CB for B lead

Sid Lambert · #1 11-18-2008, 09:11 AM

Did most of you guys put a panel mounted circuit breaker on the B lead of the alternator?

I'm thinking of moving that wire so it's not in the cockpit and using a B and C current limiter.

I will have a 5 amp field breaker and switch to disconnect the alternator if needed.

Mike S · #2 11-18-2008, 09:21 AM

Panel mounted 60a CB.

One thing that the panel mounted CB has all over the firewall mounted current limiter, is resetting in flight.

az_gila · #3 11-18-2008, 09:42 AM

Quote:

Originally Posted by Mike S

Panel mounted 60a CB.

One thing that the panel mounted CB has all over the firewall mounted current limiter, is resetting in flight.

...problem in flight, I'm not sure that resetting the breaker would be a good idea...

If it's a real problem, that's a lot of excess current floating around waiting to do damage.... or more likely, it's your CB failing...

The current limiter is more reliable (no moving parts, no electronics) and a loss of the alternator output in flight is not a disaster. That's what you have the endurance buss for.

A 60 amp short that you re-enable by pressing the breaker may do bad things...

Mike S · #4 11-18-2008, 10:24 AM

Gil, all depends on what caused the problem in the first place.

I totally agree that having a 60a potential short is not something to promote a happy feeling----if you have no idea why it is there.

My thought is that the CB at least gives you the option to try a re-set.

As a pilot, I like options when things dont go as planned.

Example: you turn on the landing lights, the CB pops. Turn off the lights, re-set the CB, and all is now good.

Sam Buchanan · #5 11-18-2008, 10:48 AM

Quote:

Originally Posted by Mike S

Example: you turn on the landing lights, the CB pops. Turn off the lights, re-set the CB, and all is now good.

Me thinks there is a problem with the wiring architecture if landing lights (or anything else equipment related) causes the B lead to blow.

I consider protection of the B lead to be for the purpose of dealing with a dead short to ground if a connector on the lead fails (or maybe an internal alternator short?). In that case, resetting a CB won't be helpful.

az_gila · #6 11-18-2008, 10:49 AM

Quote:

Originally Posted by Mike S

Gil, all depends on what caused the problem in the first place.

I totally agree that having a 60a potential short is not something to promote a happy feeling----if you have no idea why it is there.

My thought is that the CB at least gives you the option to try a re-set.

As a pilot, I like options when things dont go as planned.

Example: you turn on the landing lights, the CB pops. Turn off the lights, re-set the CB, and all is now good.

It shows that there is a problem somewhere, and all is not good.

Personally, I don't like flying a plane with "unknowns".... why did the breaker pop in the first place?

Again, in your example, if you rate the landing light "critical to flight", then you should have two of them on independant circuits.
If it's not rated that way, just leave it and land - same as if a bulb had blown, which with a 25 hr life on the certified 4509 bulbs, is a frequent happening...

...and many times the problem is a faulty CB.

Note that a high quality CB is in the $100 range -

http://www.aircraftspruce.com/catalo...klixon7270.php

How many of us are using these high quality ones?

The smaller Klixon comes in two versions, AircraftSpruce does not even sell the Mil-Spec one...

http://www.aircraftspruce.com/catalo...klixon7277.php

Mike S · #7 11-18-2008, 10:58 AM

Guys, you are reading too much into this.

Sam is right, my choice of landing light is a bad example-----it was the only high current, switched load, that I could think of quickly.

My point is that a CB gives you an option that a firewall mounted fuse link does not.

As I have said before, I like options when flying.

f1rocket · #8 11-18-2008, 11:07 AM

The other advantage of using a current limiter is that it keeps the circuit breaker and the high current wire attached to it away from the instrument panel where electrical noise could be a problem. I know that many production airplanes do this but I like to keep the "noisy" wires as isolated as I can.

N941WR · #9 11-18-2008, 01:54 PM

Quote:

Originally Posted by Sid Lambert

Did most of you guys put a panel mounted circuit breaker on the B lead of the alternator?

I'm thinking of moving that wire so it's not in the cockpit and using a B and C current limiter.

I will have a 5 amp field breaker and switch to disconnect the alternator if needed.

Sid,

To answer your question, I put an AnL fuse on the firewall and a 5 amp field breaker on the panel.

Camillo · #10 11-19-2008, 07:40 AM

Quote:

Originally Posted by f1rocket

The other advantage of using a current limiter is that it keeps the circuit breaker and the high current wire attached to it away from the instrument panel where electrical noise could be a problem. I know that many production airplanes do this but I like to keep the "noisy" wires as isolated as I can.

I did as per Bob's architecture and so with current limiter and without CB on panel (though I use a CB for field). I read the explanation above on Bob's book and FAQs, but I didn't understand it.

If current limiter opens the circuit when well above its rated amperage, unless this occurs, there is no difference with a cable directly driven to the main bus and there protected with a CB. Am I right?

Camillo