if the arc was to ground like it looked like what happen if the ground was not very good and it shorted with not much amperage 3 seconds to blow the breaker if it was like 100 amps or so it could trip very quickly. so i think that could do it because the voltage was not a smooth sign wave.
as far as the aluminum wire if a inspector is going to look at it i use anti ox. if not i tighten up to almost strip tight. honestly every time i come to a problem with a aluminum wire if the anti ox was used it failed. if it is tighten with a torq wrench it comes loose. the term that the code uses is cold flow. i have never in my 35 years experience ever had one of my wires go bad.
as far as the aluminum wire if a inspector is going to look at it i use anti ox. if not i tighten up to almost strip tight. honestly every time i come to a problem with a aluminum wire if the anti ox was used it failed. if it is tighten with a torq wrench it comes loose. the term that the code uses is cold flow. i have never in my 35 years experience ever had one of my wires go bad.