Battery main fuse rating?

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With this sort of trouble free protection I can see you are one of the fierce proponents in favor of fusing the start circuit.
Could you tell me what size is your engine?

[/ QUOTE ]I wouldn't say I'm a "fierce proponent", but KenMcCulloch asked the question about fuse size, and I offered one of a number of different opinions he received. My opinion was based on actual experience. My engine's a 45hp Volvo 2003T.
 
As it's a complicated business, using what some vehicle manufacturers use may be a good plan, if your application is no more onerous. It seems that MegaFuses are more or less optimised for this application.
The point about wire sizing is that wire is generally continuously rated, it's possible to work out I^2R losses at high current and divide by the heat capacity to get temperature rise. For instance I calculate that 1000A for about 2.5 seconds would heat a 10mmsq cable to about 150degC, the rating of some of the better automotive insulation. So the cable is surpirisingly robust, since most people (inc me) would say 10mmsq is too small for a starter cable. Maybe I'm coming round to the fuse idea...
Cheers,
 
As the resistance of starter cable is minimal the currents can be huge.Some years ago my friend was manager of a milk business and during the night one of the milk floats had a fault and it embedded itself into a double brick wall.they had to acrow the wall to make safe for removal.Have I got fuses? No, thought of fitting them a few times but never got round to it, I suppose if a huge fault occured the battery would explode any flames would be put out by the diluted acid.Never heard of a fire started by an exploding battery.but very messy.I worry more about leaving de-humidifiers on.
 
Another case of you cannot generalise.

It depends on your engine / starter motor and battery / cranking voltage. Running currents can be as high as 500 / 600 amp, with stall current 1200 amps.

Comes down to check what your starter motor spec is, then your cable, then a suitable fuse.

Brian
 
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For instance I calculate that 1000A for about 2.5 seconds would heat a 10mmsq cable to about 150degC, the rating of some of the better automotive insulation. So the cable is surpirisingly robust, since most people (inc me) would say 10mmsq is too small for a starter cable. Maybe I'm coming round to the fuse idea...
Cheers,

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Assuming a 10mmsq cable was mounted 3m from the engine bay and the + and - leads directly connected together (a direct short) assuming no resistance loss in any connections or in any fuse block ,battery switches , or any internal resistance in the battery ( all of which is impossible) the current would be limited to about 1000A by the resistance in the cable alone. The cable would have dissipate 12000 watts.
Its hard to believe the cable would remotely stand this sort of abuse.
If I have gone wrong somewhere forgive me they are forecasting force 9 to 11 (depending on what forecast you believe) at my anchorage tonight so I am a little distracted.
 
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Assuming a 10mmsq cable was mounted 3m from the engine bay and the + and - leads directly connected together (a direct short) assuming no resistance loss in any connections or in any fuse block ,battery switches , or any internal resistance in the battery ( all of which is impossible) the current would be limited to about 1000A by the resistance in the cable alone. The cable would have dissipate 12000 watts.
Its hard to believe the cable would remotely stand this sort of abuse.

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I haven't checked your numbers, but they sound plausible. Based on a quick mental calculation, 12KW into this cable will heat it at just over 100 °C /second if the 3m is the total 'round trip' distance, or about 60 °C/s if each wire is 3m (6m total) - please excuse rounding. This gives a second or two for any protective device to act before the cable is permanently damaged, and significantly longer before it gets hot enough to start a fire. 'Fault' ratings for cable and equipment are completely different from continuous ratings, as you might know.

For the starting circuit only, I share your concerns about striking a balance between a marginally rated fuse which will fail randomly due to fatigue, and a generously rated fuse which will offer little protection, and am undecided whether to fuse this or not. Others' experience with the MegaFuse is reassuring in this context.

For all other circuits, however, I wouldn't have any hesitation in adding a battery fuse, and it is something I plan to do this winter. The rating will be whatever the wire to the main fuse-board is capable of taking. (The windlass already has both fuse and over-current trip).

Each type of fuse has its own peculiar current / time relationship for it to blow. FWIW, I think you're worrying too much about your '1000A' spike (to paraphrase) - this is so brief that I doubt that even a semiconductor fuse (which is extremely quick to blow) would react to it, so a 'normal' metal fuse won't even see it.

0.02p

Hope your boat is OK!

Andy
 
Well cars for many years have not had fuses in the starter circuit.
Questions that arise are can the battery provide enough current for log enough to cause the cable to incandesce. What would happen if the cable were red hot apart from making a lot of smoke from the insulation burning. What are the likelihood of a short? Probably a lot less than that in a car.

So if the battery is near the starter motor and you don't have those 1,2,both switches remote from the battery and starter and if the cables are mounted away from flamable material then you are on the way to a safe system. Every switch and fuse you fit increases the liklihood of high resistance and failure which in the cas of a boat can present its own dangers.

it is a pity that most starters now have a solenoid which also acts on the drive gear. If the solenoid (start relay) can be mounted remotely then the obvious place is near the battery. then no problem.
olewill
 
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1) Well cars for many years have not had fuses in the starter circuit.....................................

2) Questions that arise are can the battery provide enough current for log enough to cause the cable to incandesce...................

3) What would happen if the cable were red hot apart from making a lot of smoke from the insulation burning.

4) What are the likelihood of a short? Probably a lot less than that in a car.................



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1) Well a car isn't a boat, you don't sleep and live in a car and there are often many difficulties installing batteries and cabling in boats way above any car. There is much more movement and vibration in a boat as well

2) The insulation may or may not burn depending on what it's made of, but a much more important question is what it might be in contact with in the boat. plus the battery could explode

3) See 2

4) I would suggest there is a great likelihood of a short circuit. Marine installations are subject to massive amounts of movement and vibration which can lead to metal fatigue, chafe and terminals becoming undone. Starter motors jam as well because of corrosion and that can be catastrophic too.

Being in the real world means accepting that many marine installations are poorly installed and maintained even by professionals. Marine electrical installations are often one off or short run and are often done by poorly skilled workers with inadequate resources and materials unfit for the job.
 
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Being in the real world means accepting that many marine installations are poorly installed and maintained even by professionals. Marine electrical installations are often one off or short run and are often done by poorly skilled workers with inadequate resources and materials unfit for the job.

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That is a good point. I am generally content to leave a good boat-yard to do just about anything on a boat except gas and electrical work. I find that the extra effort to get it done right is a very rare thing and definitions of 'done right' are extremely variable. There is no point in specifying good kit if it is lazily installed.

Specifying adherence to a code seems little help either. I know a boat in which the plumbing was modified (I suspect) to meet a requirement that underwater through hulls can be shut off, the result was a dramatic reduction in safety, but compliance with the code!
 
Great topic!

It seems that the existance of a fuse is to protect against accidental short circuits maybe caused by a cable vibrating against an earthed object.

If the starter solenoid is located next to the battery, the majority of the cable connecting to the starter will normally be disconnected.
 
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If the starter solenoid is located next to the battery, the majority of the cable connecting to the starter will normally be disconnected.

[/ QUOTE ]True, but not relevant to most people. Since the 1960s, most new engines have been fitted with pre-engaged starter motors which have an integral solenoid.
 
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