A connecting sleeve generally used for a bus bar connection is produced from an insulating elastic material, often an elastomer material, having insulating properties which are deteriorated by partial discharges and decrease over the operating time of the switchboard system. According to the invention, so-called partial discharge measuring methods must be carried out, inter alia also in the UHF range (UHF: ultra high frequency), in order to identify damaging effects on the insulating material in time. For the reliable and simple detection of the measuring signals, the connecting sleeve (M) has an outer, electroconductive surface (OA) which is earthed, and an inner, electroconductive surface (OI) to which the voltage potential of the bus bar (S) is applied, in addition to a coupling electrode (KE) which is integrated into the insulating material (I).