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The incidents
Power Distribution Failure - 29th May 2000

Events leading up to the incident

Prior to the start of the excavation of the trench representatives from the groundwork contractor, the excavation contractor and BP walked the length of the proposed trench on 17th April in order to identify any difficulties/obstacles likely to be encountered during the excavation and to mark the route. The cable route selected required a trench to be dug between two groups of cables, and above a third group of cables. The area was scanned using a detector prior to commencement of the excavation work.

Digging for the trench commenced on 18th April with the intention of excavating a trench 600mm wide and 900mm deep or until the cable tiles for the 33kV cable were reached.  Several squads excavated the trench simultaneously along the length and the work was carried out over a three week period and completed on 5th May.

Despite the earlier instructions from the "toolbox talk" on the 20th April a BP employee observed operatives from the excavation contractor using a clayspade to the sides of the trench at a depth greater than 300mm. Another "toolbox talk" was subsequently given to reinforce the message of safe working practice.

The 11kV cable was due to be installed in the trench on 8th May but was delayed. Following further inspections some additional trench work was carried out by the excavation contractor (8/9th May and 18/19th May).

On the 25th May a cable-laying operative from the cable-laying contractor observed a damaged tile and cable while laying rollers in the trench in preparation for laying the cable which had been rescheduled for 30th May. At this stage however he did not report the damaged cable in the belief that the cable was dead and it had already been reported.

Supervision during the excavation period was limited. The supervisor from the excavation contractor provided daily but not continuous site supervision since he was required to organise and supervise permits for several teams in addition to this trench work. Similarly the supervisor from the groundwork contractor and the BP Construction supervisor had other commitments and did not provide continuous supervision. In particular the BP Construction supervisor first visited the site of the excavation on 20th April since he had not been made aware by BP Works Projects Group that the excavation work had started.

Details of the incident

A serious loss of electrical power occurred at 18:07 p.m. on 29th May when an earth fault occurred on the 33kV feeder cable that ran underground from No.1 to No.5 electrical sub-stations. The occurrence of the fault was recorded by Scottish Power who reported a fault current in the order of 3000 amps for just over one second.

The fault should have been detected by the 33kV circuit breaker in No.1 electrical sub-station. If this had happened supplies would have been maintained to No.5 electrical sub-station through two other feeders and power supply to the Oil Refinery area would have been maintained. However the 33kV circuit breaker had previously been disabled and as a result the fault was cleared by the two 33kV circuit breakers in No.2 switchboard that fed No.1 switchboard. No.2 switchboard supplied a wider area and as a result mains power was lost to No.1, No.5 and No.10 sub-stations. This had a major impact on the operations of the Complex.

Figure 1 - Power Distribution Grid