Safety relays and control modules differ from conventional relays in that they have force-guided (positive-driven) contacts. Three relays are used in a safety relay module. One relay checks the condition of the other two and signals a failure. In addition, safety relays and control modules may be configured as either a single channel or dual channel.  Single channel safety relays are capable of accepting only one normally closed (NC) input. When used in safety circuits, they are unable to detect a short circuit failure or a failure of the monitored input to change state. Dual channel safety relays ensure safety system integrity even if one logic circuit fails.

Important output parameters to consider when specifying safety relays and control modules include maximum current rating, maximum AC voltage rating, maximum DC voltage rating, maximum AC power rating and maximum DC power rating. The current rating is the maximum switching current rating for the relay or module. The AC voltage rating is the rated AC voltage for the module. The DC voltage rating is the maximum DC voltage switching rating for the relay / module. The AC power rating is the maximum power rating of the switched line. The DC power rating is the maximum power rating for switched DC voltage. Safety relays and control modules with other output parameters are also available.

Safety relays and control modules have normally open, normally closed and changeover contacts. Normally open contacts are a relay contact pair that is open when the coil is not energized.  Normally closed contacts are a relay contact pair that is closed when the coil is not energized. Changeover contacts for safety relays and control modules are typically Form C, single pole, double throw (SPDT).

Safety relays and control modules may be equipped with either an AC or DC power supply.  The ratings and approvals can be Type 1, Type 2, Type 3 or Type 4.  Type 1 safety relays and control modules may stop functioning after a single fault. Type 2 devices may experience loss of function if a fault appears between two test cycles. Type 3 safety relays and control modules have a safety function that is maintained in the case of a single fault.  Type 4 relays have a safety function that is maintained in the case of multiple faults.