Traveling through O'Hare airport recently to catch (no - more likely to wait for) a plane and seeing banks of monitors flashing their woeful tales of canceled and late flights reminded me of a case of a few years back. A monitor manufacturer's attorney called me to report (with just a bit of hyperbole) that an employee, had been attacked and injured by a monitor she was inspecting! My charge was to determine the cause of her injury. At the manufacturer's plant, with other parties present, I saw the subject monitor and observed monitor manufacturing. Various components and the cathode ray tube were installed and the completed chassis moved to a test station. An inspector powered up the unit, fed in a calibration signal, and aligned the monitor for proper color and image.
Scene of the Crime
The injured inspector testified that she turned the monitor with one hand and reached with her other hand to adjust several control potentiometers. As her arm came close to a wire from the high voltage (HV) supply to the picture tube, she was severely shocked. She jumped back from the wire and fell, pulling the monitor onto her body with resulting deep muscle injuries, and severe cuts and scratches. Her lawsuit named the power supply manufacturer and other component suppliers. In turn, they sued the monitor manufacturer, citing improper and careless assembly and test methods.
With some of the control potentiometers and the beam alignment magnets close to a wire carrying high voltage, I suspected her shock came when she touched that wire or a live terminal. A careful, nine-year employee, she had no previous shock injury. She claimed she followed the same procedure as always, but that the monitor "came at her." Although I suspected that she might have become careless working near the high voltage lead, I left open the possibility that the HV lead insulation was defective.
Video monitors are essential for everything from viewing industrial processes to video games to flight schedules at the airport. The television-type picture tube used in many monitors requires 20,000 to 30,000 volts to accelerate a fluctuating electron beam to the inside face of the glass envelope. The beam then strikes a field of phosphorescent dots or bars. Depending on its photochemistry, each bar or dot produces one of three primary colors when excited by the beam. An image is produced as the magnetically or electrostatically aimed beam sweeps both horizontally and vertically striking a particular dot or bar to produce a color, while instantaneous beam current controls brightness. A power supply furnishes the required high voltage, which is sent to the cathode ray tube by a conductor not unlike a high-quality spark plug wire.
The Investigation
In this case, the HV supply was purchased as a pre-assembled unit in a metal cage or "dog house," with a high voltage wire and connector attached. I reviewed witness statements, production records, monitor technical specs and assembly procedures, and wire specifications. Through my client, the manufacturer's attorney, I requested a test of the HV power supply wire on the subject monitor. With all parties present, the subject monitor was tested. The high voltage wire described above was found to have such severe electrical leakage that visible sparks jumped from random places on the wire to the probe of a high-voltage meter. With mutual agreement, this wire was removed and subjected to tests specified in Underwriters Laboratories (UL) procedure. The wire failed all tests. HV power supply specs called for UL listed wire that could only be UL labeled after passing severe tests such immersing an energized wire in water to measure insulation integrity. I concluded that unlabeled wire got into the production stream where the high voltage supplies were manufactured.
The Smoking Gun
By way of the discovery process, I learned of documentation that revealed unlabeled wire was used on the power supply in the subject monitor. At the offshore plant of the HV power supply builder, an inspection revealed stocks of unlabeled rolls of high voltage wire, evidently used when supplies of UL labeled wire ran low. Serial numbers of faulty HV power supplies allowed a recall of monitors having those potentially dangerous HV power supplies. Samples of HV wires from those monitors also failed the UL tests.
I prepared my opinion report citing the various wiring standards, monitor manufacturing procedures, and the HV wire failures. After agreeing to let my client, the monitor manufacturer, out of the case, the HV power supply manufacturer ultimately settled with the injured inspector. n
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