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21What are “positive-guided” or “force-guided”relays, and why are they preferred over conventionalrelays when designing safety systems?Positive-guided relays feature N.O. and N.C. contactswhich operate interdependently. For such relays, the N.O.and N.C. contacts can never be closed simultaneously. Inthe event one of the contacts welds closed, the other contactscannot change state. For example, should one ormore of the N.O. contacts weld/stick shut when closed, theN.C. contact(s) will remain open with a minimum gap of0.5mm.This unique feature is desirable in machine safety circuitswhere “fail- to-safe” and/or “single component failure controlreliability” is desired. The positive relationship (interdependentoperation) between N.O. and N.C. contacts permitself-checking/monitoring of the performance of thesedevices. Such relays provide a higher level of safety systemintegrity and reliability.A simple illustration of the interdependent function of positive-guidedcontacts is shown in Figure 9.FIGURE 9POSITIVE-GUIDED VS. CONVENTIONAL CONTACTS373