System for monitoring gas insulated substations

A diagnostic measurement system for gas insulated electrical substations, comprising means (20) for automatically monitoring a plurality of UHF couplers (11) fitted to pressure vessels in the substation, the monitoring means (20) comprising means (21B) for detecting individual partial discharge events, means (21C) for characterizing detected events according to amplitude and time of occurrence and for representing each characterized event in a data format, said characterizing means (21C) being responsive to said detecting means (21B), means (21D) for communicating data representing characterized events from said characterizing means to an analyzing means (25), the analyzing means (25) comprising (a) pre-stored data representative of known pre-fault conditions, and (b) comparative means for comparing the pre-stored data with the communicated data, and for issuing a warning signal in the event of identity, and means for compressing the data representing a plurality of detected events from each of said plurality of UHF couplers into a single data word for communicating to the analyzing means (25).