Faulty load disconnector
In a robust cable network, with very few faults detected over time, supplying a city center, a short fault event occurred. Having no overhead line on the affected feeder, and the event being very short, a faulty component in a secondary substation rather than a cable fault was suspected.
A team was sent out to inspect the substation on the affected feeder. A load disconnector showed traces of flashover in one of the phases. PD measurements with hand-held equipment were carried out in the station to verify the ocular inspection. The faulty component turned out to be the arc chamber on the load disconnector.
The load disconnector was replaced with a newer type of disconnector. The work was planned and entailed no cutting of the electricity for customers. Without early warning this fault could have led to a trip of the entire substation, leaving thousands of customers without power for between 30 minutes and over an hour.