During a test conducted on one of the generator groups of a diesel power plant, a single-phase fault appeared at 11:10 am on the low-voltage cable of the step-down transformer output located at the -6 m level in the transformer gallery. The cable drawn ignited and fire spread to the upper cable paths. The 4-person crew, notified by the fire detection system, put out the fire with carbon dioxide extinguishers. The Internal Emergency Plan was activated at 11:25 am, at which point the authorities and classified facilities inspectorate were informed and all generator groups shut down; moreover, a head count was taken of onsite operators to ensure all had assembled at the designated meeting point. By 1 pm, the carbon monoxide concentration in the machine room had returned to zero and the emergency plan was lifted at 1:55 pm. Fire-fighters left the scene at 2 pm after proposing to re-inflate the plant’s self-breathing apparatuses using their compressor in order to restore complete autonomy for a potential subsequent intervention.

A total of 10 employees, slightly intoxicated by the smoke, were evacuated to hospital for medical exams. Five cable paths were affected, with 120 cables either burned or damaged over 2 linear metres. Full plant operations started back up on 26th April, 4 pm. This incident did not cause an electrical outage, as electricity supply could be rerouted, in particular from the island’s dam turbines.

The test on the alternator’s vacuum curve recording triggered this accident; the test consisted of sending an excitation current into the alternator and then measuring the ratio between this current and the alternator output voltage. The test, performed by 2 agents on a zero-torque engine, was run with an open circuit-breaker on the step-down transformer. The cable was found to be live yet uncharged, hence without any electrical protection. This was the 2nd such test at the power plant; the 1st was conducted without encountering any problems. Four factors led to the hypothesis of cable ageing due to electrically or mechanically-induced fatigue: 1) strong electrical charges on this section of cable over the past several years, especially when fire broke out on the electrical panel in 2004 (ARIA 28565); 2) the cable ignited over its curved part in the direction of the panel, which was potentially more vulnerable; 3) the cable had been installed 30 years prior and therefore was much more heavily charged than a networked cable due to the presence of strong charges on the generator groups; and 4) a recent rat extermination campaign could have foreshadowed an alteration in cable insulation due to rodents. Any link between the fire and human causes was excluded.

In addition, the monitoring system designed to notify the control room technician should a fault arise was not triggered, leading to the conclusion that this system’s grounding was defective due to a connection problem.