Between 28 August 2011 and 8 September 2011, five 33kV OIP bushing failures occurred at JBB Ali Grid Station belongs to Oman Electricity Transmission Company (OETC). Three failures resulted in a complete loss of supply and around 130,000 consumers were reported with a lack of electricity. OETC assigned a third party investigator to conduct a power failure investigation in order to obtain a possible root cause for these failures. The investigation approach or process that were followed included the following phases: ·Phase 1: Facts finding and preliminary analysis·Phase 2: Destructive dismantling and laboratory investigation through.·Phase 3: Root cause analysis. From the investigation and buildup of hypothesis, no single most likely cause of failure could be identified. Metal migration and semi-conductive copper sulphide migration in the oil that led to surface contamination of insulation in the stress zone is considered causal factor for bushings insulation degradation. Copper sulphide in bushing was detected caused due to corrosive Sulphur in oil. There has to be another factor influencing the weakened insulation breakdown. The source of this factor can be found in the suspected frequently occurring earth faults in the 33 kV line feeders, leading to a line voltage to ground of 33 kV and transient over voltages due to switching off the faulted feeder. All the OIP bushings were supplied in the same patch filled with the same oil and operated in identical service conditions so degradation of insulation as chemical process can be considered in similar state. Electrically all bushings were subjected to identical voltage and load related stresses as transformers are operated in parallel. Thus combinations of these two factors are considered as causal factor for frequent breakdown of multiple bushings.