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Thread: AFCI Test Button - What does it do?

  1. #1
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    AFCI Test Button - What does it do?

    This post was made to add to a discussion that has come up from time to time as to what an AFCI breaker does when its test switch is pushed. Many here are familiar with what the GFCI test switch does but I have not seen anything on the test function of the AFCI.

    I have read several articles on the AFCI but have been unable to get great detail or much of anything about how the test switch worked.

    In an attempt to gain some understanding I disassembled a QO115AFCI and began examining the circuits inside and performing tests. My intent is to share experimentally derived information about the inner workings of the one particular AFCI that I examined. I do not claim to be an expert on AFCIs nor do I hold that all of my work is flawless or 100% accurate.

    Here is a photo of the AFCIs insides:



    Click for large version

    The black coil on the center right may be referred to as the “Injection/arc sense” coil. The white coil to the right will be referred to as the “leakage sense” coil.
    As seen in the picture only the red wire “hot” is inside of the black core. Both the hot and the neutral wires are inside of the leakage detect core.

    The black coil has two windings measured at roughly a 7:1 (dc resistance) ratio. A partial schematic below shows that one winding (bc) is used to inject a signal for test purposes. This coil connects to pin 8, an output, of the microcontroller. The other coil (ab), with a higher winding count senses current in the hot conductor , or currents injected by the injection coil.



    The test button is simply an input to the microcontroller. When the test button is pushed the microcontroller does not produce a “true arc fault condition”. Instead it “injects signals” into the (bc) winding of the black coil. The signals injected there are then coupled into the (ab) winding. Several op-amp filters are connected to the (ab) winding that then feed multiple inputs to the microcontroller for analysis.

    Since it would be prohibitive for this device to produce an actual high current arc it instead tests its ability to sense certain frequencies that are used in the analysis of an arc signature. Since this black core is crucial to the ability to sense an arc fault the integrity of it is what is tested. It was found that when the black coil was physically removed from the circuit board that the circuit breaker test button would no longer work (indicating a problem). However I could remove the red wire from the middle of the core and the breaker would still test ok!


    The output of the microcontroller that feeds the (bc) coil was then monitored while the test button is pressed and the results are shown below:


    When the test button of the AFCI is pressed the microcontroller outputs this waveform from pin 8 that lasts approximately 25 milliseconds.

    From this it can be seen that the microcontroller puts out three distinct frequencies. First it outputs a signal at approx. (1Khz) , it then changes to a higher frequency of approx. (10Khz) , and finally back down to (2.5Khz).
    Each frequency band lasts for 1/3 of the total or equal to a one-half cycle duration at 60hz.

    This sequence lasts for 25 milliseconds and the breaker then trips. Note this is less than the 5 half- cycles I have read were required as a minimum to sense a true arc.

    Based on this limited data, on this one product example, it appears that the test button confirms the integrity of its internal circuitry by simulating the high frequency aspects of a true arc.

    It then couples these high frequencies into its sense circuitry to assure that those circuits are capable of registering the high frequency content of an arc.

    I performed a lot more testing after this to the point where I was able to simulate an arc and trip the breaker. I could share more if there is any interest.
    I am afraid I may have already put too much into one posting and bored you all :smile:
    Last edited by iwire; 03-08-09 at 06:44 PM. Reason: Removed repeated text by request / Reduce picture size
    - Resistance is Futile ..... (if less than < 1 ohm) -

  2. #2
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    Quote Originally Posted by ELA View Post
    I am afraid I may have already put too much into one posting and bored you all :smile:
    Ahh what a great nap. :grin:

    Seriously, just make it so it doesn't trip for no good reason and I'll be happy.
    There are two kinds of people - those smart enough to know they don’t know, and those dumb enough to insist they do.-----Margery Eagan

    Open shop since 1988

  3. #3
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    It looks like it would be easy to convert one of these breakers into a shunt-trip breaker, if one were so inclined.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #4
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    I have changed lights for years by just turning the switch off. Now with AFCI, even with the switch off it will trip the breaker. Neutral to ground I guess.

  5. #5
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    aha, just as I thought. the test button is simply connected to the triflex capacitor.

  6. #6
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    I don't know if the Line diagram is correct to the picture, as presented.

    The white barrel you show in diagram but isolated but is electrically shown interconnected in the picture.

    Is the Test botton via the picture is a dual use application, releasing off the Ground screw to swing or slap the other plate or move in this gap in the plate or insure moment of the white barrel, A charged rod into a magnetic field, I don't, I have to study this more later, and as usuall read more.
    = : )

  7. #7
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    Quote Originally Posted by cadpoint View Post
    I don't know if the Line diagram is correct to the picture, as presented.

    The white barrel you show in diagram but isolated but is electrically shown interconnected in the picture.

    Is the Test botton via the picture is a dual use application, releasing off the Ground screw to swing or slap the other plate or move in this gap in the plate or insure moment of the white barrel, A charged rod into a magnetic field, I don't, I have to study this more later, and as usuall read more.
    Cadpoint,
    Your comments are tough to follow?

    It is difficult to see in the picture but the green screw does not have anything to do with the test switch.
    - Resistance is Futile ..... (if less than < 1 ohm) -

  8. #8
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    Quote Originally Posted by ELA View Post
    It is difficult to see in the picture but the green screw does not have anything to do with the test switch.
    Agreed; it appears to be an adjustment screw.

    The button spring looks to be a contact, labeled J8 on the PCB.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  9. #9
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    The following is a capture of the AFCI test feature of the Ideal 61-165 Suretest. This is only a part of entire test which lasted 126.4ms if I remember correctly.

    Bill

  10. #10
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    Look at the wires look at your diagram, The white objects power looks like the key!

    Frankly in respects to your diagram the "trigger" looks engaged at three points not in series as shown in your line diagram.
    Again, It looks like your diagram is more parellel than in series that use
    these component devices.

    The white wire is what will see the induction of the AF, the coil in front of that is the base line coil to cause everything else.

    I might be totally wrong!

    Where's the power coming from or goes to, you can't float the coil as you showed it cause it doesn't look like the photo.

    Ode to Dad, he always stressed the "Road map of electronic's" if you can't get there, well you just can't. Can I bank on a draftsman eye, well it's been awhile!

    Sorry, I'm so stark! ...
    = : )

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