Re: Over Current Protection On Pos Battery Connection
I can't believe Sailnet just dumped my whole post and I need to type it again!
I usually don't get involved with electrical threads, too many boat electrical experts here already. However I decided this one needed comment. I have an extensive electrical background, growing up in the business with my dad. He was great teacher, and taught me why as well as how. In the early years house wiring was knob & tube and you'd often hear "This house is a fire trap, it still has that old knob and tube wiring! We need to replace it all." Yet nothing could be further from the truth. The conductors were spaced 12" apart, supported by ceramic insulators, with soldered connections, how could any of that short out or start a fire? My younger brother though managed to burn out his first apartment when he fastened up a piece of loose paneling with a large nail, driving it right through a modern cable.
A boat has low voltage wiring, the wires are often spaced far apart, so how do we get a short circuit to start a fire? Same as in house wiring, short circuits are not the most likely source of ignition. A high resistance connection is the real hazard. A connection dropping a couple of volts at a couple of amps can produce a whole lot of heat, just like a 25 watt soldering iron can. The problem is, that a fuse can't protect your house or your boat against a high resistance connection causing a fire. The bad connection is actually limiting the current in the circuit, often to far less than the fuse rating. Fuses have time/current curves too. A 30 amp fuse will never trip with a 30 amp load. It might take several MINUTES to trip at 40 amps, and even some very long seconds at 100 amps!
A fuse right at the battery can protect you against that errant wrench that somehow finds its way directly across the main buss connections. However the fact is that with both wires insulated, low voltages, and wires running though insulators like wood and fiberglass, the chances of a short that would be cleared by a fuse is very low.
Boat wiring is low voltage, so currents are quite high to get the watts needed. Voltage drop is therefore a problem, so very large wires are the rule rather than the exception. To get the best protection you should therefore size the fuses for the load, not the wire size. A short on a 30 amp rated wire with a 30 amp fuse might not ever blow a 30 amp fuse. However it could generate a whole lot of heat at that bad connection you didn't even know about, which kept the current from blowing the fuse. So fuse should be sized for the total load, not the wire size. It wouldn't be unusual to have a five amp load at the bow of your boat that had a #10 wire nominally rated at 30 amps, to keep the voltage up for that long run. However a 6 amp fuse would keep the load working reliably, and limit to a great extent the possibility that a bad connection could overheat, especially if a short occurred.
You need to be alert for high resistance connections that lead to fires. Keep flammable materials away from electrical connections. Watch for poor performance on radios, winches, lights, windlasses, that signal a poor connection when you already know the wire sizes are okay. Check connections using a volt meter, while the load is turned on, you need current to test. A good connection should show nearly 0 volts when the probes are touched across a connection over a distance of an inch or two. A bad connection may show a fraction of a volt, and possibly many volts across the connection.
Hope this helps,
Gary H. Lucas