Fusing the starter circuits. Yea or nay?

[Portage_Bay]

ABYC says you don't have to. Some of greater knowledge and experience than I suggest you should. I may put fuses in the starter circuits on my next maintenance cycle.

My story. I had just buttoned up the cooling systems on a pair of Cummins 6BT 5.9 engines. Fire 'em up one at a time and look for leaks. Port engine had a leak on it's cooler end cap. Cooler is located on the inboard side. Easily remedied. So time to check the starboard engine. I don't want to crawl across a running engine to get to the cooler so I get in position, head lamp on, tools at hand and signal my sweetie to start the engine. Out of my left peripheral there's a massive spark. A bolt of lightning it seems. I nearly jumped out of my skin. I had her shut it down. Twisted myself into a new pretzel position to keep an eye on the starter and had her start it, count to 3 and shut it down. Confirmed sparks were flying out of the solenoid.

Now, why am I interested in fusing the staring circuit? Well on a car long ago I the starter fail staying engaged. Starter failure? Solenoid failure? Bendix failure? Who knows. It caught fire and burned the electrical cables. Don't know which caused what to burn but I do know it was substantial damage and near loss of the car. On a car I can pull over to the side of the road and get out. On a boat? Nope.

Might I wind up with a blown fuse and a starter circuit that doesn't work. Yup. But it's fair warning. Find the fault, fix it, replace the fuse. Lost time, a bit annoyed and busted knuckles. But no fire.

 

[Flatswing]

You are correct, most boat fires are electrical and high amp circuits are a frequent cause. However because of the unique OCP demands for these high amp circuits, it’s not mandated by ABYC & not typically done but no reason you shouldn’t. Similarly large house banks will need high AIC OCP also.
A few caveats:
You will need a high AIC Rating fuse like a Class T or JLLx fuse with a high enough amperage rating for your starter. Best to measure the constant current draw for your 6BT so you have enough “ headroom” for those rare occasions where you need to keep cranking. Basic DC clamp meters max at 400A, so you may need to buy or borrow a big a** meter.
Blue Seas’ highest rating Class T fuse and holder is 400A. I needed a 600A JLLN for the almost 500A constant current draw for my JD6068’s. These are readily available online but you may need to make your own holder and cover above 400A.
Too expensive to carry a lot of spares for Class T so I just made up short 4-0 jumper cables to bypass the fuses in an emergency (once the reason for OCP blowing was identified & corrected).

 

[Delta Riverat]

How about switches instead of fuses?

I have 3 switches in the helm electrical panel for each engine and the generator. High current, connected right to the battery cables.

 

[Flatswing]

I believe most trawlers have/had battery disconnect switches for safety to to work on the alternators and starters. They do not provide the overcurrent protection from a dead short that Portage Bay was seeking.

 

[Fleming]

You will need a high AIC Rating fuse like a Class T or JLLx fuse with a high enough amperage rating for your starter. Best to measure the constant current draw for your 6BT so you have enough “ headroom” for those rare occasions where you need to keep cranking. Basic DC clamp meters max at 400A, so you may need to buy or borrow a big a** meter.
Blue Seas’ highest rating Class T fuse and holder is 400A. I needed a 600A JLLN for the almost 500A constant current draw for my JD6068’s. These are readily available online but you may need to make your own holder and cover above 400A.
Too expensive to carry a lot of spares for Class T so I just made up short 4-0 jumper cables to bypass the fuses in an emergency (once the reason for OCP blowing was identified & corrected).

You don't need a class T or high AIC rated fuse. The need for high AIC rating is based on the size of the battery bank, ABYC has a chart. I believe it's over 800 Amp hour battery bank or something close to that before you need to go up to the class T. The idea being that the larger the bank the larger the short circuit current, but a start battery alone can't short circuit over 3 or 4 thousand amps. For a simple start battery you can use a mrbf fuse with a 5,000 amp AIC.

 

[Flatswing]

You don't need a class T or high AIC rated fuse. The need for high AIC rating is based on the size of the battery bank, ABYC has a chart. I believe it's over 800 Amp hour battery bank or something close to that before you need to go up to the class T. The idea being that the larger the bank the larger the short circuit current, but a start battery alone can't short circuit over 3 or 4 thousand amps. For a simple start battery you can use a mrbf fuse with a 5,000 amp AIC.

Quite correct about the AIC for single start banks BUT: MRBF fuses 12v max is 300 A. If your starter’s continuous draw is much higher (as it can be with larger diesels), you have to move up to ANL (possibly) or Class T/JLLN to get a fuse with sufficient 12v A rating and a holder that works well in the typical trawler environment. Rod Collins has written a nice article on OCP (Marinehowto.com) that also discusses this.

 

[Delta Riverat]

I believe most trawlers have/had battery disconnect switches for safety to to work on the alternators and starters. They do not provide the overcurrent protection from a dead short that Portage Bay was seeking.

It would make it possible to turn off the current and avoid the car starter incident he described. That caught my eye too as I was a first hand witness to the same event in a parking lot. Engine wouldn't stop cranking, smoke coming up and burning insulation dripping down. We called 911 and told them to get out of the car. Fire Co. showed up and crow barred the battery cable off while it was still cranking. Yeah, not pretty.

 

[Portage_Bay]

It is indeed over current protection that is the concern. Yes, I have switches. But they are in the engine room near the start bank. If things get "interesting" I don't want to be in there. I looked into remote switches but apparently according to my local expert they will fail in the same way a fuse without a high enough AIC rating will fail. The other option is long cable runs to outside the engine room. I don't want to do that.

 

[Portage_Bay]

It would make it possible to turn off the current and avoid the car starter incident he described. That caught my eye too as I was a first hand witness to the same event in a parking lot. Engine wouldn't stop cranking, smoke coming up and burning insulation dripping down. We called 911 and told them to get out of the car. Fire Co. showed up and crow barred the battery cable off while it was still cranking. Yeah, not pretty.

I was a bit more blunt. Told them dropping the web app was a bad business decision.

 

[SteveK]

How many amps do you need to make spark. Unless dead short what would 600 amp fuse do?

 

[psneeld]

I used the concept ABYC adheres to main DC wiring runs that are short and don't have to be fused because the chances of a short are greatly reduced... as safety conscious that ABYC is, I would bet their risk management on that determination was fine. So I used the 7/40/72 inch rule for the starter to battery circuit.

On/off switched for runaway starters for me was always good enough.

 

[EngNate]

For about $200 get an industrial surplus 3 pole 200-250 amp circuit breaker, buss the three poles together. This will do the job without the voltage drop, or replacement hassle, of a fuse. What I have seen done with fuse was two ANL-500's stacked with spacers between.

 

[Steve DAntonio]

It would make it possible to turn off the current and avoid the car starter incident he described. That caught my eye too as I was a first hand witness to the same event in a parking lot. Engine wouldn't stop cranking, smoke coming up and burning insulation dripping down. We called 911 and told them to get out of the car. Fire Co. showed up and crow barred the battery cable off while it was still cranking. Yeah, not pretty.

I carry a small cable cutter in every car for this reason.

 

[Steve DAntonio]

A fuse may have no effect on a starter that fails to disengage. In some cases the stuck engagement is a contact issue in the solenoid, removing power would correct that, however that would need to be done manually, with a battery switch. If the engaged starter is a mechanical issue, the Bendix is jammed, the fuse and or battery switch will have no effect, although shutting down the engine would resolve the issue, assuming you know it was occurring, in most cases operators don't know the starter is stuck engaged.

Making certain the positive cable run to the starter from the battery is well-protected, and most importantly of all that it in no way touches the engine/transmission or any parts thereof, is probably more practical than installing a fuse. This is a common fault I encounter, as recently as a couple of days ago, see photo. This starter cable is touching the exhaust manifold. Although it is water cooled, if the engine overheats it could easily get hot enough to melt the insulation, or just chafe over time, and create a short. I also point out that split loom isn't armored, it only buys you chafe time, it is not chafe prevention, and it will melt just like insulation.

Unfortunately builders have gotten carried away with split loom, it is now way over-used. I took a 5 gal spackle bucket of unnecessary loom off of one 450 HP engine alone. There's no need to loom negative and bonding wires...