New! - Norman Equipment Co.

22What are “redundant” logic circuits, and whatare their benefits in safety circuit applications?“Control reliability” implies that the safety device or systemis designed, constructed and installed such that thefailure of a single component within the device or systemshall not prevent normal machine stopping action fromtaking place … but shall prevent a successive machinecycle from being initiated. To achieve this, safety controllersare typically designed with dual logic circuits,each of which can provide safety circuit checking/monitoring.These functionally-equivalent logic circuitscross-monitor each other, as well as checking thesafety circuit for component failures, short circuits, opencircuits, etc.Since these controllers detect faults in the safety circuitcomponents and interconnection wiring to effectmachine shutdown, such “redundant” self-monitoring circuitsenhance safety system reliability. In so doing theyprovide a higher level of safety for the machine operatorand maintenance personnel.To heighten the integrity and reliability of these units,SCHMERSAL engineers have had each of the redundantlogic circuit microprocessors programmed by a differentsoftware specialist … thus reducing the probabilityof a simultaneous logic-circuit malfunction due to aprogramming error.Use of such safety controllers, in combination with safetyinterlock switches, tamper-resistant coded-magnetswitches, and emergency cable-pull switches enablescontrol engineers to achieve the “single component failurecontrol reliability” required by OSHA, ANSI, and internationalmachine guarding safety standards/guidelines.23What characterizes “fail-to-safe” operation?“Fail-to-safe” safety devices are designed such that a componentfailure will cause the device to attain rest in a safecondition. This term is generally applied to electronic safetyinterlock systems using non-mechanical presence orposition sensors (such as reed switches, proximity switches,et al) and/or safety controllers. Such controllers areoften designed to feature redundancy, self-diagnostics,and positive-guided contacts.374