It only resets when the reset signal changes from 'high' to 'low'. This prevents the operator from 'fooling the system' by using a screwdriver to keep the reset line 'high'. An electromechanical switch is said to be positively driven when the actuator acts directly on the switching element via a non-resilient member. This forces disconnection of the contacts since the switching action is not dependent on springs. Once one of the contacts weld, the relay cannot be restarted.
Redundancy Describes a system where more than one device is used to ensure reliability.
For example, a safety PLC that runs its program concurrently on two identical processors is redundant. A method of determining the level of danger that a machine operator is exposed to. Self-checking A control circuit that periodically performs tests on itself to assure correct functionality. Semiconductor output An output of a relay or controller that uses semiconductors to switch the output voltage. A PSS output which uses a protected semiconductor output with feedback to switch 24VDC to a load.