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Power Site Survey: A Case of "What Went Wrong"

Apr 1, 1998 12:00 PM, Ken Michaels

Beware of consultants and companies that may have a financial stake in the resolution of your power quality problem. Also, make sure their personnel follow recommended investigative techniques/measurement practices.

Many of us in the power quality industry learn a great deal about safety practices and technical issues by reviewing our own successes (or failures) as well as those of others in the field. Unfortunately, when seemingly qualified people in the power quality industry make a mistake, it results in the customer wasting time and money in attempting to solve the problem. The following case study illustrates this point.

Identifying the problem A company was having operational problems with computer workstations fed from partition wiring in a particular area of its office building. In some cases, equipment operators complained that their systems were shutting off and coming back on again. Others noticed an "electrical smell" coming from the partitions themselves.

While inspecting a panelboard that fed the partition wiring, an electrician measured a large amount of AC current on an equipment grounding conductor and noticed part of the conductor's insulation had burned. The electrician also informed the owner that he suspected "electrical noise" or harmonics and suggested the company call a power quality expert.

Conducting the initial testing The owner employed the power quality investigative services of a consulting firm to determine the source of the problem. Their investigative approach included the following steps: * A one-week observation of the workstation tenants to verify the problem. * One week of power line monitoring at the building's 120V/208VAC main service entrance. * An analysis of the grounding system for the branch circuits by testing with three-lamp circuit testers.

The consulting firm then submitted a report with the following conclusion: The equipment grounding system for the office area's partition wiring was inadequate, as was the AC power from the utility.

Consultant's solution The firm recommended the owner install uninterruptible power supply systems (UPSs) at each outlet feeding electronic equipment. In its report, the consulting firm offered a special discount if the owner bought a certain number of units. Suspicious of the recommendation, the owner opted to retain the services of a second consultant.

A more effective analysis. The second consultant made the following tests, measurements, and inspections: * Visual inspection of equipment's surrounding environment. * True rms voltage measurements. * Wiring polarity checks. * Ground and neutral conductor impedance measurements. * Improper neutral-to-ground bonding. * Harmonic current and voltage. * Power line monitoring.

Here's what the above testing uncovered * The presence of high levels of harmonic load currents as well as line-to-neutral voltage waveform distortion at some of the partition outlets. This is one of the many symptoms possibly caused by resultant harmonic currents where non-linear loads are powered via partition wiring. * The neutral and equipment ground conductors were reversed at the outlet box where the cord, or "whip," fed the partition wiring. This reversal accounted for high levels of previously measured AC current on the equipment ground at the panelboard. * High impedance connections along the hot, neutral, and equipment grounding conductor paths. * Satisfactory AC power delivered by the electric utility. However, cyclic three-phase loads fed from the same panelboard as the partition wiring caused the rms voltage to decrease sharply at each of the loads.

The conclusion: The combination of harmonic load current, various loose connections found throughout the partition wiring, and the neutral-to-ground reversal (which caused the load current to rely ona minimum-sized equipment grounding conductor path) contributed to voltage waveform distortion. Ultimately, this caused a decrease in the rms voltage for the equipment.

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© 2012 Penton Business Media, Inc.

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