Guide to Electrical Engineering Symbols: Thermal Relay, Contactor and Motor

In the field of industrial automation and electrical engineering, a thorough understanding of standardised symbols (IEC 60617) is essential for reading and designing professional electrical diagrams. This article details the key symbols used in motor protection.

1. The Thermal Cut-out Symbol (F1)

The thermal relay consists of three bimetallic strips connected in series for power control and auxiliary contacts (95–96 NC and 97–98 NO) for control.

• IEC symbol: A rectangle crossed by three broken lines (representing the bimetallic strips).
  Function: Protection against prolonged overloads.

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2. The Contactor Symbol (KM1)

The contactor is a power-handling device controlled by a coil (A1-A2).

3. Interactive Diagram: Direct Motor Start

Here is a realistic simulation of a complete industrial system. Note the physical connection between the push-button switch, the KM2 contactor and the thermal protection relay.

See also: For a detailed guide on setting the trip current (Ir), please refer to our expert guide on How Thermal Relays Work.

Note: Article written by the Electrotuto team of experts to support students studying for a BTS in Electrical Engineering.