To fuse or not to fuse.. that is the question!

[Grahambda]

What are your thoughts on whether engine cranking and thruster/windless batteries should be fused? Getting mixed responses; one camp feels that they can draw so much it will blow a fuse when you need it most, and another camp says absolutely fuse- to stop the wire melting. Check out PYC Jeff Cote Youtube vid (PYC has some great info, worth subscribing to his channel)

 

[DavidM]

Fusing engine starting circuits is exempted by ABYC. Thrusters/windlasses are not.

David

 

[Comodave]

In my boat it would depend on what engines I had. I have fused gas engines with 300 amp fuses with no problems. Diesels draw more so I would probably not. But with diesels it would be a good idea to put the starter cables in a flex conduit to help protect them from chafing and then shorting.

 

[O C Diver]

IMO, part of fusing a starting battle depends on what else is attached to it and how long the cables are. If your start battery is also the house battery (some start their engine off the house bank), then yes there needs to be a fuse.

In my situation, the start battery cables are less than 2' long and double or triple ought cable. The wires aren't going to melt and nothing is going to compromise them.

Ted

 

[Action]

I am in the camp of no fuse. Run in a conduit or protective covering. And well supported with end connections covered.

I have done some vehicle restorations where a large displacement and high compression engine was difficult to re-start after a hot soak. (usually on a hot day) That scenario may not exactly play out on a vessel. And larger cable diameter and a (Non-metallic) heat shielding sleeve around the cable solved the issue. In addition the heat shielding did double duty as additional protection around the cable. If you will a replacement for a conduit.

The starter cables when heat soaked had additional electrical resistance. The cable to the starter passed very close to the exhaust manifold. While the starter had it's shield to protect from heat, I added cable coverings. Along with up sizing the cable diameter.

 

[High Wire]

I have been using 300 amp MBTF for 6 years on my Perkins 6 cyl diesel. Haven’t blown a fuse yet. The reason for the fuse is on Monk 36s, the standard wire arrangement is both engine AND DC power panel is fed by the same cable that is more than 6 ft long. Every inch of positive conductor is also sheathed by me as extra security.

 

[Don L]

My thoughts are to FUSE.

No way a proper system should ever blow a fuse sized for the cable. It is a blow the fuse or burn down the boat choice!

PS p=- does it matter if you engine starts while the fire spreads through the boat?

 

[Action]

PS p=- does it matter if you engine starts while the fire spreads through the boat?

It would if you need to get to the shore urgently.

 

[Mambo42]

I completely changed the system on my boat. In the past had the batteries of the windlass and the bow thruster standing in the ER and there was about 8 to 9 meter of cable, running through a bus bar to the windlass and the bow thruster.
Now I placed 2 x Optima yellow 75 Ah batteries right next to the bow thruster and windlass, no fuse, but with a battery switch so I can isolate the batteries if I need to. The whole system is run through the pilothouse, where the switch basically activates a relay, which then allows power to both systems. just for good measure I used 120 mm2 to power the windlass (24 V 1700 W) and also 120 mm2 for the Vetus bow thruster. 70 mm2 would have been enough, but we decided to over engineer it a bit, so as to avoid the cable becoming warm/hot during prolonged operation and to ensure I have no drops during operation.
Running that amount of Amps through a fuse may cause problems in the future, that is why I avoided it.

 

[Don L]

It would if you need to get to the shore urgently.

is not that ""The horses got out, shut the barn door!"

 

[Senojev]

Fusing Starter Battery

Rod Collins has a good article supporting fusing of starter batteries.

https://marinehowto.com/battery-banks-over-current-protection/

 

[Comodave]

I completely changed the system on my boat. In the past had the batteries of the windlass and the bow thruster standing in the ER and there was about 8 to 9 meter of cable, running through a bus bar to the windlass and the bow thruster.
Now I placed 2 x Optima yellow 75 Ah batteries right next to the bow thruster and windlass, no fuse, but with a battery switch so I can isolate the batteries if I need to. The whole system is run through the pilothouse, where the switch basically activates a relay, which then allows power to both systems. just for good measure I used 120 mm2 to power the windlass (24 V 1700 W) and also 120 mm2 for the Vetus bow thruster. 70 mm2 would have been enough, but we decided to over engineer it a bit, so as to avoid the cable becoming warm/hot during prolonged operation and to ensure I have no drops during operation.
Running that amount of Amps through a fuse may cause problems in the future, that is why I avoided it.

Bow thrusters are not excepted from the no fuse by ABYC. Here is an excerpt from Rod Collins website addressing this instance.

The argument a few of the members of the ABYC committees make is that starting motors draw too much current and are difficult to fuse. Ok, if this is the argument, why don’t we take a look at devices such as DC powered bow thrusters. A bow thruster is not a cranking conductor so by the ABYC standards that connection to the thruster bank must be fused.

 

[TowLou]

Last check I did was on a cat 3512 24v system, 2- 8ds, 2 years old and it pulled every bit of 915 amps at 24Vdc. All the capacity of the batteies. Pulled so much makes the positive wire jump and tense for a few seconds. Pretty neat and made me jump the first time seeing it out of the corner of my eye.

It's tough to fuse if you did 90% of battery CCA for a start circut. Can't fuse total capacity as the battery does lose over time.

Everything but the starters. Good crimps and wires with maintenance should give no worry for the starter not being fused. Plus gonna need every last amp to get that motor going.

Of all the starters on engines I can say I have not seen a fuse from positive to the selnoid and no fuse on the direct negative to the frame and block.

 

[MYTraveler]

Last check I did was on a cat 3512 24v system, 2- 8ds, 2 years old and it pulled every bit of 915 amps at 24Vdc. All the capacity of the batteies.

If the starter is going to pull all the amps the battery can delivery, then a dead short won't pull more and the fuse wouldn't blow in any circumstance other than its failure to handle the intended load. So, in that circumstance fusing is pointless as it introduces the possible failure of a critical circuit no offsetting benefit. But, on boats, like mine, where the battery bank can delivery far more amperage than can be demanded by the starter, fusing offers a significant benefit (protection against a dead short) and should be incorporated, even if not required by ABYC.

 

[TowLou]

If the starter is going to pull all the amps the battery can delivery, then a dead short won't pull more and the fuse wouldn't blow in any circumstance other than its failure to handle the intended load. So, in that circumstance fusing is pointless as it introduces the possible failure of a critical circuit no offsetting benefit. But, on boats, like mine, where the battery bank can delivery far more amperage than can be demanded by the starter, fusing offers a significant benefit (protection against a dead short) and should be incorporated, even if not required by ABYC.

Thats when starting with your switch in all. Is there fusing between all the battery banks? Why put that load on the house bank to start the motor?

 

[Senojev]

A couple of points.

The short circuit amperage of an 8D battery is around 7500 amps. All of the good crimps, tight connection, and maintenance will not protect a cable in the event of a short.

Starter cables are often undersized for their task. The ampacity of a 4/0 cable is 375A so pulling over 900A, although for a short duration, is overloading that cable significantly.

Fusing a circuit that pulls over 900A is not easy but a slow blow 800A fuse should provide adequate protection.

In lieu of fusing, extra precautions to protect starter cabling such as conduit or non flammable sheathing and good support should be taken.

 

[PM]

I would not fuse the main engine starting battery, but I would make sure the cables are large enough to stand the draw and that the battery and cabeling are isolated for only engine starting.

Everything else should be fused. If you have to "lay on" the thruster long enough to blow a fuse you should review your docking procedures.

pete

 

[Jc180]

Long standing philosophy has been to give priority to emergency propulsion. This includes alarms for such things as low oil pressure and high cooling water temperature vs trips (shutting off engine), as well as no over current protection (fuse or breaker) in starting circuit. Furthermore, the starting batteries are positioned near the engine to keep cables short and the cables are sized to handle the high inrush currents to minimize likely hood of faults (failures).

It seems this philosophy has been forgotten or is being eroded by the up coming generation of boaters.

Lastly, retired now, but spent career as an electrical engineer in power plants. Over current devices do indeed spuriously fail in normal service.

 

[Steve DAntonio]

Fusing engine starting circuits is exempted by ABYC. Thrusters/windlasses are not.

David

Ditto, no question about fusing everything except engines. Frankly I'm surprised windlasses and thrusters are even a question, it's mandatory.

I'm encountering more and more builders who are using fuses on start circuits. I have mixed feelings. If the fuse and wire are sized properly, nuisance blowing should not be an issue. Regardless, the positive cable run for the starter must be well-protected, and for ABYC compliance it must not touch the engine or any part of the engine, in any way.

More here https://stevedmarineconsulting.com/over-current-protection/

 

[Circumnavigator]

I'm surprised no one has mentioned road vehicles -- I've never seen a fuse on a automotive starter and that's a much more difficult environment -- more abrupt bumping and shaking, more salt, and the occasional accident.


I fuse everything but starter motors.


Jim

 

[DennisB1]

One of the issues when adding fusing in a high amp draw line is voltage drop. In testing that I did when I was working each interface a close coupled fuse assembly (<7inches from battery) would be the resistance between each interface in the device. There appears to be 4 intefaces on a Blue sea fuse. Each interface would be in the area of 1/1000 ohms resistance or <4/1000 ohms for the device. This is pretty low resistance, however if you are drawing 1000 amps which my Cat 3208 12v start does it amounts to about 3- 4 volts using ohms law.(v=IR). You need special equipment to make this resistance measurement which I no longer have access to.
So on engine starter circuits I would not use a fused line. For house loads I would use fused lines.

 

[CarlF]

Please read the Rod Collins piece posted. As Rod says, ABYC is the BARE MINIMUM. Both he and Nigel Calder feel it is always best to fuse starter circuits. Who among us settles for the minimum when greater safety is easy and inexpensive?

If your engine is more than 250hp it gets harder. But I’ve fused my 110hp Yanmar start circuit with a 300amp Blue Sea MRBF fuse for 10 years and never had a “nuisance” blow.

Rod Collins has a good article supporting fusing of starter batteries.

https://marinehowto.com/battery-banks-over-current-protection/

 

[Comodave]

I fused a gas engine boat with MRBF battery mounted 300 amp fuses and never had a problem with blown fuses. It would be difficult to fuse my 450HP diesels though.

 

[motion30]

The 3208 cats in my boat draw well over 600 amps while cranking

 

[DDW]

One must also keep in mind that dead shorts that draw 300A are very rare. You can arc weld 1/2" steel with 300A - and not blow the fuse. The jacket wearing and some strands touching a ground are likely to arc, but not likely to blow a 300A fuse.

The idea that if I have a 300A fuse in the cable, it will absolutely prevent a fire is just not supportable. It will prevent some fires. Probably not the most likely ones. In a starter circuit, it is the cable terminations or connections that are most likely to develop high resistance, these can catch fire and will never blow a fuse (they are the fuse...).

 

[Patrick4002]

to fuse or not to fuse ?

starter motors are normaly never fused, all other users well,

the problem with starter moters is the high start current my toroflow detroit diesels took 900amp normal and 1200 amps when warm, its for a short period but to fuse these is crazy your fuse has to bee so high that it will hardly protect annything. thursters, inverters ga max 450amp and should be protected, Best regards, Patrick

 

[Steve DAntonio]

One must also keep in mind that dead shorts that draw 300A are very rare. You can arc weld 1/2" steel with 300A - and not blow the fuse. The jacket wearing and some strands touching a ground are likely to arc, but not likely to blow a 300A fuse.

The idea that if I have a 300A fuse in the cable, it will absolutely prevent a fire is just not supportable. It will prevent some fires. Probably not the most likely ones. In a starter circuit, it is the cable terminations or connections that are most likely to develop high resistance, these can catch fire and will never blow a fuse (they are the fuse...).

You make a good point, high resistance connections, which generate heat, are especially dangerous as they will not trip a fuse or breaker, they simply represent a load, whether it's doing work, like turning a starter, or create a great deal of heat. While not DC, high resistance shore power connections are a great example, they can overheat and burn and never trip a breaker.

I would not condemn a start circuit that did not have a fuse, and by the same token I would not condemn one that had a fuse provided it was properly executed. On small engines it's pretty easy, larger engines can, as others have noted, get tricky.

I routinely encounter poorly routed positive starter cables, draped around engine blocks and over bell housings on a regular basis, these are a fire waiting to happen. This cable run must be the most protected and well-inspected aboard any vessel.

 

[ronobrien]

I am in the camp of no fuse. Run in a conduit or protective covering. And well supported with end connections covered.

I have done some vehicle restorations where a large displacement and high compression engine was difficult to re-start after a hot soak. (usually on a hot day) That scenario may not exactly play out on a vessel. And larger cable diameter and a (Non-metallic) heat shielding sleeve around the cable solved the issue. In addition the heat shielding did double duty as additional protection around the cable. If you will a replacement for a conduit.

The starter cables when heat soaked had additional electrical resistance. The cable to the starter passed very close to the exhaust manifold. While the starter had it's shield to protect from heat, I added cable coverings. Along with up sizing the cable diameter.

That hot soak hard start (cranking) is also an indicator of a bad starter. Just a thought for anyone experiencing that issue. It may seem fine when cold, but hard (cranking) when hot.

 

[ronobrien]

I see other similar posts that well protected conductors to an isolation switch is the best approach. Not fused but have the ability to isolate the starting circuit. I have read here of a couple incidents that the starter shorted and started cranking on its own.

 

[Patrick4002]

not always

You make a good point, high resistance connections, which generate heat, are especially dangerous as they will not trip a fuse or breaker, they simply represent a load, whether it's doing work, like turning a starter, or create a great deal of heat. While not DC, high resistance shore power connections are a great example, they can overheat and burn and never trip a breaker.

I would not condemn a start circuit that did not have a fuse, and by the same token I would not condemn one that had a fuse provided it was properly executed. On small engines it's pretty easy, larger engines can, as others have noted, get tricky.

I routinely encounter poorly routed positive starter cables, draped around engine blocks and over bell housings on a regular basis, these are a fire waiting to happen. This cable run must be the most protected and well-inspected aboard any vessel.

a difficult crank with a hot motor is not always a bad starter, when a motor is hot the piston seals are better and makes a higher presser build up in the cilinders, sure with big blocks 6liter or more.

You can also look if your starter motor is a real heavy large one or more like a small spraypaint can, the large ones mean that the motor need a lot of power to cranc.

You can not look at the output power of the motor, for example my marine crusader V6 motors(detroit diesel) where only 170hp but 6 liter motors and had a very large starter and took 900+ Amp of current, my carterpillars V8 450hp 10liter have a starter witch is a 3 of the size and take a lot less current

if your not shore wat you have, ask a technical person to take a look at your situation

Best regards,
Patrick