I'd have to say the earliest depiction of lethal exploding consoles was in TWOK, though it was only fake-lethal in the simulator.
I have an impression there was a TOS episode where somebody -- I want to say Uhura -- had their hands burned by an electrical surge from their console. But I'm not at all sure that happened.
Maybe it was Chekov who got burned when V'ger hit the Enterprise in TMP?
But that is interesting that the lethal exploding consoles seem to start after TWOK.