Fuse For Starter Circuit?

[Dougal]

After all that starter grief I had with my 4.108 that you guys helped me with, I now have to get a fuse back in that circuit. It previously had a whopping 250amp baby fitted, which I now believe is too big. Total + & - run of what I think is mostly 50mm2 cable is aprrox 22ft but I've no idea what the actual normal draw should be on those ancient starter motors. Should i look at reducing the cable to a more realistic 35mm2
Edit; Theres a small piece of 35mm2 cable in there so I guess it should all be treated as such?

 

[matthewriches]

You’re fusing the 35mm cable in that case, unless you can swap that to 50mm2?

Mega fuse holder or similar should be fine. No need for any large T class ceramic stuff in your case. Although you’ll see most older boats don’t fuse their starting circuits at all.

 

[Dougal]

I know many don't bother, but some years ago while on the River Ouse in York, we watched in horror, as a BRAND NEW GRP craft took less time than it took the fire brigade to arrive, to go down in flames when the starter failed to disengage, and there was no fuse to blow. The white hot starter cable lit the GRP in sbout 5 seconds. However many times they put out the flames with CO2 bottles, it simply reignited when supplied with fresh air. The owner got off but it could very easily been different. It didnt help that in despair, he threw on the used bottles!

 

[knuterikt]

The blue sea calculator
A 200
duration 1 minute
V 12
length 7 meters
give 50mm2

fuse 200-250 A

 

[William_H]

If we look at cars they have had unfused starter circuits for 100 years. It does not seem to have become necessary to fuse the high current circuit. This would be partly because a fuse large enough to carry the starter surge current would be far to large to protect the wire and partly because the average car battery could not provide enough current to over heat the wire. (to any great extent) Also generally the wiring is short and well supported and so a short to the metal body is unlikely. Also a serious fire is unlikely to beak out from a red hot wire.
Now all of these factors are a variable quantity on a boat. We may have a large capacity and high current capability battery. The cable may be quite long and close to flammable grp. We may not be able to mount and suspend the cable in clear air to avoid shorts or a fire starting from a hot cable. So some safety standards for boats do specify all circuits be fused but it seems to me most private boats do not have a fuse in the starter circuit.
I wonder what the real fault in the story Dougal relates was. I would think a fault in the starter unlikely to draw huge currents. If a pinion sticks in the ring gear and the engine starts the starter motor will over speed and destroy itself but not draw much current. If it sticks and engine does not start it may continue to crank drawing starter motor normal current possibly over heating wires. But then hopefully you have a battery switch to turn off.
So owner needs to decide if a fuse is appropriate given that any fuse or switch in battery line to starter is going to be a source of faults and voltage drop. olewill

 

[Dougal]

Whatever the actual cause, we were easily within 6ft of the fire at one point while trying to use our CO2 and Halon. The white hot glowing battery cable was clear to see even through the smoke. That was really spooky to witness. The fire officer also confirmed the fire was caused by "a severe electrical fault, probably a direct short, excasibated and fed by two fuel cans on the side deck".
The ods on those events are no doubt pretty slim, but having seen just how astonishingly fast the fire became totally out of control, I'll be keeping my fuse thanks

 

[knuterikt]

If we look at cars they have had unfused starter circuits for 100 years. It does not seem to have become necessary to fuse the high current circuit. This would be partly because a fuse large enough to carry the starter surge current would be far to large to protect the wire and partly because the average car battery could not provide enough current to over heat the wire. (to any great extent) Also generally the wiring is short and well supported and so a short to the metal body is unlikely. Also a serious fire is unlikely to beak out from a red hot wire.
Now all of these factors are a variable quantity on a boat. We may have a large capacity and high current capability battery. The cable may be quite long and close to flammable grp. We may not be able to mount and suspend the cable in clear air to avoid shorts or a fire starting from a hot cable. So some safety standards for boats do specify all circuits be fused but it seems to me most private boats do not have a fuse in the starter circuit.
I wonder what the real fault in the story Dougal relates was. I would think a fault in the starter unlikely to draw huge currents. If a pinion sticks in the ring gear and the engine starts the starter motor will over speed and destroy itself but not draw much current. If it sticks and engine does not start it may continue to crank drawing starter motor normal current possibly over heating wires. But then hopefully you have a battery switch to turn off.
So owner needs to decide if a fuse is appropriate given that any fuse or switch in battery line to starter is going to be a source of faults and voltage drop. olewill

I have seen results of fire caused by shorting the cable between starter and battery, not nice.
The cable had been chafing against the engine mounts and insulation was worn through and battery got grounded....
In cars the cable between battery and starter in short with less risk of shorting, starters can also fail.

 

[zoidberg]

The cable had been chafing against the engine mounts and insulation was worn through....

I'm installing battery-starter cabling at present, so this is relevant.

Suggestions, please, for suitable anti-chafe sheathing material...?

 

[matthewriches]

Conduit?

 

[PCUK]

Conduit?

I use the split stuff for ease of installation.
Also I always include an easily accessible isolating switch in the starter wiring. Thought that was standard common sense!

 

[Dougal]

I'm installing battery-starter cabling at present, so this is relevant.

Suggestions, please, for suitable anti-chafe sheathing material...?

Good luck. I hope it goes well for you. It can be a difficult process on your own. I think I'd always use conduit now when possible for that circuit. I use the semi flexible round plastic stuff form B&Q etc. Doesn't look pretty but gives peace of mind.

I'm using the (some would old fashioned) 1,2&Both switch. As its only mounted about 16" from the battery bank, I haven't also used a separate isolator switch. As William said earlier, just one more set of connections to fail.

 

[PaulRainbow]

If we look at cars they have had unfused starter circuits for 100 years. It does not seem to have become necessary to fuse the high current circuit. This would be partly because a fuse large enough to carry the starter surge current would be far to large to protect the wire and partly because the average car battery could not provide enough current to over heat the wire. (to any great extent) Also generally the wiring is short and well supported and so a short to the metal body is unlikely. Also a serious fire is unlikely to beak out from a red hot wire.

Sorry Will, but that is all just wrong. Average smallish boat battery cable is 35mm square, a 240 amp fuse will protect the cable and will not blow under starter loads. There is more than enough energy in a starter battery to get the battery cable hot enough to start a fire. Even 2.5mm cabling will start a fire.

 

[PaulRainbow]

After all that starter grief I had with my 4.108 that you guys helped me with, I now have to get a fuse back in that circuit. It previously had a whopping 250amp baby fitted, which I now believe is too big. Total + & - run of what I think is mostly 50mm2 cable is aprrox 22ft but I've no idea what the actual normal draw should be on those ancient starter motors. Should i look at reducing the cable to a more realistic 35mm2
Edit; Theres a small piece of 35mm2 cable in there so I guess it should all be treated as such?

Cable run is irrelevant, you are fusing to protect the cable, in your case the 35mm section. The existing fuse is correct for the 35mm cable and i would not down rate it, there is no point. I would, however, replace the 35mm section with 50mm cable, for neatness and consistency, if nothing else. Also, if you have mostly 50mm cable you may as well enjoy the lower voltage drop and stick to all 50mm.

 

[Dougal]

Cable run is irrelevant, you are fusing to protect the cable, in your case the 35mm section. The existing fuse is correct for the 35mm cable and i would not down rate it, there is no point. I would, however, replace the 35mm section with 50mm cable, for neatness and consistency, if nothing else. Also, if you have mostly 50mm cable you may as well enjoy the lower voltage drop and stick to all 50mm.

Agreed. Why someone chopped a piece of 35mm is beyond me. Maybe a previous owner needed a battery jumper cable;-)

This whole damn boat is a total mess when it comes to wiring. 'One day' I rip the whole lot out and start from scratch. Well, maybe one at a time lol;-) With you lot holding my hand;-)

 

[<152587>]

I'm installing battery-starter cabling at present, so this is relevant.

Suggestions, please, for suitable anti-chafe sheathing material...?

More to the point, forget the sheath, clip, clip and clip again.

 

[lw395]

More to the point, forget the sheath, clip, clip and clip again.

Absolutely.
Eliminate chafe by eliminating movement except where necessary.
Use clips etc to ensure cable ends are not stressed.

 

[alan_d]

I have recently fitted a battery fuse. Here's why:

I had just got back to my home mooring and was returning to the cockpit having secured the mooring strop when I became aware of an unusual smell, a bit like burning sugar. My first thought was that the engine was overheating and I looked over the transom to check the cooling water, but that seemed fine.
Then all the electrics went off.
I opened the lid of the engine compartment (possibly unwisely) and was greeted by acrid fumes, but no sign of flames. I quickly closed the lid again and went to turn off the battery, but the 1-2-Both switch was jammed. I got a spanner with a view to undoing a battery terminal, but when I opened the battery compartment I was greeted by more acrid fumes. By the time I had disconnected the battery the fumes had dissipated and I could see that the insulation had completely melted/vaporised on the negative battery lead and part of the negative battery post had melted.

What had happened was that the heavy cable between the 1-2-Both switch and the starter motor had been rubbing against the speed-control quadrant on the engine and had worn through the insulation, causing a short circuit between the engine and the starter cable (always energised when the battery is switched on as it is a pre-engaged starter with the solenoid mounted directly on the starter motor). The last straw had been when I closed the throttle after arriving at the mooring, nipping the starter cable in the speed-control quadrant and causing a major short-circuit, melting/vaporising the insulation on the starter cable between the point of the short-circuit and the 1-2-Both switch. The circuit was only broken when the spindle of the 1-2-Both switch got so hot it melted its plastic support and the rotating part of the switch came away from the fixed contacts (which is why I could not turn it when I tried to isolate the battery). I don’t know if this is a design feature or just an accident, but it does seem as if the switch acted as a high-current fuse, and without it I might well have had a fire on my hands.

I was initially puzzled as to why only the negative battery lead had melted its insulation while the positive leads were unaffected, but I then discovered that the positive leads were 50sq.mm while the lead from the battery negative to the engine block was only 35sq.mm, as was the starter lead (although the engine wiring diagram said 50sq.mm). I have now fitted a new 1-2-Both switch, replaced the burned-out cables with 50sq.mm tinned marine quality leads, and in the best tradition of stable-door shutting I have fitted a 200 Amp battery fuse.

 

[zoidberg]

I have recently fitted a battery fuse....... in the best tradition of stable-door shutting I have fitted a 200 Amp battery fuse.

Just for avoidance of ambiguity ( as the bishop said to the trans actress ), is that a fuse rated to BLOW at 200Amps - or for 200Amps continuous, with a higher 'blow' rating....?

 

[alan_d]

Just for avoidance of ambiguity ( as the bishop said to the trans actress ), is that a fuse rated to BLOW at 200Amps - or for 200Amps continuous, with a higher 'blow' rating....?

Can't tell you that, but it is one of these: https://www.furneauxriddall.com/acatalog/Mega-Fuse---Per-Fuse-Mega_Fuse.html#SID=32

 

[matthewriches]

Almost all (but please check with your supplier or manufacturer tech docs as old fuses could show the blow rating), MEGA fuses will blow at double the "rating". So a MEGA labelled at 200A for CONTINUOUS USE will BLOW at 400A, not considering any environmental factors.

Can't tell you that, but it is one of these: https://www.furneauxriddall.com/acatalog/Mega-Fuse---Per-Fuse-Mega_Fuse.html#SID=32