3 hourly EEs & a supervisor were working on Kiln 6 preheat plug up at Stage 3. They'd been lancing upper section about 45 min. They decided to try the blaster cannons on the 5th floor. One EE left floor 6 & went to floor 5 to initiate blast. He didn't call on radio to make sure everyone was clear. The injured EEs were standing in front of kiln door when he fired the cannon.