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AFDDs - are they sensitive to upstream arcing?

AJJewsbury
It's occurred to me that if AFDDs work by spotting distortion in the a.c. current waveform that's passing through the device, then they're likely to see the same effect if series arcing is occurring upstream (with a decent sized load downstream).


If that's the case a decent DNO's crackle could trip out a significant number of final circuits if they happened to be loaded at the time.


Or are AFDDs cleverer than that? (e.g. do they look for an undistorted incoming voltage as well as a distorted current?)


As many of the dimming and flickering I've seen has been down to DNO loose connections, there could be a lot of nuisance tripping with these new devices if my worry is correct.


  - Andy.
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    Good point about the purple curve (possible artifacts of sweep rate of a spectrum analyzer).  I hadn't paid much attention to that plot, since in brochures I'd not be confident it's even from real data rather than an artist's impression.  I hope the block diagram of the device is fairly true to life, e.g. about measuring current in one conductor only, and not measuring voltage. 

    The actual AFDDs are presumably based on simple filters and rectification in the analog part before the microcontroller, so I don't see what their trouble would be until their output near-saturates all the time due to non-arc disturbance, so that the correlation of HF detection to ac cycle wouldn't be seen. I see nothing about a level of external disturbance that would be needed for this, and don't recall any immunity requirement in the standard. 

    This immunity is an interesting line to test, but I suspect the AFDDs would work as intended in "most" circumstances ... the bigger problem I see in justifying them is the familiar lack of evidence of the level of benefit they'd bring even if able to catch all/most arcing types of fault.
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    Good point about the purple curve (possible artifacts of sweep rate of a spectrum analyzer).  I hadn't paid much attention to that plot, since in brochures I'd not be confident it's even from real data rather than an artist's impression.  I hope the block diagram of the device is fairly true to life, e.g. about measuring current in one conductor only, and not measuring voltage. 

    The actual AFDDs are presumably based on simple filters and rectification in the analog part before the microcontroller, so I don't see what their trouble would be until their output near-saturates all the time due to non-arc disturbance, so that the correlation of HF detection to ac cycle wouldn't be seen. I see nothing about a level of external disturbance that would be needed for this, and don't recall any immunity requirement in the standard. 

    This immunity is an interesting line to test, but I suspect the AFDDs would work as intended in "most" circumstances ... the bigger problem I see in justifying them is the familiar lack of evidence of the level of benefit they'd bring even if able to catch all/most arcing types of fault.
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