John M. Tobias, Ph.D., PE, Electrical Engineering Consultant ::::
Case Description: A new county Emergency Services Center (ESC) was constructed and placed into service. This ESC was responsible for taking 911 emergency calls, dispatching and coordinating fire, police and rescue responders. The electric power system included a three-phase 80 kVA Uninterruptible Power Supply (UPS), capable of powering the essential equipment of the ESC for some time to enable transferring power to a backup generator. During final installation, the UPS functioned to supply the essential equipment from its batteries, resulting in severe over voltages, damaging equipment downstream and nearly caused a fire in the ESC.
Analysis: UPS are always active in modern designs for backup power so that there is no need for switching. In this implementation, incoming alternating current utility power is constantly converted into direct current which charges the UPS batteries. The direct current supply is also converted back into alternating current to supply downstream equipment continuously. A bypass is also provided to take the UPS out of service for maintenance.
Forensic testing revealed that when the UPS switched from its normal operating mode to its battery mode, voltages to utilization equipment reached nearly 400 volts, where 120 volts was the nominal supply voltage.
Inspection of the UPS, which included analysis of the utility power system supplying the building, comparison of the installation to manufacturer instructions and analysis of the UPS event log, revealed that the UPS inverter had a ‘floating neutral’ and could not determine a ground reference for the synthesis of alternating current from direct current. Ground reference (essentially a zero volt reference) is essential because the circuit otherwise had no way to determine the output supply voltage. Consequently, the control logic varied the output, ‘hunting’ for the correct voltage. The result of the investigation was that a key component within the UPS that provided a neutral reference to the ground circuit through an isolating transformer was not installed. Utility supply to the ESC was three-phase, three-wire with ground derived from the main panel, but supply into the UPS was also a three wire (e.g. ‘delta’) system. When the UPS operated on batteries, the supply side ground was no longer in circuit.
Opinion: The UPS vendor/installer did not correctly install the UPS in consideration of the overall ESC power system. The general contractors did not exercise effective supervision and coordination between the subcontractors, contributing in this major error. Commissioning testing to check all modes of operation of the power system prior to connecting mission equipment was not conducted.
Result: The analysis and engineering opinion provided effective legal support for insurance subrogation to recover damages for the $750,000 value of the damaged equipment, and labor costs for reinstallation. Follow up work included devising and conducting commissioning testing to ensure correct function of the ESC power systems.
John M. Tobias, Ph.D., PE, Electrical Engineering Consultant with DJS Associates, can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.