3 hourly EEs & supervisor were working on Kiln 6 preheat plug up at Stage 3. They'd been lancing upper section for about 45 min. They decided to try blaster cannons on floor 5. 1 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 cannon.