Failure Analysis of a UPS

An answering service company experienced failure of its computers. Around 9 PM, the UPS (uninterruptible power supply) unit came on, that is, it applied the ac voltage generated from the backup battery, and stayed on although apparently there was no interruption in the normal power supply from the utility system. As a result of the simultaneous feeding of the backup power and utility power, the computer network supplied from the UPS was irreparably damaged. Most likely, a transient overvoltage occurred, exceeding the allowable voltage levels of the computer equipment. This consultant was asked to perform failure analysis and, possibly, to testify in court, should the answering service company sue the UPS manufacturer for compensation.

It must be mentioned, that the manual of the UPS unit specifically mentions that "for safety reasons" the UPS unit should not operate with the simultaneous supply from the utility line and backup battery. This is understandable, since the resultant output voltage waveforms differ significantly. One of the fundamental rules of electrical engineering prohibits parallel connection of two different voltage sources, because of the hazard of high balancing overcurrents between these two sources. Such overcurrents, in presence of inductances in the system can, in turn, cause transient overvoltages.

The alleged failure was caused by the battery-fed inverter producing the supply ac voltage for the computer network. The inverter was activated by a transient voltage dip in the utility line and not deactivated when normal operating conditions were restored, or it became active under normal operating conditions of the utility line.

Switching transistors (power MOSFETs IRZ44) of the inverter were controlled using a pulse width modulation technique implemented in the modulator 3524B on the so-called Daughter Board. The modulator supplied switching signals to gates of transistors. In turn, the modulator was activated by the INV_ON signal coming from the utility voltage sensor on the so-called Mother Board. The INV_ON signal was transmitted from the Mother Board to the Daughter Board via Pin 5 of Connector J13. An analysis of the flow of logic signals has indicated that if the connection were imperfect, the inverter would become active independently of the value of the signal coming from the utility voltage sensor.

To confirm results of the analysis, an experimental investigation of the UPS unit was performed. The UPS unit, reportedly the same in which the alleged failure had happened, was obtained from the manufacturer. The unit was properly installed and powered, and a 300-W load (two 150-W light bulbs) was fed from it. Normal operation of the UPS was observed, with the utility voltage both on and off. However, when the contact at Pin 5 of Connector J13 was purposefully broken, the inverter kept operating even when the UPS unit was powered from the utility line.

Expert Analysis

At completion of the performed investigation it was concluded that the UPS unit was not fail-safe when operated from the utility line. When the connection at Pin 5 of Connector J13 was imperfect, the inverter received an ON signal, no matter what was the condition of the utility line. The loss of contact could have happened due to temperature changes, mechanical disturbance, or an assembly error during manufacturing.

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