Battery main fuse

[boatmike]

While upgrading my charging system I am questioning the wisdom of fitting a main fuse and where to put it. Several publications show a fuse of 100/120 amp in the line from service battery to services. Victron show a "max 100 amp" fuse protecting the VCR. One publication I have suggests putting one in the main negative lead arguing that it protects both batteries which is novel. Others don't show a fuse at all. None show a fuse in the engine positive start line. This leads me to ask "why have one at all?" All services are individually fused so any equipment shorts after the distribution panel are catered for at much less than 100 amps so all a fuse at the battery terminal protects against is a short in the power line up to that point. It does appear that no publication suggests a fuse at the starter battery so why have one at all on the service battery? I can see that Victron consider protection of the VCR might be advantageous but the chances of the relay seeing 100 amps is surely remote? how many of you have a main fuse at all and if so where and why?

 

[simonfraser]

no main fuse at the battery in cars / mowers either ...

 

[GHA]

After redoing the main battery cables I'm erring against, with the fat cables either inside trunking or well supported where there's no real chance of getting damaged in a knock down. The ABYC don't require them within certain lengths , think this is it.
12B switch -

 

[Boathook]

Engine doesn't have a fuse for the starter cables, but I then have a 75 amp circuit breaker protecting the main cables up to my distribution board / switch panel where all the individual circuits are then fused.

 

[pvb]

It's always a good idea to have a fuse in the main battery cables, close to the batteries. In the event of a short, this will prevent a fire. On my last boat, I fitted MegaFuses with appropriate ratings for the cables. Once, when re-fitting the alternator, I foolishly connected the cables wrongly; when I turned the main switch on, the MegaFuse blew instantly. My current boat came from the factory with fairly extensive fusing.

 

[BabaYaga]

This leads me to ask "why have one at all?" All services are individually fused so any equipment shorts after the distribution panel are catered for at much less than 100 amps so all a fuse at the battery terminal protects against is a short in the power line up to that point. It does appear that no publication suggests a fuse at the starter battery so why have one at all on the service battery?

I think they are both well worth having, mainly because of the proximity of the positive charging and/or starter wire to the engine block.
I have a 250A fuse close to the starter battery and a 80A fuse close to the house bank. There is an additional 250A fuse in the emergency starting link, for starting from the house bank.
Not so rare to find these suggested in various diagrams, a random example here:
https://www.keoghsmarine.com.au/bep...age-sensing-relay-140-amp-113668-sur-710-140a

 

[PaulRainbow]

All cables need to be protected, if using fuses that would mean a fuse at the domestic battery, rated for the cables, that would commonly be starter sized cables to allow emergency starting of the engine, so something like a 240a fuse (assuming 35mm cable, for example). If, however, the battery cables are suitably protected from short circuit, there is no requirement for a fuse. For instance, my own batteries are fitted in a wooden box, which is inside a wooden sofa base, the isolators are fitted in the end of the sofa, it's impossible for the cables to short, so no fuses. The majority of boats that i work on don't have battery fuses and don't need them. Where the installation involves cables routed through an area where a short might be possible, fuses are fitted at the batteries. For instance, where batteries might be installed in an engine room, or pass through it. Metal hulled boats always get fitted with battery fuses.

From the domestic isolator to the distribution board the cable would have a lower rating, so this should be fused according to the cable rating. From the distribution board, each circuit is fused accordingly.

When fitting the VSR, if there is a possibility that either of the cables going to the VSR could short they should be fused. If both could short, they both get fused, those fuses are protecting the wiring, plus the VSR. If neither cable could short, there should still be a fuse in one of the cables to protect the VSR from overload.

 

[PaulRainbow]

I have a 250A fuse close to the starter battery and a 80A fuse close to the house bank. There is an additional 250A fuse in the emergency starting link, for starting from the house bank.

This doesn't look correct. The starter fuse is correct (assuming 35mm cables), but the other two fuses look wrong. The cable to the domestic isolator should also be 35mm and be fitted with a 250a fuse to allow emergency starting. The domestic distribution feed should be connected to the isolator and be fitted with a fuse rated for the cable. There is no need for an additional fuse in the emergency link, as both batteries are fused.

View attachment 80730

 

[BabaYaga]

the other two fuses look wrong.

View attachment 80730

I don't think so, it is just that my charging setup may not be according to the common standard. That however should not the subject of this thread.
My point to the OP was that it is well worth fusing also the house bank. If this should be used for emergency starting there must be adequate wiring and a fuse of similar size to that of the starter battery.
If there is, as in my case, a separate wire which caters for charging from alternator, mains charger and for domestic loads, this might be dimensioned to reduce voltage drop (so a bit over sized) but can be fused according for the largest current, in most cases that from the alternator. Therefore the 80A fuse (also close to the battery).

 

[PaulRainbow]

I don't think so, it is just that my charging setup may not be according to the common standard. That however should not the subject of this thread.
My point to the OP was that it is well worth fusing also the house bank. If this should be used for emergency starting there must be adequate wiring and a fuse of similar size to that of the starter battery.
If there is, as in my case, a separate wire which caters for charging from alternator, mains charger and for domestic loads, this might be dimensioned to reduce voltage drop (so a bit over sized) but can be fused according for the largest current, in most cases that from the alternator. Therefore the 80A fuse (also close to the battery).

The diagram below is typical for the OPs intended setup, including the VSR. The two cables from the batteries would be fused according to the cable rating. It doesn't show charging sources, but these can be connected directly to the batteries or to the battery terminal of the isolator switches, whichever is more convenient. Each source should have it's own cable and fuse. Domestic loads wouldn't normally be connected to the battery or the isolator switch battery terminal, as these are permanently live, doing so would generally negate the need for the domestic isolator, as it would be bypassed.

 

[boatmike]

All cables need to be protected, if using fuses that would mean a fuse at the domestic battery, rated for the cables, that would commonly be starter sized cables to allow emergency starting of the engine, so something like a 240a fuse (assuming 35mm cable, for example). If, however, the battery cables are suitably protected from short circuit, there is no requirement for a fuse. For instance, my own batteries are fitted in a wooden box, which is inside a wooden sofa base, the isolators are fitted in the end of the sofa, it's impossible for the cables to short, so no fuses. The majority of boats that i work on don't have battery fuses and don't need them. Where the installation involves cables routed through an area where a short might be possible, fuses are fitted at the batteries. For instance, where batteries might be installed in an engine room, or pass through it. Metal hulled boats always get fitted with battery fuses.

From the domestic isolator to the distribution board the cable would have a lower rating, so this should be fused according to the cable rating. From the distribution board, each circuit is fused accordingly.

When fitting the VSR, if there is a possibility that either of the cables going to the VSR could short they should be fused. If both could short, they both get fused, those fuses are protecting the wiring, plus the VSR. If neither cable could short, there should still be a fuse in one of the cables to protect the VSR from overload.

Got all that Paul. In my case the battery + cables are very short and are not at risk of shorting. They terminate in a double pole switch that when ON connects the engine battery to the starter and alternator directly. The chance of a short here is pretty low therefore unless I do something stupid when playing with my wiring. The second pole of the switch connects the house battery directly to a fused distribution panel and again the cable is fully insulated and in a trunk. Normally I would not see this as a significant risk either. All downstream stuff from here is protected by individual fuses. Considering the VCR, current only flows through it when charging so the only risk I can see here to the VCR is if I am charging and have a dead short. When not charging it is OPEN so no risk. I guess it is possible to have a short internal to one battery when charging that will therefore be connected to the other through the VCR but this is the only situation I can see requiring a fuse ........ What have I missed????

 

[PaulRainbow]

Got all that Paul. In my case the battery + cables are very short and are not at risk of shorting. They terminate in a double pole switch that when ON connects the engine battery to the starter and alternator directly. The chance of a short here is pretty low therefore unless I do something stupid when playing with my wiring. The second pole of the switch connects the house battery directly to a fused distribution panel and again the cable is fully insulated and in a trunk. Normally I would not see this as a significant risk either. All downstream stuff from here is protected by individual fuses. Considering the VCR, current only flows through it when charging so the only risk I can see here to the VCR is if I am charging and have a dead short. When not charging it is OPEN so no risk. I guess it is possible to have a short internal to one battery when charging that will therefore be connected to the other through the VCR but this is the only situation I can see requiring a fuse ........ What have I missed????

That all looks fine Mike. If the VSR wiring cannot be shorted, the only risk to the VSR is an overload. The risk is pretty small, but i do always fit a fuse, choice is yours though.

 

[boatmike]

Thanks Paul, that's pretty well as I saw it. Nice to confirm though.

 

[BabaYaga]

In my case the battery + cables are very short and are not at risk of shorting. They terminate in a double pole switch that when ON connects the engine battery to the starter and alternator directly. The chance of a short here is pretty low therefore unless I do something stupid when playing with my wiring.

What have I missed????

You may not have missed anything, but consider that when the switch is closed your battery cable effectively reaches all the way to the starter and alternator. Should one of those connections vibrate loose and the cable fall off the consequences could be serious.
Not very likely of course...

 

[William_H]

I like the idea of following the requirements of a standard. However you must judge the physical risk of a short circuit in any part of the wiring compared to the rating of the wire and the likely damage (further fire) from the wire getting red hot.
As said cars have been around for many years with many examples and this experience seems to point to a fuse regime similar to ABYC standard as shown earlier. ol'will

 

[boatmike]

In life it is is impossible to avoid all risks. Proper risk management is based on a calculation of likely risk not avoiding all possible risk. How many people always wear a life jacket on deck, carry an in date serviced liferaft, have up to date flares, fire extinguishers etc etc. A good question to ask perhaps is " regardless of the obvious risk of cocking up when working on the boat wiring how many of us have ever had a main battery or VCR fuse blow in normal use?"

 

[lw395]

By definition, fuses rarely blow in 'normal use', particularly main fuses.

 

[boatmike]

By definition, fuses rarely blow in 'normal use', particularly main fuses.

By definition eh? Who's definition might that be? My point was unless you do something stupid rewiring your boat or some item of equipment fails (and in this case any bit of kit downstream protected by a separate fuse doesn't count) the chance of blowing any high amp fuse on the batteries is pretty small unless you have bad insulation or dodgy switchgear and cable runs are relatively short. I was asking quite sincerely if ANYONE has actually blown a main battery on VCR fuse in "normal use" meaning without doing something daft when working on the boat electrics without disconnecting the batteries first. I suspect the answer is no......

 

[William_H]

A couple of months back a boat in our yacht club marina had a pretty scary electrical fire. There was no fuse fitted and I believe it was the wire alternator to battery which chafed through the insulation due to poor attachment. So yes a fuse would have saved the problem bu then so would decent mounting of the wiring. ol'will

 

[coopec]

Something I was not aware of