More than one protective device in a circuit?

[alpinelakespilot2000]

Quick question that will say a lot about what I don't know about electricity. If anyone would be willing to humor me, I'd appreciate it...

I'm generally following Bob Nuckoll's recommendations for wiring. I'm planning to use fuses to protect most everything. The fuse blocks will be located right under and behind the passenger side of the panel and accessible by a pull-down tray. However, I don't generally plan to mess with them in flight. For those items that I do want to be able to disable in flight (flaps, boost pump, autopilot, and perhaps alt. field), I intend to use a few circuit breakers.

One of the advantages of going the Nuckolls route it seems, is that the fuse blocks also serve as the bus bars. However, if I'm going to use breakers on at least those four circuits that I mention, it seems like I'd have to create another bus bar or two.

To avoid having to do this, is there any reason I couldn't have each of these four items draw power through the fuseblock (with an appropriate fuse) and then run it to the circuit breaker that I intend to put right by the switch for the item that I want to be able to take offline? This would add one more connection in a circuit with a corresponding failure point and greater resistance, but would it pose a significant problem in my intended application? It seems like this would be a really clean installation otherwise.

Alternatively, is it easier just to add another bus bar (or two) over on the pilot side?

Again, I have a lot to learn here, so please be nice in your responses .

Thanks.

 

[Dave Cole]

A fuse and a switch is enough

For the system you are describing, I would use the fuse for circuit protection and use the switch to take the item offline. I don't see where the addition of a circuit breaker adds any functionality if you already have a fuse and a switch.

 

[Ironflight]

I know I'm not addressing your question Steve, and for that I appologize, but I am wondering what you're protecting for if you are going to put the breaker right next to the switch? I assume that the only thing between them will be a very short length of wire, so if you are worried about the item failing on, why not just flip the switch off? I guess you are protectign for a switch failure....but given the reliability of quality switches, you might be making the overall system less reliable by adding the extra component. Just something to think about if you hadn't already.

Paul

 

[alpinelakespilot2000]

Thanks for your responses, Paul and Dave. I think I see your point. I'm trying to express a couple different concerns:

Basically, yes, one of my concerns was a failed switch on something that could run away from me (flaps, boost pump, or electric primer for example). I agree that probably the switches are quite reliable and that my concern is generally unwarranted. At the same time, should I be concerned about a "worst-case" scenario? If so, would that worst-case scenario be that the switch might fail in the on position requiring the pilot to (inflight) reach down over and under the passenger side of the panel to pull a fuse to stop the flap motor? Am I comfortable with accepting that worst-case?

The autopilot, on the other hand, has no switch (except perhaps a power on-off button on the face--I know the Trio does; I'm not sure about the lower-end Tru-Traks). Should I be comfortable having the autopilot on a fuse and just relying on the autopilot's built-in power switch to shut it off if it decides to run away? (Again, I'm probably not giving enough credit to the quality of the a/p built-in switch. I recognize that.)

Trying to think about all of the potential failure modes and then determine the ones that really matter and how to address them is kind of fun, but it sure makes my head hurt!

Thanks again.

 

[Jim P]

TT AutoPilots

If you take a look at the DigiFlight II documentation, it does reference an AP master switch which controls the AP controller, and both pitch and roll servos.

Not too sure about the single axis controllers though.

Jim

 

[kentb]

Hello Steve,

I too tried to follow Bob's suggestions. I only ended up with two CB in my electrical system. One for the Alt. field coil and the other for the stdby alt.
Everything else is fused. There were some items in my panel that didn't have a way to turn off so I put in a switch for them.
The things that I added switches for are:
AFS 2500 engine monitor
BMA EFIS Sport
BMA EFIS lite

I didn't add switch to turn off trim motors (your can fly with the trim in the wrong position for awhile).
I also didn't add switch for the flaps. If it fails most likely it would fail to turn on the flap motor. If the flap switch were to fail while I was commanding a flap change and the motor continue to run, then I must be either taking off and retracting the flaps or on downwind and starting to deploy the flaps. Either of these case I can just go ahead and land.

Kent

 

[Jekyll]

Steve:

You and I seem to be pacing each other. I just wired up my panel last week and faced the same situation. In my case (Z-13/8 with fuses), I decided I wanted CBs for my trims. They are the only CBs I have besides my SD-8 and main alt field CBs.

You have 2 choices, run the power feed directly off the stud from your bus and use the CBs by themselves or, run from a fuse to the CBs. My E-bus is about 20 inches from my trim CBs which is too far for an unprotected wire so I doubled the fuse size (to a 2 amp) and fed both trim CBs from there (1amp CB for each). I ran 2 wires off the fuse, one to each CB but after I tied up all the harnesses I had one of those epiphany moments and realized I could have gotten by with 1 wire that daisey chained the 2 CBs.

Double the protection is no problem - the belt and suspenders approach. Just be sure you don't have an unprotected wire. I guess if you have a shortage of fuse holes, you could come off the fuse block stud with a fuseable link and then to your CB. I just might go back and do this to keep a fuse slot open for future expansion. I've been forced to double several fuses for low power, non critical items.

Jekyll

Edit: I personnaly didn't want switches in place of CBs for trim. I wanted the immediate recognition of my 2 CBs within the switch row. No need to search for a switch when the adrenalin spikes, just grab the CBs and pull. After I calm down, I'll push the unaffected CB back in. The trim may start out minor but it can grow guickly.

 

[gmcjetpilot]

You can do that, but..... and fusible links

You can piggy back off the plastic car automotive ATC fuse block and put a large fuse in to drive your 4 CB protected circuits, however you have limits to what each fuse position can handle in the plastic fuse block. You have to look the specs up. BUT if you loose that one fuse you loose all your CB protected items. However its unlikely you would blow that one big fuse if its rating is over all of the circuits CB protection ratings.

IF.....you are not going to have all 4 CB's protected circuits on one fuse and the capacity is more than enough in ONE fuse slot, say 25 amps, and all the circuits add up to less than that, say 20 amps max (both in operation OR all CB's trip ratings), than sure you can do it. That is if you are willing to possibly loose all 4 items with one fuse. It is unlikely all 4 CB protected items will short at the same time and blow the fuse however.

Besides if you POP a CB or fuse do you want to reset. Well may be and that is why you have the concept you mention, resettable CB's. To put a fuse in the mix does distracted a little from the goal but not totally. If the FUSE slot can handle more than enough to drive the 4 CB's with a good margin (15-20% at least), than its an option. CB's have some tolerance. The direct approach mentioned above is probably a little better. However none of the items are show stoppers really.

It would be best to just split off the HOT wire from the battery, one to the plastic fuse block and one jumper to your CB's. A simple brass or copper strap tie between the CB's makes a dandy buss.


There is that dirty little secret that no one talks about. Most people run a HOT wire from the battery to their bus in the plane. That wire is NOT fuse or CB protected run of wire. If you do fuse it, it is the SINGLE line or failure point that can take the whole system down, so most people leave it unprotected. However if that ever grounds it will smoke and burn. If you have a big wire, unprotected, make sure it is not near other things (that will burn). Look at Bob's Z drawings, there is no protection from BATTERY to MAIN BUSS. The thinking is this is a well protected heavy wire is pretty safe, and you can always turn the battery master off if you see or smell smoke (and you will).

There is a solution, Bob N talks about I think, fusible links. I would consider a fusible link for any HOT wires you run around to other buses, like from the plastic fuse block to the CB's.

For those who don't know what a fusible link is, its a small section of lower gage wire in a run of large gage wire, wrapped in some fire protective sleeve (fiberglass), so if a BIG wire grounds it will burn the fusible link section out first and not burn anything. Even with its small gage, it does not cause too much voltage drop because of its short length. The key is that sacrificial length of wire is in a safe spot and wrapped is fire proof material (like a fiberglass material).

You could go with a BIG fuse for your HOT battery buss wire, but since the shorting of the hot wire is thought to be a very unlikely event, fusible links are used (a sort of last ditch just in case fail safe if the worst happens). They also tend to not blow unless there is a dead short.

Cars have fusible links. You can buy them pre-made at auto stores but be sure you cover them in a fiberglass sleeve. I probably would not use a car fusible link since they are likely made with material that can give off hazardous fumes. However the amount of fumes are probably low. It would certainly be better than a whole length of HOT burning 6 awg tefl wire. Bob talks about using 20 or 22 awg aviation tefl wire for fusible links on his web site. Here's his how to: http://www.aeroelectric.com/articles/fuselink/fuselink.html

 

[jcoloccia]

IF.....you are not going to have all 4 CB's protected circuits on one fuse and the capacity is more than enough in ONE fuse slot, say 25 amps, and all the circuits add up to less than that, say 20 amps max (both in operation OR all CB's trip ratings), than sure you can do it. That is if you are willing to possibly loose all 4 items with one fuse. It is unlikely all 4 CB protected items will short at the same time and blow the fuse however.

George made the key point here. If you tap it off one fuse, you can loose a lot when that fuse goes pop. Theoretically you could make it so that the downstream breaker always pops before the fuse but in practice it's difficult to gaurantee that unless the fuse is WAY bigger than the breakers. For example, what will pop first given a 100 amp short circuit current - a 60 amp fuse or a 15 amp breaker? Probably the breaker but I'm not sure. I could figure it out, probably, but I wouldn't even bother. It's one of these weird things that you don't see very often so I don't have good instincts for it.

THe only thing I'd add is if the battery's forward of the firewall, I wouldn't run and additional wires through the firewall just for this. You could just run the wire from the battery to the fuse block, and then branch off and run it to the bus bar for your circuit breakers. Maybe someone else will point out a problem doing this but I can't think of one off the top of my head.

I understand why you want the circuit breakers instead of a switch. It's because a switch looks like it's meant to go "on" and "off" and it's weird to leave switches in the "on" state (AP, flap motor etc, you'd always just leave "on" unless you had a problem). Seems like a small thing but that would bother me too.

 

[alpinelakespilot2000]

You can piggy back off the plastic car automotive ATC fuse block and put a large fuse in to drive your 4 CB protected circuits, however you have limits to what each fuse position can handle in the plastic fuse block. You have to look the specs up. BUT if you loose that one fuse you loose all your CB protected items. However its unlikely you would blow that one big fuse if its rating is over all of the circuits CB protection ratings.

IF.....you are not going to have all 4 CB's protected circuits on one fuse and the capacity is more than enough in ONE fuse slot, say 25 amps, and all the circuits add up to less than that, say 20 amps max (both in operation OR all CB's trip ratings), than sure you can do it. That is if you are willing to possibly loose all 4 items with one fuse. It is unlikely all 4 CB protected items will short at the same time and blow the fuse however.

Thanks George and John. If I wire it the way I'm currently thinking, each of the four circuit breaker protected items would come off their own independent fuse assuming I have the fuse slots available. Thus, I shouldn't have to worry about one blown fuse taking out four items.

One other thing I'm trying to avoid, again, is another bus bar just for 3-4 circuit breakers that will be scattered around the pilot side of the panel. (I'm hoping to locate whatever breakers I use right next to the item I'm trying to take offline so that it is completely intuitive in time of emergency. Sort of along the lines of thinking Jekyll expressed at the end of his post. If they were all going to be in a neat little row, adding another bus bar wouldn't be a big deal.)

Thanks again though to everyone. I'm learning a lot and revising my thinking as I go. I'm still trying to decide whether I even need something other than a very reliable switch to disable things like the flaps, boost pump, primer, autopilot etc. Like Paul and Dave mention at the top of this thread, if the switches rarely fail is it worth the hassle of giving myself the ability to disable those items in flight?