Bad ground wire

You have a short circuit somewhere. That could be the problem. Start there.

A bad connection could cause an arc, which would burn out the wire at the location of the arc.

There's not supposed to be a ground wire at the Alternator. There should be 2 plug-ins and a cable to a large nut for direct battery power. If you try to charge a dead battery with an alternator, you will wear out or burn up the Alternator. It is intended to maintain the charge - not bring it back from the dead. You'll really need to do some investigating if you think you have a draw. Hook up an Ammeter and check for a draw. Remove fuses one by one and determine which circuit the draw is on. Disconnect the components on that circuit one by one until the draw is gone. Replace the failed component.

on my 1991 f150 ford pickup. my black wire that's going to my battery cote fire why and I know its something to do with the ground

I have a 1993 F-150 with the 5.0 (302) motor. I need to replace the negative cable and it grounds behind the smog pump. Its impossible to reach w/o going thru the hassle of removing the pump...Are they're any other areas where the main cable can be safely grounded to

Ditch the smog pump anyway