"Its a mis-truths that breakers protec wire, thats merely how they are installed by the boat builder, but not the only way things should be."
Wow! I am not sure if you fail to understand the physics or the concept
Any OCPD protects the circuit from TWO possible occurrences, a ground fault, in which the purpose of the OCPD is to trip out under the resultant maximum current draw and thus prevent the fire potential of a burn-through. The second purpose is to trip (or blow) when the current in the circuit exceeds the wire ampacity. Both are important, not just one or the other. One is theoretically infinite amps and the other is simply the wire getting too warm as a result of the circuit drawing too much current. This second condition does not have to imply a circuit is faulty but that there is too much total load on the wire.
The best example of the second case is in the typical 120VAC receptacle circuit. One can go on forever stringing duplex receptacles on a circuit, as long the total amp draw does not exceed the wire rating then the circuit is safe. Plug
in a light here and there and the circuit perimeters are not exceeded. Plug
in two blow dryers and the 20 amp circuit would trip out. A good example of this in 12 volts is the lighting
circuit. It does not protect the individual light or wires within the fixture, only the wire runs between the fixtures are protected by the OCPD.
If an OCPD is used in a dedicated device it can then be sized to protect the device, such as a motor
control circuit. BUT, if there is more than one application such as a transformer to supply the lower control voltage used in the controls, then that transformer circuit must be individually protected.
If the fuse is such an antique device why does the NEC prohibit the use of circuit breakers for OCPD in most motor
control circuits? Must be a bunch of qualified electrical
engineers bought off by the fuse people!
I have personally seen expensive breakers operating in clean conditions that would not trip at a current even close to the rated capacity. Plus or Minus 25% is a typical operating range of a used breaker. I am not saying that I would not have breakers on my boat because I do have and use them. I am saying that they are not yet the simple cure-all that you make them out to be.
I will tend to agree that any overcurrent protection device will not prevent circuit failure in delicate electronics. Crowbar circuits work but only if the OCPD works in turn. In the case of motor control the fuse will always be better than the breaker unless the breaker is custom made for the application. The big safety
of the fuse in an electronic device is protection from reversed polarity. I am man enough to admit that I have gotten pos and neg crossed in an installlation and was darn glad the little fuse popped!