Structure and Function of Safety Relays

Today's safety relays are distinguished primarily by their technological design:

• Classic contact-based relay technology
• With electronic evaluation and contact-based volt-free outputs
• Fully electronic devices with semiconductor outputs

Nothing has changed in the fundamental requirement that safety relays must always be designed in such a way that – when wired correctly – neither a fault on the device nor an external fault caused by a sensor or actuator may lead to the loss of the safety function. Technological change has advanced the development of electronic safety relays, which offer much greater customer benefits: Electronic devices are non-wearing, have diagnostic capabilities and are easy to incorporate into common bus systems for control and diagnostic purposes.

Structure and Function of Safety Relays

The typical design of a first generation safety relay in relay technology is based on the classic 3 contactor combination. The redundant design ensures that wiring errors do not lead to the loss of the safety function. Two relays (K1, K2) with positive-guided contacts provide the safe switch contacts. The two input circuits CH1 and CH2 each activate one of the two internal relays. The circuit is started via the start relay K3. There is another monitoring circuit between the connection points Y1 and Y2 (feedback loop). This connection is used to check and monitor the position of actuators which can be activated or shut down via the safety contacts. The device is designed in such a way that any faults in the input circuit are detected, e.g. contact welding on an emergency off/emergency stop pushbutton or on one of the safety contacts on the output relay. The safety device stops the device switching back on and thereby stops the activation of relays K1 and K2.