On 29th May 2000 at 18:07 p.m. all power was lost to No. 1, 5 & 10 electrical substations that supply electrical power to the North Side of the Complex which contains the Oil Refinery, various chemical plants, utility plants and logistics facilities.

As a result, emergency shutdown of the Oil Refinery and the chemical plants on the North Side occurred and the utility plants were also affected due to a loss of power to the main cooling water pump systems. (There was some smoky flaring visible as a result of the emergency shutdown.)

In addition because of the duration of the power failure, a controlled shutdown of some other facilities elsewhere on-site (some chemical plants on the South Side and the Kinneil operations) was also necessary because the supply of steam for the correct operation of the flare system could not be maintained.

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:

The Earth Fault
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.

Circuit Breaker Failure
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 power distribution failure had the potential to cause fatal injury and environmental impact, although no serious injury occurred, and there was only short term impact on the environment.

KEY ISSUES:
• Systems of work;
• The clarity and adequacy of instructions;
• The adequacy of supervision;
• Operatives behaviour;
• BP planning processes;
• Risk assessments carried out by the contractors;
• Details in the method statements;
• Inconsistent and different methods of application of the permit-to-work (PTW) system;
• Procedures, systems of work and test equipment for the testing of the 33kV circuit breaker;
• Implementation of maintenance policies.

Image Credit: HSE