A well organised post as always. You explanation about the difficulty in extinguishing a dc arc is quite lucid and understandable because of the absence of current zero in dc quite unlike in ac where the current goes through natural current zero. This natural current zero helps in allowing a time lapse necessary for recombination of ions and this makes the restriking voltage somewhat higher than the what would have been needed in the absence of natural current. As a result after few cycles, the arc dies off.

Thanks for keeping up to your reputation in delivery a quality content. I wish I could post of just one particular thing the way you do. Well done :)