# Klixon circuit breakers as switches

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
My two cents; first, use a CB or fuse sized to protect the device, not the wire. Many times I've heard that the CB should be sized to protect the wire under maximum load, while the device (or load) on that wire is drawing only a fraction of the current that the wire can manage. That makes no sense to me... if you protect the device, you've by definition protected the wire...
Let's start by knowing that we have been putting electrical systems in airplanes for about a century now. Power generation for lighting, avionics, starters, fans, and so on.

A lot of really smart people have worked on airplanes in that century. Lots and lots of innovation in every area of flying machines.

Circuit protection has been thought about a bunch over that century by these smart people.

Do any of us really think that in all that effort over all that time by all those smart people, that protecting the device with a fuse instead of protecting the wiring with a fuse might have been overlooked completely? I suspect not. With the choices on circuit protection boiled down to: Protect the device from burning; Protect the wiring from burning; or a Combination, I suspect that it has been thought through.

I suspect that a more likely event (orders of magnitude more likely) is the scheme of sizing wire for adequately low line loss and low temperature build up, then sizing the circuit protection to keep us from burning those wires came out of a lot of painfully acquired experience. And there is evidence. The regs have bits and pieces scattered about that drives us to the standard approach. Yeah, the regs do not apply to us, but they are still a pretty good idea, as they have largely been written in the blood of victims. A fire in the cockpit is a nightmare I would rather leave out of my flying experience.

Building avionics to live despite the slings and arrows of life in an airplane has been mandated for a while now. Document for certified airplane avionics is DO-160 and any successors, then there are Military specs as well. Our experimental systems do not rigorously have to meet these specs, but look through Dynon and GRT's web sites, and you will find that they do. These documents specify minimum spikes, surges, and noise that each type of device must with stand without failures and allow immediate return to functionality when the event ceases. It also addresses faults and failure trapping inside the device. The levels spec'd are pretty impressive. For 14 volt systems, spikes of 300 volts for 100 microseconds, surges of 40 volts for 100 milliseconds, 20 volts for 1 second, it goes on in the areas of noise and faults too. The spec is met by internal design of devices through a combination of sturdiness, filters, and other protective schemes. If the hardware is good per the specs, you are unlikely to size your circuit protection device adequately to protect the device. And if the device fails in a way that would cause it to draw excess current for the ship, you are not going to keep it from being broken with a fuse or circuit breaker, but you can keep it from lighting your airplane on fire.

Do I see harm in the approach posited above? Well, one might succeed in using lower amp rated fuses or circuit breakers and have a workable airplane. I do not think that one will have any increased risk of fires, but you might need to run a lot more wires and circuit protection to protect each device at its level, with all of its attendant complications to design, build, install, use, and service. The increase in wire footage and connections does carry some risk of increased failure exposure with decreased dispatch rates and increased repair work. I suspect that in developing your scheme to protect the devices, you wlll get nuisance interruptions and have to up size your circuit protection. You may even find that you approach the nominal scheme as you do so.

In short, this proposed scheme is unlikely to protect the devices it is intended to protect and is likely to make for a poorer airplane.

Please read the AeroElectric Connection. At least then you are going onto whatever path you choose with conscious decisions.

Billski

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#### dtnelson

##### Active Member
Help me understand: protect the device from what? You might minimize the amount of smoke that leaks out following a failure, but how can you prevent a failure in the device by using an external breaker?

BJC
Great question - the device has failed, which trips the breaker, so how am I protecting the device?

The answer is the same reason that every fused (or breaker'ed) device you've ever owned has a fuse/CB rated specifically for that device - it's to prevent further damage, destruction, smoke or fire by stopping the current flow as soon as it's exceeded whatever the device normally needs.

Thanks,
Dave

#### dtnelson

##### Active Member
Let's start by knowing that we have been putting electrical systems in airplanes for about a century now. Power generation for lighting, avionics, starters, fans, and so on.

A lot of really smart people have worked on airplanes in that century. Lots and lots of innovation in every area of flying machines.

Circuit protection has been thought about a bunch over that century by these smart people.

Do any of us really think that in all that effort over all that time by all those smart people, that protecting the device with a fuse instead of protecting the wiring with a fuse might have been overlooked completely? I suspect not. With the choices on circuit protection boiled down to: Protect the device from burning; Protect the wiring from burning; or a Combination, I suspect that it has been thought through.

I suspect that a more likely event (orders of magnitude more likely) is the scheme of sizing wire for adequately low line loss and low temperature build up, then sizing the circuit protection to keep us from burning those wires came out of a lot of painfully acquired experience. And there is evidence. The regs have bits and pieces scattered about that drives us to the standard approach. Yeah, the regs do not apply to us, but they are still a pretty good idea, as they have largely been written in the blood of victims. A fire in the cockpit is a nightmare I would rather leave out of my flying experience.

Building avionics to live despite the slings and arrows of life in an airplane has been mandated for a while now. Document for certified airplane avionics is DO-160 and any successors, then there are Military specs as well. Our experimental systems do not rigorously have to meet these specs, but look through Dynon and GRT's web sites, and you will find that they do. These documents specify minimum spikes, surges, noise, and faults that each type of device must with stand without failures and allow immediate return to functionality when the event ceases. The levels spec'd are pretty impressive. For 14 volt systems, spikes of 300 volts for 100 microseconds, surges of 40 volts for 100 milliseconds, 20 volts for 1 second, it goes on in the areas of noise and faults too. The spec is met by internal design of devices through a combination of sturdiness, filters, and other protective schemes. If the hardware is good per the specs, you are unlikely to size your circuit protection device adequately to protect the device. And if the device fails in a way that would cause it to draw excess current for the ship, you are not going to keep it from being broken with a fuse or circuit breaker, but you can keep it from lighting your airplane on fire.

Do I see harm in the approach posited above? Well, one might succeed in using lower amp rated fuses or circuit breakers and have a workable airplane. I do not think that one will have any increased risk of fires, but you might need to run a lot more wires and circuit protection to protect each device at its level, with all of its attendant complications to design, build, install, use, and service. The increase in wire footage and connections does carry some risk of increased failure exposure with decreased dispatch rates and increased repair work. I suspect that in developing your scheme to protect the devices, you wlll get nuisance interruptions and have to up size your circuit protection. You may even find that you approach the nominal scheme as you do so.

In short, this proposed scheme is unlikely to protect the devices it is intended to protect and is likely to make for a poorer airplane.

Please read the AeroElectric Connection. At least then you are going onto whatever path you choose with conscious decisions.

Billski
Hi Billski - and thanks for a well thought out commentary. You are right that "Circuit protection has been thought about a bunch over that century by these smart people.".

Here's what the installation manual for my Garmin GTX-650 says:

2.4.7.1 Circuit Protection

Circuit protection devices for the GTN and GMA 35 must be push-pull manually resettable circuit breakers (e.g. Klixon 7274 or 7277 Series circuit breakers). Refer to Figure E-4 for GTN circuit breaker ratings or Figure F-2 for GMA 35 circuit breaker ratings. The circuit breakers must be labeled as specified in the interconnect diagrams in Appendix E and Appendix F and the circuit breakers must be readily accessible to the pilot.

A single circuit breaker must be dedicated to the GTN Main and NAV power inputs as shown in Figure E-4. A single circuit breaker must also be dedicated to the COM input for the GTN 635/650/750. See Figure E-4 for more information. Do not combine more than one unit on the same circuit breaker.

2.4.7.2 Power Distribution – Single GTN, Aircraft Weight Less than 6000 Pounds

When one GTN is installed in aircraft with a maximum certified gross takeoff weight of less than 6000 pounds, the GTN should be connected to the avionics bus. The NAV/GPS and COM circuit breakers (Reference Figure E-4) must be connected to the same avionics bus. When the GTN is the second NAV/COM unit being installed in the aircraft, the GTN should be connected to the avionics bus. The NAV/GPS and COM circuit breakers (Reference Figure E-4) must be connected to the same avionics bus. The GTN and other NAV/COM must be grounded at separate ground terminal/stud locations on the aircraft. The power and ground wiring for the GTN should be routed separately from the power and ground wiring for the other NAV/COM. This method, shown in Figure 2-11, will maximize system redundancy if the ground connection for one radio fails.

The CB/fuse requirements for the device (per E-4) are 7.5A (for the Nav side) and 10A (for the Comm side).

Going from memory, I think I wired both with either 18 or 16 gauge wire. I for sure used the specified CB ratings... not what the wire could handle.

I'm aware of Bob Nuckoll's book(s), and the guy clearly has great experience; far more than I (I am an EE, but my background is in microelectronics & IC design). There is only one reason I can think of that Bob makes his recommendation; and that is that down the line, if someone adds another load onto an existing wire, the total load can overload the wire. And that does happen; in particular in commercial applications where the panel changes over time.

Finally, for point of reference, this is (at least) the 5th airplane I've wired. All have been perfectly reliable. I've got over 1600 hours now in my Velocity without any fires, smoke, or reliability issues (other than the switch CB issues I mentioned before).

Peace - and thanks again for a great discussion!
Dave

#### bmcj

##### Well-Known Member
HBA Supporter
A fire in the cockpit is a nightmare I would rather leave out of my flying experience.
Been there, done that.

#### rv7charlie

##### Well-Known Member
As Billski and others have said, CBs & fuses protect wire; not devices. "Circuit Breaker"..... Voltage drop due to resistance almost always becomes critical *long* before the wire is in any danger from heat. Wire is sized for current, and voltage drop due to length, and protection is sized to protect the wire. EX: look in your home's CB panel and try to find the 0.1A breaker that protects your cellphone charger wall wart. On the wild assumption that you found it, did it magically move to the kitchen circuit when you moved the wall wart from the bedroom to the kitchen? What about that 1500 watt coffee pot plugged into the same outlet?Similar issue with two or more co-dependent devices in the a/c; EX: virtually every a/c has all the strobes on one circuit. How do you protect a device (a single strobe) with circuit protection, without overloading that protection by having multiple strobes on the circuit? Every device should provide its own internal protection, period, full stop.

The reason for AC & DC ratings for contacts is because of arcing as the switch opens and resultant burning of the contacts. 120V (or 240, or 480) will make a much higher energy arc at a given current than 12 or 24 V. Offsetting that (somewhat) is the fact that the voltage 'goes to zero' twice per cycle, while DC never varies (we hope).

A switch designed for *120V DC* will have a much faster 'snap action' type opening time to minimize the duration of the arc, than a switch designed for the same current at 120 *AC*. The reason you often see a switch with both 120V AC & 12V DC ratings is the fact that 12V produces a *much* weaker arc than 120V. Virtually all dual rated switches will have a 120V AC current rating, and the 12/24V DC rating will be the same current. People a lot smarter and better educated than me have pointed out that even if the switch doesn't have a DC rating stamped on it, it's reasonable to assume that it will handle the same current at 12V DC that it will handle at 120V AC.

Back to the OP's specific question: It'll 'work', but I'd be uncomfortable about pullable CBs as switches in my a/c, if they're controlling really critical circuits with no backup. The cycle counts are 'mean time to failure' numbers; they don't mean yours won't fail a lot earlier.

Of course, I'd also be really leery about having *anything* that's really critical that didn't have a backup, that would force me to fiddle with it instead of flying the plane. Years ago, an airliner crashed because all 3 crew members got so enamored with a non-critical CB, all 3 forgot to fly the plane. Troubleshooting should happen on the ground. That's one upside to fuseblocks. If they're out of reach, you're not tempted to fiddle with them in flight. The FAA rule on spare fuses is an anachronism; a holdover from a much less sophisticated era, where you needed two magnetos because one of them would frequently fail, the glass fuses used at that time were notoriously unreliable themselves (I was alive during the latter part of that era), and where a backup radio for a C-120 weighed 40 pounds instead of 3 pounds. (And where the FAA didn't have enough experience to realize that a 3-crew airliner could crash because all 3 crew members would get fixated on troubleshooting instead of flying.) And if you read the rule carefully, neither access nor spares are required if the circuit isn't critical to flight.

Charlie

#### rv7charlie

##### Well-Known Member
In response to the Garmin install docs: Remember, Garmin (and many other avionics mfgrs) are writing for two groups; their legal departments and the FAA (for type-certificated a/c). Many of the 'requirement's are just common sense, to make everything more reliable and to make it coexist well with other avionics. But be assured that a 10A circuit breaker will not stop a fire within the case of a Garmin radio. That number is to ensure that the circuit has *enough* juice to operate; not to protect the radio.

#### Marc Zeitlin

##### Exalted Grand Poobah
<A bunch of really good stuff>...
That's one upside to fuseblocks. If they're out of reach, you're not tempted to fiddle with them in flight.
Exactly. That's why mine are under the seats. Couldn't screw with them in flight even if I wanted to.

The FAA rule on spare fuses is an anachronism...
Agreed, although I keep spare fuses in my plane because if/when one blows, I want to be able to troubleshoot/replace when I land at my leisure because no single circuit is critical to the safety of flight.

And if you read the rule carefully, neither access nor spares are required if the circuit isn't critical to flight.
Bingo.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
I keep spare fuses in my plane because if/when one blows, I want to be able to troubleshoot/replace
If they are the automotive blade style you might consider adding a low amp CB to your flight kit. They make a really nice diagnostic tool for those times you aren't 100% sure you have isolated the problem. Once the CB no longer trips swap in the proper fuse.

#### rv7charlie

##### Well-Known Member
Good tip; certainly worth keeping one on hand even if it's for home use. Simpler than dragging out a variable voltage/variable current power supply (and less expensive, if you don't already own the power supply).

#### 12notes

##### Well-Known Member
Log Member
Aircraft Spruce has them too:

for about $30 -$50 each, depending - not $200. I used to have one of these in my panel to turn on the Nuckolls design "Emergency Bus". Always worked fine, and with a life of 6K - 10K cycles, would have more than lasted the lifetime of the plane. Thanks, this is what I'm looking for. When I searched, all I saw were the Klixon brand of this type of circuit breaker, which are all over$200 each.

#### 12notes

##### Well-Known Member
Log Member
My personal risk assessment is a little different. If I'm landing at an isolated grass strip and the radio goes, then, no, I wouldn't reset the breaker. However, if I'm in the pattern at an uncontrolled airport with 7 other planes, one of which is a student pilot on a right pattern instead of left like everyone else, and you have a Cirrus that's been silent since calling a 10 mile final to a different runway than everyone else is using 4 minutes ago (actual situation I've been in), then if the radio breaker tripped, I would definitely prefer the option of trying to get the radio working. My judgement is that mitigating the elevated risk of being involved in a mid-air collision in that situation is more important than the very low risk of there both being a problem serious enough to cause the wiring to catch fire immediately and the breaker not tripping again if reset. I'm not stupid enough to hold the breaker in.

Your risk assessment may be different, as are your experiences and flying environment. Different situations require different responses, I don't like removing options, even if they are not desirable in general, they may be desirable in extreme situations.

HBA Supporter

#### BJC

##### Well-Known Member
HBA Supporter
Would need a spreadsheet to optimize.
Yup, prices vary quite a bit, and illogically, to me. I wasn’t thinking of splitting up the order for lowest total cost, just lower between ACS and Allied, or other favorite source, for the total order. Ty-Raps, for example, are much cheaper (in quantity, and I generally buy more of everything than I think that I need to avoid reorders and shipping costs) at Allied than ACS.

BJC

#### 12notes

##### Well-Known Member
Log Member
Yup, prices vary quite a bit, and illogically, to me. I wasn’t thinking of splitting up the order for lowest total cost, just lower between ACS and Allied, or other favorite source, for the total order. Ty-Raps, for example, are much cheaper (in quantity, and I generally buy more of everything than I think that I need to avoid reorders and shipping costs) at Allied than ACS.

BJC
Splitting up the order will probably kill any savings with the multiple shipping charges. Just finding the cheapest one supplier with the lowest price for the total order is what I was implying needed the spreadsheet.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
My personal risk assessment is a little different. If I'm landing at an isolated grass strip and the radio goes, then, no, I wouldn't reset the breaker. However, if I'm in the pattern at an uncontrolled airport with 7 other planes, one of which is a student pilot on a right pattern instead of left like everyone else, and you have a Cirrus that's been silent since calling a 10 mile final to a different runway than everyone else is using 4 minutes ago (actual situation I've been in), then if the radio breaker tripped
you go to your number 2 COM. If you only have one, and you feel a radio failure is really a threat, you would be covering our judgement with a hand held radio already set up.

For everyone reading this and learning, stuff breaks. When stuff breaks, It should not be a threat to life and limb. If something must be upgraded, it is because we are tired of the nuisance, not because it almost got one of us killed. We prevent threats to life and limb by smart design.