Power supply exploding

Today at work i was working on a computer that required me to install a copy of windows xp on it. I plugged it in and then went to turn it on and realized that I had forgotten to flip the switch. I reached back to the power supply, and when a flipped the switch, a loud bang and a big flash followed by a cloud of smoke came out of the power supply. Fortunately the computer was not damaged, although the power supply is going to be RMA'ed. The power supply was an Enermax noise taker 420 watt. For the rest of the day I was hesitant to touch any power buttons for fear of getting zappped.