I feel like throwing in a bit of relay logic I ran into on a job that was done wrong. Cable-driven wagon (aka a platform with wheels) in a production of "A Christmas Carol." Operator-cued, but with safety limit switches at each end of the track that would cut the motor power (the motor was self-braking).
The upstage limit switch, as I discovered afterwards, was wired to supply power to a relay that broke the motor connection. And they'd supplied this power by running an extension cord along the floor under the pinrail. Which got kicked out of the outlet by a railman during tech rehearsal.
So of course the next time they ran the wagon all the way upstage, it over-ran the limit switch and smashed into the back wall -- crushing the very same relay box that was supposed to prevent this.
Wiring to the other side of the relay would have, of course, prevented this. I've noticed, though, how many people can only think on the positive side. How the thing is "supposed" to work or how it works when it is working correctly. The scientific equivalent of only looking at data that proves the hypothesis. Takes training and effort to properly plan so your devices fail safe instead of catastrophic.
