Battery fuses

[Peter]

Just completing a new battery installation, separate batteries for starter and "houses". And I would like some comments on whether one should fit high amp rated fuses and isolation switchs after the batteries before feed goes to the distribution bus bar and the starter. If yes, what do they protect, the battery or the panel/starter motor?

Peter

 

[pvb]

Yes!

You certainly should fit some fuses, ideally as close as possible to each battery bank. The Mega Fuse Block by Blue Sea Systems is excellent, and there's a similar one made by Littel. They take Mega fuses, available from 100A to 300A.

The fuses protect the wiring (and ultimately the boat!). In the event of a fault creating a short circuit, the huge current which could flow would overheat the wiring, potentially causing a fire. By putting the fuse as close as possible to the battery, you protect the maximum length of wire.

If you have proper fuse protection, the isolator switches can be positioned in a more convenient location, away from the batteries.

Remember too that any electrical devices which are permanently connected to the batteries (bilge pump, radio, navtex, etc) should each be individually fused (again, as close as possible to the battery).

 

[softop]

I agree if you want help with specifying fuses etc call Merlin on 01202 697979 they are the Blue Sea Distributor.

 

[VMALLOWS]

Re: Yes!

This is certainly perfection (and possibly essential if you are considering installations with enormous battery capacity). However it is certainly not normal practice in boats (or cars) to fuse the starter. As you say, the fuses would need to be extremely hefty (and expensive) and well installed to avoid unaceptable voltage drop. It is also common practice not to fuse the alternator output, though this would be more practical if desired (though a fuse failure might blow your alternator without additional measures).

Some people even prefer to route the starter directly to the cranking battery, avoiding any loss at the isolating/battery selector switch. I prefer to route everything from the battery(s) via a switch (located close to the batteries) so that everything is isolated when the boat is unattended. Of course, if you have bilge pumps, alarms etc these will need a separate fused feed.

 

[pvb]

You don\'t need an enormous battery...

You don't need an enormous battery to start a fire. An ordinary 12volt starter battery has more than enough stored energy to set fire to your boat. Most boat starter motors only take around 100A, so fusing the circuit with a 150A fuse isn't a major exercise, especially if you're in the process of rewiring.

As many boats now have some sort of charge-splitting arrangement, which connects the alternator output to more than one battery circuit, a single fuse failure wouldn't damage the alternator, and a simultaneous failure of both fuses is unlikely.

 

[peixe]

I have just fitted 250 amp fuses in both the cranking battery circuit and the services battery circuit (I have arranged things so that the services battery can, if necessary operate the engine starter). The engine is a Bukh 20 in Vancouver 27. The fuses are in the positive leads right at the batteries. They protect the heavy battery cables which if they came adrift and shorted to earth would, in the absence of a fuse surely produce much mayhem.

The fuses and fuse-holders were bought from On Board Power (www.onboardpower.com). Mega fuse holder £4.62, fuses £2.75. Not exactly eye-wateringly expensive...

Do it ! Good luck.

 

[LadyInBed]

Re: 250 Amp fuse

Yes as you say, you will be protected against a heavy duty cable floating around in the battery compartment, but surely if your cables exit the battery box, head off in different directions in separate ducting - Neg to engine block and Pos to 1/2/both/off switch, how could your dead short occur? (apart from dropping a spanner in the battery box). Putting another couple of terminal connections (to the fuse) is IMO creating more of a risk.
Secondly, unless I have forgotten something, to blow your 250A fuse your wiring resistance would have to be less than 0.05 ohms.
As you will have realised, I am not drawn to the idea of a big battery fuse. Equipment and lighting circuit fuses should be enough.

 

[peixe]

Re: 250 Amp fuse

This is in reply to NigelB - I'm new here and not too sure how things work on the forum.
Maybe my wiring is not as neat & tidy as you think...supposing the positive cable from the cranking battery to the starter solenoid fell off onto the block ?
I do not follow your comment referring to the resistance being less than 0.05 ohms in order to blow the 250A fuse; I imagine that the resistance of a short length of 9mm dia battery cable must be negligible and if shorted to earth would certainly blow a 250A fuse.

Ideally, surely every circuit should be fused ?

 

[LadyInBed]

Re: 250 Amp fuse

Please don't think I am getting at you. Discussion is what this forum is about.

I don't think a battery fuse is nessary, reading other threads, other people do.

Yes, I accept your scenario. But even then, isn't the starter motor down at the bottom of the engine, and there is a starter solenoid in the way. A few locking washers and Nylock nuts will also offer good protection.

There, I've stuck my head above the parapit, so I'm now waiting for the flack!

Oh, one other thought, do you carry a spare fuse? ;-)

 

[peixe]

Re: 250 Amp fuse

NigelB

This is not intended as flack !

Think I understand your comment re resistance being less than .05 ohms – this obviously limits the current that can flow; however the resistance of battery cable is about 0.04 ohms/100metre. Assume a cable length of 1 metre, that the battery voltage falls to 10 and the resistance of the fuse is the same as the cable (I have seen a figure for this but cannot lay my hands on it) then by Ohms Law the current that will flow in a short circuit is 10/2*(0.04/100) = 12,500 amps. This is of course a big oversimplification and ignores other resistances such as the battery itself etc but I think shows that a 250A fuse will blow.

I think the reason that battery fuses are not commonly fitted is that only recently have big fuses been cheaply available. I decided to fit them as the pros outweigh the cons (what are they ?) and they are cheap and easy to fit. In the end of course it is an almost philosophical decision. If you are looking for things to worry about then there are probably much more important matters such as hitting a submerged container or de-zincified skin fittings falling off...

Of course I carry spare fuses – several !

Incidentally I hope this post inserts itself in the correct place after NigelB's last message - if not then what the hell should I do ?

 

[LORDNELSON]

I would be interested to know how you arrived at the size of fuse (250 amp) most of the books I have referred to are silent on the size of a main fuse for a boat

 

[Guest]

The reply from pvb is the one to take note of. Short circuits can happen in unexpected conditions, but often a degree of human error is present.

During a short circuit, to all intents and purposes, the size of the current flow is limited by the capacity of the battery bank. If the current flow is through a thin wire, that wire will quickly overheat. The insulation will melt and then the wire itself will melt, with the obvious fire risk. Thicker wires will take longer to melt or, if thick enough, may not melt at all but the heat created will still be very dangerous.

To calculate the size of fuse required at the battery, first calculate the maximum current that your battery bank will need to supply with all of your loads switched on. Add 50 % and then choose the nearest available fuse value.

 

[peixe]

It took many phone calls to discover what current the starter motor might draw. Eventually - (taking an average of the many guesses I was given) - I decided that 150A would be about right for a Bukh 20 (disregarding the initial inrush - which the slow-blow fuse should cope with). The fuse size of 250A should ensure that the starter does not blow the fuse but limits the current to an amount that the battery cable can safely take.

So, a bit of guesswork, (as you say the books, while recommending a fuse, will not say how big). In practice it works.

 

[HaraldS]

The probability of ever needing that fuse is low. But then you also hope you'll never need that fire extinguisher or the life raft. The sum of these things determines the safety level of your boat.

I fitted separate 355A fuses to all four battery bank supply cables (70 mm2).
I calculated how much heat the cable can safely dissipate. I'm loading the cable with under 10 Watts per meter in this case before the fuse would blow. For that cable this is on a surface of 300 cm2, so it will never get really hot.

As I said, very unlikely to get a short, but candidates for problems are alternators and starter motors, which are most ofen unfused.

I know of a case where somebody opened the battery switch while the engine was running, which blew out the diodes in the alternator, then he realised his fault and closed it again, causing a short through the broken diodes of the alternator and the cabling went to flames. Because of the short the contacts of the battery switch had been welded together so he couldn't turn it off any more. Eventually the alternator cable (35mm2) gave in and he was able to stop the fire before it was too late.

I'm using strip fuses with a cermic body, they are around £5 and you can get them up to 500A. There are also holders for them, or you can place them between two posts 60 mm apart. Posts can be between 8 - 10 mm thick.