What is Safety Relay? Why is a Normal Electromechanical Relay not Considered Safe?

• One of the key components in an electrical panel is a relay, an electromechanical switch that operates its mechanical contacts when electrically energized. 
• In essence it serves as a contact between two circuits and divides them. 
• Relays come in different varieties each ideal for a particular use. 
• A safety relay is unique among them all because of its simple design and easy to use functionality. 
• Because of their small size and excellent dependability these relays are used extensively. 
• They are crucial parts of settings like machinery and power plants where safety functions are vital. 
• An overview of safety relays their functions and applications is given in this brief article

What is a Safety Relay?

• Relays intended for use in industrial or machine settings for safety purposes are known as safety relays. 
• It functions in the presence of dangers to lower risk to a manageable degree. 
• The safety relay monitors particular functions as necessary and upon detecting an error initiates a dependable and secure response. 
• These relays ensure safe equipment operation are easy to use and have a long service life while adhering to safety standards.
• The image below illustrates a safety relay.

• The primary function of a safety relay is to halt movement in a secure and controlled manner, monitor the position of movable guards, manage emergency stops, and interrupt closing movements during access.

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Working Principle of a Safety Relay

• The working principle of a safety relay involves detecting faulty contactors, actuators, and wire breaks by sending electrical pulses through the wiring. 
• Safety relays have mechanically coupled contacts; if a normally open (NO) contact remains closed, then a normally closed (NC) contact cannot be closed. 
• This design ensures that sets of contacts are not welded together, and wire breaks are detected by measuring current flow. 
• These relays are essential for promptly shutting down in an emergency and for consistently monitoring signals from safety devices.

Fault Detection of a Safety Relay

• Four main fault types are detected by safety relays: timing problems faulty safety actuators faulty contactors and wire breaks. 
• By passing electrical pulses through the wiring and monitoring current flow they are able to quickly and accurately detect problems such as wire breaks and welded contact sets. 
• They can also identify malfunctioning actuators or contactors. 
• Another advanced fault detection method that safety relays use is timing. 
• For instance in a safety actuator with redundant contact sets the relay will deactivate the auto-reset feature if the contacts dont close within a predetermined window of time. 
• In a variety of applications this guarantees a high degree of safety and dependability.

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Safety Relay Wiring Diagram

Below is a wiring diagram for a safety relay, illustrating the connection process with a dual-channel emergency stop button. Here is a step-by-step guide on how to wire it:

Powering the Safety Relay

• Connect a 24V DC power supply to terminal a1 of the safety relay.
• Connect the ground (GND) to terminal a2.

Connecting the Emergency Stop Button

• The emergency stop button has two normally closed (NC) contacts.
• Connect the first NC contact of the emergency stop button between terminals S11 and S12 of the safety relay.
• Connect the second NC contact of the emergency stop button between terminals S21 and S22 of the safety relay.
• This setup allows the safety relay to monitor both channels (channel 1 and channel 2) for any activation of the emergency stop button.

Monitoring and Resetting

• Once the emergency stop button is pressed, the safety relay will detect the change in state (opening of NC contacts) on both channels.
• To manually reset the safety relay, connect a push button that has a normally open (NO) contact.
• Wire the NO contact of the reset push button to terminals S33 and S34 of the safety relay.
• This push button allows for a manual reset of the safety relay once the emergency condition is cleared.

Connecting to a Master Control Relay or Contactor

• Connect the normally open (NO) contact of the safety relay to the master control relay or master control contactor.
• Use terminals 13 and 14 on the safety relay for this connection.
• When the safety relay is in a safe state (not detecting any faults or emergency stop activation), the NO contact between terminals 13 and 14 will close, activating the master control relay or contactor.
• This setup guarantees that the linked equipment or system will only function when the safety relay is in a safe condition. 
• This wiring diagram will show you how to integrate a safety relay into your control system so that it can monitor certain functions detect emergency stop conditions and keep your industrial setup highly reliable and safe.

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Operation of a Safety Relay with Master Control Contactor for 3-Phase Motor

Here is an expanded step-by-step guide on the operation of a safety relay with a master control contactor connected to a 3-phase motor:

Powering On the Safety Relay

• Activate the safety relay by providing a 24V power supply. This action will turn on the power LED, indicating that the relay is powered.

Resetting and Activating the Master Control Contactor

• Press the reset push button. This action will cause the master control contactor to be turned on by the safety relay.
• The normally open (NO) contacts of the master control contactor will close, allowing 3-phase power to be supplied to the motor, thus starting the motor.

Monitoring Emergency Stop Push Button

• The safety relay will start monitoring the contacts of the emergency stop push button on channels 1 and 2.
• The emergency stop push button is wired to terminals S11 and S12 for channel 1, and S21 and S22 for channel 2.

Activating Emergency Stop

• Press the emergency stop push button. This will open the contacts on channels 1 and 2 (S11, S12, S21, S22 terminals).
• The LEDs for both channel 1 and channel 2 will turn off, indicating that the channels are open.
• The safety relay will open its contacts at terminals 13 and 14, deactivating the master control contactor.

Turning Off the Master Control Contactor

• When the contacts of the safety relay (terminals 13 and 14) open, the master control contactor will be turned off.
• The NO contacts of the master control contactor will open, cutting off the 3-phase power to the motor, and stopping the motor.

Resetting the Emergency Stop

• Reset the emergency stop push button. In this wiring configuration, the safety relay does not automatically reset.
• To reset the safety relay, press the reset push button again.

Resuming Normal Operation

• Once the reset button is pressed, the safety relay will start monitoring the contacts of the emergency stop push button on both channels 1 and 2.
• The master control contactor will turn on again, closing the NO contacts and supplying 3-phase power to the motor, thus resuming motor operation.

This detailed operation sequence ensures that the safety relay provides a reliable method to control and monitor emergency stop functions, enhancing the safety and reliability of industrial machinery and systems, particularly those involving 3-phase motors.

Why is a normal electromechanical relay not considered safe?

A normal electromechanical relay is not considered safe for critical safety applications due to several reasons inherent to its design and operation:

Mechanical Wear and Welding

• Normal relays use mechanical movements of metal contacts, which can lead to wear over time. 
• After repeated operations, these contacts may weld together due to arcing or physical damage. 
• If welding occurs, the relay may fail to open contacts when required, potentially allowing machinery to continue operating when it should stop, especially in emergency situations like pressing an E-STOP button. This failure can place operators in hazardous situations.

Reliability and Service Life

• Electromechanical relays typically have a limited service life, often measured in millions of operations. 
• However, under heavy usage conditions (e.g., hundreds of operations per day), their lifespan can shorten considerably (3-5 years). 
• This lifespan limitation increases the risk of unexpected failures over time, compromising safety.

Compliance with Safety Standards

• Many national and international safety standards, such as those in Europe and America, prohibit the use of simple relays and contactors in hazardous machinery due to these reliability concerns. 
• These standards emphasize the need for safety devices that reliably perform safety functions without risk of failure.

Safety Relay vs. Normal Relay

Here’s a detailed comparison between a safety relay and a normal relay:

What is the difference between a safety relay and a normal relay?

Functionality

• Safety relays are designed with specific safety functions in mind, ensuring that they can reliably handle critical safety tasks like emergency stops and fault detection.

Size:

• Due to their role in safety-critical applications, safety relays are often larger and more robust than normal relays, which are typically smaller and used for general switching tasks.

Cost

• The higher cost of safety relays reflects their specialized design and enhanced safety features, whereas normal relays are simpler in construction and thus more economical.

Application

• Safety relays are indispensable in environments where safety is paramount, such as in industrial machinery and hazardous operations. 
• Normal relays, on the other hand, are versatile and find use in various automation and control applications where safety considerations are less critical.

This comparison emphasizes the specialized nature of safety relays in assuring industrial safety and the broader applicability of normal relays in general control and automation tasks.

Safety Ratings  in selecting safety relays

When selecting safety relays, it’s crucial to consider their safety ratings, categorized into four types according to the EN954-1 standard:

• Category 1: Stops functioning immediately after a single fault.
• Category 2: May lose function between two test cycles if an error occurs.
• Category 3: Operates in the presence of a single fault.
• Category 4: Maintains normal operation even in the presence of several faults.

Advantages of Safety Relays

Safety relays offer several advantages over standard relays:

• Consistency: They are more reliable compared to standard relays.
• Cost-Effectiveness: Generally less expensive than other types of safety devices.
• Simplicity: No software programming required for operation.
• Safety Enhancement: Provides higher security by controlling or de-energizing components.
• Protection: Helps protect machinery and operators, reducing maintenance and replacement costs.
• Activation: Can be activated manually or automatically.
• Operating Characteristics: Typically operate with an operating time of 45ms and a recovery time of 1s.
• Ambient Temperature: Can operate in ambient temperatures ranging from -20°C to 55°C.

Disadvantages of Safety Relays

However, safety relays also have some drawbacks:

• Complex Wiring: Wiring can be challenging, especially in large systems.
• Troubleshooting: Once the system is down, troubleshooting and fault finding can be difficult.
• Rewiring Needs: Complete rewiring may be necessary if changes are required later.
• Operation Speed: Relatively slower operation compared to other electronic safety devices.
• Environmental Sensitivity: Susceptible to environmental factors such as noise and varying conditions.
• Noise Generation: Can produce audible noise during operation.
• Current Limitations: Typically used in circuits with lower current requirements.

Applications of Safety Relays

Safety relays find wide application across various industries and scenarios:

• Detect faults in input contacts within safety circuits during earth faults.
• Used in automatic control circuits for electromechanical switching to prevent hazardous switching operations.
• Enhance machine safety ratings by monitoring safety input devices and halting machine operation in unsafe conditions.
• Commonly used in safety gates, emergency stop circuits, light curtains, safety mats, two-hand control systems, interlocked gates, and foot-operated switches.
• Provide protection against electric shocks and prevent equipment overheating in daily applications.
• Applicable in both simple and complex security solutions, meeting functional safety standards requirements.

Global Standard Classification of Safety Relays

Safety relays are classified under a global standard system known as the Harmonized System of Nomenclature (HSN). Developed by the World Customs Organization (WCO), the HSN code for safety relays is 85364900. This classification ensures that safety relays are systematically categorized for customs, trade, and regulatory purposes globally, facilitating their identification and classification in international trade transactions.

What is Programmable Safety Relay Module?

• A Programmable Safety Relay Module (PSRM) is an advanced safety device used in industrial settings to monitor and control safety functions like emergency stops, safety gates, and light curtains. 
• It combines the reliability of safety relays with programmable logic for flexibility in setting up complex safety protocols. 
• PSRMs can self-diagnose, integrate with larger control systems for real-time monitoring, and meet stringent global safety standards. 
• They are cost-effective compared to full safety PLCs and are used widely in automotive, pharmaceutical, packaging, and heavy machinery industries to ensure operational safety.

Frequently asked Questions

Where are safety relays required?

Safety relays are required in various industrial applications where ensuring the safety of personnel and equipment is critical. Some common places where safety relays are used include:

• Machinery Safety: To monitor emergency stop (E-STOP) buttons, safety gates, light curtains, two-hand controls, and other safety devices.
• Process Control: In environments where hazardous conditions can arise, safety relays ensure safe shutdown or operation of critical processes.
• Automated Systems: To prevent unsafe conditions during automated operations, ensuring machines stop or function safely when required.
• Robotics: Ensuring safe interaction with robotic systems by monitoring safety zones and emergency shutdowns.
• Conveyor Systems: Monitoring for jams, overloads, or other unsafe conditions to prevent accidents.

What is K1 and K2 in a safety relay?

In a safety relay, K1 and K2 typically refer to the two separate channels or circuits within the relay:

• K1: Channel 1 of the safety relay, which monitors and controls the safety functions connected to it.
• K2: Channel 2 of the safety relay, providing redundancy or additional monitoring capability for critical safety functions.

These channels ensure that safety functions are independently monitored and controlled, enhancing reliability and ensuring that safety measures are always in place even if one channel experiences a fault.

Why do safety relays have two channels?

Safety relays often have two channels (K1 and K2) for several reasons:

• Redundancy: Having two channels allows for redundant monitoring of safety functions. If one channel fails or detects an error, the other channel can still maintain safety operations.
• Safety Integrity: Dual channels increase the safety integrity level (SIL) of the safety system. They provide a higher level of confidence that safety functions will operate correctly, even in the presence of faults.
• Diagnostic Capability: Two channels enable diagnostic checks and self-monitoring of the safety relay. They can detect internal faults or failures in external safety devices more effectively.
• Compliance: Many safety standards and regulations require redundancy in safety systems to ensure reliable operation and compliance with safety norms.