An electrical power interface of a vehicle, in particular a utility vehicle or a rail vehicle, comprising a first power contact (11; 31) that can be brought into contact with a second power contact (21; 41) to produce an electrical power path; an auxiliary contact (14; 34) that is arranged so as to be electrically insulated from the first power contact (11; 31) and that is arranged with respect to the first power contact (11; 31) in such a way that, when the first power contact (11; 31) and the second power contact (21; 41) are brought into contact with one another to form the electrical power path, the auxiliary contact (14; 34) likewise makes contact with the second power contact (21; 41), wherein the auxiliary contact (14; 34) is connected electrically in parallel with the first power contact (11; 31) via a measurement path (3, 4); and a measurement device for capturing the state of an electrical contact-connection between the first power contact (11; 31) and the second power contact (21; 41) that is configured to determine a voltage drop across the electrical power interface and/or a variable correlating with the voltage drop via the measurement path (3, 4), wherein the measurement device is a voltage measurement device that measures the voltage difference between the auxiliary contact and the first power contact, characterized by a plug that can be brought into contact with a second power contact that is configured as a socket, wherein both the first power contact and the auxiliary contact are in the form of a portion of a plug element of the plug and are connected to one another via an electrically insulating portion of the plug element. a first power contact (11; 31) that can be brought into contact with a second power contact (21; 41) to produce an electrical power path; an auxiliary contact (14; 34) that is arranged so as to be electrically insulated from the first power contact (11; 31) and that is arranged with respect to the first power contact (11; 31) in such a way that, when the first power contact (11; 31) and the second power contact (21; 41) are brought into contact with one another to form the electrical power path, the auxiliary contact (14; 34) likewise makes contact with the second power contact (21; 41), wherein the auxiliary contact (14; 34) is connected electrically in parallel with the first power contact (11; 31) via a measurement path (3, 4); and a measurement device for capturing the state of an electrical contact-connection between the first power contact (11; 31) and the second power contact (21; 41) that is configured to determine a voltage drop across the electrical power interface and/or a variable correlating with the voltage drop via the measurement path (3, 4), wherein the measurement device is a voltage measurement device that measures the voltage difference between the auxiliary contact and the first power contact, characterized by a plug that can be brought into contact with a second power contact that is configured as a socket, wherein both the first power contact and the auxiliary contact are in the form of a portion of a plug element of the plug and are connected to one another via an electrically insulating portion of the plug element.