In the late morning, a fire broke out in one of the electrical cabinets of a transformer room at a wastewater treatment plant. The transformer powers the pyrolysis installations. It also feeds power to a secondary transformer supplying utilities (energy, compressed air, etc.) to the sludge drying installations. The fire detection system signalled the outbreak of fire. Once the personnel determined that it was not a false alarm and that the fire could not be controlled locally, the internal emergency plan was activated at 12 p.m. The fire brigade arrived at the scene at 12:25 p.m. After the on-call maintenance supervisor had disconnected the electrical power supply, they were able to bring the fire under control with a dry chemical extinguishing agent (powder) at around 2:30 p.m. First responders left the scene at 3:30 p.m. No injuries were reported. The operator encountered difficulties in putting the sludge drying installations back into operation. This resulted in a reduction of the incoming flow of the wastewater so as not to overload the sludge production and treatment process. Two generators were delivered and commissioned the following day at 4 p.m. Normal activity resumed the following evening, although five days were needed to discharge the sludge. The damage was significant and totalled more than €100,000: electrical cabinets damaged by the heat and fire-extinguishing powder, main low-voltage switchboard destroyed, and operating losses associated with the shutdown of the sludge drying and incineration system.

A battery failure caused the outbreak.

The expert-led investigation determined that an explosion occurred on a capacitor bank in one of the electrical cabinets. The resulting fire then spread to the adjacent cabinets. The explosion followed a heating up of the bank. The origin of the temperature rise may have been due to a failure of one of the bank’s eight steps, four of which had been in operation for more than seven years. The bank’s manufacturer recommends preventive replacement of the steps every five years. However, within the scope of its maintenance contract with the operator, only steps that are out of service are replaced. Another source of possible overheating may have been increased loading of the battery, although it is dimensioned for this. This loading follows the operation of the transformer on a single busbar instead of two for a period of five months as a result of modifications made to the high-voltage power supply.

The operator reinforced the risk analysis conducted during transient operating phases while work is underway. The supplier was contacted in an effort to understand and re-evaluate its step maintenance practices.