Ground, or Earth as is known across the pond, was sometimes questionable.
Coming to pick up your plot, and touching the frame of the plotter, probably caused stored energy to ground to the human - static electricity build up. When multiple plots were run without the big bag of water touching the plotter as they picked up their plot, a path to ground through a circuit would leave the "computer" locked. A reset might work, and failing that, lift the floor tile and press the magic red button. Red button = a big path to ground/earth = static electrical charge dissipated from the plotter chassis.
Back in the 80's we had an office worker who had plugged their ground requiring Unix terminal into an ungrounded extension cord. Every half hour we received a call of "Core Dump" messages and "get your ass over here and fix this thing". On-site technicians duly changed RAM and processor modules - with a half hour window before the next "Core Dump" phone call. My site visit, to see what was really going on discovered the ungrounded terminal - a new extension cord was procured and problem solved.
Biting the hand that feeds IT
