Safety Relay : Working, Wiring diagram, Ratings, HSN Code & Its Applications

One of the most important components used in an electrical panel is a relay. A relay is an electromechanical switch that is electrically energized to operate its mechanical contacts. Basically, it separates two circuits and works as a contact between them. There are different types of relays available and each relay is used for a specific application. So a safety relay is one of the types of relay which has a clear structure and is very simple to operate. So these relays are used broadly because of their high reliability, compact design, etc. These are becoming essential components where safety functions are essential like power plants or machines. This article gives brief information on a safety relay – working with applications.

What is Safety Relay?

A relay that is used to execute safety functions within a machine or industry is known as a safety relay. This relay will work in the occurrence of a hazard and it reduces the risk to an acceptable range. Once an error occurs, then this relay will initiate a reliable & secure response, and also each relay will monitor a specific function. These relays are efficient as well as simple in achieving safety standards which results in the secure operation of any equipment & also provides long service life. The safety relay picture is shown below.

Safety Relay
Safety Relay

The safety relay function is to stop a movement in a secure & controlled manner, monitor the movable guards’ position, emergency stops, and interrupt a closing movement throughout the access.

Working Principle of Safety Relay

A safety relay working principle is to detect faulty contactors, actuators, and wire breaks by transmitting out electrical pulses using the wiring. Safety relays include mechanically connected contacts, such that if a NO (normally open) contact remains closed, then an NC (normally closed) contact cannot be closed. This relay ensures for sets of contacts are welded & wire breaks by simply measuring the current flow. These relays are very helpful in reliably monitoring the signals from security devices & very quickly turn off in an emergency.

Fault Detection

Generally, safety relays detect four kinds of faults wire break, faulty contactor, faulty safety actuator & timing.

A safety relay is used to detect wire breaks as well as faulty actuators or contactors by simply transmitting out electrical pulses throughout the wiring. So these relays check wire breaks & for welded contact sets by simply measuring the current flow. So this is all that can be done with timing.


Timing is one more type of fault detection technique used by safety relays. The best example of this is the redundancy within contact sets of a safety actuator. If the two contact sets within the relay don’t close within a less time interval then auto-reset will not be allowed.

Safety Relay Circuit

The whole set-up with three relays by all the available contacts like a complete unit is normally known as a safety relay. The safety relay circuit diagram is shown below. Here, the safety contact is connected in between two A & B points. The supply would be around 110V AC most probably.

The reset push button is connected between C & D points. Both the E & F terminals are simply connected to the controller like the PLC controller for monitoring purposes whereas G & H terminals are simply connected within the safety line to perform on the final contractors for feeding the motor.

Safety Relay Structure
                         Safety Relay Structure


Once the AC or DC supply is given to the circuit then all three relays K1, K2 & K3 will be de-energized. The terminals connected within the safety line must be open and these terminals are used for monitoring. To activate the safety relay, the contact of the safety device has to be closed to make points B & point ‘C’ live, after that the reset push button will be pushed.

When this reset push-button is pushed, the K3 relay will be energized due to point ‘D’ becoming live. Once the K3 relay has energized, then it simply closes its NO (normally open) contacts which activate the K1 & K2 relays. So this can cause K1 & K2 relays to activate & self-latch throughout their self-latching contacts.

Once the reset push-button is opened, the K3 relay will be de-energized, although K1 & K2 relays are still energized. So the EF terminals as well as GH terminals are closed. Once the contact of the safety device opens, then it will make point B dead. So the K1 & K2 relays de-energize, therefore opening the connection among EF & GH terminals, and as a result, opening the safety line & tripping the main contactor. Here, the capacitor is required to make sure that the relay ‘K3’ has an off-delay, to provide K1 & K2 relays sufficient time to activate & self hold.

Safety Relay Wiring Diagram

The safety relay wiring diagram is shown below. Now we will see how to connect a safety relay with the dual channel emergency. To power ON the safety relay, we need to provide 24V DC at the a1 terminal & the a2 terminal is connected to GND. After that, we need to connect both sets of normally closed contacts with the emergency stop button. The first contact of an emergency button is connected between S11 & S12 terminals whereas the second contact is connected between S21 & S22.

Wiring Diagram of Safety Relay
Wiring Diagram of Safety Relay

Now the relay starts to monitor the NC contacts of the emergency star push button at channel1 & channel2. After that, we need to connect a push button to manually reset the safety relay and we can connect normally open (NO) contact of the push button at the S33 & S34 terminals of the safety relay. Next, we can connect a master control relay or master control contactor with the safety relay. We use normal open (NO) contact of the safety relay terminals 13 & 14 to activate the contactor.

Operation of Safety Relay

Now let’s activate the safety relay by providing a 24V power supply then the power LED turns ON. If we press the reset push button then the master control contactor will be turned ON by this relay. After that, start monitoring the contacts of the emergency stop push button at channel1 & 2. Now we press the emergency stop push button then it will open up channel1 and channel2 at the S11, S12 & S21 S22 terminals of the safety relay, and both LEDs of channel1 and channel2 go off.

When the contacts of the safety relay like 13 and 14 open up, then the master control contactor will be turned OFF. Let’s reset the emergency star push button, then the safety relay does not reset automatically in this wiring configuration. To reset, we have to push the reset push button once. As soon as we push the reset button, both channels channel1 & channel2 will start monitoring the contacts of the emergency star push button and again master control turns ON.

Safety Relay Vs Normal Relay/General Relay

The difference between a safety relay and a normal relay includes the following.

Safety Relay

Normal Relay

A safety relay is a device used to implement security functions. A normal relay is an electrically operated switch, used for controlling a high-power circuit with a low-power signal.
These relays are available in big sizes. These relays are available in small sizes.
In this relay, C contact is not available. In this relay, C contact is available.
A safety relay includes force-guided contacts like locked, positive, or captive-guided contacts. Normal relays include electrically conductive metal pieces.
Safety relays are available in specific colors like yellow. Normal relays are not available in a specific color.
As compared to normal relays, safety relay dimensions are high like 17.5 mm, 22.5 mm, etc. As compared to safety relays, these relays’ dimensions are fewer.
The safety relay includes different functions like switching, indication & protection.


The normal relay is mainly used for just switching within control circuits.
This relay is mainly used for the arrangement of switches This relay is mainly used for the connection of contacts.


Compared to a normal relay, this relay is up to 15 times more expensive Normal relay is not expensive.
These are used in security applications These are almost used in any automation application.

Safety Ratings

While selecting safety relays, unique specifications like safety ratings need to consider. So products may be rated as one of four types or categories which are defined by the EN954-1 standard. Buyers should decide the safety needs of their applications in advance & choose a product with a minimum determined rating. The relays with high safety ratings are typically high cost.

  • First-category devices may stop working after a single fault. So, these products are simply designed with fixed components & principles to decrease the occurrence of a fault.
  • Second-category devices may experience a loss of function if an error occurs in between two test cycles.
  • Third-category devices operate in case of a single fault.
  • Fourth-category relays maintain normal operation in case of several faults.


The advantages of a safety relay include the following.

  • Safety relays are more consistent as compared to standard types.
  • These are not expensive compared to other types of relays.
  • These are very simple.
  • It doesn’t need software programming.
  • These relays provide higher security to strengthen or de-energize components.
  • These relays are helpful in protecting both the machinery as well as the operator, so avoiding. maintenance otherwise replacement of equipment.
  • It has automatic and manual activation.
  • Its operating time is 45ms.
  • Its recovery time is 1s.
  • Its ambient temperature ranges from -20˚C – 55˚C.


The disadvantages of safety relays include the following.

  • Wiring is difficult on big systems.
  • Once the system is down then hard to charge & fault find.
  • It needs complete rewiring if changes require to be made later.
  • Operation speed is low.
  • It can be simply affected by environmental factors.
  • These relays can produce noise.
  • Relays are used in circuits that use less current.


The applications of safety relays include the following.

  • Safety relays detect faults at the input contacts within the security circuit in the occurrence of an earth fault.
  • Generally, these relays are utilized in automatic control circuits.
  • These are electromechanical switching devices mainly used to avoid failure within dangerous switching operations.
  • These are used on machinery & equipment to enhance the security rating of a machine
  • These relays monitor safety input devices consistently & prohibit the operation of the machine if any dangerous conditions are noticed.
  • These are applicable in security applications.
  • The common applications of safety relays mainly include safety gates, e-stop circuits, light curtains, safety mats, two-hand control, interlocked gates & foot-operated switches.
  • These are used in daily life for protection to avoid electric shocks & also avoid overheating of equipment.
  • These are applicable in both simple as well as more advanced security solutions once safety devices require to be checked based on the functional security standards requirements.

What is the Hsn Code of Safety Relay?

HSN (Harmonized System of Nomenclature) is used for classifying goods in a systematic way. This code was simply developed by the WCO (World Customs Organization) which is considered the global standard when it comes to giving names to goods. The HSN Code of Safety Relay is 85364900.

What is the Purpose of a Safety Relay?

Safety relay purpose is to protect both the operator & the machinery by avoiding replacement expensive equipment as well as maintenance.

Thus, this is all about an overview of a safety relay. These relays are the most frequently used components in safety systems because of growing regulations & attempts to defend operators from hazards. These relays detect failures in input & output devices and also internal failures; These are just single components in the safety control systems. All components within the control system should be selected & applied properly to attain operator protection in the desired range. Here is a question for you, what is a protective relay?