Introduction: Why Industrial Relays Remain the Backbone of Automation
Industrial relays are electrically operated switches that provide isolation, amplification, and reliable signal control in countless automation systems. Despite the rise of solid‑state devices, electromechanical relays continue to dominate applications where robustness, visual status indication, and complete galvanic isolation are required. This guide follows real‑world engineering challenges and delivers modular, fact‑based answers. Whether you are designing a PLC panel or maintaining mining equipment, you will find actionable guidance.
What Is an Electromagnetic Relay? Core Components and Functions
An electromagnetic relay uses a coil to generate a magnetic field, which mechanically opens or closes contacts. The core components are:
- Coil (control side) – receives the control voltage.
- Armature – moves when the coil is energised.
- Contacts – normally open (NO), normally closed (NC), or changeover (CO).
- Spring return mechanism – returns the armature when the coil is de‑energised.
Key function: Electrical isolation between low‑power control circuits (e.g. PLC outputs) and high‑power load circuits (e.g. motors, heaters). This isolation protects sensitive electronics from voltage spikes and ground loops.
Application-Oriented Classification of Industrial Relays
3.1. Signal Relays for Low-Power Control
Signal relays are designed for low‑power switching, signal transmission, and interface control. They are often referred to as a signal relay switch, signal relay, small signal relay, or low signal relay. Their typical current rating is below 5A, and they have a compact size for PCB or dense panel mounting.
- Switching sensor signals
- Logic level isolation
- Interface between microcontrollers and field devices
Example models (cross‑reference):
- Omron LY series (e.g. LY1‑0‑DC12, LY2‑0‑AP‑DC12)
- Finder 56 series
- IDEC RH series
- Hongfa HF‑13F
- Song Chuan SCLA
Product spotlight: BLY‑2C – a compact DPDT signal relay for general‑purpose control.
3.2. PLC Interface Relays (DIN Rail Relays) for System Integration
A PLC relay (also called DIN rail relay or part of a relay system) is built to interface between PLC outputs and field devices. These relays mount on DIN rails and often include LED indicators, test buttons, and suppression modules.
Why use them?
- Fast installation in industrial cabinets
- Easy replacement without rewiring
- Integrated status LEDs for troubleshooting
Examples:
- Schneider Zelio RPM series (RPM22P7)
- Weidmüller DRL270
- Magnecraft 78R series (W78ARCSX‑9)
3.3. Power Relays for High-Current Industrial Loads
Power relays handle higher loads such as motors, heaters, and industrial equipment. Their current range is typically 10A to 30A or more. They feature reinforced contacts and higher insulation ratings.
Typical use cases:
- Direct motor control
- Heating element switching
- Industrial solenoid valves
Scenario-Driven Relay Selection (Problem → Solution)
The following table summarises common automation problems and the recommended relay solution.
| Scenario | Problem | Solution | Why? |
|---|---|---|---|
| PLC output expansion | PLC cannot drive solenoids/motors directly | PLC interface relay (DIN rail) | Isolation, overload protection, easy maintenance |
| Signal switching | Need to switch low‑current sensor/logic signals | Signal / compact relay | High sensitivity, low coil power, small footprint |
| Mining trucks (high vibration) | Continuous vibration, dust, temperature swings | Industrial sealed relay + locking socket | Vibration resistance (IEC 60068), dust‑proof housing |
| Factory automation panels | Need modular, serviceable system | DIN rail relay system | Fast installation, integrated accessories (LED, suppression) |
Recommended models per scenario:
- PLC expansion → Zelio RPM2, Weidmüller DRL270, Magnecraft 78R
- Signal switching → Omron LY2, Finder 56, IDEC RH, Hongfa HF‑13F, Song Chuan SCLA
- High vibration → Plug‑in relays with locking sockets; avoid unsecured PCB relays
- Factory panels → DIN rail relays with LED and test button
Technical Specifications: A Structured Reference Table
The table below provides a quick reference for key electrical and environmental parameters.
| Parameter | Typical Industrial Range | Notes |
|---|---|---|
| Coil voltage | 5V / 12V / 24V DC | 24V DC is most common for PLC systems |
| Contact rating | 5A / 10A / 16A / 30A | Resistive load at 250V AC |
| Contact type | SPDT / DPDT (Form C) | Changeover contacts preferred for flexibility |
| Operating temperature | -40°C to +70°C | Industrial standard |
| Vibration resistance | 10–55 Hz | Typical; mining requires higher spec (IEC 60068) |
| Electrical life | ≥100,000 operations | At rated load |
| Mechanical life | ≥10 million operations | Unloaded |
| Insulation resistance | >100 MΩ | Between coil and contacts |
| Dielectric strength | 2,000–5,000 VAC | 1 minute |
Protection features to look for:
- Surge suppression (diode for DC, RC or varistor for AC)
- Sealed or dust‑proof housing for harsh environments
How to Choose a Power Relay for Industrial Use: Step-by-Step Decision Framework
Follow these five steps to select the correct power relay.
Step 1 – Identify load type
- Resistive (heater, incandescent lamp) → standard relay
- Inductive (motor, solenoid, contactor) → relay with higher rating + arc suppression
Step 2 – Calculate load current
- Measure steady‑state and inrush current.
- Choose a relay with contact rating ≥ 1.5 × load current (safety margin).
Step 3 – Match control voltage
- Most PLCs provide 24V DC outputs. Ensure coil voltage matches.
Step 4 – Evaluate environment
- High vibration → socket + locking clip or DIN rail with retaining bracket.
- Dust / moisture → sealed relay (e.g. washdown or mining grade).
- Wide temperature range → verify -40°C to +70°C rating.
Step 5 – Decide mounting method
- PCB mounting → for compact, integrated designs.
- DIN rail mounting → for industrial cabinets and easy maintenance.
Example: A 24V DC PLC output switching a 2A inductive solenoid in a vibrating conveyor → choose a 5A+ DIN rail relay (e.g. Weidmüller DRL270) with a diode suppression socket.
Cross-Brand Relay Mapping Table
The table below helps you find equivalent relays across major manufacturers.
| Function | Omron | Finder | IDEC | Hongfa | Song Chuan | Schneider | Magnecraft | Weidmüller |
|---|---|---|---|---|---|---|---|---|
| Signal relay | LY series | 56 | RH | HF‑13F | SCLA | – | – | – |
| PLC relay | LY / G2R | 55/56 | RJ | HF | SCLA | RPM2 | 78R | DRL270 |
Note: Always verify coil voltage and contact configuration before substitution.
Key Takeaways for Engineers
- Signal relays (including small signal relay and low signal relay) are used for low‑power control and switching – ideal for sensor signals and logic interfaces.
- PLC relays (DIN rail relays) are designed for system integration and protection – they provide isolation, LED status, and easy replacement in control cabinets.
- Power relays handle high‑current industrial loads – always apply a 1.5× safety margin for inductive loads.
- Environmental conditions (vibration, dust, temperature) are critical selection factors – do not overlook sealing and mechanical locking in harsh environments.
Summary
In summary, the correct selection of an industrial relay requires a balanced trade‑off between load characteristics, control signal compatibility, mounting method, and environmental robustness. Signal relays (including small signal relays and low signal relays) excel at low‑power logic and sensor switching due to their high sensitivity and low coil consumption, while PLC relays (DIN rail relays) are purpose‑built for system integration, offering LED indication, test buttons, and easy replacement within a relay system. Power relays should only be used for high‑current loads with an adequate safety margin. Environmental factors such as vibration, dust, and temperature swings are equally critical; harsh conditions demand sealed enclosures and mechanical locking. By following a structured, scenario‑driven approach, engineers can achieve reliable, safe, and maintainable automation designs.
Direct Answers to Common Engineering Questions (FAQ)
Q1: What relay is used in PLC systems?
A: PLC systems typically use interface relays (DIN rail relays) such as Schneider Zelio RPM, Weidmüller DRL series, or Magnecraft 78R. These relays isolate PLC outputs and enable safe switching of higher loads.
Q2: Which relay is suitable for high vibration environments?
A: Use industrial plug‑in relays with secure sockets or DIN rail relays that feature high vibration resistance and mechanical locking. Sealed relays with reinforced construction are preferred in mining and mobile equipment (e.g. mining trucks).
Q3: How to choose a power relay for industrial use?
A: Select based on load type, current rating (≥1.5× margin), coil voltage compatibility, environmental conditions, and mounting method. For inductive loads, ensure arc suppression and higher durability contacts.
Q4: Can I replace a signal relay with a power relay?
A: Not recommended. Signal relays (including small signal relays and low signal relays) have lower coil power and higher sensitivity. Power relays require higher coil power and may not respond correctly to low‑energy signals.
Q5: What is the difference between a PLC relay and a general‑purpose relay?
A: PLC relays are a subset of general‑purpose relays but are optimised for DIN rail mounting, include status LEDs, test buttons, and often have socket compatibility for fast replacement.
