The HSE concluded that the power loss which occurred on the 29th May 2000 was caused by an earth fault on a 33kV underground power cable between No.1 and No.5 sub-station and the failure of the 33kV circuit breaker in No.1 sub-station to trip and clear the fault.
The source of the earth failure was not immediately apparent. The cable which failed was situated in the bottom part of the excavated trench, almost in the side wall of the trench and only protruded from its protective cable tile over a short length.
The fault was ultimately cleared by two 33kV circuit breakers in No.2 electrical sub-station resulting in power loss to No.1, No.5 and No.10 electrical sub-stations.
The immediate cause of the power distribution failure was a combination of two direct causes:
Forensic evidence indicated that the earth fault was caused by physical damage to the cable from an air powered tool known as a clayspade. The clayspade equipment was operated by a number of different personnel during the construction of the trench and the cable was protected over the majority of its length, except in the location where the damage occurred, by the protective cable tile system. All personnel were aware of the responsibility to report any problems during the construction of the trench but none were reported. As a result of the damage the integrity of the lead sheath on the cable was breached, allowing water ingress, weakening of the cable’s insulation and the resultant earth fault.
Despite interviewing all the personnel who worked in the trench and who operated the clayspade there was a lack of corroborative evidence for the HSE to prove beyond reasonable doubt who struck it and when. However the investigation concluded that the cable was most likely damaged during digging of an adjacent trench for installation of a new 11kV cable to be run from the No.5 electrical sub-station to a new facility on the Chemicals area of the Complex on or around the 18th April.
The cable fault described above should have caused the 33kV circuit breaker in No.1 electrical sub-station to operate and clear the fault. However it failed to operate because its earth protection relay had been disabled by two small sections of plastic (cable ties with the ends cut-off) inserted in the connections between the relay and its current transformer. This meant that the earth fault protection relay was disabled and would not operate.
The HSE identified no evidence to suggest that the relay was disabled for malicious intent or in conjunction with the failure of the 33kV feeder cable. It is possible that it occurred during performance testing of the relays. Work activity was recorded as having taken place in 1999, 1993 and in the 1980s. Certainly in the 1980s there is evidence to suggest that the appropriate connector block was not available for the relay tests and that temporary plastic inserts would have allowed the testing to take place. If this was the case then they should have been removed after testing since the consequence of leaving them in-situ was to disable the functionality of the trip system. It is however difficult to envisage how the plastic inserts would not have been noticed during the secondary injection testing of the relay which was undertaken in 1993.