The invention relates to a disaggregation apparatus (4) for being used in a multi-group electrical network (5), which comprises multiple electrical groups (1, 2, 3), each comprising one or more appliances (11, 12, 13, 21, 22, 23, 31, 32, 33). A determination unit (7) determines an electrical group (1, 2, 3) comprising an appliance, of which an operational state has been changed, based on first changes in mains voltages (V1, V2, V3) measured while the operational state change occurs, second and third changes in the mains voltages (V1, V2, V3) measured while switchable loads (1n, 2n, 3n) are switched, and the resistances (R1n, R2n, R3n) of the switchable loads (1n, 2n, 3n). Thus, a misdetection due to an operational state change of an appliance in another electrical group may be avoided and the accuracy of disaggregation may be improved in a multi-group electrical network (5).