Electrical switching unit including a shaft for supporting at least one mobile contact in an electrical switching unit, said unit being housed in an insulating housing and including at least one unipolar assembly and a control mechanism, said unipolar assembly including an arc quenching chamber, said chamber enclosing a fixed contact, a mobile contact supported by said shaft for supporting the mobile contacts, and an arc quenching device, said shaft for supporting the mobile contacts being able to be brought, by said control mechanism, manually or automatically, into a position for opening the mobile contacts of the various subassemblies, in which position the fixed contacts and the mobile contacts are separated, or what is termed a closed position of the unit, in which the mobile contacts are respectively in contact with the fixed contacts, said shaft including an elongated one-piece component forming a shaft, said component being made of an insulating material, said shaft being mounted so as to be articulated with respect to the base of the unit about an axis extending substantially parallel to its longitudinal direction, said shaft including means (45,46) for mechanical linking to the abovementioned control mechanism by way of transmission means, said shaft including, for each phase or neutral subassembly (2,3,4,5), means (20 to 24 and 53 to 58) for holding and positioning the mobile contact (7,47) and means (34,35,36,37 and 63 to 66) for holding and positioning the contact pressure spring (18,61) associated with the mobile contact (7,47), said shaft being mounted so as to be articulated with respect to the housing by way of two axles forming bearings that are situated on the two opposite ends of said shaft, the abovementioned mechanical linking means being situated close to one of the ends of said shaft, said shaft including, on either side of each of the areas intended to receive a mobile contact and its associated contact pressure spring, parts whose shape is intended to ensure stiffness of the shaft and transmission of the mechanical torque all along the shaft, and what are termed rearming means (40,41) formed by particular shapes of the shaft and able to allow rearming, after opening upon a fault, of the various thermal trips belonging to the various abovementioned phase subassemblies (2,3,4), respectively, characterised in that said unit includes a thermal protection device including firstly what is termed an active bimetal able to produce heating when an overcurrent occurs above a certain level above the nominal current value, this heating leading to thermal tripping of the control mechanism of the unit in such a way as to bring about opening of the contacts, and secondly what is termed a compensation bimetal that is intended to compensate for the variations that changes in the ambient temperature bring about in the temperature of the circuit, the compensation bimetal strip being mounted so as to be able to rotate about what is termed a first fixed axis belonging to a support of the unit, the two bimetal strips (42,43) being joined to one another at one of their ends by way of a connection device, this connection device being deactivated when an overcurrent occurs, this deactivation leading to actuation by the compensation bimetal strip of the control mechanism, and of the rearming means (40,41) that are able to return the connection device to the activated position when the contacts are opened, and in that the axis of rotation of the mobile contact-bearing shaft and the axis of rotation of the compensation bimetal strip are brought close to one another and extend substantially parallel with respect to one another, and in that the mobile contact-bearing shaft includes what is termed a first part interacting with what is termed a second part that is joined to the compensation bimetal strip so as to rotate therewith, these two components (40,41) being arranged with respect to one another such that, after tripping of the mechanism, when the mobile contact-bearing shaft is driven in the direction of opening of the contacts, what is termed the first part drives what is termed the second part in rotation, and therefore drives the compensation bimetal strip in a direction opposite to that that led to the mechanism being tripped, until the two bimetal strips (42,43) are reconnected to one another.