What Is A Safety Relays? Safety Relay Types, Functions, and Applications

Safety relays are specialized devices composed of multiple relays and circuits, designed to complement each other’s abnormal defects, thus achieving low failure rates and high safety standards. They are widely used in mechanical control systems that require high safety measures to protect operators from potential hazards. Below is a detailed introduction to safety relays:

Overview of Safety Relays

Safety relays are control components formed by combining multiple relays and circuits, designed to reduce malfunctions and failure probabilities by compensating for each other's defects, thus improving overall safety. Their primary objective is to protect operators exposed to different levels of mechanical risks.

Functions

The primary function of safety relays is to safely and controlledly stop motion, monitor the positions of movable guards, initiate emergency stops, and interrupt movement during the access process.

Working Principle

Safety relays typically use a dual-channel signal design, meaning the relay only works when both channel signals are normal. During operation, electrical pulses are transmitted through the circuit to detect faults in contactors, actuators, and wiring. If any signal from either channel is interrupted, the safety relay will immediately stop output until both channels are restored and reset, ensuring that the system can quickly and reliably cut off control circuits in case of potential danger, thus protecting personnel and equipment.

Main Types

Safety relays come in various types based on working principles and applications, including electromagnetic, thermally sensitive reed, and solid-state relays.

· Electromagnetic Relays: Composed of iron cores, coils, armatures, and contact springs, these relays operate through electromagnetic induction to open or close contacts.

· Thermally Sensitive Reed Relays: Utilize thermally sensitive magnetic materials to detect and control temperatures, and the magnetic force generated by a permanent magnetic ring drives the switch to operate.

· Solid-State Relays: These relays employ isolation devices to ensure electrical isolation between input and output, featuring compact size, long life, and high reliability.

Applications

Safety relays are primarily used to protect operators and machinery by preventing costly equipment replacements and maintenance work. Therefore, they are one of the most commonly used components in safety systems. They can detect faults in input and output devices as well as internal faults. However, they are just a part of a comprehensive safety control system, and all components must be correctly selected and applied to achieve optimal operator protection.

Example Application: The PNOZ S4 safety relay, for instance, features two safety channels and can be configured for single or dual-channel operation (with or without short-circuit detection between contacts). It supports various reset/start methods, including automatic reset/start, manual reset/start, falling-edge monitored reset/start, rising-edge monitored reset/start, and reset/start with start testing. These features make the PNOZ S4 flexible for various safety control scenarios, meeting diverse safety needs.

Fault Detection

Safety relays detect four types of faults: wiring faults, contactor faults, actuator faults, and timing faults. By sending electrical pulses through the entire circuit, safety relays can detect wiring faults and welded contact groups. Timing is another fault detection technique used, such as in the redundancy of actuator contact groups. If two contact groups inside the relay do not close within a short period, automatic reset is not allowed.

Operation

Usage Requirements:

1. After emergency stops are lifted, machines must not restart suddenly to prevent accidental incidents.

2. In case of a safety circuit fault, the machine's power supply must be quickly cut off to ensure system safety.

3. Dual-Channel Design: Though dual-channel design is necessary, it must be cross-checked to ensure all safety circuits are fully disconnected, and the operator should manually restart when necessary.

Protective Measures: Protection relays are a special type of relay designed to monitor parameters such as current, voltage, or temperature in a circuit and take action, like cutting off the circuit, when an abnormality is detected. Protection relays are crucial for the safe operation of electrical systems.

Standards and Codes

The design and use of safety relays must comply with several international safety standards to ensure their reliability in high-risk environments. Common standards include EN ISO 13849-1, which assesses machine safety systems and defines the performance level (PL) of safety relays, such as PL c, PL d, and PL e. IEC 62061 evaluates systems based on safety integrity levels (SIL), with safety relays often classified as SIL 1, SIL 2, or SIL 3. The UL 508 standard in the U.S. outlines safety requirements for industrial control equipment. Additionally, TÜV certification is frequently seen with safety relays, indicating rigorous testing and safety approval.

Various manufacturers have specific model codes for their safety relays, often indicating the relay’s function and safety level compliance, such as the PILZ PNOZ X series, SIEMENS 3TK28 series, or Schneider Electric XPS series. These codes help users select safety relays suited to specific application requirements.

Differences Between Safety Relays and Regular Relays

Safety relays offer significant safety advantages compared to regular relays. They meet stringent international safety standards (e.g., EN ISO 13849, IEC 62061) and ensure reliable circuit disconnection in emergency or fault situations, thus safeguarding both operators and equipment. Safety relays typically incorporate redundancy and self-monitoring features, allowing them to continue functioning even if one circuit fails. Their fail-safe function ensures that systems enter a safe state in case of a fault, making them ideal for high-risk environments like industrial automation and robotics.

In contrast, regular relays are mainly used for basic electrical control and lack advanced features such as redundancy and self-monitoring, making them unsuitable for applications requiring high safety standards. Although regular relays are more economical and easier to install and maintain, they are not ideal for environments with strict safety requirements. The choice between safety relays and regular relays should be based on specific application needs, considering factors like safety requirements and cost.