I agree with you Dave- I worked with a customer who produced ice cream and they could not afford to have the ice cream stop moving through the pipe line that was refrigerated with amonia because the pipeline and the manifold would freeze solid. They could tolerate a single fault and then empty the line and clear the fault under scheduled maintenance. Jon's theory while is not popular could be used to detect faults and not shut down the system.
I'm kinda looking at the information in 250.21. In 250.21(3)d, it talks about ground detectors in the control circuit when the continuity of the control circuit is required. When you look at item number (5) there is no indication the detectiors are required. I agree with the comment that it makes sense to have them.
