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From: (no name) (no email)
Date: Mon Dec 08 2003 - 02:08:08 EST
In a message dated 12/7/2003 3:19:05 PM Eastern Standard Time,
writes:
>
> Thanks for your reply. How do I determine the size of the fuse for a
> starter cable? Can I find out the amount of amps that the starter will
> draw and then add a factor for when the engine in cold or dragging? Seems
> if I pick too large of a fuse it would defeat the purpose. And too small a
> fuse could block the starter when I might need it the very most.
>
The problem here is that the starter typically needs 200 amps, for a small
engine, to 750 breakaway/500 spin, for my 6V-53. This is a lot of power, more
than enough to start a dandy fire so you cannot stop a fire with a current
limiting device here. It's a good thing to severely limit combustible materials
in the engine room. My view is that the engine starting battery and wires be
so well designed and installed there would be little chance of a fire starting
there. There is the option of installing a high current switch on the
positive lead, near the battery to limit exposure. A remote control for this switch
would be good. The battery should be in a box so that nothing could fall on
the current conductors to cause short circuits (aka: smoke and fire). The
wires should be short and have a significant copper cross-section. Two 1/0 wires
have less resistance than one 2/0 wires (if I recollect..) and are easier to
bend. Welding cable will be more vibration resistant because it has lots of
very fine wires which tolerate bending better but must be strongly protected
from corrosion of the little wires. Larger gauge cable strands offer the
opposite properties. It's your choice.
> Also, is it realistic to think that I could adequately put in circuit
> breakers within the house system to eliminate almost all chances of
> electrical fires? Or is it a matter of percentages where the circuit
> breakers eliminate most, but not all, fire potentials?
>
Please understand about circuit breakers or fuses.
These current limiting devices are last ditch limiters and do well against
hard, heavy, shorts. They are sized, by code, to trip before the wires get too
hot to start a fire or even damage their insulation. However, there are
plenty of other parts of the circuit that can get quite hot enough to release the
magic smoke without tripping the breaker. The one that started the last fire,
in Styrofoam insulation, on this vessel was a very small wire, speaker wire
going to a fan in a berth. The disconnected hot end got on a ground (with
assistance from Frumi, the ship's cat) and soon the little wire heated up enough to
ignite the Styrofoam. Black, greasy, smoke and low smoldering flames. I put
it out and cleaned up the mess. Since the wires were thin, the current
through them was limited, limited to below the rating of the breaker protecting
them, thus allowing a current sufficient to overheat the wires, but not
sufficient to pop the breaker. True, I should have added a fuse, (I do use inline
fuses routinely) to this branch line sufficient to protect the thin line.
However, this is just an example, the only one I have to show. This same effect
could happen in every part of the circuit. A small wire in a fan or light
fixture, smaller even than the supply line properly fused, could overheat from an
abnormal load, a breakdown of materials, or corrosion of a connection. If the
heat rises sufficently and reaches flammable materials, fire results, and the
breakers never trip.
On my boat I have a big, fat, 1/0 twisted pair running fore and aft from the
house batteries and protected with a 300 amp circuit breaker. Opening this
breaker will shut down all 12 volt circuits except the engine starting circuit.
(There is something to be said for an emergency electronics circuit with
independent battery) All house 12 volt loads branch off this buss with inline
fuses protecting the positive feed wires between the buss to the equipment.
(except, of course, those pesky berth fans). When I put the proper sized breakers
or fuses protecting the wires I release no magic smoke from the wires. T
My only electrical fire happened because I did not protect that branch
circuit with an inline fuse as I do in most other branches. If I had a fuse sized
to the conductor sizes the fire would have never happened.
Norm
S/V Bandersnatch
Lying St. Augustine
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