Battery terminal post fuses.

[Huttoft]

Have been looking around for an easily installed and effective way of individually fusing all the boat's batteries.
Came across these. The fuses come rated in 25amp increments, from 25a up to 300a.
Not as cheap as I would wish at over 15 Pound for all the bits to install on each battery, and I've 6 batteries.
Spare replacement fuses would be around 4 Pound each.
The attraction to these would be that they are compact and easily installed and just as easily uninstalled if needed. (This could be if a fuse blowing fault was detected and remedied but there were no spare fuse links.)

http://www.benzworld.org/forums/unimog/1665955-battery-terminal-fuses.html

http://uk.rs-online.com/web/p/autom...C696E6B2C323030412673633D592677633D4E4F4E4526


Has anybody any info on these or anything they might think might be an alternative?
I have 3x130ah house, 2x130ah windlass/bow-thruster and 1x120ah for the engine start-up.
Would anyone like to make suggestions for the rating of the fuses?
Any advice would be gratefully received.
Good luck.

 

[pvb]

They'd be fine, although it's not normal to fuse every single battery unless you're not confident about the cabling. I have a MegaFuse on the domestic bank, and one on the start battery.

PS When I was a kid, I used to get dragged off to Huttoft Bank most Sundays. Parents had a caravan right behind the sea wall at Sutton (or maybe Trusthorpe). The abiding memory is of being cold and damp. It's the only place I've ever seen children paddling in wellingtons!

 

[Huttoft]

That wouldn't have been circa 1953 would it?
I've been told that was quite a wet time of it around there.

 

[pvb]

That wouldn't have been circa 1953 would it?
I've been told that was quite a wet time of it around there.

I'm not that old! And the sea wall was built after the floods.

 

[abraxus]

I've also been looking at fuses for my batteries and have a question.

I've looked at Blue Seas Systems fuses and holders in the 300-400 amp range and they come to around £40-£50 all in for each terminal. However Littelfuse megafuses and holders are about £12-£14 a set.

Is there any reason why I shouldn't use the Littelfuse stuff or am I missing something vitally important that's covered by the big price difference?

 

[pvb]

Is there any reason why I shouldn't use the Littelfuse stuff or am I missing something vitally important that's covered by the big price difference?

Not at all, MegaFuses are excellent products.

 

[savageseadog]

I've also been looking at fuses for my batteries and have a question.

I've looked at Blue Seas Systems fuses and holders in the 300-400 amp range and they come to around £40-£50 all in for each terminal. However Littelfuse megafuses and holders are about £12-£14 a set.

Is there any reason why I shouldn't use the Littelfuse stuff or am I missing something vitally important that's covered by the big price difference?

I haven't studied any of the products in detail but there are general principles that will apply.

1) Use the lowest rated fuse you can for the maximum potential load with a margin for extreme conditions
2) Ensure the wiring will carry the full fault current for the time it takes to blow the fuse
3) If the load is inductive or surging then use a surge resisting or delay type fuse. That way it will only blow when the rated current is exceeded for a significant fault condition.
4) Starter motors are highly inductive
5) A 20-30HP engine starter will take about 250A max initial load.
5) Fuses designed for automotive purposes will normally be well suited to boats

 

[pvb]

I haven't studied any of the products in detail but there are general principles that will apply.

1) Use the lowest rated fuse you can for the maximum potential load with a margin for extreme conditions

The purpose of the fuse is to protect the wiring, so the choice of fuse should be based on the cable rather than the load.

 

[abraxus]

Thanks pvb and savageseadog

My question then woud be what size fuse?

My battery cables to the swtich are 50mm2, which I understand is rated for 345 amp. My initial thought therefore is a 300 amp fuse, but the blow rating is 600 amps, so is that too big for the cable?

Additionally my engine is a 260hp 5.7l, so would it be big enough for that and not blow when starting? According to the specs I've managed to find the min battery requirement for the engine is 350 Cold cranking amps and I believe the specs for the starter motor are 120A, 1.7KW.

So, based on the above, what would be an appropriate fuse to protect the 50mm cable and not blow when starting the engine?

Thanks again.

 

[pvb]

My battery cables to the swtich are 50mm2, which I understand is rated for 345 amp. My initial thought therefore is a 300 amp fuse, but the blow rating is 600 amps, so is that too big for the cable?

The cable's current rating depends on the quality of its insulation, and is a maximum steady current which won't damage the insulation. The cable is capable of carrying much higher current. MegaFuses will accept higher current than their rating for a short time. In the example you give, a 300A MegaFuse with 600A current will blow in 1 to 15 seconds. However, you're protecting the cable against a short circuit, in which case the current would be far higher than 600A and the fuse would blow much sooner.

Additionally my engine is a 260hp 5.7l, so would it be big enough for that and not blow when starting? According to the specs I've managed to find the min battery requirement for the engine is 350 Cold cranking amps and I believe the specs for the starter motor are 120A, 1.7KW.

So, based on the above, what would be an appropriate fuse to protect the 50mm cable and not blow when starting the engine?

I'd use a 400A or 450A MegaFuse.

 

[William_H]

I would have concerns about having a fuse right at the terminal. Simply because this is where corrosion tends to build up if neglected. I realise fuses for safety should be as close to the battery as possible. I would think however that a stand alone fuse holder would be better situated a bit away from the battery. ie 40cms.
A fuse is fitted as a weak link to fuse in a safe envitonment if the wiring carries excess current (a short circuit). The fuse is intended to open before the wire gets so hot that it causes damage to itself and the structure or indeed starts a fire.
Now with light wires set into cabin lining etc it is important to protect the circuit from overheat and fire. Wires can easily chafe or be cut by structure or screws such that a short circuit is possible. (and some equipment can fail in a way that they show a short circuit internally).
However with your wiring of the battery and starter cables. It is often seen as an aceptable risk that wires if short well supported and mounted away from flamable material (GRP structure) should not be fused.
So you may consider that you make make a short +ve to -ve unlikely, then accept that if the current does exceed the cable capacity and the cable gets red hot before melting and parting that this could happen without burning the boat. Millions of cars were built when the -ve is the metal body of the car, the live wire was heavy and went to the starter motor or solenoid and another wire went from starter to the fuse box all not protected. A GRP boat with individual +ve and -ve is far less likely to get a short circuit.
However another aspect is that the smaller car batteries may not be able to provide enough current to melt the heavy starter wire.(before going flat) Many boats have a lot more curent supply capability. I also recognise that many safety design standards for boats insist on complete fusing. (wether that is realistic or not )
So you decide on your own fusing arrangements. good luck olewill

 

[abraxus]

The cable's current rating depends on the quality of its insulation, and is a maximum steady current which won't damage the insulation. The cable is capable of carrying much higher current. MegaFuses will accept higher current than their rating for a short time. In the example you give, a 300A MegaFuse with 600A current will blow in 1 to 15 seconds. However, you're protecting the cable against a short circuit, in which case the current would be far higher than 600A and the fuse would blow much sooner.




I'd use a 400A or 450A MegaFuse.

Thanks pvb, I'll go with 400A and see how I get on. At around a fiver each I can cope with the cost of stepping up if they're not enough.

 

[savageseadog]

The purpose of the fuse is to protect the wiring, so the choice of fuse should be based on the cable rather than the load.

I think that point was covered by my second one.

 

[pvb]

I think that point was covered by my second one.

Not really.

 

[bozlite]

On the Smartguage website (http://www.smartgauge.co.uk/sb_fusing.html) it says:

"Fuses should never be put in the battery compartment unless the batteries are certified as being suitable for use in habitable spaces. Which means they will not give off explosive gasses during charging. Generally this means gel batteries and AGM batteries only. All other types give off explosive gasses during normal charging."

Doesn't this make it impossible to fuse batteries at the terminal posts (unless you're using gel or AGM)? Is there a real risk with standard lead acid batteries? Any views?

 

[David2452]

On the Smartguage website (http://www.smartgauge.co.uk/sb_fusing.html) it says:

Doesn't this make it impossible to fuse batteries at the terminal posts (unless you're using gel or AGM)? Is there a real risk with standard lead acid batteries? Any views?

No, the fuses in question (seemingly) and specifically the Blue Sea Systems MRBF (certainly) which are of a similar if not identical pattern are sealed and ignition protected, I imagine Smartgauge alude to things like Mega fuses, in which case they are correct.

 

[bozlite]

No, the fuses in question (seemingly) and specifically the Blue Sea Systems MRBF (certainly) which are of a similar if not identical pattern are sealed and ignition protected, I imagine Smartgauge alude to things like Mega fuses, in which case they are correct.

Of course! So that's why they're safe for petrol engine installations? Should've realised that. Thanks for helping me on my (pretty steep) learning curve.