flashover vs electric arc

flashover and electric arc both are nouns.

Word	Noun	Adjective	Verb	Adverb
flashover	Yes	No	No	No
electric arc	Yes	No	No	No

As nouns, electric arc is a hypernym of flashover; that is, electric arc is a word with a broader meaning than flashover:

flashover: an unintended electric discharge (as over or around an insulator)
electric arc: electrical conduction through a gas in an applied electric field

Other hypernyms of flashover include arc, discharge, electric discharge, spark.

flashover (noun)	electric arc (noun)
an unintended electric discharge (as over or around an insulator)	electrical conduction through a gas in an applied electric field