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SAFETY CONTROLLER SELECTION CRITERIAIt helps the selection process to view a safety controller ashaving four basic characteristics, each determined by theapplication requirements. This approach can be applied toany safety controller.FOUR BASIC SAFETY CONTROLLER CHARACTERISTICSSupply VoltageInputMonitoringFeedback &Reset CircuitOutputSUPPLY VOLTAGE REQUIREMENTSSelect your supply voltage. While many voltages are possible(24VDC, 24VAC, 24VAC/DC, 115VAC and 230VAC),24 VDC gives the most flexibility since virtually all controllersare available in this voltage. Also, since a transformerand rectifier are not required, this unit generally isless expensive and smaller than a 115VAC model.It is important to recognize that the safety controllersupply voltage is always converted by the controllerelectronics to 24VDC for internal operation and forpowering monitored input devices. Hence monitoredinput devices need only be rated for 24VDC.INPUT MONITORING REQUIREMENTSThe first step is to determine whether you need singlechannel (up to Safety Control Category 2) or dual-channel(Control Category 3 & 4) operation.Single-channel systems must monitor one N.C. positivebreakcontact. Dual-channel systems can monitor1 N.O./1 N.C. or 2 N.C. contacts. Generally, these needto be isolated dry contacts since most controllers will viewC-form contacts as a short circuit. Remember that 24VDCis supplied by the safety controller for monitoring thesecontacts.Safety controllers are also available for monitoringnon-potential free contacts (such as PNP outputs fromlight curtains). Models are also available that allow usersto field select the monitored contact configuration.Another consideration is crossed wire detection (a shortbetween channels). This requires special circuitry in thesafety controllers and is required for Safety ControlCategory 4 safety control systems.FEEDBACK & RESET CIRCUIT REQUIREMENTSSafety controllers with feedback capability can alsomonitor control relays and motor contactors with positiveguidedcontacts. Such feedback is required for SafetyControl Category 3 & 4 systems. A NC auxiliary contact iswired into the feedback loop (with or without a reset (start)button) to detect welded contacts in these external controldevices. The safety controller detects the existence of aweld when the relay shuts down due to a power loss oropen machine guard and prevents a restart.In order to reset the controller, the feedback loop must beclosed (at least temporarily). If the NC auxiliary contactstays open due to a contact weld, the controller cannot bereset.Safety Control reset can be automatic or be achievedusing the edge of a 24VDC reset signal. With automaticreset the controller will automatically reset (outputs close)when the machine guard is closed. Alternately, a reset orstart button (manual reset) can be added to the feedbackloop if desired. The controller only needs to momentarilysee a 24VDC signal at the feedback terminals to reset.With a monitored-manual reset, some type of pushbuttonis required. For monitored-manual resets, the feedbackloop circuitry is designed so that it needs to see a 24V to0V transition (trailing edge) in order to reset. This methodof reset is generally required when a person can actuallyget inside a machine guard (where they would be at risk ifthe equipment should automatically restart when theguard closes).OUTPUT REQUIREMENTSDetermine the number and type of safety controller outputsrequired for machine control elements and signaling.Following are the typical types of safety controller outputs:A. N.O. safety enable circuits — eitherinstantaneous or timed.B. N.O. or N.C. auxiliary relay contacts — theseare not to be used for safety functions, butonly for annunciation/signaling.C. Semiconductor outputs for annunciation.8329