Fusible Link and the Starter Motor

[fastd]

Does a fusible link wire behave differently than a mega-fuse block or a resettable circuit breaker?

[patrickt]

Originally Posted by fastd

Does a fusible link wire behave differently than a mega-fuse block or a resettable circuit breaker?

A fusible link and a "normal" fuse both burn up when they are subject to a current that exceeds their safety limit, and they look like a little sparkler or fireworks when they burn up. This might be an issue for you if you happen to have your fusible link right next to the gas tank's vent tube, or if you're dealing with a boat where a fire is always a concern. The mega-fuse block is just a fuse block that is also covered to protect against something like this. When fuses burn up you have to take them out and replace them. When a circuit breaker blows, it can either reset itself and then retrip if the fault is still there, or stay off if it requires manually resetting. In deciding whether you want to add a fusible link off, say, your alternator, look at the line going from the alternator back to the fuse block and battery and ask yourself two questions: 1) How likely is it that wire could rub through on something, or in some other way have a serious fault to ground; and 2) If that happened, how bad is it likely to be? Remember, your alternator stops putting out current when your engine dies, or your electric system fails. Not so with your battery. As a side note, faults to ground often telegraph themselves long before they become "burn my car down" type problems by burning the wires back some, the same way that the overhead conductors in the power lines running through your neighborhood will burn back encroaching tree limbs. Sometimes it will trigger a breaker, sometimes it will do nothing, sometimes it'll shut your lights off and then a recloser will bring them right back up. But I don't want you relying on that (the smell of smoldering insulator has a very distinct aroma), if you are nervous about it all, just go ahead and put a fusible link on your alternator's BATT output and make it about 80% of the rated capacity of your alternator (assuming of course 80% is still well above your total amperage load of your lights, fans, radio, etc.). The biggest danger on the line that you realistically face is having the wire run through a grommet-less hole in the firewall or an un-shunted ammeter that somehow manages to ground itself to the dash. Batteries in the trunk are a little different as well since you have longer runs, more voltage drop and more chances to rub against something and fault to ground... like the vent tube for the gas tank.