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42Why should safety controllers be used withsafety interlocks/E-Stops?Safety controllers increase the reliability of the machineguarding safety system. Their ability to detect safety circuitfaults, and shut down the machine until the fault iscorrected, greatly heighten the safety level.43What is the difference between a single anddual-channel safety controller, and whenshould each be used?A single-channel safety controller is capable of acceptingonly one (normally-closed) input. When used in safetycircuits they are unable to detect a short-circuit failure inthe interconnection wiring, or a failure of the monitoredinput to change state.A dual-channel safety controller is capable of acceptingtwo inputs; one to each of its two, redundant self-monitoringsafety circuits. When used in safety circuits theyare typically capable of detecting interconnection wiringfaults (such as short-circuits, open circuits, and groundfaults) or a failure of one of the monitored input(s) tochange state. As such they provide a higher level of safetythan single-channel units.Single-channel safety controllers are suitable for relativelylow levels of risk assessment (e.g. EN 954-1 SafetyCategories B, 1 and 2). Dual-channel units are appropriatewhen designing “control reliable” safety systems —that is, systems in which a single component failure willnot prevent normal machine stopping action from takingplace, but will prevent a successive machine cycle frombeing initiated.44How do I decide which safetycontroller to use?Safety controller selection is usually based on:(1) the type of inputs being monitored (e.g. E-Stops,interlock switches, light curtains, coded-magnet sensors,et al).(2) the number of inputs being monitored.(3) the number and type of outputs required from thesafety controller (e.g. number of parallel outputs fromthe module’s positive-guided relays and the numberof auxiliary/signaling outputs).(4) the need/desire to monitor the integrity of the positive-guidedcontacts in the controlled output device(e.g. motor contactor, control relay, et al).(5) the level of safety desired (this is usually determinedby a structured risk assessment).These application parameters will normally narrow, andsimplify, the choice of safety controller to one or twounits.384