How big should my starter motor fuse be?

[prv]

I think the argument would be that the cable is securely mounted to the starter and so very unlikely to come off. Adding a fuse simply moves the potential for a fault along the cable a few feet, to where the battery cable is connected to the fuse. So actually it probably doesn't result in a system that is any safer.

The fuse in the battery compartment is not surrounded by lots of metal at earth potential.

In my case, the link from positive post to fuse is about an inch long (two ring terminals back to back) so cannot reach the negative terminal whatever happens. The only way to get a short would be to open the battery box and add an additional piece of metal.

Since the most likely "additional piece of metal" is a spanner, I have a dedicated battery spanner which is wrapped in insulated tape over its whole length except the business end.

Pete

 

[David2452]

Marine rated battery fuses from Blue Sea are great, I use them quite a bit, they fit directly to the battery terminal and protect the entire cable, they are available from 30a up to 300a.
Aquafax stock them if you can’t find them locally.

http://bluesea.com/viewresource/640

 

[ianj99]

The problem which has so far been overlooked in this thread is that the starter cables are not designed to carry the starter current for more than seconds or they will overheat.

Startera do require hundreds of amps, depending on the size of engine obviously and how cold it is.
Therefore fitting a fuse rated at say 500 amps indefinitely & 750amps for a few seconds, is not much good for protecting cables rated at 200amps.
(35mm cable is rated at 225amps for 125C temp rise)

I.e the cables insulation will have melted and allowed it to possibly short out & cause a fire before the fuse blows.

This is why fuses are not fitted in starter circuits.
If you insist on fitting a fuse then you will need to uprate the cables to an impracticable size for the fuse to be of benefit. eg >150sqmm

http://www.energy-solutions.co.uk/cable_conductor.html

Ian

 

[pvb]

The problem which has so far been overlooked in this thread is that the starter cables are not designed to carry the starter current for more than seconds or they will overheat.

Startera do require hundreds of amps, depending on the size of engine obviously and how cold it is.
Therefore fitting a fuse rated at say 500 amps indefinitely & 750amps for a few seconds, is not much good for protecting cables rated at 200amps.
(35mm cable is rated at 225amps for 125C temp rise)

I.e the cables insulation will have melted and allowed it to possibly short out & cause a fire before the fuse blows.

This is why fuses are not fitted in starter circuits.
If you insist on fitting a fuse then you will need to uprate the cables to an impracticable size for the fuse to be of benefit. eg >150sqmm

http://www.energy-solutions.co.uk/cable_conductor.html

Ian

I think you're confusing "allowable" current with "real world" current. In a short circuit situation, the current through a 35 sq mm cable will instantly blow a 500A fuse. Try it if you don't believe me!

 

[ianj99]

I think you're confusing "allowable" current with "real world" current. In a short circuit situation, the current through a 35 sq mm cable will instantly blow a 500A fuse. Try it if you don't believe me!

In 'real world' situations, there isn't always a direct short!
Ian

 

[pvb]

In 'real world' situations, there isn't always a direct short!

You'll need to explain that a bit more....

 

[agurney]

I'll be ordering some fuses from West Marine shortly but I'm unsure what size fuse for my starter motor to starter battery and I'm not sure how to measure this. The engine is a Perkins Prima M60 (59 hp). I have a Perkins service manual but not the specification of the starter motor itself. I know there was some talk of applying the corrrect fuse to the WIRE rating as opposed to the rating of the amps from the source. Does that apply here?

I would expect to see a fuse on the starter solenoid, but not on the cable between the starter and battery that it's switching.

 

[MrCramp]

I am with pvb on this, much better to try and be safe rather than sorry!

 

[halcyon]

In 'real world' situations, there isn't always a direct short!
Ian

In the real world there is always a direct short.

Back in the 90's I got blamed by Sealine for a faulty battery charger causing a big fire on a 365. Tracked it down to a genset retro fitted with NO fuse in the starter motor feed cable, not us.

A minor fault in a auto pilot motor controller had caused a cable to over heat, it got hot enough to melt the insulation on cables next to it. These were the positive and negative feed to the gen set starter motor, and the hydraulic steering plastic pipe.

The minor fault melted the oil pipe releasing oil into foam sound deadening, and then melted the starter cables. This shorted 300 amphour of battery bank via 35 sq mm cables.

The cables glowed red hot for there total length, melting all the other cables, setting the plywood bulkhead alight, and igniting the oil soaked foam. this then ignited the fibre glass and the back half of the boat was alight.

So yes you can get a major short, it may start as a small one, yes you can get a nice fire, and YES YOU NEED FUSES IN ALL BATTERY SUPPLY CABLES, as close to battery as possible.


Brian

 

[mitiempo]

You are fusing for a short circuit, not an overload. ABYC allows a fuse to be up to 150% of the wire's ampacity, although I try to stay with 100% whenever I can. I fuse starter circuits on engines up to 70 or so hp on a regular basis and have not had nuisance blows of fuses. I prefer either ANL fuses or the MRBF post mounted fuses (which act like ANL fuses) - they are able to handle much more than their rating for the time it takes to start a small engine.

How are your batteries switched? Is the starting circuit dedicated to the engine only or are you able to use the start battery for the domestic loads as well?

 

[demonboy]

Brian - this is up in the air at the moment as I am re-wiring and re-considering my battery set-up but as it stands I still have a separate battery for starter. Currently I am not able to parallel my two sets of batteries.

 

[ianj99]

You'll need to explain that a bit more....

How can you guarantee that a short will result in sufficient current to blow the very large fuse?

In 40years, I've never seen a starter circuit on a UK boat or vehicle with a fuse or breaker.

Instead, look at possible places where a short could occur such as by chafing, for exampe, and fit mechanical protection to prevent it and also clean and tighten all the connections.
More fires result from loose and dirty connections than from dead shorts.

 

[pvb]

How can you guarantee that a short will result in sufficient current to blow the very large fuse?

In 40years, I've never seen a starter circuit on a UK boat or vehicle with a fuse or breaker.

Instead, look at possible places where a short could occur such as by chafing, for exampe, and fit mechanical protection to prevent it and also clean and tighten all the connections.
More fires result from loose and dirty connections than from dead shorts.

I think Halcyon's post might help you to understand the need for fuses on batteries. The fact that you've apparently never seen a fuse fitted doesn't mean it's not a sensible measure to take.

 

[halcyon]

How can you guarantee that a short will result in sufficient current to blow the very large fuse?

In 40years, I've never seen a starter circuit on a UK boat or vehicle with a fuse or breaker.

I think you will find all new boats have fuses on batteries, if only to protect the builder from legal action, not counting approval requirements.

Brian

 

[halcyon]

I think Halcyon's post might help you to understand the need for fuses on batteries. The fact that you've apparently never seen a fuse fitted doesn't mean it's not a sensible measure to take.

I tell you, it's a eye opener to see a length of bare 35 sq mm wire with a ball of copper on the end. It had got hot enough to melt the copper conductor.

Brian

 

[aluijten]

I've looked it up in the manual. The starter motor of a Volvo Penta 2040 is rated at 2kW.
So at 12 volt that'll be 166 Amp, but more likely the voltage will drop to 9 or 10 volts. At that voltage the amp would be 200-220 (at same power level).
So a Fuse of 250A would be OK is my guess for this engine. Still an impressive fuse though.

 

[ianj99]

If the op feels safer with a fuse fitted then that's fine.

Otherwise a few others are presumably suggesting that we should all rush out and install one...

Would a poll not be good idea first to see just how many members have actually suffered the consequences of not having a fuse...?

Do insurance companies require a starter fuse be fitted, or have they been known to refuse a claim for fire if one wasn't fitted?

I'm not disputing for one moment what might happen in the event of a short, simply that in the 'real world' such events are statistically unlikely.

Ian

 

[pvb]

I'm not disputing for one moment what might happen in the event of a short, simply that in the 'real world' such events are statistically unlikely.

So, as a marine & auto electrics & electronics engineer, you're recommending that fuses needn't be fitted as it's "statistically unlikely" that there might be a problem? How about fire extinguishers - are they a good idea, despite fires being statistically unlikely? Or lifejackets?

 

[VicS]

I've looked it up in the manual. The starter motor of a Volvo Penta 2040 is rated at 2kW.
So at 12 volt that'll be 166 Amp, but more likely the voltage will drop to 9 or 10 volts. At that voltage the amp would be 200-220 (at same power level).
So a Fuse of 250A would be OK is my guess for this engine. Still an impressive fuse though.

You are right to assume the lower voltage but figures I have for a 810 watt Bosch starter motor are

185 to 220 amps at 9 volts loaded ( ie cranking)

400 to 490 amps at 7 volts locked

A 2 kW motor is likely to take more than double these currents.

 

[ianj99]

So, as a marine & auto electrics & electronics engineer, you're recommending that fuses needn't be fitted as it's "statistically unlikely" that there might be a problem? How about fire extinguishers - are they a good idea, despite fires being statistically unlikely? Or lifejackets?

Talk about being argumentative!

You've have no commonsense if you need me to answer that question.

If the authorities or insurance companies had evidence to show that not having a starter circuit fuse was risking injuries or increasing insurance claims, don't you think they'd have made such a fuse a mandatory retrofit or even just 'recommended' ?