Circuit protection B-lead questions

Scott Hersha has a thread on here re; a midi fuse tp protect the alternator b lead.

Didn't want to hijack his thread, but it got me thinking;

Looks like most people put their B-lead current limit solutions on or near the firewall, if they bother to install one at all.

What exactly are we trying to protect here? If the b-lead is shorted out, it seems to me that it's more likely to be up under the engine somewhere. If that's the case, doesn't a firewall located limiter only prevents the battery from dumping too many angry amps into the short? It does nothing to prevent the alternator from happily melting the wire where the problem is more likely to actually be, i.e. between the alternator and the firewall.

In a perfect world, shouldn't the B-lead fuse/breaker/whatever be as close to the alternator as possible, in order to protect more of that fat wire as it snakes back to the firewall? Am I fundamentally wrong is some way that I'm not seeing?

I guess all this rambling leads me to 3 questions;

1. How much protection is a firewall mounted B-lead fuse actually affording a system? Especially if you already have a ANL fuse between your Batt contractor and precious firewall aft stuff.

2. If the answer to #1 is "not much" is it even worth considering installing one?

3. If I decide to press ahead and install one anyway, is there such a thing as an inline midi fuse that could be installed closer to the alternator, or is that just introducing a failure point with not much return?

As always, all opinions welcome.

The purpose of protecting that wire is to keep your battery from completely dumping itself into the alterntor, should the alternator short internally.

OK, comprehension has occurred

It 100% seems like the way to go with the consideration that a faulty alternator could kill the battery, but just for the sake of argument, what happens if the B-lead wire chafes and shorts against the engine case between the alternator and a ANL located on the firewall? Will the alternator happily puke out 60 amps until the wire vaporizes?

Not trying to be problematic here, just genuinely trying to look at this thing from all angles.

Desert Rat said:

OK, comprehension has occurred

It 100% seems like the way to go with the consideration that a faulty alternator could kill the battery, but just for the sake of argument, what happens if the B-lead wire chafes and shorts against the engine case between the alternator and a ANL located on the firewall? Will the alternator happily puke out 60 amps until the wire vaporizes?

Not trying to be problematic here, just genuinely trying to look at this thing from all angles.

Click to expand...

Depends on the alternator and regulator design - if it is sensing voltage AT the alternator output, then as soon as the fat wire is broken the voltage will rise off the chart and the overvoltage on the regulator will cut it off. If your voltage sense wire is on the battery side of the break, then yes it may continue to try and push current.

The alternator "B" lead fuse should be as close to the battery as practical without carrying starter motor current. If the alternator or its B lead short to ground, that fuse will blow, thus protecting the rest of the electrical system from the short circuit. If the "B" lead shorts to the engine, then yes, the alternator will continue to output 60 amps or whatever its rated current is. The "B" lead will not vaporize because it is a large cable rated for the full alternator current. The "B' lead is unlikely to short out if installed using good workmanship. An alternator is not capable of generating much more than its rated current. It is self current limiting.

Alt field switch

As another layer of "protection", an alt field switch on the panel should allow the pilot to turn off the alternator if there is a problem.