Electronic trip unit for a low-voltage circuit breaker including a receptacle for a module, and a module for connection to the electronic trip unit

An electronic trip unit is for a low-voltage circuit breaker of a three-phase AC circuit. The electronic trip unit includes a housing with a front, rear, left-hand, right-hand, top and bottom face; a controller to compare current measurement values of the three-phase AC circuit with at least one of current limit values, current period limit values and time period limit values and to output a tripping signal, for interrupting the three-phase AC circuit, upon the values being exceeded. The electronic trip unit further includes a receptacle to provide a connection between the electronic trip unit and the module, the module being receivable by the left-hand, right-hand, top or bottom side face of the housing. At least one function of the electronic trip unit is activated upon the module being received. The front face of the housing includes a viewing window. At least a portion of a received module is visible.