Were these 4ways or 2pole switches? If they're 2 pole, and there was 480V being run through these, the fault downstream (I'm assuming) probably found its weakest link being the switch contacts not being rated for the fault voltage.. But you say a faceplate screw caused the fault which would be at 277V if all was grounded properly, or did more than one wire get damaged?
The switches are Hubbell, you can make out their oval insignia above the toggles on the yokes...
Yup, Marc's right. (I told the answer on the chat, which may have helped ) 2 Poles, Hubbell, with a label on the side "2 Pole for single circuit only".
The installer chose to disregard the label, and ran 480V through them. The fault was 277V, caused by 8-32 in the corner of a 4-S box. The switch on the left took the fault, and its failure sent this "Jack and the Beanstalk" effect into high gear.
There were 18 of these switches installed (that's 36 circuits). The customer had me remove all of them They will be replaced with contactors. (BTW, None of the 20 Amp breakers tripped)
[This message has been edited by electure (edited 11-18-2005).]