Control reliable is a term we use to describe a safety circuit that is not prone to failure through a single fault. This is achieved through dual channel wiring as opposed to single channel. What does this mean?
Let’s say you have a robot inside a fence with a door that can only move if the door is closed. There is a mechanical switch that the door hits upon closing, telling the robot it’s safe to start. This setup can be easily defeated by someone pressing their hand on the switch, making the robot think the door is closed when it isn’t. Also, a short circuit or mechanical failure in the switch can cause it to stay closed!
This is what we mean by a single failure can result in a loss of the safety function.
Now, picture that there are two switches instead of one, positioned far enough away from each other such that one person can’t force them closed at the same time. Both switches need to be closed for the robot to move, so if a fault occurs on one door switch the system will be able to detect it, since one is open while the other is closed! One door switch is a backup for the other.
This is what we mean by dual channel and it’s one key concept of a control reliable circuit.
