You're a genius JonnyB, never would have thought of that.
Gunslinger, you also were on the right track. The labels were already on the switches on the 1st floor, so no need to go back downstairs to label them. The riddle is solved by labeling the lights A,B, and C to go with the switches (which are already labeled.)
There is nothing about how the electrician gets out of the building...that's not part of the riddle. So the locked doors don't matter, long as there's a fire escape or awning to jump onto.
He waits for the key and spends the night there.
Capt. Apathy, I misread the problem, though my answer would have put the guy on the first floor where he could go home after work.
Les
Wait a minute, scratch the above. I see it now. He goes through the second floor door to the outside stairs. The door locks behind him and the building is secure. Man exits the building and goes on his business.
Good riddle.