POWER CREWS WERE CALLED TO REPLACE A GSCS BLOWN TRANSFORMER. CREW MEMBER & MINE EE PROCEEDED TO OPEN A 12KV AIR SWITCH LOCATED APPROX. 7'FROM THE GROUND. A 9'LADDER WAS POSITIONED UNDER SWITCH HELD IN PLACE BY THE MINE EE. CREW MEMBER OPENED THE SWITCH & WAS ATTEMPTING TO INSERT A HOLDING PIN WHEN THE HANDLE SWUNG UPWARD STRIKING HIM UNDER THE CHIN.