Safety Relay

A safety relay is a simple electrical device that performs the same function as a normal relay but has a more specialized design to implement safety functions in a hazard area and reduce the risk to an acceptable level when a fault occurs., so its  makes a safe and reliable response to the machine and human.

Let us see the working principle of both relay for better explanation.

Safety Relay

The Safety relay basically works on a forcibly guided contact mechanism which is different from normal relay.
Here is the electrical diagram of a safety relay as we can see that it has two channel and four terminal relay(S11,S12,S21and S22). its mean it can monitor two safety circuits S11 and S12 is an input for channel this channel can be connect with a door safety switch of a machine or a light curtain ( laser safety curtain). Terminal S21 and S22 is a input channel too it can also connect with any safety device. Terminal S33 and S34 is a start signal for the safety relay if we give 24Vdc signal on this terminal once, it will start monitoring channel A1 and A2. Terminal 13,14,23 and 24 is connect to contactor and contactor is connected with load (motor). in case one of the safety channel A1 and A2 breaks the safety relay will turn open the terminals 13,14,23 and 24 and these terminal connected with contactor then the contactor will cut the power to the load and the all machine will stop.

Common Relay or Common Relay

The Common relay basically works on when the relay coil is energizes the contact move from normal stat to the energizes state. ( NO to NC )
Below the electrical diagram of 1NO and 1NC relay contact, terminal 11 is COM, 14 NO and 12 NC and and terminal A1 and A2 for energizes the coil.
Terminal A1 connect to the plc and A2 connect with 0Vdc once the coil energized the terminal 14 become NC ( terminal 14 and terminal 11 is connect with contactor) and the contactor will running mode then load become on active mode.

Difference between a safety relay and an ordinary relay

Safety relayCommon Relay or Common Relay
The main purpose of a safety relay is to ensure that protection is provided to the machine as well as humans.The main purpose of a simple relay is to control the field instruments and switch the machine on or off.
Safety relay monitors safety equipment such as light curtains, safety gates, non-contact safety sensors, three-position instruments, emergency stop buttons.Relay is an electromagnetic switch so we can connect where we want to control the circuit so we can connect like contactor,capacitor.