Case Summary:
A wastewater treatment plant experienced a power failure after working hours. This resulted in shutoff of the plant’s influent pumps, which lift the wastewater from the incoming stream into the processing areas of the plant. The power outage was detected by the plant’s computer SCADA, which attempted to autodial out and notify the plant operator of the issue. The emergency autodialer system failed due to a lack of power, and as a result no one was notified of the power outage. When workers arrived at the plant the next morning, the influent building was completely flooded with wastewater.
Purpose of Investigation:
The plant had recently undergone a major upgrade, including the addition of a large generator, new HVAC and pumping equipment, and new electrical equipment. Encorus Group was contracted to collect evidence from the site, including two 1200A circuit breakers and the uninterruptable power supply (UPS) for the phone system, in order to determine the reason for their failure. During the collection process, the condition of the items and their installation were also observed.
Analysis and Conclusions:
During the site investigation and collection of evidence, it was observed that the main circuit breaker (MCB) was installed without a neutral current transformer (CT), and the wires intended to connect the CT had been left hanging from the MCB. The plant generator also had incorrect ground-fault settings. Additional observations included incorrect breaker adjustment settings and an evident lack of maintenance to the UPS serving the fiberoptic phone lines, with its batteries being more than ten years old. It was also observed that a large hole had been cut between the wet well and the dry well, and a duct installed in the hole for ventilation, contrary to what had been specified for this situation. Review of the power study performed in advance of the plant upgrade project showed that ground-fault analysis had not been performed, and breakers were not properly coordinated in the study or specified in the subsequent design. When the main breaker of the plant was tripped due to a fault in an in-roof heater, these situations compounded to allow unchecked wastewater flow in the unoccupied plant, resulting in over a million dollars in cleanup and repair costs.
Recommendations:
Engineers must always ensure that adjustable circuit breakers are set correctly, and that ground-fault is carefully considered when determining the correct settings for protective coordination. Owners, operators, and maintenance personnel must ensure that UPS batteries are replaced regularly, and units inspected regularly.
