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Incident took place at OPP substation at 9. 31

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1 Incident took place at OPP substation at 9. 31
Incident took place at OPP substation at hrs on the 13th of Feb 2007. Six persons were injured (4 from OPP and 2 from EPC contractor) One person died at Sohar hospital later in the day 6.6 KV switchgear (incomer to Main Transformer ‘B’) was damaged in the incident

2 Emergency Response Team responded immediately and conducted a search and rescue operation
Incident declared as level 2 ROP Civil defense and SRC called immediately and they arrived at 10 a.m. Site declared normal at hrs

3 Summary of the incident
On 09:31:38 13th February 2007, ground fault through the human occurred during rectification work on the earth switch inside the voltage transformer compartment of the 6.6 KV incomer ’B’ The technicians have isolated the Circuit breaker of the 6.6kV Incomer B (secondary of the main ‘B’ transformer) and then tried the earth switch which did not close. Then they opened the 6.6kV voltage transformer cabinet door overlooking the isolation of 33KV primary of the transformer. After racking out the Voltage Transformer, the person approached inside the cabinet compartment of incomer B to rectify the Earth Switch mechanism while the bus was live and the accident took place.

4 Single line diagram of Incoming power
CB (0BF1) TR1B VT ES 6.6KV FEEDER CB TIE CB (0T) CB (0AF1) CB (1A) TR1A 33 KV TIE CB (1T) 33KV FROM SRC SUBSTATION INCIDENT LOCATION CB (0B) CB (0A) CB (1B) 33 KV primary not isolated 6.6KV incomer

21 22 23 24 25 26 27 28 29 30 31 32 33 6.6kV CB (1B) RACK OUT 09:21:54 Minutes AM 09: 6.6kV SWGR Fault Signal 09:31:38 INCIDENT TIME: FEB :31:38 33kV CB (0BF1) Trip Open FIRE ALARM SIGNAL (S/S) 09:32 12 13 14 15 16 17 18 19 6.6kV CB 1T CLOSE 6.6kV CB 1B OPEN 09:13:15




9 Root Causes for the accident
Transformer primary 33KV power supply not isolated hence 6.6 KV incomer to the switch gear was live. Opening of 6.6 KV switch gear bottom voltage transformer door and removal of transformer and side cover to rectify earth switch without valid work permit and under live conditions.

10 Causes contributing to the accident
Isolation of 33 KV primary supply to transformer ‘B’ which was mentioned in the work permit was not accomplished and Earth switch for 6.6 KV incomer switch gear not working. Logic provided by ABB vendor in the 6.6 KV incomer ‘B’ earth switch was incorrect and not matching with construction drawings. This incorrect logic was also preventing the earth switch from closing. (Post observation) Maintenance procedure was not followed strictly. Opening the Voltage Transformer door, removal of Voltage transformer and rectifying the earth switch without hazard analysis and work permit. Assumption that primary has been isolated and Misconcept that breaker cabinets for motor and incomers are same. Inadequate supervision and control of the job.

11 Recommendations Permits to work system to be followed strictly.
Tool box talk to be carried out before starting the job. Problem analysis to be done before performing rectification work. Logic design provided by ABB to be reviewed and rectified. Maintenance procedure to be followed strictly. Implement access control on the bus / voltage transformer doors when it is in live condition. Instructions on the secondary and incomer primary breakers that isolation else where is required for power OFF. Use of High voltage tester before accessing bus related work. Additional training for electrical team on ABB switchgears. Implement additional control measures for performing work on High voltage systems.

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