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Noah
Joined: 02 Jul 2009
Posts: 18
Location: Rhode Island
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 Posted: Wed Apr 21, 2010 7:11 pm Post subject: Alternator field feed - fuse vs. fusible link |
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I understand why a fusible link is specified in line with the 5A breaker for alternator field feed. The breaker is designed to trip when the regulator grounds the feed in case of bus overvoltage. And I understand that the fusible link protects the wire from the bus to the breaker from a dead short. But if you're using the B&C fuseblocks, why not just stick a 30A fuse in there in place of the fusible link? Seems like it would also be robust, the breaker would still trip in case of overvoltage, and you wouldn't need to fabricate a custom fusible link?? In my case I have several wires being ganged together at the fuseblock terminal and this approch would eliminate one of them!
I know I must be missing something here, just don't know what!
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Noah Forden
RV-7A
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Bob McC
Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON
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| Posted: Wed Apr 21, 2010 8:36 pm Post subject: Alternator field feed - fuse vs. fusible link |
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The "dead short" on this circuit when (if) the crowbar circuit operates could quite easily "blow" the 30 Amp fuse before or while tripping the 5A breaker. The time constants for breakers and fuses are significantly different. To do as you suggest would work but at the risk of having to replace a fuse before being able to bring the alternator back on line. The suggested method has been well thought out and all failure modes taken into account. It's the way it is for good reason. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
Just a quick anecdotal story to illustrate. Many moons ago in electric motors lab, a fellow student made a wiring error when connecting a motor for "plugging" it to a stop. (Plugging a motor involves removing the line power then shorting the motor to cause it to stop quickly) Long story short he connected the plugging relay across the three phase line instead of across the motor. When the "stop" button was pressed, the 25 amp test bench breaker tripped, the 125 amp lab supply breaker tripped, the 400 amp breaker supplying the whole floor of the school tripped, and the 800 amp main incoming breaker to the building tripped. Maybe unusual, but happened.
Bob McC
[b]> --
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Bob McC
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Noah
Joined: 02 Jul 2009
Posts: 18
Location: Rhode Island
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| Posted: Thu Apr 22, 2010 3:09 pm Post subject: Re: Alternator field feed - fuse vs. fusible link |
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Great response Bob, thank you. That certainly drives the point home!
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Noah Forden
RV-7A
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rckol
Joined: 14 Nov 2008
Posts: 36
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| Posted: Thu Apr 22, 2010 6:48 pm Post subject: Re: Alternator field feed - fuse vs. fusible link |
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Just to play the Devil's advocate:
The breaker is part of an overvoltage protection circuit, not a conventional B lead breaker prone to nuisance popping. Why would anyone be resetting this breaker in flight after what is most likely an overvoltage event?
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