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Thread: CKT Breakers

  1. #11

    Join Date
    Aug 2011
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    Alabama
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    1,957
    Quote Originally Posted by martymayes View Post
    For an experimental, fuses are way underrated. I'd go with the auto blade type before I put in CB's.
    I did.

    But until I started reading this thread, I didn't even know there were such things as self resetting breakers.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  2. #12

    Join Date
    Aug 2013
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    36
    I'm a former Stinson owner and replaced the re-setting CB's in my plane when I rebuilt the panel. Fast forward about 20 years and I installed spade fuses in my EAB. My current practice when flying certified airplanes is if a circuit breaker trips in flight I do not reset it until I'm back on the ground. My EAB is strictly a VFR aircraft so it was an easy decision to install fuses since I'm not going to replace any in flight. Compared to CB's fuses are lighter, smaller, more reliable and cheaper. What's not to like. There are lots of options out there, this is the one I chose but I'm not suggesting it's right for everyone.

  3. #13
    DBurr's Avatar
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    Aug 2011
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    9
    As an engineer, I design with PPTCs regularly. If you use them in an aircraft, do read the data sheet carefully. In particular they have some characteristics you have to keep in mind. One is that they don't cut off the voltage and current to the load like a fuse or breaker, they simply go to a high-resistance state. A tripped PPTC can still deliver a few hundred milliamperes or more to a load. Secondly, they may not return to the same low-resistance state they were originally in for some time after the load is removed. There can be a residual higher resistance that may last for some time (minutes, hours or days). Also note that these are temperature-driven devices: they have a 'hold' current at which they are guaranteed not to trip and a 'trip' current where they are guaranteed to trip. The spread can be some number of amps, eg, 3A hold and 8A trip. The more amps flowing through the PPTC, the hotter it gets, and at some elevated temperature it trips. The hotter the device is to begin with (think parked on an Arizona ramp in the summertime) the lower the current will be when the PPTC trips. (Fuses also need to be derated for temperature, but typically not at levels seen in aircraft.)

    Nothing wrong with PPTCs when used appropriately. They'd be fine, for example, to protect servo or trim motors from temporary overloads. I'm not sure they'd be as useful for protecting the wiring to, say, an intercom. An intercom is unlikely to trip a fuse or breaker unless it fails catastrophically, and in that instance I'd rather just have power cut altogether rather than have the PPTC cycling power on and off to a smoking intercom.

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