What is Safety Relay?

Ⅰ. What is safety relay?

1. Definition

The so-called "safety relay" is a combination of numerous relays and circuits that complement each other's abnormal defects in order to create accurate and low malfunction relay complete performance, with the lower the error and failure value, the higher the safety factor. The "safety relay" becomes a "faultless relay" as the height increases, rather than a regular action when a failure occurs. It has a contact structure that is forced-oriented, ensuring safety in the event of contact fusion. It differs significantly from standard relays. Figure 1 illustrates this.

In a safety circuit, a safety relay is an essential control component. It accepts safety inputs and, based on the internal circuit's judgment, outputs a switch signal deterministically to the device's control circuit. Safety relays are all dual-channel signal types, to put it simply. The safety relay can only work normally when both channels' signals are normal; during the working process, if any of the channels' signals is disconnected, the safety relay will stop outputting. All of the channels' signals are normal, and they can function correctly after being reset.

Safety relays function as essential control components within safety circuits, accepting safety inputs and, based on internal circuit evaluation, outputting deterministic switch signals to control circuits. These relays operate on a dual-channel signal principle, requiring both channels to function normally for proper operation. If either channel's signal is interrupted during operation, the safety relay immediately ceases output. Normal functionality can only be restored after all channel signals have returned to normal and the system has been properly reset (Eaton, 2021).

2. Requirement

Safety relays must fulfill several critical requirements to ensure proper protection:

(1) The machine cannot resume abruptly after the emergency stop is released.

(2) In the event that the machine's safety circuit fails, the machine's power supply can be turned off.

(3) The machine cannot be restarted if the safety circuit fails.

Dualization by itself will not suffice.

Duplication is required, but it must also meet certain conditions, such as mutual inspection of the duplex circuit, confirmation that all safety circuits have been severed once, and the operator's ability to initiate the operation if necessary. In addition, when the input switch wiring is short-circuited or the wire sheath is damaged and grounding is conceivable, it is important to prevent the machine from starting unexpectedly.

In fact, the safety relay is integrated with other components to simplify the design of the safety circuit, and the basic emergency stop circuit and the safety circuit constitute the circuit module known as the safety relay module. In practical applications, safety relays are integrated with complementary components to streamline safety circuit design. The combination of basic emergency stop circuits and safety monitoring functions constitutes the comprehensive protection module known as a safety relay module.

3. Usage

It's utilized in the construction of a safety circuit with input to confirm the machine's safety, and then it's used to control the input of the contactor, etc. Safety relays are primarily employed in the construction of safety circuits that verify machine operational safety before enabling control of contactors or other power-switching devices. They are widely used in applications where personnel protection is paramount, including:

• Emergency stop systems

• Gate monitoring

• Light curtain integration

• Two-hand control devices

• Perimeter guard monitoring

• Position monitoring systems

Ⅱ. How does safety relay works?

1. Electromagnetic

Iron cores, coils, armatures, contact reeds, and other components make up electromagnetic relays. A particular current will flow through the coil as long as a certain voltage is given to both ends of the coil, resulting in electromagnetic phenomena. The armature will overcome the return spring's draw force and attract to the core, thus driving the armature, thanks to electromagnetic force. The moving contact (usually open contact) is brought together with the static contact (typically closed contact). The electromagnetic attraction will vanish when the coil is de-energized, and the armature will return to its original position under the reaction force of the spring, releasing the moving contact and the original static contact. This pulls in and releases to meet the circuit's goal of conducting and switching off. The relay contacts that are "usually open" and "typically closed" can be identified as follows: The "normally open contact" is the static contact that is in the off state when the relay coil is not activated; the "normally closed contact" is the static contact that is in the on state when the relay coil is energized. It's what's known as a "typically closed contact."

Electromagnetic safety relays contain essential components including iron cores, coils, armatures, and contact reeds. When voltage is applied to the coil terminals, current flows through the coil, generating electromagnetic force. This force causes the armature to overcome the return spring's resistance and attract to the core, actuating the contacts. This electromagnetic mechanism provides the basic switching functionality while specialized design features ensure safety performance (Siemens, 2020).

2. Thermal dry

Reed Thermal Reed Relay is a novel type of thermal switch that detects and controls temperature using thermal magnetic materials. A temperature-sensitive magnetic ring, a constant magnetic ring, a dry reed tube, a thermally conductive mounting sheet, a plastic substrate, and various other components make up the device. The magnetic force generated by the constant magnetic ring powers the switching action of the thermal reed relay, rather than coil excitation. The temperature control properties of the temperature-sensitive magnetic ring decide whether the constant magnetic ring can give magnetic force to the reed tube.

Safety relays employ dual-channel monitoring architecture to achieve high reliability. This redundant approach requires two independent signal paths that are continuously cross-checked. Both channels must be in agreement for the relay to maintain its enabled state. If either channel detects an irregularity or if the signals become inconsistent, the relay immediately transitions to its safe state, typically de-energizing the controlled equipment (Schneider Electric, 2023).

3. Solid state

A solid-state relay has four terminals, two of which are used as input terminals and the other two as output terminals. To provide electrical isolation between the input and output, an isolation device is utilized in the middle.

A defining characteristic of safety relays is the implementation of forced guided (mechanically linked) contacts. This design physically prevents normally open and normally closed contacts from being in the same state simultaneously, ensuring that contact welding or mechanical failures can be detected. International standards (such as EN 50205) define specific requirements for these contact arrangements to guarantee reliable safety performance (Pilz, 2021).

Ⅲ. Safety relay wiring diagram

The wiring configuration of safety relays varies according to application requirements and relay model specifications. However, most safety relay installations follow these general principles:

1. Power Supply Connection: Safety relays require a stable power supply, typically 24V DC or specified AC voltage.

2. Input Device Wiring: Safety input devices (emergency stop buttons, gate switches, etc.) connect to designated input terminals, usually configured in dual-channel arrangements.

3. Feedback Loop: Many safety relay installations incorporate a feedback loop that monitors external contactors or devices.

4. Output Connections: Safety outputs connect to the control circuit of the machine or process being protected.

5. Reset Function: Depending on the application requirements, automatic or manual reset options are configured.

When implementing safety relay systems, it is crucial to follow manufacturer-specific wiring diagrams and installation guidelines to ensure proper functionality and compliance with relevant safety standards.

The safety relay's internal circuit is as follows:

The following are the wiring schematics for the two applications of safety relays:

Ⅳ. How to use safety relay？

Such relays are widely found in electrical equipment control systems in our daily work, particularly imported equipment from other countries. The equipment cannot operate normally until the failure is eliminated or the failure is not confirmed, which is especially true when the equipment fails suddenly. This is to avoid the device's sudden activity from endangering the person or the equipment when it fails.

As an example, consider the PNOZ V safety relay. The internal control circuit of the device is depicted in the diagram below:

The following are the main features of the safety relay:

What is the best way to connect the power supply? A1 and A2 are power terminals in the diagram; A1 is linked to 24V+, while A2 is connected to 0V.

During normal operation, the requisite switching conditions between S11 and S12 and S11 and S22 must be connected in the control input circuit. It's usually a contact or a button contact.

A matching reset condition must be connected between S33 and S34 in the reset circuit. The condition between Y1 and Y2 is also part of the reset circuit, and both conditions must be established simultaneously.

So how do they work？

A. It is pointless to ensure that the input circuit is energized if the safety relay needs to be closed (that is, K1, K2, K4, and K5) (that is, terminals S12 and S22 are energized). It must also meet the standards for K3 to close, in addition to S12 and S22 with electricity.

B. K1, K2, K4, and K5 are all de-activated, and the reset circuit is energized, if K3 is satisfied to be closed (that is, the terminal Y2 is energized). That is, S33 to S34 are both switched on at the same time, as are Y1 and Y2.

2. Example: The control loop is shown in the control circuit drawing of a certain piece of equipment as follows:

Connect the upper and lower diagrams as follows: The input circuit is already energized if the external emergency stop button is pressed and the K11 loses power, even if the external emergency stop switch is reset, however the fault confirmation button on the control box 190SP1 must be pressed. The internal relay on the K11 will then be activated.

In conclusion, the PNOZ safety relay is capable of meeting the requirements. To safeguard personal and other safety, when the emergency stop is released, the machine cannot resume abruptly (you must click on the fault confirmation, that is, the fault reset can be re-powered).

Frequently Ask Questions

1 What does a safety relay do?

A safety relay is a specialized device that monitors safety-critical components and circuits to ensure workplace safety. These relays:

- Monitor the state of safety devices such as emergency stops, safety gates, light curtains/guards, pressure mats, and mechanical interlocks

- Provide feedback on whether the circuit is in a safe condition to operate

- Look at and evaluate time delay safety functions

- Ensure reliability in all conditions to prevent accidents 

- Help maintain compliance with industry standards and regulations for workplace safety 

- Function as an effective means of preventing hazardous situations 

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

- Functions: Safety relays include multiple functions like switching, indication, and protection, while normal relays are mainly used for just switching within control circuits

- Physical size: Safety relay dimensions are larger (typically 17.5 mm, 22.5 mm, etc.) compared to normal relays which have smaller dimensions

- Design purpose: Safety relays are specifically designed for safety-critical applications with redundancy and monitoring capabilities, whereas normal relays are general-purpose switching devices

- Reliability requirements: Safety relays must function flawlessly in all conditions to prevent accidents, demanding higher reliability standards

3 What is K1 and K2 in a safety relay?

K1 and K2 typically refer to the two internal contactors or relay elements within a safety relay. This dual-contactor design provides redundancy, which is a fundamental aspect of safety relay construction. 

4 How do I know if my safety relay is bad?

Based on safety relay functions, potential indicators of a bad safety relay might include:

- Failure to reset after a safety condition has been cleared

- Intermittent operation of connected safety devices

- Visible damage to relay components

- Failure to respond to safety input signals

- Error indicators or diagnostic LEDs on the safety relay showing fault conditions

For definitive diagnosis, consulting the manufacturer's documentation for specific troubleshooting procedures is recommended, as safety relays must "function flawlessly in all conditions".

Conclusion

Safety relays represent a crucial element in modern industrial safety systems, providing reliable protection through redundant design and specialized monitoring capabilities. Their ability to detect faults and transition to safe states makes them indispensable in applications where personnel safety is paramount. Understanding the principles, proper installation, and maintenance requirements of safety relays is essential for ensuring effective protection in industrial environments.

By implementing safety relays according to best practices and manufacturer guidelines, organizations can enhance workplace safety while maintaining operational efficiency and regulatory compliance.

References

Eaton Corporation. (2021). Safety relay modules: Technical guide and application handbook. https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/safety-relays/safety-relay-technical-guide.pdf

Gruhn, P., & Cheddie, H. L. (2019). Safety instrumented systems: Design, analysis, and justification (2nd ed.). ISA.

Pilz GmbH & Co. KG. (2021). Safety relays PNOZ - Operating manual. https://www.pilz.com/en-INT/products/operating-monitoring/safety-relays

Rockwell Automation. (2022). Safety functions and safety systems according to IEC 61508 and IEC 62061. https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/safety-wp002_-en-p.pdf

Schneider Electric. (2023). Machine safety guide: Legislation and standards. https://www.se.com/ww/en/download/document/STD_Machine_Safety/

Siemens AG. (2020). Safety evaluation tool for the IEC 62061 and ISO 13849-1 standards. https://support.industry.siemens.com/cs/document/109738831/

International Electrotechnical Commission. (2018). IEC 61508-2:2010 Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 2: Requirements for electrical/electronic/programmable electronic safety-related systems. IEC.

International Organization for Standardization. (2015). ISO 13849-1:2015 Safety of machinery - Safety-related parts of control systems - Part 1: General principles for design. ISO.