Circuit breakers

[Rhino]

I don't think that's what that means (although having each servo on its own fuse is, IMO, a good idea). It means that a failure in one component cannot cause a failure in another. The terminology in aerospace is Fault Containment Region (FCR), and the boundaries of an FCR are determined (partly) via tools such as FMEAs.

You are correct. I'm also a bit curious where that came from. According to the Code of Federal Regulations, there is no Part 23 Sec 23.1329.

Federal Register :: Request Access

www.ecfr.gov

I'm guessing the "parts list" in his link has something to do with that, but I'm not sure. I know I can't find that verbiage anywhere else when I search for it. A failure in one component in a system causing a failure of another component is not the same as simply affecting the operation of another component. Regardless, he does refer to what is often called a cascade failure, and it doesn't call for multiple breakers, fuses, etc. A single one of any of those components could fulfill that need. So could physical overrides and shear pins.

We should probably also note that an autopilot disconnect switch is not a true physical disconnect at all. It provides no true protection against runaway conditions in autopilot or servos. It is merely a momentary switch that signals an autopilot that the pilot wants it to disengage. It does not physically disconnect the autopilot. A runaway autopilot may simply ignore that signal, and runaway servos would not be affected by it at all. Breakers, fuses and physical disconnect switches on the servos provide the ultimate runaway protection, because they prevent a runaway condition from affecting the flight controls no matter where the runway condition originated.

 

[Marc_J._Zeitlin]

I'm with Bob Nuckolls re using fuses instead of breakers.

I would add space for more switches and/or breakers than you think you will need but for me fuses are in Bussmann 15600 fuse panels behind the instrument panel.

Love the 15600 series fuse panels - that's all I've used for any of my electrical installs. Simple, easy to install, reliable.

• The AP servos get a 5A pullable breaker that feeds them both. The way I read the IM, Dynon wants a pullable breaker or a fuse and switch even though there is a disconnect button (AP Disengage/CWS). I don't see it now but I read somewhere that if Skyview is booted up while the servos are unpowered there will be a non-resettable error so I used a pullable breaker so there is no switch to remember to turn on.
  
  
• The SV-AP-PANEL trim motor section gets a 5A fuse. A smaller fuse could be used but I reduce part numbers by not using fuses smaller than 5A.
  
  
• I used a 7-1/2A fuse for the HDX because when I add all the loads described in Table 3 of the IM I get 4-1/2A. Divide this by 0.75 nuisance blowing factor and I get 6A so I go with the next available size up, 7-1/2A.
  
  
• The Dynon radio gets a 5A fuse that feeds the transceiver and the control head.
  
  
• The SV-XPNDR-261 and SV-ADSB-472 share a 5A fuse.

So the fuse has to be large enough so that it will allow for as much current as the device will ever pull, certainly, and having a 1.33 safety factor is fine. But you don't say what wire gauge you're using for for any of these devices, and the fuse is there to protect the wire, not to protect the device. If you never use anything smaller than 22 GA wire (and I don't, if I can help it), then a 5A fuse is just barely acceptable, per table 11-9 of AC43.13-1B, but only if you're using 150C rated wire.

The issue sometimes is that purchased components (Ray Allen stick grips, for example) use 24 GA wire, in which case you really don't want to use anything larger than a 3A fuse. And Cthulhu forbid that some fool vendor uses 26 GA wire for something...

So, there are more factors than you've discussed here - maybe you've accounted for all of them, and if so, that's great. But you didn't indicate that. Also, fuse sizes (which then drive wire gauge requirements, and vice versa) are clearly indicated for each component in the Dynon Skyview Installation Manual. You're always welcome to be more conservative, of course, at the expense of weight.

 

[peter9545]

FWIW

I installed a panel mounted toggle switch which can disconnect power to the servos. I thought in the event of an emergency flicking the switch would be preferable to pulling breakers or switching things off in the VPX. I labelled it 'AP master'. Usually it just stays in the on position.

 

[mesae]

Go with whatever you feel comfortable with, after all , that's what Experimental is about. Me, I tried to follow FAR 23 on this subject because that's the sorta guy I am.

https://www.risingup.com/fars/info/part23-1329-FAR.shtml

I think this sub clause tells me to have a breaker, a fuse or some wire cutters as last resort ;P

(g) There must be protection against adverse interaction of integrated components, resulting from a malfunction.

ECBs do that.. They provide the same basic overcurrent protection that physical CBs and fuses do.

 

[Marc_J._Zeitlin]

I installed a panel mounted toggle switch which can disconnect power to the servos. I thought in the event of an emergency flicking the switch would be preferable to pulling breakers or switching things off in the VPX. I labelled it 'AP master'. Usually it just stays in the on position.

I have the same, but it stays in the "OFF" position for both takeoff and landing. The last thing one needs during either phase of flight close to the ground is a servo grabbing an axis and doing something unexpected, or just fighting the pilot. Unlikely? Sure - but what's the cost of doing so? Zero. It's not like I'm going to be able to turn the A/P on and think it's doing something when there's no power to the servos, so at worst, turning on the A/P takes another 1.3 seconds to turn on the power to the servos, then push the A/P button.

 

[John Bright]

... If you never use anything smaller than 22 GA wire... then a 5A fuse is just barely acceptable, per table 11-9 of AC43.13-1B, but only if you're using 150C rated wire.

... The issue sometimes is that purchased components (Ray Allen stick grips, for example) use 24 GA wire, in which case you really don't want to use anything larger than a 3A fuse. And Cthulhu forbid that some fool vendor uses 26 GA wire for something...

Re my use of 5A fuses minimum to reduce part numbers. I neglected to mention I’m speaking of protection for feeders that are usually 20 awg and never smaller than 22 awg. I prefer to use 20 awg but oftentimes purchased harnesses use 20 awg.

Let's look the case of a 26 awg wire in a purchased device:

• The feeder from a 5A fuse is 22 or 20 awg with the 26 awg downstream of that.
• 26 awg is conventionally fused at 2A and melts at 21A.
• 26 awg could conventionally be protected by a 30 awg fuselink which melts at 10A
• A 5A fuse melts at less than 10A so it opens with less energy than a 30 awg fuselink.
• Something one wouldn’t do is protect the 22 awg feeder with a 26 awg fuselink without a fuse at the other end of the feeder to protect the downstream 26 awg.

I note an example from residential 120V AC wiring:

• A 20A CB is connected to a 12 awg feeder to multiple 15A receptacles.
• A table lamp is plugged into one of the receptacles with 18 awg lamp cord, the smallest allowed by code.
• 18 awg is rated 10A and melts at 82A.
• If the 18 awg lamp cord experiences a short it will open the 20A breaker.

To me the saying "the fuse protects the wire, not the device" is a simple rule of thumb...

• ... and will keep one out of trouble in every case if voltage drop in long runs is also considered.
• It seems like a good rule if one cannot control others adding more loads at the other end of the feeder.
• The smallest feeder I use is 22 awg 150C Tefzel M22759/16 or 150C Tefzel M27500 TG which is conventionally fused at 5A. Downstream of the 22 awg feeder a hypothetical 26 awg wire can short to ground and melt a 5A fuse.
• What I might be doing is putting the guts of a device at risk of greater damage than would otherwise be the case if an internal failure occurs and the manufacturer spec'd a 2A fuse and I use a 5A fuse.

I like rules but I'm not 100% by the book because the book is written in some ways CYA. I do read "the book" so to speak and have studied AC 43.13 tables 11-3, 11-6, and 11-9.

 

[Stevec]

I have used a Tyco W31 switch circuit breaker which combines both functions. Seems to do the job ok.

 

[airguy]

I have used a Tyco W31 switch circuit breaker which combines both functions. Seems to do the job ok.

They will - for a while - but they are prone to failure due to the internal copper braid current carrier flexing on each actuation. I've lost 2 in just over 1600 hours in my airplane. I'm not saying don't use them - I'm saying don't use them on flight-critical circuits. Anything you can't live without (literally) needs separate dedicated switches and breakers, IMHO.

 

[Stevec]

I leave the autopilot power turned on all the time so I guess that will reduce the chance of an internal breakage. I have never flown an aircraft yet where you have to completely power off the autopilot for any particular phase of flight. I am happy to follow Dynons advice , installation manual Revision AJ page 8-1, and leave power connected to the autopilot system whenever the Skyview is powered on.

 

[RV8JD]

I also had two Tyco W31 5 amp switch breakers fail, the one that originally failed and the replacement one failed shortly thereafter. I then bit the bullet and purchased a Klixon 5 amp switch breaker to replace it. No problem since, and that was awhile ago.

There is an FAA SAIB on the W31 Tyco/ Potter Brumfield switch breakers: SAIB CE-13-22:

Dynamic Regulatory System

drs.faa.gov