The relay at the top connects the AC hot line to the rest of the circuitry, with a feeble red LED to show when it’s live:
The driver lives on the Low Voltage Interface board:
The GX270’s front-panel hard drive LED now serves to indicate when the AC power goes live.
I’d originally intended to turn the AC on when the Arduino gains control, but after seeing those pictures, I think it’ll remain disabled unless there’s a call for motor motion.
The interlock switch closes when the case opens, grounding the transistor base and disconnecting the AC power.
Of course, you can cheat by simply unplugging the switch, so it’s not failsafe. If you want failsafe, you need a normally closed switch in series with the collector; that’s not what Dell used as a chassis intrusion switch. That’s my story and I’m sticking with it.