When looking through a manual for a safety relay, I began to wonder why the coils of the load contactors (K3 and K4 on pages 14-15) are always connected to two different NO contacts of the safety relay?

Why not just connect K3 and K4 to a single safety contact, in parallel? Since the safety contacts are internally a series of redundant monitored contacts, this doesn't seem like it would compromise the system's safety in any way.

  • \$\begingroup\$ K3 and K4 appear to be relay coils that are wired externally to the safety relay and are used to monitor all phases that are switched hence two separate relays are required for monitoring two phases and three would be required for three phase monitoring. \$\endgroup\$
    – Andy aka
    Nov 6 '18 at 13:03
  • \$\begingroup\$ @Andyaka So those are not two redundant contactors, each capable of switching off all the phases of the load? \$\endgroup\$ Nov 6 '18 at 13:08
  • \$\begingroup\$ "Why not just connect K3 and K4 to a single safety contact, in parallel?" Maybe because the the Safety Relays contact maximum ratings? \$\endgroup\$
    – Tyler
    Nov 6 '18 at 15:01
  • \$\begingroup\$ @Tyler Of course, but you wouldn't expect that to be the default. This kind of a configuration is in every safety relay datasheet I saw. Of course, I'm not an expert so I didn't see many of them. \$\endgroup\$ Nov 6 '18 at 15:50

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