What is a Relay?
A relay is an electromechanical or electronic switching element that uses a small electrical signal to switch a larger current or voltage. In industrial automation, a relay is used to isolate, amplify or logically process control signals.
Relays are indispensable in switchgear cabinets, control panels and machine safety, both as a basic component and in special variants such as safety relays.
🧠 How does a relay work?
- Control current energises the coil
- A low-voltage signal (e.g. 24V DC) energises an electromagnet
- This moves an internal switching arm (similar to a switch)
- Main contacts switch the load
- The contacts make or break the power circuit
- Suitable for AC or DC loads
- Contact types
- NO (Normally Open) → closes when energised
- NC (Normally Closed) → opens when energised
- Changeover contact (NO + NC in one)
Relays provide electrical isolation between the control circuit (low power) and the power circuit (high power).
🏭 Use in industrial automation
- Driving actuators (motors, valves, lamps)
- Interface relays between PLC outputs and field equipment
- Time relays for delay or pulse behaviour
- Monitoring relays for current, voltage, temperature or level
- Safety relays for emergency stop, gate or light-curtain applications
Used in:
- Control cabinets, MCCs, panel building
- Machines, conveyor systems, process plants
🔍 Types of relay
| Type | Use |
|---|---|
| Electromechanical | Standard industrial relay (clicking sound) |
| Solid State Relay (SSR) | Silent, wear-resistant, faster switching |
| Safety relay | Certified for safe shutdown |
| Time relay | Switches after a set delay |
| Monitoring relay | For protection against deviating values |
SSRs have no moving parts and are ideal where there is frequent switching.
🔐 Safety considerations
- Use force-guided contacts for safety-critical functions
- Avoid overloading and select appropriate contact ratings
- Provide flyback diodes for inductive DC loads
- Protect both control and load sides with fuses or a circuit breaker
- Integrate with Lock-out Tag-out procedures and work permits for any work
For safety functions (e.g. emergency stop), always use certified safety relays or a Safety PLC.
📌 In summary
A relay is a versatile switching element that is indispensable in industrial electrical engineering. Whether for simple control or critical safety applications, choosing the right relay is essential.