System comprising a household appliance and a mains cable, wherein - the mains cable comprises an appliance-side interface unit, which is connected via lines to a main-side interface unit in order to connect the mains cable to an electrical supply network; - the household appliance comprises an appliance interface for the connection of the mains cable and a control unit; wherein - the appliance interface is configured to form a plug-in connection with the appliance-side interface unit of the mains cable; - the appliance interface comprises power-contact parts, which are configured to form power connections (211, 221) with power-contact parts of the appliance-side interface unit, in order to supply the household appliance with electrical energy from an electrical supply network; - the appliance interface is configured to indicate a quality of the plug-in connection between appliance interface and appliance-side interface unit by means of a signal on a signal line; and - the control unit is configured to prompt a measure relating to the quality of the plug-in connection in the household appliance as a function of the signal; and - the appliance-side interface unit of the mains cable comprises power-contact parts, which are configured to form power connections (211, 221) with power-contact parts of the appliance interface; and wherein - the appliance-side interface unit and the appliance interface have measuring-contact parts, in order to form a first measurement connection in parallel with a first power connection of the power connections (211, 221), and the appliance-side interface unit comprises first means, which are configured to acquire information relating to a transfer resistance of the first power connection via the first measurement connection, and to provide said information via the signal line, characterised in that - the appliance interface comprises a light source, which is configured to emit light; and that - the appliance interface comprises a phototransistor, which is configured to couple the signal line to a reference line or uncouple it from the reference line as a function of an acquired quantity of light; and that - the appliance-side interface unit and the appliance interface are embodied such that the quantity of light from the light source, which is acquired by the phototransistor, depends upon a degree of overlap between the power-contact parts of the appliance-side interface unit and the power-contact parts of the appliance interface, and - wherein the appliance-side interface unit comprises a reflective layer, which is configured to reflect light from the light source to the phototransistor, if the degree of overlap between the power-contact parts of the appliance-side interface unit and the power-contact parts of the appliance interface reaches or exceeds a predefined threshold value for the degree of overlap. - the mains cable comprises an appliance-side interface unit, which is connected via lines to a main-side interface unit in order to connect the mains cable to an electrical supply network; - the household appliance comprises an appliance interface for the connection of the mains cable and a control unit; wherein - the appliance interface is configured to form a plug-in connection with the appliance-side interface unit of the mains cable; - the appliance interface comprises power-contact parts, which are configured to form power connections (211, 221) with power-contact parts of the appliance-side interface unit, in order to supply the household appliance with electrical energy from an electrical supply network; - the appliance interface is configured to indicate a quality of the plug-in connection between appliance interface and appliance-side interface unit by means of a signal on a signal line; and - the control unit is configured to prompt a measure relating to the quality of the plug-in connection in the household appliance as a function of the signal; and - the appliance-side interface unit of the mains cable comprises power-contact parts, which are configured to form power connections (211, 221) with power-contact parts of the appliance interface; and - the appliance-side interface unit and the appliance interface have measuring-contact parts, in order to form a first measurement connection in parallel with a first power connection of the power connections (211, 221), and the appliance-side interface unit comprises first means, which are configured to acquire information relating to a transfer resistance of the first power connection via the first measurement connection, and to provide said information via the signal line, characterised in that - the appliance interface comprises a light source, which is configured to emit light; and that - the appliance interface comprises a phototransistor, which is configured to couple the signal line to a reference line or uncouple it from the reference line as a function of an acquired quantity of light; and that - the appliance-side interface unit and the appliance interface are embodied such that the quantity of light from the light source, which is acquired by the phototransistor, depends upon a degree of overlap between the power-contact parts of the appliance-side interface unit and the power-contact parts of the appliance interface, and - wherein the appliance-side interface unit comprises a reflective layer, which is configured to reflect light from the light source to the phototransistor, if the degree of overlap between the power-contact parts of the appliance-side interface unit and the power-contact parts of the appliance interface reaches or exceeds a predefined threshold value for the degree of overlap.