Motorschutzschaltern

Motorschutzschalter, or motor protection switch, are protective devices used in electrical installations to safeguard motors from damage caused by overloads and short circuits. They are designed to interrupt current to a motor when abnormal conditions occur, helping to prevent winding damage, overheating and fire risks. They are typically three-pole devices used in three-phase systems.

A Motorschutzschalter combines overload protection and short-circuit protection in a single unit. The overload protection is usually thermal, employing a bimetal element that heats with motor current and trips after a set time proportional to the overload duration. The short-circuit protection is magnetic and instantaneous, tripping when current exceeds the device’s instantaneous rating. Many devices offer adjustable overload settings and different trip curves (for example B, C or D) to match motor type and starting behavior. Some variants use electronic protection with microprocessor control, adding features such as phase failure detection, phase sequence monitoring, undervoltage protection and self-test.

In typical installations, the Motorschutzschalter is mounted in series with the motor and is often used together with a contactor and a motor starter. It can be reset automatically or manually after tripping, depending on the design. In larger systems, separate overload relays or electronic motor protection relays may be used to enhance protection and diagnostics.

Standards and selection: In Europe, motor protection switches generally conform to IEC 60947-4-1 and related standards; regional variants exist elsewhere. Selection is based on motor nominal current, service factor, starting method, ambient temperature and enclosure conditions. The device is set to permit normal running current while tripping under overload or fault conditions.