Finn Lassen wrote:

"crowbar" here means "shorting to ground". I haven't followed later discussions where Bob N. changed from that  method (on the field lead) to using a relay in the output lead, if he indeed did.

I don't see how the alternator can put out any voltage when its field winding is shorted.
B+ shorting to the field (bypassing regulation circuitry) seems to be fairly common with internal regulators so the only way to deal with that OV condition is disconnect B+.  I've seen on his list where he says (or does not correct other people who say?) that the same condition is possible (albiet rare) with externally regulated alternators.  Any direct connection between B+ and F inside the alternator will cause an OV (amoung other things) condition that can only be dealt with by disconnectng B+, hence the crowbar.  It seemed a bit of a stretch to me for this to happen with an externally regulated Alt, but that's what I read on his list.

I couldn't have made this up :o) ... Jim S.

Finn

Jim Sower wrote:

Finn Lassen wrote:

Yes, you'll need a relay in series with the "fat" wire, IF you have an internally regulated alternator that does NOT stop putting out voltage when the "thin" wire is disconnected. Looks like Bernie's alternator will not need that relay. No need to introduce an unneeded additional component - another point of possible failure.
According to Nuckolls, even alternators with external regulators can run away and that's why he recommends the crowbar OV protector.  Disconnecting the "thin" wire won't always stop the alternator from putting out.  You have to disconnect the "fat" (B+) line to be assured of an end to an over voltage condition ... Jim S.
Finn