Safety relays are devices that generally perform safety functions.[1]​ The term safety device is used as a synonym for safety relay.

Relays and contactors controlled machines and installations at the dawn of safety technology. In dangerous situations, the actuator was simply disconnected from the power. These types of security systems could be manipulated in the event of a failure, thus neutralizing the security function. Considering how to avoid this, special relay circuits were initially designed, e.g. e.g., a combination of 3 contactors.[2][1][3]​.

Safety functions (e.g. emergency stop, protective door or stop monitoring) are intended to reduce the risk of a dangerous situation to an acceptable level by means of appropriate resources. Examples of security functions of this type:.

They allow safety relays to monitor specific functions. Through connection with other safety relays, they guarantee complete supervision of a machine or installation.[1]​ They meet the requirements of EN 60947-5-1, EN 60204-1 and VDE 0113-1.[4]​.

Safety relays mainly differ in technological structure:

The structure of the safety relays must be such that, based on correct wiring, a failure of the device itself or an external failure caused by a sensor or an actuator cannot cause the loss of the safety function.[5]​.

A normal switching relay uses a coil and the mechanical movement of metal contacts to connect and disconnect the load. After many switching cycles, the metal contacts can end up soldering. If this situation has occurred and the operator presses the emergency stop button, the machine would continue working. The consequence would be a dangerous state for the operator. For this reason, many European, American, national and international safety standards and regulations prohibit the use of simple relays and contactors in dangerous machines.[6]​.

The typical structure of a first generation safety relay is based on the classic 3-contactor combination. The redundant structure ensures that connection failure will not cause the loss of the safety function. Two relays (K1, K2) with positively guided contacts provide the safe switching contacts. Of the two input circuits CH1 and CH2, each controls one of the two internal relays. The circuit is reset by means of the reset relay K3. Between the connection points Y1 and Y2 there is another monitoring circuit (feedback circuit). This connection is used to control and monitor the position of the actuators that are activated or deactivated through the safety contacts. The device is designed to detect faults in the input circuit such as For example, the "welding" of a contact of the emergency disconnection button or of a safety contact of the output relay. The safety mechanism prevents the device from being reconnected and, therefore, from activating relay K1 and K2.[6]​[7]​.