I too will also agree that failure rates of 30A and 50A cords and recepticals are low. The big differances being repeatitive use, and contact area on the whipes of the receptical being much larger and under much more tension.
I wouldn't worry so much about 30's and 50's.
Worry about the 15's and 20's is substantiated as use and abuse is more predominant, with minimal contact area.
