Tesla that is not the arc of this discussion the sparks are a good illustration of the challenge for DC switches and interruption of current flow.
At 24 VDC it is impossible to create a self sustaining arc where the air actually changes to plasma and becomes a conductor.
The AC voltage threashold is around 250 VAC and a lot of laboratory time is being spent to prove it as there are some that maintain a 208 volt system cannot sustain an arc.
The shower of sparks that is the result of a hot wire touching a ground has an arc component but the spark is about the air gap required to interrupt the current.
I am not too happy about my explanation either as it is a difficult nuance to explain.
