Alternator B-Lead Circuit Breakers

Russ Erb

Originally published June 1998

(The following is an e-mail question I sent to Bob Nuckolls of The Aeroelectric Connection and his response.)

Our local Chapter 1000 electrical guru raised the question of why people put large breakers on the B-lead of the alternator. His point was that it is impossible for the alternator to put out in excess of its rated amperage (excess voltage, of course, is possible).

I thought about it for a while, and came up with this: Is the purpose of the B-lead breaker to protect against the power windings or the diodes shorting to ground and thus the battery popping the breaker?

(The response:)

You are entirely correct . . . the fuse/breaker is used to protect the b-lead wire from BATTERY current should a diode or wire short in the alternator. This is such a rare event these days, I've moved b-lead protection out of the cockpit and onto the firewall in the form of an 80 amp fuse for a 60 amp alternator.

- Bob Nuckolls


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Revised -- 14 March 1999