Power Distibution Board - Broken stud

[FrankJ]

So where did it fail? Is it at the button of the stud where it transitions to the square? Does the nut turn freely on the stud? Was the stud bent or the threads damaged?

You are never going to get a grade 8 fastener in this application. You can teach the difference without going into metallurgy.

Occasionally even the best of us make mistakes. I would like to say it wasn't so.

[Richard Wallace]

The PD shanks are fragile, when compared against steel M6 screws. The PDB doc (here) states, on p.7, that the shanks can shear if torqued to more that 3.9 N-m. That's about four pounds of force at the far end of an eight inch wrench. Not nearly as much torque as is typically recommended for steel M6 screws -- about 10 N-m for Class 8.8 or 15 N-m for Class 10.9.

However, the OP clearly stated that the person with the wrench was a veteran electrical mentor guy. I agree this one is probably just bad luck. One of the best students on our team had similar bad luck a couple of years ago.