nigelmercier
RIP
- Joined
- 20 Jun 2007
- Messages
- 16,234
- Location
- Live in Kent, boat in Canary Islands
My isolators are in the negative, but not the fuse.
fuse size
- Thread starter rosewood
- Start date
My isolators are in the negative, but not the fuse.
Seatrout
Well-Known Member
Hilarious! Thanks for cheering me up Alistair. BTW my batts are modulated to the correct polarisation this season. Of course that may change?
William_H
Well-Known Member
The question of fusing of the battery wire is a complex one. In older cars the heavy wire going to the starter and also often to the alternator is not fused. This seems to be based on the idea that the small battery can not provide enough current for long enough in a short circuit situation to really cook the heavy wire. it is also I think based on the idea the wire is short and not likely to contact any metal negative part. We did not seem to get a lot of fires in a system where negative of the metal body makes it much more likely to get a short +ve to _ve.
So it is in many boats that the starter motor wiring is not fused. The relatively heavy wire alternator out to battery may also not be fused. Though more often we have a big switch to disconnect the battery when the boat is not in use. It is however common to have the lower current circuits and therefor the wiring fused. usually with a fuse for each service. Based on the idea that if one circuit fuses the others will remain OK. But it also means that circuits with low current drain can have light wire and an appropriate light fuse while heavier drain circuits have a heavier wire and hevier fuse.
Remember a fuse is there to stop the fire that can result from a short circuit causing excess current in a wire until it gets red hot.
So unless you are driven by safety standards like BSS or commercial coding of boats.then you can decide on particular fusing based on the likely hood of a short circuit +ve to -ve and the likely hood of the wire getting red hot and the likely hood of this hot wire starting a fire. So it might seem reasonable to have a heavy wire from a small battery well supported in mid air going to the bus bar to individual fusesfro lights etc and having a heavy wire similarly supported going to the solenoid on the starter motor. With or without a isolation switch. Many might disagree with this approach but many boats for a long time have been so set up. Certainly if you are going to fuse a starter wire you will need a very high current fuse to protect the large wire needed to minimise volt drop. With a similar heavy wire to the bus or a lighter wire with another fuse.
good luck olewill
PS a friend had a small boat with a mast head nav light. He attached an old car battery then stood back to check if the light could be seen. Unfortunately the mast had squashed the wire short circuiting +ve to _ve so the wiring from battery to the mast base got very hot. It happened to be touching new sails laying on the bunk these burnt and he very nearly lost the boat in a few minutes. fuse would have saved the sails.
So it is in many boats that the starter motor wiring is not fused. The relatively heavy wire alternator out to battery may also not be fused. Though more often we have a big switch to disconnect the battery when the boat is not in use. It is however common to have the lower current circuits and therefor the wiring fused. usually with a fuse for each service. Based on the idea that if one circuit fuses the others will remain OK. But it also means that circuits with low current drain can have light wire and an appropriate light fuse while heavier drain circuits have a heavier wire and hevier fuse.
Remember a fuse is there to stop the fire that can result from a short circuit causing excess current in a wire until it gets red hot.
So unless you are driven by safety standards like BSS or commercial coding of boats.then you can decide on particular fusing based on the likely hood of a short circuit +ve to -ve and the likely hood of the wire getting red hot and the likely hood of this hot wire starting a fire. So it might seem reasonable to have a heavy wire from a small battery well supported in mid air going to the bus bar to individual fusesfro lights etc and having a heavy wire similarly supported going to the solenoid on the starter motor. With or without a isolation switch. Many might disagree with this approach but many boats for a long time have been so set up. Certainly if you are going to fuse a starter wire you will need a very high current fuse to protect the large wire needed to minimise volt drop. With a similar heavy wire to the bus or a lighter wire with another fuse.
good luck olewill
PS a friend had a small boat with a mast head nav light. He attached an old car battery then stood back to check if the light could be seen. Unfortunately the mast had squashed the wire short circuiting +ve to _ve so the wiring from battery to the mast base got very hot. It happened to be touching new sails laying on the bunk these burnt and he very nearly lost the boat in a few minutes. fuse would have saved the sails.
Poignard
Well-Known Member
I have just been looking at the wiring diagram for the BUKH dv10/20 engines and there is no fuse in the cable from the battery to the starter; just a switch. My engine was installed by a BUKH agent in 1982 and has never had a fuse in that circuit.
I'd be interested to know whether other boat owners have fused starter cables and if so, what size fuse they have
I'd be interested to know whether other boat owners have fused starter cables and if so, what size fuse they have
John the kiwi
Well-Known Member
This is the critical point that many seem not to understand.
The other point is the location of the fuse in relation to the power source.
I have reasonably large cross section cable from battery to distribution board. The wire is sized for realistic minimum volt drop including possible future additions.
The fuse is sized to suit present day demand but most importantly is as close as possible to the source of power. (In this case the Battery.)
e.g.
Imagine i have a positive and negative running from Battery to distribution board 2 metres away and the fuse is at the board end of that loop.
Mechanical damage such as a spare anchor sitting on top of wires between the battery and the fuse and abrading the insulation could short circuit the wires but the fuse, being outside this new accidental circuit, would offer no protection at all!
Take Care.
halcyon
Well-Known Member
Back in the late 1980's went to look at a Sealine 365 with the helm position burnt away. To save a long story, a gen set had been fitted with a starter feed taken from the service battery isolator switch with no fuse fitted. A fault developed in a piece of equipment, causing a current higher than it's cable rating, the cable got hot melting the hydraulic steering pipe and the two gen set starter feed cables. Resulting in a dead short across 200 / 300 amphour battery bank and fire, not from gen set, but a totally unrelated circuit.
The fuse in the engine feed is to protect you and your boat, not the engine.
Brian
Poignard
Well-Known Member
What you and John say makes perfect sense but then why doesn't the BUKH wiring diagram include one? It seems odd they would deliberately exclude a relatively low cost safety feature.
halcyon
Well-Known Member
Maybe because it's not a technical requirement, you are allowed to fit your own safety devices though.
It's your boat and you say,
Brian