Input and Switching Components for Safety Circuits

Increases in awareness and technology are driving motor control towards safety oriented circuits.  There are fundamental differences between control circuits and safety (fail-safe) circuits.  Control circuits are all about turning loads On and Off as required, whereas a fail-safe circuit ensures that the load is in a safe state, or switched off, when a safety related stop command, such as an E-Stop, gate switch, light curtain, etc. is present.

Safety circuits require that the devices that initiate the fail-safe Stop have positively driven contacts.  A positively driven contact is designed so that the opening of a Normally Closed (NC) contact is ensured then the device is operated, even if the contact is welded, according to standard EN 60947-5-5.  Positively driven contacts are marked with the following symbol

All normally closed input and feedback contacts used in a safety circuit must be marked with this symbol.  Similarly, contactors used in a fail-safe circuit use “mirrored” contacts that ensure that a NC auxiliary contact will not be closed at the same time as the Normally Open power poles.

It is important to note that contactors used for switching loads On and Off in a safety circuit must also incorporate positively driven normally closed contacts.  Typically, traditional NEMA contactors are not suitable for use in safety circuits.  Similarly, not all IEC contactors are suitable for use in safety circuits.  We catalog a line of “Safety Contactors”  that include integral auxiliary contacts, as required by the US automotive industry.  However, be aware that all Siemens 3TRT contactors are suitable for use in safety circuits.

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