At around 1:15 a.m., coke-oven gas was vented to atmosphere when the electricity powering a 225 kV transformer at a power station supplying a steel mill was lost. Electricity is supplied to the steel mill by three 150 MVA transformers from a power station that decrease it from 225 KV to 63 KV. Two substations supply electricity to the units. Each substation has two step-down transformers (63 KV to 20 KV).

When the blast furnace was started back up following maintenance, one of the 225 KV/63 KV step-down transformers began heating up. Per procedure, the operator connected it to the backup transformer in order to disconnect the overheating transformer. In doing so, the 63 KV output circuit breaker tripped, cutting the steel mill’s power and thereby shutting off the coke-oven gas extraction system and diverting the gases to the stacks to be vented. These gases contained carbon monoxide, hydrogen sulphide, methane and other VOCs such as benzene. An estimated 66 t of methane, 6 t of benzene and 31 t of carbon monoxide were released.

The emergency response unit was operational at 2:00 a.m. The internal emergency plan was implemented at 2:26 a.m. and 100 people were ordered to shelter in place. Air quality measurements were collected The shelter-in-place order was lifted at 4:30 a.m. At 5:10 a.m., power was restored to one of the two main transformers. This allowed the coking plant (including two gas extractors) to operate at 50% capacity. At 6:07 a.m., the coking plant’s stacks were closed. The site was switched back to the backup transformer due to a fault on the second transformer’s circuit breaker. The internal emergency plan was lifted at 6:30 a.m.

Following the event and at the request of the inspection authorities for classified facilities, an external company carried out a survey of the electrical facilities that was focused on equipment reliability. The company made many recommendations on the servicing and maintenance of these facilities. The accident scenario, which had been excluded from the safety report due to the absence of observed lethal or irreversible effects, was added to the next version in view of the visual impact and the high concentrations of carbon monoxide and hydrogen sulphide at ground level.

A similar event had occurred at the site the previous month (ARIA 51898).