fuses v circuit breakers

[thallock]

Fuse status panel

For those of you who are using fuses, but are concerned about being able to detect when the fuse has blown, you can do what I did. That was to design a fuse status board that shows the status of all fused curcuits. It's not that hard to design, and it was cheap and light weight.

Also, I should mention that for some fuse types, it is not always readily apparent when a breaker has blown. My last airplane was a Cherokee, where a few of the CBs were blocked by the control wheel. Even if they blew, they did not pop out all that far.

Tracy.

 

[Captain_John]

Tracy, I believe the best way to determine that a fuse has blown is the fact that something doesn't work.

From there we have 2 types of troubleshooting. Airborne and ground based. Airborne, step 2 is live without it. This is true with CB's or fuses.

Ground based, we can remove the load from the circuit and locate the short. This is also true of CB's or fuses.

CB's are fine, I just don't think they are any better than fuses. In fact, there are more failure points in them than fuses. They are more convenient than fuses, but that comes at a cost.

I am hiding my fuses out of reach of the pilot. No need to be resetting or playing with wires AND flying a plane simultaneously.

CJ

 

[thallock]

CJ,

I respectfully disagree. The best way to determine that a fuse has failed is not the fact that what it was connected to does not work. I would rather be notified of this when it fails, not when I go to use it. We can argue all day whether we should troubleshoot things in the air or on the ground, but when the failure occurs, I want to know. This may influence my decision as to whether I want to continue the flight or land as soon as possible.

It might be very useful to know that your lights are not getting power, before you rely on them to show you the runway when you go to land. You might want to divert to a better lighted field in this case. Or perhaps you have battery backup on a piece of avionics, so when the fuse has blown (for whatever reason), it might be nice to know this and start looking for an airport before the backup battery runs down. This is what a fuse status panel gives you.

Tracy.

 

[Captain_John]

Tracy, that's cool. I can agree to disagree. This is why we fly experimental category. We do it OUR WAY!



I feel that at the preflight is a good time to become aware of a malfunction. Odds are if you will need it soon, you will spot it on a functional preflight. Actually visually inspecting each and every fuse link is not the intention.

A CB will give you a nice, inflight notification of idle systems like a flap motor, navigation lights or avionics cooling fans. I consider that unneccessary with our systems. Stuff like that really isn't THAT important to me.

CJ

 

[kevinh]

It might be very useful to know that your lights are not getting power, before you rely on them to show you the runway when you go to land. You might want to divert to a better lighted field in this case. Or perhaps you have battery backup on a piece of avionics, so when the fuse has blown (for whatever reason), it might be nice to know this and start looking for an airport before the backup battery runs down. This is what a fuse status panel gives you.

Actually, if you look at it from the perspective of what is more likely to fail:

• landing light vs. fuse protecting wire to landing light
• fancy GPS doodad vs. fuse protecting wire to doodad
• Flap motor vs. fuse protecting wire to flap motor

In all of these cases the device is far more likely to fail than the wire+fuse system. In fact, I just looked at the schematic for my plane and this applies to ANY device.

In that case, having some sort of indicator that a fuse has blown has little benefit - you need to test the actual device if the device is critical to flight.

It could be argued that there are some device failures that would fail as a short to ground - as an EE (of sorts - I mostly do software now) I can say that those failures are far less likely than an internal fault that would result in an (undetected) open circuit.

The one exception: the overvoltage protection circuit on my plane works by creating an intentional short to ground. This is one of the reasons that the alternator field current is the only breaker in my plane.

 

[thallock]

I still stand by my belief that it is better to know about a failure when it occurs, than to wait to find out. So, when a fuse blows, the status panel will tell you by changing color from green to red. The fact that there are other failure modes that are not detected is irrelevant. The CB is not going to tell you that a device has failed due to an open, either. Also, one of the purposes of the fuse status panel is to tell you before you go flying that all fuses are powered.

Tracy.

 

[Captain_John]

Also, one of the purposes of the fuse status panel is to tell you before you go flying that all fuses are powered.

Tracy.

True, but that is what the prefiht does with no complexity, failure points or weight whatsoever.

CJ

 

[szicree]

CJ,

I respectfully disagree. The best way to determine that a fuse has failed is not the fact that what it was connected to does not work. I would rather be notified of this when it fails, not when I go to use it.

This sounds good, but the breaker won't blow until you put power to the circuit.

 

[thallock]

Someone pointed out that with fuses, you do not know the status of them if they are out of sight. If you put them behind the panel it is hard to check them prior to flight, so they do not get checked. I responded that this is easily solved by a simple fuse panel monitor board, that is very light, and is not a failure point, since it is not in line with the circuit being fused. This can easily be checked before flying, and even during flight. It solves the problem that people seemed to be concerned about.

I will be usiing fuses for almost all of my wiring, if not all. I will have a fuse status panel that lets me know exactly which fuses are conducting, and which are not. It may not be needed, but it makes me feel good. I have not decided yet, whether I will have at least one block of fuses behind a panel that can be opened during flight.

As far as I am concerned, this topic is dead.

Cheers,
Tracy.

 

[tonyjohnson]

IFR

I am grateful for all the thoughts and obsevations provided so far on this thread. I should have added to my initial question the fact that I intend to equip my RV for IFR flight.

The observation was made that the analysis of cb v fuses might change given the mission, that is IFR v VFR.

For those of you who favor fuses generally, would you change you mind on some items if the mission profile included IFR? If so, what IFR essential items would you put on cb's?

Thanks again for all the input, it is extremely helpful.

 

[Captain_John]

Nope, I am planning an instrument ship. I still am going fuses.

Just as in VMC I am not going to do any electrical work, I will have even less time to do it while shooting a precision approach.

Tony, no change in my opnion.

CJ

 

[Captain_John]

Oh, and Tracy... I didn't mean to offend. I hope you didn't take it that way.

CJ

 

[Davepar]

Sort of a side comment: I've seen a few side-by-side slider guys that put the fuse blocks on a shelf that swings down between the panel and subpanel. It's a slick idea, but on a tip-up I'm a bit afraid of water going straight down into the fuse blocks. Even a few drops of rain would not be a good thing.

 

[gmcjetpilot]

Why is the question

It also should be appreciated that Cbs aren't there for repeated resetting. Only ever once. In fact, one maker of Commercial Airliners (the one Americans don't like a whole lot) took all the Cbs out and stuck them in the Avionics Bay for the ground engineers to reset.Pete.

That is because they are putting them near the local BUS. Also those items are NOT used anywhere but the ground. It would not make sense to run individual wires from the cockpit and install fuses there. If you visit a jet cockpit there is no room for more CB's. In the B767 there is an acre of fuse in the overhead and more in the lower right side wall. There is no room for more CB's. Each Galley (kitchen) has there own CB's. The flight critical ones are on the flight deck.


So I get it, but it is not relevant, except they are CB's NOT Fuse's.

One of the draw backs of a fuse is you need to carry them around and fish them out and stick them back in. From AUTO experience you have to take them out to see if they are blown.

I had to reset a CB on a 19 seat commuter plane. The VOR was not was not working on a VFR arrival. On the ground a CB was out and was reset. VOR fixed.

 

[n5lp]

...I had to reset a CB on a 19 seat commuter plane. The VOR was not was not working on a VFR arrival. On the ground a CB was out and was reset. VOR fixed.

This begs the question. Where was the malfunction? Circuit breaker or VOR receiver? Fuses are pretty darn reliable.

 

[N941WR]

My reason for selecting CB's over ATO fuses was simple.

Some day I may wish to sell my RV and it will most likely have to go through a pre-buy inspection. There is a good chance that neither the buyer nor the person performing the inspection will know very much about home builts and the use of ATO fuses.

I can hear the inspector telling the prospective buyer, "Stay away from that plane, he used automotive parts in the electrical system."

Yes, the CB's were much more expensive. Yes, they are heavier. Yes, I can pull everyone of them from the left seat. Yes, they are standard aircraft CB's. Yes, adding things later may be more difficult. bla, bla, bla.

I hope they will pay off in the long run.

That's my story and I'm sticking to it.

 

[Jconard]

So,

If the fuse to your recognition lights or collision fail at night, and you cannot replace the fuse, what do you do. I suppose you land the plane, but every moment you fly without lights you are putting every other plane in the airspace at risk, which is why I think there is actually a requirement for night flight related to carrying the fuses at night.

A Lighted monitor panel is an extra hot wire to each and every indicator light...see any additional risk there?

 

[Tom Maxwell]

Fuses are good!

My take on this comes from military and private industry experience of 33 years. I've worked with everything from low voltage circuits to radar power supplies of over 70KV at 50 amps. I can honestly say that I have never ever ever seen a fuse fail. I am sure there have been some due to manufacturing flaws and whatnot, but I have never personally seen one fail on its own. If a fuse does open it is because of one thing - too much current going through it. If there is too much current going through the fuse, then either the load is amuck or an overvoltage condition existed causing excessive current through the load. In either case we want the fuse to blow and stay blown until the situation has been checked out and remedied.

Fuses seldom blow at all, and the probabilty of one blowing on its own is miniscule. Think back to the last time you had a fuse go bad in your car(s) without something in the load causing it to blow. It just doesn't happen enough to even consider the possibility in electrical circuit design. On the other hand, I have seen numerous circuit breakers go bad on their own.

If something does go amuck in the load, then the last thing you should be doing is bringing the circuit back online in flight. I actually view a fuse tucked away under a panel as safer than a circuit breaker at the ready. It is much too easy to reset the circuit breaker and then adopt the mentality of, "awe I'll try it just one more time to see if it might stay closed." This is just human nature. If the button or switch is there, we are going to try to push it or flip it.

I have seen numerous circuit breakers trip, and then when turned back on, the circuit or something in the load goes up in smoke. This is not a good scenario in a flying aircraft! If a fuse blows or a breaker trips, the first course of action should be to leave the circuit off until the load can be checked out.

Either fuses or circuit breakers will work just fine. But I am in the camp that says when one blows/trips in the air there is a reason and things should be left alone until the plane is on the ground. If s fuse or breaker is protecting a life or death circuit, then backup should be considered in the design of the circuit in the first place.

Of all things in the cockpit, few are truly life threatening. The probability of enough equipment going offline at one time to create a life or death situation is remote, especially if each device is individually protected with its own fuse or circuit breaker. It just doesn't happen very often unless an entire panel goes up in smoke in which case resetting a breaker is going to be the last thing on our minds.

A bigger issue that should be considered in system design is the power sources themselves and how to design redundancy into the alternator(s)/battery(ies) systems. These sources of power are far more likely to fail than a load device or the distribution system. Equipment internal batteries, dual aircraft batteries, dual alternators, handheld radios, flashlights, et el are all better responses to a potential blown fuse or tripped breaker than trying to replace the fuse or reset the breaker in flight. In other words, I don't view the option of being able to reset a breaker in flight as any kind of safety feature at all.

If you subscribe to the above philosophy then fuses are a great choice because they are cheaper and lighter. If you don't subscribe to the above philosphy then circuit breakers are fine even though they do cost more and are heavier.

A couple of points made:

I agree with the poster who said that not checking every fuse does not constitute poor preflighting. On the contrary, checking for the absence of a tripped breaker without checking the equipment on the other end of those breakers could be interpreted as poor preflighting.

The fact that anecdotal evidence exists that resetting a breaker did resolve a problem (i.e. flaps worked again), does not mean that resetting breakers in flight should be a routine procedure. It may work sometimes, but the one time it doesn't can kill you. If at all possible, a better choice is to put the plane on the ground and then if you want to reset the breaker, go ahead. But then again, if it does reset, then what? Does it mean the aircraft is safe to fly again? Maybe, maybe not! How many times should we go through this trip/reset routine before we finally start checking things out?

 

[fodrv7]

None

Not quite George.
Airbus didn't just move some Cbs (The ones you call ground ones). They moved them ALL. If one trips it is annunciated.
Of course it is possible because of the redundancy in a modern Airliner.

On a 19 seat commuter without a second VOR, a Cb is need to have.

On a DAY VMC RV?

Pete.

 

[rv8ch]

Fuses are good!

...If something does go amuck in the load, then the last thing you should be doing is bringing the circuit back online in flight. I actually view a fuse tucked away under a panel as safer than a circuit breaker at the ready. ...

It's amazing how lucid other's comments can be when they agree perfectly with my own opinions!

 

[szicree]

It's amazing how lucid other's comments can be when they agree perfectly with my own opinions!

I thought exactly the same thing when I read Tom's post, but was afraid to say it.

 

[rodrv6]

That is because they are putting them near the local BUS. Also those items are NOT used anywhere but the ground. It would not make sense to run individual wires from the cockpit and install fuses there. If you visit a jet cockpit there is no room for more CB's. In the B767 there is an acre of fuse in the overhead and more in the lower right side wall. There is no room for more CB's. Each Galley (kitchen) has there own CB's. The flight critical ones are on the flight deck.


So I get it, but it is not relevant, except they are CB's NOT Fuse's.

One of the draw backs of a fuse is you need to carry them around and fish them out and stick them back in. From AUTO experience you have to take them out to see if they are blown.

I had to reset a CB on a 19 seat commuter plane. The VOR was not was not working on a VFR arrival. On the ground a CB was out and was reset. VOR fixed.

Check out the cockpit of a 777. There is only a small area of c/b's on the overhead panel. The vast majority have been moved down to the electronics bay, including the ones for the flight control computers and other systems that are vital to flight. We don't want flight crews resetting breakers, that's best left for us on the ground. The critical systems have redundant backups which may not be a bad idea in our planes.

 

[gmcjetpilot]

Don't know

This begs the question. Where was the malfunction? Circuit breaker or VOR receiver? Fuses are pretty darn reliable.

May be it comes under the title of STUFF HAPPENS? There was no inflight emergency, still had a second VOR/LOC receiver, making a visual approach. The CB was reset on the ground and flew another 6 legs with no problem. For commercial applications CB's are all you find. Also the ability to pull the CB plays an important roll to maintenance crews. Obviously a RV is not bound by economics and dispatch reliability.

Not quite George.
Airbus didn't just move some Cbs (The ones you call ground ones). They moved them ALL. If one trips it is annunciated. Of course it is possible because of the redundancy in a modern Airliner.

On a 19 seat commuter without a second VOR, a Cb is need to have. Pete.

Oh Airbus, figures. Great Instead of taking CB's out of the cockpit, I wish they have spent more time bolting their vertical stabs on better so they don't fall off and figure out how to make their landing gear go down while not rotating 90 degrees to the aircraft center line. I am just kidding. I am sure it's a great idea to have all the circuit protection centrally located, outside the cockpit, saving wiring, weight and cost of routing the CB's in the cockpit. The warning system adds some wiring I am sure, but sure it saves manufacturing cost as well.

Check out the cockpit of a 777. There is only a small area of c/b's on the overhead panel. The vast majority have been moved down to the electronics bay, including the ones for the flight control computers and other systems that are vital to flight. We don't want flight crews resetting breakers, that's best left for us on the ground. The critical systems have redundant backups which may not be a bad idea in our planes.

Well if Boeing is doing it than that is better..........ha ha ha ha. I agree pilots should not reset CB's, however there are a few non-normal checklist that require a CB to be pulled on the B757/767, but I am sure that is easy to solve or eliminate. I'll admit CB's accidentally "POP", pulled out when someone physically hits or snags them. It happens, so sure, taking them out keeps them from getting tripped accidentally also. Of course as you know sometimes a CB cycle fixes a FMC fault (like unplugging your PC). Again I agree it's a maintenance function, but maintenance would just give us their blessing and we did it: POP, WAIT, PUSH IN, FIXED.

Back to RV land......... none this matters and in the end they all use CB's. I guess the only point is you don't NEED to reset most circuits faults in flight. I will buy that.

 

[Captain_John]

Tom Maxwell! You made a GREAT POINT!!!

Not everyone realizes the IAC (Instantaneous Current) on a short circuit! When a short occurs, the Instantaneous current can be in the TENS of THOUSANDS of amperes!!! Of course this is only for a nanosecond. But nevertheless, it is there!

A fuse, like you stated... is MUCH faster to react than a CB!

A CB (depending on if it is thermal or magnetic) will take longer than the fuse to respond to the overcurrent situation.

Your wiring system will be cooked less with a fuse, especially if it is a high resistance short to ground. A CB will see that as additional load if it is brief enough.

On the upside, a nice, hot, shorted BLOW UP... the bad section would be easy to locate!

CJ

 

[osxuser]

I put one vote in the push-pull CB crowd. All my reasons have probably been mentioned before in this thread

 

[Kevin Horton]

If the fuse to your recognition lights or collision fail at night, and you cannot replace the fuse, what do you do. I suppose you land the plane, but every moment you fly without lights you are putting every other plane in the airspace at risk, which is why I think there is actually a requirement for night flight related to carrying the fuses at night.

If the fuse blows, you would do the exact same thing you would do if the recognition light bulbs failed, which is a much more probable failure.

If the fuse is properly sized, there shouldn't blow unless there is a good reason - i.e. electrical short, etc. Resetting popped CBs or replacing blown fuses is asking for an electrical fire. I'll do that experiment on the ground, not in flight.

 

[gmcjetpilot]

Cook that wire

Tom Maxwell! You made a GREAT POINT!!!

Not everyone realizes the IAC (Instantaneous Current) on a short circuit! When a short occurs, the Instantaneous current can be in the TENS of THOUSANDS of amperes!!! Of course this is only for a nanosecond. But nevertheless, it is there! ..........Your wiring system will be cooked less with a fuse..... CJ

I appreciate your point, a fuse is faster, but I am going to call a big fat so what. CB's operate as a function of the overload, and they can act in fraction or 2-4 seconds if the overload is only twice CB's rating. A dead short will pop in 0.05 sec. I don't know about Tens of Thousands of amps, but it does take time to get a wire HOT. It just does not happen in a nanosecond.

The good news is the operating current of a wire is way less than it's "fuse" or melt current by at least 5 times, often more. Typically we size wires well under their max operating rating, to keep voltage drop or temp rise to a min. We also size CB's rating well under the wire's continuous current capacity.

The bottom line is the CB will POP quickly, before the wire is even warm much less HOT. It's all about proper sizing of the CB and wire.

This CB time delay characteristic may be an advantage for some devices. One that comes to mind is the infamous "crow-bar" aeroelectric promotes, which is also used in B&C's voltage regulator.

The comment that fuses "will cook (the wiring) less" is subjective. I'm not sure about fuse reaction time, but it is more than a nanosecond. You may be correct that a fuse can react faster than a CB, but a CB acts fast enough. I am not sure what the difference 0.05 seconds or 0.10 seconds makes to the wires temp?

 

[Captain_John]

I don't know about Tens of Thousands of amps, but it does take time to get a wire HOT.

George, I agree with most of what you say. You are just gonna hafta take my word for it!



Google up Instantaneous Current and Circuit Breakers if you feel better about it.

Most Class 1 Devices have 5 - 10,000 IAC ratings. It is our industry standard for UL and CSA ratings of Overcurrent Protection Devices.

Electrons don't behave any differently in Industrial, Commercial and Residential structures than they do in planes, aside from the sine wave structure.

Oh, and it is amazing how much difference a split second means to a shorting conductor. I have seem MANY a breaker allow significantly more inrush current throught it's terminals on ground fault situations.

George, I intentionally short out CB's almost DAILY. I know what the characteristics of the ground fault situations are and it's subsequent arc blast. I have seen electrical explosions first hand both with higher voltages and longer durations. I teach proper installations and safety. I am a wireman and an instructor... Electrical and OSHA.

When it comes to spooling up oil burners, I will defer to you. Trust me on this one.

CJ

 

[Captain_John]

...oh, but like I said earlier, CB's are MUCH more convenient. They SHOULD be use for this purpose!

Each has it's place. Choose between them for the benefits that each one has!

CJ

 

[Mel]

Here's a meaningless thought: Circuit breakers are tested at the factory to see if they work properly. Are fuses?

 

[jcoloccia]

Here's a meaningless thought: Circuit breakers are tested at the factory to see if they work properly. Are fuses?

LOL. This would make for LOW yield

I dunno...maybe they do a continuity check, or something.

 

[WildBill]

fuses vs breakers

From reading the many posts, there seems to be stronger general disagreement here between what I perceive as the 'electrical design' crowd and the professional crew crowd.

I think the original poster was further inquired about best practices circuit protection (vfr/ifr).

I think the best solution (especially for ifr aircraft) is to educate yourself about your options without that circuit, and the risks (of reseting), and use CB's, so the choice is available to you then.

No one says you have to reset; but if things are looking real bad without that circuit, I want the option.

Wierd things have been known to happen in (increasingly electric) aircraft.

 

[Captain_John]

Ha! Fuses can only be checked once.



But... they always check out fine.

CJ

 

[Kevin Horton]

No one says you have to reset; but if things are looking real bad without that circuit, I want the option.

Wierd things have been known to happen in (increasingly electric) aircraft.

Just curious - what circuits do you have that are important enough that you would need to do a reset in the air? What are your plans if the item powered by that circuit has some other failure?

If some item is important for safety of flight, you really need to have a backup in case it fails.

There was an interesting incident with one of our King Air A100s a few years ago. The windshield heat CB popped in flight. The crew wisely left it popped, and reported the snag to maintenance. The maintenance guy, without doing any other troubleshooting, decided to push the CB in and turn the windshield heat back on. The power connection at the corner of the pilot's windscreen started arcing, and it started an electrical fire.

If you have a CB or fuse that regularly trips, that indicates a design problem. The CB or fuse isn't big enough to handle transients. The CB or fuse size needs to be increased, which means the wires it protects might need to bumped up a size, to ensure the wire can safely handle continuous current at the CB or fuse rating.

If you have a CB or fuse that just pops in flight, leave it alone until you are on the ground, unless you need the circuit bad enough that you are willing to risk an electrical fire.

I expect a bunch of guys will chime in now, and say that they've been pushing CBs in all their life, and they've never had an electrical fire, so this is all just a load of BS. Well, in aviation, you need to be able to learn from the misfortunes of others, as you won't live long if you have to make all the mistakes yourself.

 

[rv8ch]

Learn from others...

Well, in aviation, you need to be able to learn from the misfortunes of others, as you won't live long if you have to make all the mistakes yourself.

Very true. I'd expand this concept of learning from other people's mistakes beyond the aviation world to every day life!

Just as another data point, I'm going all fuses.

 

[Yukon]

Alternator Field CB's

Just curious - what circuits do you have that are important enough that you would need to do a reset in the air? What are your plans if the item powered by that circuit has some other failure?

If some item is important for safety of flight, you really need to have a backup in case it fails.

There was an interesting incident with one of our King Air A100s a few years ago. The windshield heat CB popped in flight. The crew wisely left it popped, and reported the snag to maintenance. The maintenance guy, without doing any other troubleshooting, decided to push the CB in and turn the windshield heat back on. The power connection at the corner of the pilot's windscreen started arcing, and it started an electrical fire.

If you have a CB or fuse that regularly trips, that indicates a design problem. The CB or fuse isn't big enough to handle transients. The CB or fuse size needs to be increased, which means the wires it protects might need to bumped up a size, to ensure the wire can safely handle continuous current at the CB or fuse rating.

If you have a CB or fuse that just pops in flight, leave it alone until you are on the ground, unless you need the circuit bad enough that you are willing to risk an electrical fire.

I expect a bunch of guys will chime in now, and say that they've been pushing CBs in all their life, and they've never had an electrical fire, so this is all just a load of BS. Well, in aviation, you need to be able to learn from the misfortunes of others, as you won't live long if you have to make all the mistakes yourself.

Kevin,

If CB's are as troublesome as depicted , why is everybody entrusting their alternator field to one? Surely a 60 amp fuse could be found? Why would one ever want to reset such a large and hazardous breaker???

 

[Captain_John]

John, I gotta see Kevin's side here.

...and I see what you are saying too!

Like I said earlier, each device has it's benefit/drawback.

The CB can be "switch rated" and you know that occasionally you need to cycle the field of the alternator. That is the benefit of a CB.

It is a matter of selecting the right tool for the job. Kinda like using a socket wrench when you have an adjustable or vica-versa. Both will work, just which one do ya wanna use.

CJ

 

[jcoloccia]

Kevin,

If CB's are as troublesome as depicted , why is everybody entrusting their alternator field to one? Surely a 60 amp fuse could be found? Why would one ever want to reset such a large and hazardous breaker???

Because the field doesn't carry 60 amps. The point of putting a breaker on the field is so when the OV protection crowbars, the circuit breaker pops. In this case, it's more or less safe to keep resetting it (at least if the OV protection is working properly). Because of the crowbar, it doesn't matter that the breaker takes some time to pop...as soon as it crowbars, the field drops to 0. The circuit breaker popping is just the icing on the cake and keeps you from draining batteries, melting wires, etc.

Nobody's said CB's a troublesome. For myself, this isn't an all or nothing thing as if they're somehow in competition with each other. Once again, I'll use both and put them were appropriate.

 

[WildBill]

reseting

>"Just curious - what circuits do you have that are important enough that you would need to do a reset in the air? What are your plans if the item powered by that circuit has some other failure?"

Are you telling me you have never seen a checklist that called for doing something with a breaker?

Threre's too many variables, and is highly dependant on the airplane and situation. My point was that since noone can be absolutely sure of systems in every possible scenario, that its good to have options (especially in imc conditions).

In your story about the windshield heat you left out the most important parts of the story, such as: what were the flight conditions (ice obscuring windhield)?, was there some place warmer to go?, are their 2 windshield heat systems (left/right), can you see out the side windows?, can you fly the airplane using only side windows? etc

Lets say you were behind the 8 ball on all of those, and didn't push the breaker in, then crashed, and someone tested the circuit and it worked long enough to clear the ice for a landing?

Don't make it sound like I'm going to reset breakers for a silly reasons like I want to accelerate past the cold screen "chicken test" airspeed.

 

[gmcjetpilot]

Blame the device not the CB, and the future

There was an interesting incident with one of our King Air A100s a few years ago. The windshield heat CB popped in flight. The crew wisely left it popped, and reported the snag to maintenance. The maintenance guy, without doing any other troubleshooting, decided to push the CB in and turn the windshield heat back on. The power connection at the corner of the pilot's windscreen started arcing, and it started an electrical fire.

If you have a CB or fuse that regularly trips, that indicates a design problem.

Kevin I agree if a CB trips the pilot has the option to NOT reset.

However I question if your story relevant. Something does not sound right with your logic.

A quick check of Beech Aircraft window heat shows where the problem is, the window heat. A FUSE would not have saved the day, and IF the CB did not trip a fuse would not trip.

Lets say it was protected by a FUSE. It blew if flight. Than later on the ground the maintenance guy popped in a new fuse, the same thing would have happened. Also the current draw of cool window and hot window may be quite differnt. There is more to it than what you're letting on.

As far resetting a CB in flight, in this case if they would have reset it and sparks appeared I am sure they would have turned the window heat back off. However I am pretty sure the CB would not have re-set in flight.

A CB is a thermal device and accumulates heat and will trip over time with just a slight over load. In your example, if the pilots did attempt a CB reset, it would have re-tripped, since the CB was already hot. Later on the ground it was totally cool. Still the problem is NOT the CB it's the window heat.

The problem is the device, the window heat, which can arch and spark under the rated current limit of the protection.

To imply somehow a CB is NOT as good as a fuse because of this example is just unconvincing. It is interesting. It brings up the nuance of how CB's work and some devices, but I think the CB did just what it was suppose to do very well. I kind of blame maintenance for just pushing the CB in. From all the NTSB reports I ref below, this is not a rare event.

The window heat problem was not a CB issue. A fuse would have done the same thing and may have not blown in the first place (see list of NTSB reports below).



Some facts about CB's
CB's incorporate a trip free design, meaning they can't be reset with a fault. They mechanically will not engage or latch. You can't even override them by holding them in.

As I mentioned CB's are thermal devices. A fuse might take a slight overload indefinitely. A CB will eventually TRIP even if not way overloaded, as long as it gets hot enough. That's why IF you do choose to reset a CB, you must wait a minute or two for cooling. Otherwise they'll not reset unless cooled, even if the fault is removed. That is a cool feature (pun intended ).

The maintenance guy I hope turned the window heat off or pulled the CB out he just reset as soon as he got the arch. I know of a few maintenance guys burning planes to the ground. I am fairly sure the cool window and CB aggravated the situation when the mechanic reset it on the ground.

I am totally convinced a fuse would NOT have done anything better than a CB here, except remove the ability or option of the pilots to reset. IF they would have attempted a reset, I am also fairly sure it would have not reset or popped quickly. If it did reset, the problem was obvious and they would have de-powered the circuit. It does bring up an important point, be careful resetting the CB. You don't have to reset a CB. Have you ever landed with out window heat and iced over windscreen? I did once. It was not the CB it was the hot plate window strip was open, dead. Good thing the runway was wide and the side windows where clear.

Any high amp device, window heat, coffee makers and ovens, can make smoke and sparks below the circuits CB/fuse current limit. The first thing you do in a big plane (or any plane) when you smell electrical smoke (notice I did not say fire) is turn the utility buss off, which has all non-essential and big current items: coffee maker, ovens and basically everything not needed. If you are flying a King Air you turn then window heat off.

If flying a RV and smell electrical smoke what do you do? You turn the ??????? off? Master off? Just FOOD FOR THOUGHT, what would you do?

Bottom line don't blame the protection, blame the device.

Why do CB's pop when there is no hard short?, they get hot. In the OLD days it was common to GANG several devices onto one CB, like we do in home wiring. At home we know you can blow a CB if you put too many items on at one time. Most home builders, including myself, install +20 CB's for discrete CB protection of almost every circuit. BTW, why did almost all homes and commercial/industrial wiring go from fuses to CB's?



TAKE A QUICK GLANCE AT THESE KING AIR WINDOW HEAT SHORT REPORTS

These NTSB reports sound similar to your story. Your maintenance folks must have known of this fairly common King Air window arching issue? Don't you think he would have looked for arching before pushing the CB?

arcing on the copilots windshield
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001208X07001&key=1
smoke arch's emanating from windshield
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001211X11404&key=1
smoke bottom left corner of F/O's windscreen
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001208X05914&key=1
Window arch smoke
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060504X00513&key=1
Window cracked and started to burn
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20040413X00453&key=1


PS:
Better protection would be ARCH-FAILT interrupter protection like in your bathroom. There are ARC-FAULT protection CB's for aircraft. The Arc?Fault Circuit Interrupters (AFCI) detects potentially hazardous arcing conditions and prevents catastrophic damage caused by electrical fires.

http://www.sensata.com/products/controls/arcfault.htm

An AFCI CB would likely be a good choice for the Beech King air window heat. Cost? I don't know but no doubt more expensive. Do we need it in a RV? No, but I find it interesting.

You all might remember (1998) Swissair Flt 111 (Nova Scotia, overhead wiring fire behind cockpit) and (2000) TWA 800 (Long Island NY, short if fuel tank, explosion). A fault interrupter may have saved those looses. CB's and fuse's both have limitations. The future is better circuit protection with logic built into them, like AFCI, which will be CB's not fuses. However for a single engine GA plane CB's or fuses are fine. The key is GOOD wiring practice to avoid any short in the first place. I think we can agree on that.

 

[Kevin Horton]

Kevin I agree if a CB trips the pilot has the option to NOT reset.

However I question if your story relevant. Something does not sound right with your logic.

A quick check of Beech Aircraft window heat shows where the problem is, the window heat. A FUSE would not have saved the day, and IF the CB did not trip a fuse would not trip.

You missed my point George. Earlier in this thread there were several people who said that they wanted CBs, so that if one popped in flight, they could push it back in to recover the service.

My point is that pushing a popped CB back in may be a very bad idea. It should never be necessary, if the CB is properly sized to handle transient loads, and the aircraft system design and operation considers the possibility that any piece of equipment may fail. If the aircraft is designed and operated so that any single equipment failure is a non-event, then there will never be a need to push a popped CB back in. Leave that exercise for after you land.

Lets say it was protected by a FUSE. It blew if flight. Than later on the ground the maintenance guy popped in a new fuse, the same thing would have happened.

I'll take an electrical fire on the ground over one in flight any day.

As far resetting a CB in flight, in this case if they would have reset it and sparks appeared I am sure they would have turned the window heat back off. However I am pretty sure the CB would not have re-set in flight.

Well, it reset on the ground, so I don't know why it wouldn't reset in flight.

To imply somehow a CB is NOT as good as a fuse because of this example is just unconvincing. It is interesting. It brings up the nuance of how CB's work and some devices, but I think the CB did just what it was suppose to do very well. I kind of blame maintenance for just pushing the CB in. From all the NTSB reports I ref below, this is not a rare event.

And lots of pilots think pushing CBs back in in flight is a good idea too (see earlier in this thread).

TAKE A QUICK GLANCE AT THESE KING AIR WINDOW HEAT SHORT REPORTS

These NTSB reports sound similar to your story. Your maintenance folks must have known of this fairly common King Air window arching issue? Don't you think he would have looked for arching before pushing the CB?

I'm not sure if our maintenance guy wasn't aware of those events, or if he was of the type that can't learn from other's mistakes.

 

[L.Adamson]

BTW, why did almost all homes and commercial/industrial wiring go from fuses to CB's?

Panel space and the convenience of easily killing the circuit. Or an easy reset, when such unknowns as plugging too many appliances into an outlet takes place.

Unlike too many appliances, the current draw of a specific radio, motor, light, etc. should be a known value to start with.

My RV is equiped with about 2/3rds fuses, and that last remaining third as CBs. The reasoning behind the CB's is usually to kill the circuit, such as runaway flap motor & trim servos.

And I've done both high voltage & low voltage wiring for the last 35 years. The fuse blocks just make good sense.

 

[gmcjetpilot]

How they work

Ummm, George... with all due respect, could you please expain to us how a CB works and what exactly happens inside the CB to measure the amount of current flowing through it?

I think an understanding of how it operates would shed some light on it's capabilities, strengths and shortcomings. CJ

I am not sure what you mean by all due respect?

As stated, CB's are thermal devices. Let me see if I can explain how they work? (By the way not all CBs are thermal, there are many types like electro-magnetic and electronic.)

I am not an electrical engineer (mechanical by degree) and never took one apart or know design details. However I did call Texas instrument and talked to an engineer about a question I had about max interrupt voltage capacity a few months ago.

Before I get into it here is a cool resource I googled.

http://science.howstuffworks.com/circuit-breaker.htm

(THERE ARE THREE PAGES: The second page is very cool w/ pictures and video of a CB tripping)


From my nice discussion with TI engineering the basics of the TI CB's is they use a bimetallic strip that responds to heat. Because the two metals bonded together have differnt thermal expansion rates, it bends one way when heated and another when cooled. The heat comes from flow of current. This b-metallic strip works with spring, mechanisms and contacts. The exact design I am not privy to, but it is similar to the example above. The brochure describes its function and other specs.

http://www.sensata.com/products/controls/acb-2tc.htm
(scroll down to the time, current, trip curves)

In the case of the King Air the fault was soft, not hard and allowed the CB to reset. As you know as wires heat their resistance change, so the current draw on initial power up of the window heat and after heated is differnt.


The spacific question of HOW the measure current? I am fairly sure it is a function of current and temp in the bimetallic strip. They do lots of testing and design work to "Callabrate" the CB for different current ratings. They no doubt change spring rates, mechanism geometry and the bimatallic strip for different currents.

The deflections of the bimetallic strip by the way is very small. They have mechanisms that actually does the trip or latching. Think of it as a hair trigger mouse trap. If the cheese gets to hot, SNAP!

How does the fault or trip free circuit work? I mentioned that a CB with this feature will not allow a reset if a fault still exist. This is what the specs say:

"Trip?Free
The complete line of TC series circuit breakers is trip?free. The circuit breaker cannot be maintained closed during an overload even with the actuator button held closed. "

What I think (I am guessing a lot) is as you push the button the bi-metallic strip gets the current first and remains deflected, NOT allowing the latching mechanism to close the main contacts. I could check one of my CB's for this function, but let's jsut leave it at I will not force a hold a CB closed.

That is all I know and that is my story and I am sticking to it.


The other refrence I found is from Wikipedia, but it's more about high current and very advanced circuit breakers of all kinds.
http://en.wikipedia.org/wiki/Circuit_breaker#Common_trip_breakers

I hope that helps. Again as I said they are "Thermal Devices"; they work on heating of the bimetallic strip. The metals in top end CB's I recall are fairly exotic, like tungsten and nickle? Also the TI circuit breakers are temperature compensated. How they do that I don't know exactly.

 

[tonyjohnson]

fuses with LED indicators

The observation was made that when a fuse blows, unlike a circuit breaker, you cannot tell by looking at it. I think that I have seen fuses that have an LED indicator that lights when the fuse blows.

Does anyone know anything about those? That would remove one of the downsides of a fuse. I know, you are thinking "how would I know if a fuse has blown without being able to see the fuse". A very fair question.

I am planning to build a drop down fuse board that would attach the underside of the panel and allow the fuses to be visible when it drops. My purpose is not to make it accessible for in flight fuse changes, although it would do that...but rather to make it much easier to access the fuse panel than going through the baggage compartment.

I recently saw an 8A which had a fuseblock mounted on the bulkhead forward of the panel, which was visible from the cockpit.

http://img141.imageshack.us/img141/5236/fuseblocksdown4fo.jpg


I would like to claim credit for this great idea, but I stole it from another builders web site, I would give him credit but I don't remember where I got it.

 

[gmcjetpilot]

We agree to agree, again

You missed my point George. I'll take an electrical fire on the ground over one in flight any day.

No I get your point. I said that your refrence totally illustrates that resetting a CB is not a given or should always be done. I don't agree that it should never be considered.

However there was a sub-text in your comments, at least I got, that a fuse is better or safer. I just wanted to point out that the CB worked perfectly.

Also in the fuse v CB debate this King Air scenario really did not address a major PRO or CON of either. I guess the argument is the fuse takes the choice out of the pilots hands and keeps him or her from doing something dumb. OK

The rational for a "remote" fuse box, is you don't need to reset or even consider resetting a fuse in flight. As long as a blown fuse is not flight critical, I agree. See my 1st and 2nd post on this thread. I think you will see we agree.

However in many planes one CB might handle many circuits, which was common practice in the old days. 6 or 10 CB's handled the whole plane. In some RV's you see as many as +20 CB's or 30 fused protected circuits.

With several circuits on one CB an overloaded might pop it semi-normally. CB's do have some tolerance. Some pop sooner than later. This "ganging" issue of CB's can be avoided by using discrete CB's for each circuit, which is what I did. However this is more expensive and heavy. Thus the advantage of the fuse block, low cost. You can afford to make 20 or 30 separate fused circuits. I am down with that. Light, cheap and provides the basic function, wire protection. However to imply better safety or protection with a fuse is pushing it. I agree I would prefer discrete circuit protection than ganging of circuits on one CB or fuse. A little doubling up is OK.

Panel space and the convenience of easily killing the circuit (breaker).

Unlike too many appliances, the current draw of a specific radio, motor, light, etc. should be a known value to start with.

My RV is equipped with about 2/3rds fuses, and that last remaining third as CBs. The reasoning behind the CB's is usually to kill the circuit, such as runaway flap motor & trim servos.

And I've done both high voltage & low voltage wiring for the last 35 years. The fuse blocks just make good sense.

I don't disagree with you either. If you read my 1st or 2nd post, I say just what you did. Darn can't we just stop agreeing. geeee

Please don't make me out to be hardcore CB proponent. I personally like CB's, if that's OK with you all. I also think fuses are as safe, but not safer.

Panel space is not an issue for me. I also had bought a box full of high end surplus mini CB's for cheap, which are pretty cool. So I am using them. However if I had to buy new today, I guess it would cost about $360. Where two $35 fuse blocks and a handful of fuses sounds real good. Of course like you, I would still use a few CB's on several items.

I do like having the ability to "pull" the CB. I also like: indication of a trip, convenience of NOT having to carry fuses around and being able to easily kill an individual circuit's power for what ever the reason (maintenance or suspected problem).

One scenario is a smoking avionics that will not stop smoking despite turning the power off with the knob on the front. It does happen.

To me the number one plus of plastic fuse blocks and ATO / ATC fuse is lower weight, followed by cost. It's a given that both CB's and fuse's protect the wiring equally well, at least to me. The set or not to reset debate is up to the designer (the person building the plane).

I reject arguments that fuses are better or safer. I will admit a CB is way more complicated, but they should be very reliable. You have a better chance of plugging in a 20 amp fuse where you want a 2 amp fuse than a CB not tripping when needed. I can't tell you how many cars I have found with "WHAT YOU GOT" fuses stuck in the fuse block. Heck people once put pennies or foil in their home fuses way back when.

Despite the predictions of CB's going away for fuses, I think you will find it only be popular in experimental aircraft, especially ones built per "aeroelectric". Nothing wrong with that (please read my 1st and 2nd post), but I am glad to have an electrical buss I can see and touch so to speak. It may be overkill or not critical but it makes me happy. Besides an acre of CB's on the panel looks cool.

My phylosopy is use CB's or fuses, better both, what ever you want, but make sure your wiring is top notch. DON'T have a short in the first place is the best plan.

 

[Jekyll]

I still stand by my belief that it is better to know about a failure when it occurs, than to wait to find out. So, when a fuse blows, the status panel will tell you by changing color from green to red. The fact that there are other failure modes that are not detected is irrelevant. The CB is not going to tell you that a device has failed due to an open, either. Also, one of the purposes of the fuse status panel is to tell you before you go flying that all fuses are powered.

Tracy.

Tracy: Do you have a means of testing the status panel? Do you have real-time notification of a failure in your fuse status panel so you are guaranteed not to be seeing false negatives or vice-versa?

I'm being humorous here but, where does our desire for information surpass its effectiveness. Few of us have the resources to design and install BIT, be it IBIT, PBIT or CBIT (BIT=built-in-test). We rely on verifying the systems during preflight walkaround and pre/post-start proceedures (a good checklist should include verification of all systems needed for the intended flight).

Jekyll

 

[Jekyll]

A-bombs, CBs and fuses

Here's a meaningless thought: Circuit breakers are tested at the factory to see if they work properly. Are fuses?

Are a-bombs and hand grenades? No, test samples are taken out of production batches (well, I guess not really with the a-bombs though).

Seriously, The only data the factory test of a CB provides you is that it worked once at the factory. This feeds the simplicity arguement in that mechanical assemblies have multiple failure modes. The simple fuse has pretty much 1 failure mode - burning up as a result of performing its intended function.

Jekyll

 

[Jekyll]

Fuse block access

Tony:

Good shot of how to make a buse block accessable. I've got a friend building a 7A that put a hinged door on the passenger side of the panel to hold his fuse block. Simply open the door on the panel and you have your fuses at hand.

Look in the catelogs of the major electrical houses (Digikey, All-Electronics, etc.) and you can find rear-wired fuse bocks with quick release covers that are intended for surface mounting such as on the front of a panel, console or other accessable location. These type of mountings will allow easy access to your fuses during flight. I don't think Stein, B&C or the other places we traditionally buy from carry these type of blocks.

Fuse blocks can be installed in ways that allow access during flight. A fuse block diagram can (and should) be part of your pocket checklist (you do want a diagram with the plane at all times don't you?) so you will know which fuse to replace based upon which system just stopped working.

Jekyll

 

[Captain_John]

I am not sure what you mean by all due respect?

George, I deleted my intial post because I thought it could be taken the wrong way. I am glad it wasn't and from seeing your online personality, you seem to have pretty thick skin!



Now, I haven't read your most recent tirade/novel, but will skim through it later today. How DO you find the time to write all that stuff! My ADD/ADHD won't allow me to sit that long and I am not sure people want to read such long pieces. That is why I write short.

Basically, I think an understanding of the feature benefit list will be of help to the lurkers on this thread. That is, if they are not already lost by the fact that things are now dwelling in the minutia of thermal/electrical dynamics.

I will chime back in later after my reading assignment is done!

CJ

 

[tonyjohnson]

fuse blocks

Jekyll,

Good idea about the fuseblock for visibile mounting. I found this one at a Marine supply site:

http://www.bluesea.com/product.asp?Product_Id=30995&d_Id=7463&l1=7463&l2=




It has a clear cover and storage for the spare fuses that we are required to have aboard. Also labels on the cover to identify the circuit. I don't know if I would have the space on the bulkhead for the two fuseblocks that I would need, one for main and the other for avionics.

Therefore, the drop down idea is probably still the way to go for me, but others may want to use a fuseblock like the one above which could be mounted in view of the pilot.