Tintin
Well-Known Member
A quicky.........Is it standard to fuse just the positive from a battery bank or both +ve and -ve?
Fusing +ve & -ve battery feeds
- Thread starter Tintin
- Start date
pvb
Well-Known Member
You only need to fuse one. Generally, people fuse the positive, but you could equally fuse the negative.
prv
Well-Known Member
It's probably standard, as in most common, not to fuse either of them. But a Megafuse of suitable value right next to the battery always seemed like a sensible precaution to me. Do. Ale sure you have spares on board 
Pete
Pete
David2452
Well-Known Member
MRBF direct to the battery + post is what I normally use, they take up the least room of alll the options.
sailorman
Well-Known Member
Tintin
Well-Known Member
Thanks all - what got me thinking was seeing black and red fuse holders on ASAP's website.
I saw the MRBF fuses - neat solution but at a cost, (based on Merlin Powerstore http://www.power-store.com/?id=389) - £20 for the holder and another £14 per fuse.
But having said that Powerstore seems expensive anyway - I cannot fathom how a 250a ANL fuse at ASAP (http://www.asap-supplies.com/marine/anl-fuses/anl-fuse-714440) is at £3.30, but well over £18 at Powerstroe. Similarly the ANL fuse holder (ASAP £10, Powerstore >£20).
Graham_Wright
Well-Known Member
A 250A fuse is a 250A fuse! There are slow-blow varieties which might be of interest for starter duty and the other consideration is mechanical attachment to the battery. You may not need a fuse holder.
chrisbitz
Well-Known Member
At the risk of slight Fred Drift, If you had a 200A fuse on 300A wire, and further down the line, a 35A wire shorted out, what would happen?
Would the 35A wire glow red hot, melt the insulation and set fire to anything near, or would it still blow the 200A fuse?
Would the 35A wire glow red hot, melt the insulation and set fire to anything near, or would it still blow the 200A fuse?
David2452
Well-Known Member
That shouldnt happen, the smaller cable should be protected at the interface, it is a bit misleading to use the mantra a fuse is to protect the cable, it is to protect the circuit, that obviously includes but is not limited to the cable.
JumbleDuck
Well-Known Member
Actually, fuse design is far more complicated that most people think. It's not just slow-blow and quick-blow: you can have a whole range of speeds of blowing, or fuses that blow slowly at 200A and fast at 300A, or fuses which will take one 300A surge for a few seconds and thereafter blow quickly at 200A and all sorts of things. It's done by cunning metallurgy. You may, for example, have see glass cartridge fuses with what look like blobs of solder on the fuse wire - these are different metals which diffuse into the wire when sufficiently heated and change its fusing characteristics.
Not my field, I hasten to add, but I used to work in fault current limiters and had to find out a bit about the opposition!
chrisbitz
Well-Known Member
At the risk of slight Fred Drift, If you had a 200A fuse on 300A wire, and further down the line, a 35A wire shorted out, what would happen?
Would the 35A wire glow red hot, melt the insulation and set fire to anything near, or would it still blow the 200A fuse?
haha, yes, that's absolutely true, it shouldn't happen, in a brand new boat, professionally wired, to all relevant health and safety standards.
But in a real-life 30 year old boat, that lots of people have fiddled with, and bodged and *maybe* not quite followed relevant legislation... what WOULD happen? Could a 35A wire draw 200A by getting very hot and melty? Or would it become a very inefficient heating element?
Would the 35A wire glow red hot, melt the insulation and set fire to anything near, or would it still blow the 200A fuse?
haha, yes, that's absolutely true, it shouldn't happen, in a brand new boat, professionally wired, to all relevant health and safety standards.
But in a real-life 30 year old boat, that lots of people have fiddled with, and bodged and *maybe* not quite followed relevant legislation...
what WOULD happen? Could a 35A wire draw 200A by getting very hot and melty? Or would it become a very inefficient heating element?
David2452
Well-Known Member
It would try to draw it yes, and would become very hot to the point of melting the insulation and become a fire hazzard,
William_H
Well-Known Member
At the risk of slight Fred Drift, If you had a 200A fuse on 300A wire, and further down the line, a 35A wire shorted out, what would happen?
Would the 35A wire glow red hot, melt the insulation and set fire to anything near, or would it still blow the 200A fuse?
haha, yes, that's absolutely true, it shouldn't happen, in a brand new boat, professionally wired, to all relevant health and safety standards.
But in a real-life 30 year old boat, that lots of people have fiddled with, and bodged and *maybe* not quite followed relevant legislation...
The actual behaviour of that situation is difficult to forecast. The resistance of the 35 amp wire through the short circuit back to the battery may be such that less than 200 amps would flow. So likely fuse would not blow. What is certain though is that the 35 amp wire would get very hot. Certainly the insulation would emit a lot of smoke and possibly flames. The copper conductor would get hot to the point where it would fuse itself. This point and place would depend on how easily the wire could dissipate heat. So if it is jambed in with other wires or a tight space that is where it will get hottest and so fuse(melt) there, but often this is the place it is most likely to cause further damage. ie burn GRP. So you can risk a 35 amp wire without a fuse ie from battery cable to distribution fuses provided it is short with minimal chance of shorting to negative and mounted in a way that if it got hot it would not be inclined to burn the ship. Best practice however would use a heavier wire or another fuse 35 amp at the branch off from the fat cable. good luck olewill
Tintin
Well-Known Member
Thread drift - what size fuse do you reckon for a 250amp house battery bank and a 70a engine battery ? biggest draw items on the house side being fridge, 12v TV, and radar.
davidpbo
Well-Known Member
A 200A fuse should not be protecting any part of the circuit that has cable rated at less than 200A. Lower rated cables and circuits should be sub fused with fuses of an appropriate value equal to or less (there are times when a cable of great diameter than the current drawn dictates is used to limit voltage drop) than the current the cable is rated to carry. (Broadly speaking).
Billjratt
Well-Known Member
Going back to the OP, just be aware that fault-finding can be prolonged if the are odd fuses in the negative leads. Please make a drawing of the modified circuit so the next person in line has half a chance.....
Been there.
Been there.
William_H
Well-Known Member
Probably around 20 amp fuse. But as said fuse size should really depend on the smallest wire size. I expect your wiring to be reasonable for the radar and TV. good luck olewill
abraxus
Well-Known Member
The fridge, tv and radar should be individually wired and fused at the fuse box according to their specs. Assuming you mean for the battery cable at the battery, then it's less about the battery sizes and more about what will be drawn from them, and so that should be small enough to blow before the limit of the cable and large enough to handle the starting requirements of the engine.
