected lead before the fuses or a negative side fuse close to the battery.
Click to expand...

You're missing a point Mike.... protection does not need to be a fuse, the trunking can provide protection. If the cables within the box cannot short, they don't have to have a fuse. Even new boat regs don't require a fuse in the starter circuit.
 
Last edited:
PaulRainbow said:
Not got the Cortina any more then Pete ? :)
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Ha! I drove loads of them, but never owned one. The first car I owned was a 1949 MGTC which, when I just Googled the reg number, is apparently now for sale. When I had it, the car was British Racing Green with green leather, and was in a fairly dreadful state. I sold it in the 60s to an American serviceman at a base near Harrogate. Looks like it's had a shedload of money spent on it over the years, but I don't like the red. Oh, and it didn't have a battery fuse.:rolleyes:

1949-MGTC-BFB343---2-X3.jpg
 
pvb said:
Ha! I drove loads of them, but never owned one. The first car I owned was a 1949 MGTC which, when I just Googled the reg number, is apparently now for sale. When I had it, the car was British Racing Green with green leather, and was in a fairly dreadful state. I sold it in the 60s to an American serviceman at a base near Harrogate. Looks like it's had a shedload of money spent on it over the years, but I don't like the red. Oh, and it didn't have a battery fuse.:rolleyes:

1949-MGTC-BFB343---2-X3.jpg
Click to expand...

Looks good, you should buy it.
 
halcyon said:
Mates son designs fuse boxes for cars, one thing he does is semtex fuses, when the airbag goes off it blows a link in the main battery feed, faster and safer than a normal fuse. So they may have them, last car he worked on was the new Golf platform so that may have one.
Click to expand...

That's interesting. I used to work on (the theory of) fault current limiters for power stations, where they need something which will react very quickly and ideally reset itself for the next peak of the sine wave. I was looking at superconducting limiters, but it was pretty clear that Stuffing Great Fuses were a better bet. Explosive disconnection sounds fun.
 
pvb said:
No, it was another time, another life. Today, it simply wouldn't be the same. Anyway, I have nowhere to keep it, and I'd be terrified of driving it in today's traffic; it was scary enough in the 60s!
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Seizing opportunity for boast: I'm writing this in a B&B in the hills above Interlaken, to which the DS has brought us via IJmuiden and around 1000km on Dutch, German, French and Swiss roads. Luckily it's a classic car which can keep up with modern traffic, and comfortably too. I wouldn't fancy the trip in my Herald nearly as much.
 
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