Glossary · Electrical engineering and control panels
Relay
Also known as: Electromechanical relay, Elementary relay
German: Relais
In electrical engineering, a relay is a device that switches one or more electrical circuits in response to an input signal, classically an electromagnetic coil that operates mechanical contacts.
- Control panels
In one sentence
A relay switches circuits in response to an input signal, classically a coil that operates mechanical contacts.
Example
A relay with a 24 V DC coil and changeover contact switches a 230 V indicator lamp from a PLC output.
How it applies
- Engineering: Relays provide isolation between the control and load circuits and can switch different voltages. Select coil voltage, contact configuration, contact rating for the load type and electrical life. Inductive loads need contact protection.
- Maintenance: Mechanical relays wear out; their electrical life depends on load current and switching frequency. Relays on sockets simplify replacement.
- Documentation: Show relay designations, coil and contact cross-references and ratings in the circuit diagram. The word "relay" is also used for protective relays in power systems, which are a different device class; clarify in terminology databases.
Relay vs. contactor
Relays switch signals and small loads; a Contactor switches power loads such as motors. Both are electromagnetic, but they are rated and standardized differently.