The present invention relates to a magnetic protection device, in particular for an automatic circuit breaker or a disconnecting switch, preferably for use in low voltage systems. The invention also relates to an automatic circuit breaker comprising this device. The protection device (1) according to the invention comprises a low voltage circuit breaker comprising an air gap magnetic circuit (T), provided with a first portion (10) forming a first surface (11) of said air gap (T) and a second portion (20) forming a second surface (21) of the air gap (T). The first portion (10) constitutes a fixed section of the magnetic circuit, while the second portion (20) constitutes a moving section of the magnetic circuit. The magnetic circuit also comprises a flexible portion (30) that connects the first portion (10) to the second portion (20) with continuity, forming an intermediate section of magnetic circuit interposed between the fixed section and the moving section. The third flexible portion (30) advantageously allows a relative movement of the second surface (21) of the air gap (T) with respect to said first surface (11). In its essential form, the protection device (1) comprises an actuation element (50) associated with the second portion (20) of the magnetic circuit for the purpose of contacting a trip device of a circuit breaker to which the protective device is applied.