I work with 480 every day. The word “shocked” is near and dear to my heart. :^)
The difference between 120 and 277 is you can survive about 8 seconds of getting zapped by 120 and walk away feeling pretty bad, but still walk away.
277 will turn your skin black in seconds and nobody just walks away.
http://www.mikeholt.com/technical-safety-life-threatening-stories.php