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Location of Aternator CB

Airzen

Well Known Member
Hi All,

Just wondering where folks are mounting the Alternator CB (note: this is NOT the Alt Field, but the thick/big CB for the main Alternator).

Do you mount it on the panel? Or leave it in the Engine compartment?

Thanks in advance,
Ashish
 
I believe that circuit breakers are thermal activation devices.

Makes me wonder if having one under the cowl would alter its triping characteristics.
 
I believe that circuit breakers are thermal activation devices.

Makes me wonder if having one under the cowl would alter its triping characteristics.

A fuse (the ANL part) is more reliable (less parts) and if it is mounted on the firewall it's not like you are going to reset the breaker in flight...:D
 
I put mine on the lower part of the panel with the low voltage warning light next to it which is easily viewable. I used a 70 amp breaker and 8 gauge wire for my 65 amp alternator.
 
I have often thought about completely eliminating an alternator fuse or breaker simply because its very unlikely that a short of the alternator wire would ever occur.
 
The thing I always ask folks that have this question is:

How many inches is it from your battery to that circuit breaker?

That's how many inches of big fat unprotected fire starting wire you will have in your airplane in the event of a short on that wire.

I also put an ANL fuse on my main buss feed from the master relay. That was another big fat fire starting wire that I wanted protected.

Some say that in the event of a fault that large, that the master relay will open but I don't trust that theory. In my line of work I have seen contactors weld closed with hard faults.
 
That is the basis of my thinking "whats the point in having it". In my -6 and in the -6 I fly now there is a CB in the panel for the alternator. If a short to ground occurred anywhere between the CB and the alternator, the CB would never trip, assuming somehow the battery side didn't trip the CB. Or as Brian points out, a short occurs between the CB and the battery buss. I think if one is very careful about supporting the cable properly there's virtually no possibility of a short occurring that would trip your flavor of circuit protection. Just another failure point having the circuit protection in there.
 
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alternator CB used on C172R

after sitting a couple days and nights in the rain the C172R alt CB tripped on taxi out. I reset it in the cockpit and did a runup to blow the water off. It is working properly for the past 500 hrs.
 
I bought Van's starter wiring kit when I finished my -7A in 2005. The plans sheet showed the 60A ALT CB on the panel which is where I put it. It's worked without any issues or tripping for the last 800 hrs. since then.

But, it sounds like a good idea to redo that. Which ANL do you use and where do you get it? How big of wire do you run from the ANL to the main bus I've got on my panel?


Thanks,

Mike
 
Like Bob said. It is fed from both ways, Alternator on one side, The Main buss from the other side.

What is the reason for it?

It will not prevent a fire (unless protected at both ends)

It will not protect your equipment from over voltage

Really the only purpose I see for it is to possibly protect your alternator from overload.
 
I have often thought about completely eliminating an alternator fuse or breaker simply because its very unlikely that a short of the alternator wire would ever occur.

Not only that.......... But think about the contact pressure it would take to pop a 70 amp breaker. If you had a broken wire (cable) from your battery or alternator and it was touching a ground, it would spark like mad and only be drawing 15 30 50 amps? and never pop the 70 amp breaker.

This is easy to prove with a car battery, cable and a breaker.

The safest thing you can do is install a very robust cable from your alt to the batt in a manner that will never require a fuse. And if you must install a fuse, install it on the battery end of the cable to the alternator. That's where the real BIG amps come from.
 
Most people now days use a large ANL fuse mounted on the firewall.

I'm in the process of a panel change that is affecting nearly all of my electrical.

Can you provide some info: what ANL are you using? A picture of the installation? Etc ?

Greatly appreciate the safer path !
 
Guys, the purpose of the alternator B lead overcurrent protection is not to protect from the current from the alternator. Most of us run a 70 amp or less alternator which even at full power with a dead short will not produce enough current long enough to turn the typical B lead into a fire or smoke bomb.

That overcurrent protection is there to protect against an accidental short of the B lead or in the guts of the alternator. The current source in this case is the BATTERY. The battery can dump huge amounts of current in a short time and it is fully capable of turning the B lead into a fire.

Therefore I suggest that people always install an ANL fuse as close to the battery as you can get it for both the alternator B lead and a second one on the main buss feed.

I got mine originally thru B & C Specialty products here.

and the base here.

Follow the directions there on how to size it.

I think Stein may sell these as well.

Both of mine are mounted right beside the battery on the engine side of the firewall.

atxjiv.jpg
 
Guys, the purpose of the alternator B lead overcurrent protection is not to protect from the current from the alternator....That overcurrent protection is there to protect against an accidental short of the B lead or in the guts of the alternator. The current source in this case is the BATTERY. The battery can dump huge amounts of current in a short time and it is fully capable of turning the B lead into a fire.

Right on Brian.

In the context of Brian's statement, consider the local environment for a battery cable vs an alternator B-lead. The heavy duty battery cable is generally near and/or attached to static airframe surfaces.....limited vibration or no movement at all. The poor B-lead, a smaller wire, extends from the fixed airframe to a route alongside a vibrating collection of hot metal parts, then suspended across a gap in space and terminated on a dangling chunk which may vibrate even worse. Which side of this system is most likely to short?
 
Right on Brian.

In the context of Brian's statement, consider the local environment for a battery cable vs an alternator B-lead. The heavy duty battery cable is generally near and/or attached to static airframe surfaces.....limited vibration or no movement at all. The poor B-lead, a smaller wire, extends from the fixed airframe to a route alongside a vibrating collection of hot metal parts, then suspended across a gap in space and terminated on a dangling chunk which may vibrate even worse. Which side of this system is most likely to short?

On top of that, when a circuit breaker is used, this B lead must penetrate the firewall twice, most likely the subpanel twice and gets ran all the way to the panel and back. The number of inches of wire inside the cockpit that can cause a bad day dramatically increases when a breaker is used and it is mounted on the panel.

When using a ANL fuse on the firewall, there are 0 inches of B lead wire inside the cockpit.

When using a second ANL fuse on the main buss feed, there are 0 inches of unprotected big fat fire causing wire inside the cockpit.

For shunt leads and battery busses and the like, I recommend fusible links.
 
Brian your explanation makes sense. The problem I have with an ANL is based on an experience I had about 8 months ago. I fly an all steam airplane with no low voltage warning. This was a newly purchased RV-6. One of the problems it had initially was it was really hard to start cold. The previous owner/builder just accepted it. One evening I decided to go fly and ran the battery down trying to get it started. So I decided the keyswitch wiring was the culprit, got under the panel, pulled the keyswitch, removed the jumper, and voila, it started just fine. The mag leads were reversed, so the impulse-coupled mag was being grounded. Put it back together and got it going again, the battery was strong enough to get it started. After it started I checked the ammeter and it was showing about 40A charge. I thought hmmm, ok, that should drop off soon. Do my runup and about that time it showed a little bit above zero. Well that was quick. Take off for the local flight and after about 15 minutes I start noticing all the instrument lights are low. Voltmeter showing about 10V. Hmmm, not good so I come back in and land without landing lights, flashlight is in my mouth with it pointing to the airspeed indicator. Shut down put everything away I notice the alternator B-lead breaker popped. Never had that happen. The low battery and strong charge current caused the breaker to pop (40A breaker, which is too small IMO). ANL's are simply slow-blow fuses and in a situation like this would have left me stranded somewhere because they're not commonly available. That and having them mounted on the firewall means rooting around inside the cowling. The ammeter sort of fooled me due to the way it is wired.

I think it would be a better idea to place a breaker on the firewall where the ANL is. That way there is no unprotected wire, and you're protecting the battery, and its reset-able. A simple diode/led combination wired across the breaker to an led on the instrument panel could be used to indicate a tripped alternator breaker, which would make a problem like this much more easy to diagnose in flight. One could just pull the top cowl to reset the breaker, in the event it trips.
 
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Brian your explanation makes sense. The problem I have with an ANL is based on an experience I had about 8 months ago. I fly an all steam airplane with no low voltage warning. This was a newly purchased RV-6. One of the problems it had initially was it was really hard to start cold. The previous owner/builder just accepted it. One evening I decided to go fly and ran the battery down trying to get it started. So I decided the keyswitch wiring was the culprit, got under the panel, pulled the keyswitch, removed the jumper, and voila, it started just fine. The mag leads were reversed, so the impulse-coupled mag was being grounded. Put it back together and got it going again, the battery was strong enough to get it started. After it started I checked the ammeter and it was showing about 40A charge. I thought hmmm, ok, that should drop off soon. Do my runup and about that time it showed a little bit above zero. Well that was quick. Take off for the local flight and after about 15 minutes I start noticing all the instrument lights are low. Voltmeter showing about 10V. Hmmm, not good so I come back in and land without landing lights, flashlight is in my mouth with it pointing to the airspeed indicator. Shut down put everything away I notice the alternator B-lead breaker popped. Never had that happen. The low battery and strong charge current caused the breaker to pop (40A breaker, which is too small IMO). ANL's are simply slow-blow fuses and in a situation like this would have left me stranded somewhere because they're not commonly available. That and having them mounted on the firewall means rooting around inside the cowling. The ammeter sort of fooled me due to the way it is wired.

Yes there are pro-s and con-s to almost every approach to things.

One thing about the ANL's are that they are "r e a l l y s l o w" blow fuses and if properly sized won't blow unless there is a real hard and strong short.

While there is no perfect solution, I feel the ANL's are the best thing we have at this time to take care of the job.
 
Laughing at Bob. Glad I'm not the only one who's driven around with a flashlight in their mouth. It gets really fun when you start to gag.
 
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