Connecting batteries

[Omatako]

I just read a thread from Andyball regarding a fuse issue on busbars and I'm now con-fused.

A busbar serves the function of distributing electricity to several appliances/circuits at a place where it is convenient to locate. That is, a single cable runs from the battery/ies to the busbar and then circuits run from the busbar to the appliances.

If the cable from the battery were to be fused, it would need to be a fuse that carries enough current to power all the appliances fed by the busbar simultaneously. If not, the fuse (or breaker) would fail every time the max load condition is approached. So to fuse it to the thinnest wire will be real frustrating.

Rather ensure that the feeder cable is big enough to provide safe and adequate supply to the busbars and then ensure firstly that the busbars are located so that they cannot be accidentally bridged and secondly that all appliances are appropriately fused directly after the busbar.

My busbars are mounted on a vertical surface about 100mm apart, almost impossible to accidentally bridge them

Sorry to drag up old issues but I think this important

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[claudio]

Cables from the battery to the Busbar should be fused at the battery, the fuse rating should be such that in the case of a short circuit across the busbars, the cable will be protected. The fuse is not there to protect any appliances, these should have their own individual breaker or fuse.
In the event that there is a short circuit at the busbars, your fuse will blow rather than possibly causing a fire on the boat.

Technically the starter cable should be fused as well. I have come across a situation where the cable has been in contact with an exhaust manifold, the insulation melted and the cable took on a nice red glow!!
Luckily there was somebody there at the time and he managed to switch off the supply. The vessel was filled with a pungent smell of vapourised PVC.




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[William_H]

The ideal situation is to protect the cable at the battery terminal so that the fuse will open with any short circuit or excessive current.. In practice we take a gamble that the cable is unlikely to be shorted to ground or the other wire and that if it does the resultant red hot cable will not cause a fire to spread. These are the usual criteria for starter cables from batter5y to starter solenoid in a car or boat. Don't forget the cable must also be heavy enough to carry the current without excessive voltage drop although in practice this is not normally a problem as we choose heavier wire for robustness and corrosion resistance . regards will

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[brianrunyard]

What size fuses have you got in the main battery leads?

<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard

 

[andyball]

Well I suppose ideally there'd be a fuse for each battery, one after the (insulated & more than 100mm apart) busbars
then fuses for each cable.

Point was to think very carefully about what happens if there's a short-circuit that overloads a conductor/switch to the point of it getting hot enough to start a fire.

There's some undue emphasis on protecting appliances with fuses imo, if a fire starts due to 5 meters of cable overheating,
all hidden behind trim & upholstery adjacent to other cables.....the consequences may be much more serious & expensive than if inside an appliance.

Sometimes it's not "use a small enough fuse", but use big enough wire instead.

A small point, but I've seen a few near-fires on motorcycles with this, is to make sure that
new/replacement fuses are of the right value & if designing...to think carefully about what size fuse to fit....
too small can cause problems, due to the fuse overheating after running close to, but not at
it's blow rating. Naturally fuses/holders etc should be of material that won't catch fire. Circuit breakers are better in this respect, since they trip sooner.

It's a game though, some retailers/wholesalers can't even agree on whether the fuse rating is what it blows at or what it will carry in normal use.

Final pointer, be wary of "nominal rating" for cables. one catalogue I have suggests a nominal rating of 16.5Amps for their
1 sq mm cable (with thin walled pvc insuation), just 4m of cable at this nominal rating would drop 1.24 Volts across the cable, according to same firm's voltage drop tables.

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[extravert]

My boat has a 60A fuse next to the battery. Everything goes though this, including the engine starting circuit. The engine is a 10hp petrol outboard, starting of which will take less that a diesel though.

<hr width=100% size=1>One day, I hope to own a real boat.

 

[ongolo]

I have no fuses directly on the batteries, but before the buss bars of the distribution panel I have three Knife fuses, two 125A for domestics and one 250A for the engine circuit.

I have a steel boat and there is nothing in a dedicated workshop/engine room that could burn.

The reason for the high rating is that lower ratings will not handle the current of my 2KW starter and since the two domestic batteries may be used parallel to start the engine, they need to be half of the bigger fuse.

The reason why I used the knife fuses (identical to those used in 220 V and 380V which need a special handle to remove them), that they are silver plated, I swing them out, pivoting about the lower knife only braeking the upper connection (my distribution is also vertical) and use them as a switch, they have a lugs enabling me to connect test leads and should they blow, wrapping a few turns of wire around them, will get them back into service.

The system is as solid as can be and the cost is only a fraction of what 2 x125A and 1 x250A Fuse/CB or switch would cost.

For diagnostics, observing by opening and closing whether there is a tiny spark gives the first indication of a consumption, or latent short or a leakage.

Opening the exposed fuse, allows then to connect a meter or test lamp in whatever fashion is required.

To disable all systems, simply take them out and nobody will be able to steal your boat, unless he knows the trick how easy it is to shorten them in an emergency.

But I bargain on the fact, that electrics is a little confusing and a red mains switch with a key removed will indicate much quicker where to bridge to get the boat going than empty knife contacts which only an electrician can identify for what they are.

regards ongolo

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