Battery Post Fuses!

[Colvic Watson]

Won't the alternator be damaged if the fuse blows with the engine running?

Yes, it'll blow the diodes and cost you a couple of hundred quid and a few hours of your time. But these are for domestic batteries, where a current of more than 75amps means big disasters and boy you want a fuse to blow! We have an 80 amp fuse just off the domestic bank and happy to know it's there. An engine starter battery will briefly chuck out a lot more than that so I'd need to see what fuse was needed. If there's a dead short on starting then the fuse will blow and the alternator won't be affected, if there's a dead short during running then the fuse will blow and by golly you'll be glad you fitted it!

 

[FWB]

Yes, it'll blow the diodes and cost you a couple of hundred quid and a few hours of your time. But these are for domestic batteries, where a current of more than 75amps means big disasters and boy you want a fuse to blow! We have an 80 amp fuse just off the domestic bank and happy to know it's there. An engine starter battery will briefly chuck out a lot more than that so I'd need to see what fuse was needed. If there's a dead short on starting then the fuse will blow and the alternator won't be affected, if there's a dead short during running then the fuse will blow and by golly you'll be glad you fitted it!

I have only two batteries with 1-both-2 switch. No separate start battery. Been like that for years. All the services are fused. Can't see how this set up is a problem. Aircraft have a hot battery bus bar, no cb deemed safe for flight. I think my boat is safe.

 

[theoldsalt]

It seems to me there are many boat "safety" devices being fitted these days by owners that for decades have not been a part of standard fitment on boats ( I, of course, exclude personal equipment such as lifejackets, lifelines etc ) . Items such as auto bilge pumps, auto fire extinguishers, tracking devices, burglar alarms, and yes even battery cable fuses.

There seems little justification since few serious accidents have happened without these facilities and no insurance company has stipulated their fitment or made them a condition of cover. Also few if any improve the seaworthyness of the vessel

So are owners just looking to spend money and tinker or have owners become far too paranoid?

 

[abraxus]

Why would you want to put a fuse in the battery cable? Why run the risk of damaging the alternator?

The way I see it is this.

Assuming the fuse isn't dramatically undersized, then there's no greater risk to the alternator than an unfused cable, as any situation that might cause a battery fuse to blow is just as likely to cause an unfused cable to fail. In either case the effect on the alternator is the same.

As such, it's better for any failure to be contained in a safe location, ie the fuse, than to manifest itself on a random part of the circuit. The risk therefore of damaging the alternator is, to all intents and purposes, the same as a random failure on an unfused circuit. However, the outcome of the former is known and therefore limited, whereas the later is entirely unpredictable and so the consequences are potentially unlimited.

 

[abraxus]

It seems to me there are many boat "safety" devices being fitted these days by owners that for decades have not been a part of standard fitment on boats ( I, of course, exclude personal equipment such as lifejackets, lifelines etc ) . Items such as auto bilge pumps, auto fire extinguishers, tracking devices, burglar alarms, and yes even battery cable fuses.

There seems little justification since few serious accidents have happened without these facilities and no insurance company has stipulated their fitment or made them a condition of cover. Also few if any improve the seaworthyness of the vessel

So are owners just looking to spend money and tinker or have owners become far too paranoid?

Maybe, but statistics for the US (I don't have UK figures) say that 55% of boat fires are electrical in origin, with 45% being down to DC shorts/wiring.

 

[nigelmercier]

There was another thread about these recently, Blue Sea fuse

 

[<152587>]

Not usual practice to fuse starter battery as loads can be variable with hot or cold starts and the poss of taking out alternator, however it is always good practice to fuse domestic system at source. Think of how your house is wired,

1 Fuse in the meter box sufficient to take the whole household load and a bit.

2 Fuse/breaker in the consumer unit sufficient to take the circuit

3 Fuse in the spur or plug top applicable to the appliance load

Fuses in the battery compartment are ok as long as they are ceramic or sand filled, ignition certified circuit breakers as fitted to anchor winches are a good method.

 

[Dougal]

It seems to me there are many boat "safety" devices being fitted these days by owners that for decades have not been a part of standard fitment on boats ( I, of course, exclude personal equipment such as lifejackets, lifelines etc ) . Items such as auto bilge pumps, auto fire extinguishers, tracking devices, burglar alarms, and yes even battery cable fuses.

There seems little justification since few serious accidents have happened without these facilities and no insurance company has stipulated their fitment or made them a condition of cover. Also few if any improve the seaworthyness of the vessel

So are owners just looking to spend money and tinker or have owners become far too paranoid?

Sometimes, it's a case of reading too many books (Nigel Calder?) and having the fear of god put into you, when previously, I believed things were 'reasonably' safe;-)

 

[mjf107]

The way I see it is this.

Assuming the fuse isn't dramatically undersized, then there's no greater risk to the alternator than an unfused cable, as any situation that might cause a battery fuse to blow is just as likely to cause an unfused cable to fail. In either case the effect on the alternator is the same.

As such, it's better for any failure to be contained in a safe location, ie the fuse, than to manifest itself on a random part of the circuit. The risk therefore of damaging the alternator is, to all intents and purposes, the same as a random failure on an unfused circuit. However, the outcome of the former is known and therefore limited, whereas the later is entirely unpredictable and so the consequences are potentially unlimited.

Agreed..... also you can fit an alternator protection relay, no personal experience with them, came across them looking at something else
sterling-power.com/products/alternator-open-circuit-protection-device

 

[Norman_E]

How do you propose to connect it to the starter motor without bringing it anywhere near the engine?



Why do these cables need fuses yet the others do not?

Pete

The positive lead to the starter motor does have to run to the engine, but if correctly routed so that it does not chafe on anything that is no problem. The other battery positive cables need not go close to the engine at all.
As for the battery charger, its a new Sterling Power one and the instructions included fitting fuses, though my old charger did not have any and I only fitted them because the instructions said so. Similarly I have a fuse in the positive cable from the house battery isolation switch to the inverter, but I would not fuse the main positive lead to the boat's switch panels because all circuits are individually protected by circuit breakers, nor would I put a fuse onto the engine start battery positive because of the danger to the alternator if it blew. I would never install a fuse in a battery compartment.

 

[yachtorion]

The positive lead to the starter motor does have to run to the engine, but if correctly routed so that it does not chafe on anything that is no problem. The other battery positive cables need not go close to the engine at all.
As for the battery charger, its a new Sterling Power one and the instructions included fitting fuses, though my old charger did not have any and I only fitted them because the instructions said so. Similarly I have a fuse in the positive cable from the house battery isolation switch to the inverter, but I would not fuse the main positive lead to the boat's switch panels because all circuits are individually protected by circuit breakers, nor would I put a fuse onto the engine start battery positive because of the danger to the alternator if it blew. I would never install a fuse in a battery compartment.

It's obviously quite proper that each skipper make his/her own choice on this one based on their own subjective view of the risks.

That said, for me personally, I'd probably fit these fuses, though I'd probably fit an alternator protection device as well. My battery is vented overboard, the fuses (Blue Sea at least) are ignition protected, and I believe the biggest dangers to my boat, apart from me, are gas and electricity. Both need to be constrained as much as is practical and I simply don't have enough faith in crimp connectors and plastic insulation to not want a backup plan.

But even given all that... I'm not sure I can fuse the engine positive and starter link? Blue Sea MRBFs only go up to 300A and I'm not sure if that will be enough? The engine not starting when you need it to because the fuse blows would be very unfortunate.

I think once I've got the engine back in the boat I'll do some tests with a clamp-on meter. If I find it's down in the 150Amp region (which is what I expect for a small new 2cylinder diesel) then I'll go with the fuses and just make it part of the maintenance regime to check the starter current every so often. These fuses are reasonably slow so an initial surge as the starter motor begins turning shoudn't be a problem?...

 

[lw395]

A lot of these fuses may never blow.
You need to consider the cable resistance and battery CCA curves.
A 300A (nominal) fuse will carry upwards of 450A for a very long time.
https://www.bluesea.com/products/5108/MEGA___AMG_Fuse_-_300_Amp
Your battery and cables will possibly struggle to provide 450A for more than a few seconds, particularly if there is much cable length involved. After a few seconds, the cable heats up and the resistance rises.
But a short will still be dumping enough amps into the cable to cause a fire.

You have to weigh up the actual protection you are getting vs unreliability you are adding.
I am inclined towards fusing large house banks but not starter circuits.
The main thing though is to avoid it being an issue by ensuring all cables etc. are in good nick and protected from damage.

 

[RichardS]

I am inclined towards fusing large house banks but not starter circuits.

My boat from new is fitted with a 250A fuse in the house battery circuit (540ah) but no fuse in the starter circuit.

Richard

 

[Lon nan Gruagach]

MCA coding for a 15m charter yacht insists that battery cables are either fused or double insulated. This would appear to address chafing.

A note though, particularly for all those saying a proper installation will mean no problems. Unless you inspect the entire unfused cable run on a very frequent basis you cannot be sure some foreign object isnt eating your insulation. You are very unlikely to be able to guarantee that no heavy sharp object will ever come loose and damage your cables.

Heavy duty fuse dont blow for no reason, even 13A plug fuses rarely blow for no reason. I think that too many people "fix" equipment by replacing the fuse without any investigation as to why.

 

[lw395]

My boat from new is fitted with a 250A fuse in the house battery circuit (540ah) but no fuse in the starter circuit.

Richard

But are the cables to the main distribution board (or wherever the next fuses/breakers are) capable of standing 750A for a second to blow the fuse?

 

[RichardS]

But are the cables to the main distribution board (or wherever the next fuses/breakers are) capable of standing 750A for a second to blow the fuse?

I'm not sure where the 750A comes from but I don't know the answer to your question.

Richard

 

[theoldsalt]

................. even 13A plug fuses rarely blow for no reason. I think that too many people "fix" equipment by replacing the fuse without any investigation as to why.

My personal experience is that on the few occasions I have had a mains plug fuse "blow", when replaced with a new fuse there was usually no repeat "blow". On the very rare occasion when a repeat occurred I have investigated. It seems that fuses due get "tired" and "blow" for no apparent reason.
I have found the same with circuit breakers on a distribution panel. Worrying when they keep tripping a circuit but when no fault found and the breaker replaced with new all appears to be OK. So circuit breakers seem to "tire" as well.

 

[lw395]

I'm not sure where the 750A comes from but I don't know the answer to your question.

Richard

The 750A comes from the graph of time to blow vs current on the blue sea website:
https://www.bluesea.com/products/5107/MEGA___AMG_Fuse_-_250_Amp

Not intending to criticise your boat, just saying it can get a bit complicated when the fuse value gets up towards what the supply will give!

 

[lw395]

My personal experience is that on the few occasions I have had a mains plug fuse "blow", when replaced with a new fuse there was usually no repeat "blow". On the very rare occasion when a repeat occurred I have investigated. It seems that fuses due get "tired" and "blow" for no apparent reason.
I have found the same with circuit breakers on a distribution panel. Worrying when they keep tripping a circuit but when no fault found and the breaker replaced with new all appears to be OK. So circuit breakers seem to "tire" as well.

Some fuses actually have a finite life if run above say 80% of their rated capacity. Usually 1000's of hours, but they can just 'wear out' in industrial applications.

 

[yachtorion]

A lot of these fuses may never blow.
You need to consider the cable resistance and battery CCA curves.
A 300A (nominal) fuse will carry upwards of 450A for a very long time.
https://www.bluesea.com/products/5108/MEGA___AMG_Fuse_-_300_Amp
Your battery and cables will possibly struggle to provide 450A for more than a few seconds, particularly if there is much cable length involved. After a few seconds, the cable heats up and the resistance rises.
But a short will still be dumping enough amps into the cable to cause a fire.

You have to weigh up the actual protection you are getting vs unreliability you are adding.
I am inclined towards fusing large house banks but not starter circuits.
The main thing though is to avoid it being an issue by ensuring all cables etc. are in good nick and protected from damage.

I'm inclined towards fusing everything that I reasonably can. which I think we all are so we're just debating what's reasonable.

I think I could blow the fuse, but I'm willing to be corrected. Good crimps are alleged to have a resistance equivalent to a similar length of cable, which with 25mm2 is going to be neglible. 6ft of AWG 4 has a resistance of approximately 0.001 ohms, plus about 20 milliohms for the internal resistance of the battery... so 21 milliohms worst case if we have a good strong contact with whatever is compromising our cable.

I = V / R = 13 / 0.021 = 619 amps, which is below the CCA of my battery.

They're slow blow, if my starter does use what I expect (150amps) then I could manage no problem on a 200 amp fuse, which on a dead short, going by the Blue Sea graph (https://www.bluesea.com/products/5187/MRBF_Terminal_Fuse_-_200A) ought to blow in a few seconds, but not blow just on the surge as the starter engages.