# Klixon circuit breakers as switches

### Help Support HomeBuiltAirplanes.com:

#### dtnelson

##### Active 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...

Second, use the right DC rated switch and a separate CB or fuse wherever you can. I've used circuit breaker switches in several of my panels, and I've had them fail. In one case I even had one fail resistively; in other words, not open, not closed, but it was internally dropping enough voltage to cause the com radio it controlled fail. In my current airplane, I do have some CB switches (Tyco W31 series, like this) - and I've learned that I need to have a spare or two in my onboard tool kit.

Third, use "pullable" circuit breakers like this, instead of like this. You may find yourself in a situation where you need to isolate just one device on a circuit branch instead of all of them (think of your "radio master" branch). If you have a case where you know, with some certainty, which device is failing and causing the main breaker to pop, you might consider pushing that breaker if you can isolate and remove the defective device. That's pretty much the only time I'd consider resetting a breaker in flight.

Fourth, I'm personally not a big fan of "hidden" fuses, if only because I want to know which fuse has blown so as to know what else won't work in flight.

My two cents,

Dave

#### BJC

##### Well-Known Member
HBA Supporter
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...
Size the wire to provide the current needed to operate the device (the load).

Size the over-current protection (fuse, breaker, or solid state device) to protect the wire.

There is no way to protect the load / device with the overcurrent protection because the device must have an internal failure to lead to an over-current condition.

BJC

#### BJC

##### Well-Known Member
HBA Supporter
Second, use the right DC rated switch and a separate CB or fuse wherever you can.
There are many switches that are rated for AC that perform acceptably for nominal 12 and 24 volt systems. There also are some DC rated switches that are of a quality that I would not put in an airplane.

BJC

#### bmcj

##### Well-Known Member
HBA Supporter
Why do breakers have to occupy valuable real estate? They can be mounted anywhere within reach, even out of sight. Resetting is only a matter of tactile sense... run your fingers across the row and feel for the raised breaker, then reset. That way, your switches can go where they need to be.

One other note... breakers and fuses are meant to protect against excessive (fire inducing or instrument destroying) power flow. If you trip a breaker, it’s not always a good idea to reset it inflight. Given that, why would you want to circumvent their primary purpose just to use them as switches?

HBA Supporter

#### Marc Zeitlin

##### Exalted Grand Poobah
Aircraft Spruce has them too:

#### Wanttaja

##### Well-Known Member
For what it's worth, I use automotive blade-type fuses and individual holders. These are labeled on one side, with a dot of velcro on the other. I wire them to hang behind the instrument panel near the bottom below the device they're connected to, with a matching bit of velcro to to hold them in place. There's enough wire to let them come down an inch or so to let me examine/replace them.

I don't carry spare fuses in the airplane. I figure if a fuse blows, I'll wait until I'm on the ground to trouble-shoot. Since I'm running a conventional magneto ignition, there's nothing that can't wait until I'm back home. Just have a radio (panel-mounted handheld), a transponder, and ADS-B. There's an avionics master switch and a switch for the ADS-B power.

I've also got a single Klixon-type breaker for the generator output.

Ron Wanttaja

#### Pops

##### Well-Known Member
Log Member
For what it's worth, I use automotive blade-type fuses and individual holders. These are labeled on one side, with a dot of velcro on the other. I wire them to hang behind the instrument panel near the bottom below the device they're connected to, with a matching bit of velcro to to hold them in place. There's enough wire to let them come down an inch or so to let me examine/replace them.
View attachment 94801
I don't carry spare fuses in the airplane. I figure if a fuse blows, I'll wait until I'm on the ground to trouble-shoot. Since I'm running a conventional magneto ignition, there's nothing that can't wait until I'm back home. Just have a radio (panel-mounted handheld), a transponder, and ADS-B. There's an avionics master switch and a switch for the ADS-B power.

I've also got a single Klixon-type breaker for the generator output.

Ron Wanttaja

Oh, the joys of a simple airplane.

##### Well-Known Member
There are many switches that are rated for AC that perform acceptably for nominal 12 and 24 volt systems. There also are some DC rated switches that are of a quality that I would not put in an airplane.
It should be pointed out that technically there is not way to derate an AC switch for DC rate, without either testing it or a detail design analysis, due to the fundamental differences in the design requirements between AC & DC switching.

Fuses & CB's do perform slightly differently in the time and current required to trip / blow.

One should always consider out breaks stupidity, I have heard of bored pilots closing collared breakers to see what would happen while in flight (this was on freighter conversions, always lots of capped and stowed wires with a sea of permanent collared breakers).

#### BJC

##### Well-Known Member
HBA Supporter
It should be pointed out that technically there is not way to derate an AC switch for DC rate, without either testing it or a detail design analysis,
Yup. My comment was not intended to suggest such a scheme.

BJC

#### User27

##### Active Member
Try Potter & Brumfield W28 breaker switches or
ETA 1110 Series
The ETA breaker are simple push on/push off and shows a white collar when "popped".
These are European links, but Mouser US must have something similar
P&B W28
ETA 1110 Series

#### dtnelson

##### Active Member
Size the wire to provide the current needed to operate the device (the load).

Size the over-current protection (fuse, breaker, or solid state device) to protect the wire.

There is no way to protect the load / device with the overcurrent protection because the device must have an internal failure to lead to an over-current condition.

BJC
Hi BJC,

My point is this; many people (including me) will use 20 gauge wire for many runs. The American Wire Gauge table, which we should all be using, tells us that 20 gauge wire can carry a consistent 11 amps. While considering that bundled wires, long runs, and wires under a constant load can require a larger gauge, very (very) few wires in, at least, my airplanes will be carrying 11 amps consistently.

Let's do some math to check this. Using 20 gauge wire as an example, and considering a 5 foot run, the wire will have a resistance of 0.05 ohms end to end, a voltage drop of 0.56 volts (at 11 amps), and the heat generated will be 1.23 watts/foot. This is pretty much worst case, as few of us would be running a constant 11 amps down a 20 gauge wire (at any length).

So, for many of my radios and other devices that require 2 amps or less, are you suggesting I use an 11 amp circuit breaker? Or a 10 amp breaker? I doubt it.

This was my point; in those cases, of which there are many, I'd select a 2 amp, or even a 5 amp CB before using a 10 amp CB. That would protect the device before protecting the wire. Using heavier gauge wire just means carrying around lots of extra weight. Using a larger CB than the device requires means I'm willing to cook my radio while the wire is fine.

Peace,
Dave

#### BJC

##### Well-Known Member
HBA Supporter
That would protect the device before protecting the wire.
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

#### dtnelson

##### Active Member
There are many switches that are rated for AC that perform acceptably for nominal 12 and 24 volt systems. There also are some DC rated switches that are of a quality that I would not put in an airplane.

BJC
Hi (again) BJC

Regarding switches, you are absolutely correct that quality matters whether the switch comes with an AC or DC rating.

The difference is this; Because AC current goes from 100% of the voltage, to 0%, and then back to 100% at whatever the frequency of the signal is, the duty cycle is, (going from memory), something like 70%, instead of the 100% that a DC signal presents. That's harder on a switch. Also, given that when you flip the switch on an AC signal the electrical transition point is likely not at the 100% voltage peak, AC is easier on the contactor(s) than DC is. That's why switches have a DC rating.

This is not to say that you can't, or shouldn't use AC rated switches. Just be aware of those differences.

Peace,
Dave

HBA Supporter
Yup.

BJC

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