Crowbars and circuit breakers

[Bavafa]

Can someone set me straight here please, isn?t a crow bar pretty much the same as a circuit breaker?
Or is it that crowbar protect against voltage where CB protect against Ampage?

[AlexPeterson]

Not at all. It puts a dead short on the circuit, tripping the breaker.

https://en.wikipedia.org/wiki/Crowbar_(circuit)

[Bavafa]

Originally Posted by AlexPeterson

Not at all. It puts a dead short on the circuit, tripping the breaker.

https://en.wikipedia.org/wiki/Crowbar_(circuit)

Alex,
Can one use a circuit breaker in place of a crowbar? As I understand it, a crowbar is to protect a circuit and isn?t what a CB does?

[BobTurner]

Originally Posted by Bavafa

Alex,
Can one use a circuit breaker in place of a crowbar? As I understand it, a crowbar is to protect a circuit and isn?t what a CB does?

You were closer on hour first post. Crowbars protect against over voltage, and also trip the CB. CBs are just sensitive to current.

[Sam Buchanan]

Originally Posted by Bavafa

Alex,
Can one use a circuit breaker in place of a crowbar? As I understand it, a crowbar is to protect a circuit and isn?t what a CB does?

No, a breaker trips when current exceeds the rating of the breaker, the OV module reacts to voltage.

[DanH]

A CB opens due to excess amperage.

A crowbar overvoltage device guards against excess voltage, just as its name suggests. It connects the field wire to ground when voltage exceeds its set level. The ground short causes the CB to see excess amperage, so it opens.

[Bavafa]

Awesome and thanks everyone, looks like my initial understanding was correct only unsure of.

[Toobuilder]

Just thinking out loud here....

So if one was to use a crowbar short of the field control wire, the CB pops or the fusible link burns through preventing the alternator from making power and thus, its a "hard" fault - it would take a separate and distinct action to reset. Works great as long as the field control circuit remains functional through all alternator failure modes.

However, if the OV device controls a relay on the output side of the alternator, then it seems like opening the output lead would allow the system voltage to return to normal, triggering a relay reset and output connection and subsequent reoccurrence of the OV condition. I can see a scenario where the "trip, reset, trip, reset" cycle would cause plenty of problems. Is a latching relay the solution here?

[digidocs]

Google load dump...

[Carl Froehlich]

Originally Posted by Toobuilder

SNIP
However, if the OV device controls a relay on the output side of the alternator, then it seems like opening the output lead would allow the system voltage to return to normal, triggering a relay reset and output connection and subsequent reoccurrence of the OV condition. I can see a scenario where the "trip, reset, trip, reset" cycle would cause plenty of problems. Is a latching relay the solution here?

No one would wire an alternator output solenoid as you discribe.
The output solenoid is powered from a 5amp or so breaker. The OV Crowbar module, when tripped, shorts across this breaker so it opens. When it opens, the alternator output solenoid has power removed so it opens as well, isolating the alternator from aircraft power. You must take action to reset the breaker, thus you will not get a ?cycle?.

Carl