For a series arc the waveform will be what the equipment completing the circuit from the receptacle allows. Current in a series circuit is the same at all points. If an arcing connection is in the branch circuit portion the current will still need to pass through the extended part of the circuit from the receptacle. Would the waveform be distorted the same whether the arc is in the equipment or the branch circuit wiring? It is the same current.
The AFCI is not looking at a waveform like we would see it on an O-scope. It needs some criteria to determine that an arc is present. If it is counting the jagged edges of the wave it is counting more peaks than would be present in 60 cycle. That is why I think it would be looking for a higher frequency than 60 HZ. I know it is looking at the waveform for an abnormality. I am guessing at how it determines the wave is not clean and is not going to trigger erroneously on harmonic loads. Do you have more information on exactly how this is done?
