Device for motor and short-circuit protection

Abstract

A device for motor and short-circuit protection in low-voltage plants includes a contact device (3) which is connected in series with a tripping circuit comprising two parallel-connected current branches. One of the branches comprises a first thermistor (1) serving as an overload protective means and connected in series with a second thermistor (2) serving as a short-circuit protective means. The second branch comprises a multi-turn excitation coil (4) which is adapted to influence the contact device (3) for opening thereof at an overcurrent. The resistivity of the two thermistors (1, 2) has a positive temperature coefficient in the temperature range in question. The first thermistor (1) may consist of a ceramic material or electrically conductive polymer composite. The second thermistor (2) consists of polymer composite with a extremely high content of electrically conductive filler and is, in addition, layered. This results in the thermistor very rapidly changing from a low-resistance state to a high-resistance state at a high current rate of change.

Dependencies are inferred from the claim text.