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Jiggling Can Be Dangerous

Nov 1, 2000 12:00 PM, By Thomas P. Shefchick, P.E.

Even the best design cannot overcome an improperly installed, modified, or maintained electrical system.

As he looked inside the door of a motor starter to see what was preventing the unit from closing, the electrician paused to evaluate the electrical system. He quickly identified the problem: jammed interlocks—not such a big deal, he thought. In an attempt to clear the control circuit, he simply jiggled the engagement handle of the starter. At that instant, a short circuit occurred within the starter cubicle—leaving the electrician with serious burns and causing severe damage to the starter structure.

What triggered the unexpected short circuit? By jiggling the engagement handle, the electrician caused some loose conductive materials within the starter to fall between the phases of the primary connection terminals inside the starter cubicle. Proper design, installation, modification, or maintenance could have prevented this accident.

As in most cases, the injured party wanted some answers. The burned electrician hired an attorney to seek compensation for his suffering. The attorney hired a forensic mechanical engineer, who retained me to assist with the electrical specifics of the case.

I examined the burnt starter, burnt cubicle, an unburned identical starter, the repaired starter cubicle, specifications and drawings, manufacturer’s instruction book, facility’s installation and maintenance records, and supplemental reports made by other investigators and engineers. I also reviewed applicable industry electrical standards and interviewed the electrician. Finally, I submitted a report in conjunction with the mechanical engineer to the attorney. After reconstructing the incident, here are the steps leading up to the accident.

After the motor failed to start up during a changeover of pumps, the electrician came to troubleshoot the controls of the 2.4kVAC motor starter. He removed the starter from its cubicle and operated it with an external testing rig. The starter operated correctly in its external environment. However, it would not close when he reinserted it in its cubicle. (This facility had experienced problems with the starter’s engagement mechanism ever since the equipment’s original installation.)

Here’s where the problem started. Too often, electricians try to correct an electrical problem by jiggling an electrical device, mechanism, or switch. This act sometimes expels foreign material from a jammed mechanism. Other times, this motion can cause items to fall into proper alignment for conduction of electrical current. Occasionally, jiggling causes conductive materials to fall in such a way that allows an unplanned path for current flow—causing a short circuit. Whether the current path is to ground or phase-to-phase on polyphase systems, the erroneous electrical current flow typically results in damage to equipment or injury to anyone located near the current path.

This particular engagement mechanism contained safety interlocks, to prevent operation of the starter if the unit was not properly located within its cubicle. Someone had modified the starter’s mechanism to correct a prior engagement problem. However, a misalignment still existed. The specifications and drawings of this starter required placement of electrical insulation on each phase of the polyphase system. This phase insulation was an additional cost factor, since the phase bus bars were normally mounted bare on insulators.

After examining the equipment, I found the primary electrical connection terminals in the starter cubicle didn’t have insulation between them. Indentations existed between the primary connection terminals, but the drawings didn’t illustrate nor indicate the purpose of the indentations. Installers should have mounted phase barrier insulation upon the indentations. Instead, the indentations aided in the accumulation of foreign materials between the phases. The components of the starter’s operating mechanism were defective, and someone had modified them on site.

After inspecting the repaired starter cubicle, I found particles of machined metal inside. These metal particles could have resulted from the manufacture, installation, or modification of the starter. However, the manufacturer, installer, or modifier should have removed the metal particles before energizing the equipment. Furthermore, high-voltage dielectric testing after installation, modification, or maintenance would have revealed the presence of a defect in the starter’s insulating system. The insulating defect, lack of phase insulation at the primary connection terminals, and a jammed operating mechanism united in a disastrous short circuit.

The electrical design of this system should have incorporated insulation between phases at the primary connection terminals, using notations on the design drawings and instruction books to check for its proper location. Furthermore, power isolation switches should have been included in the design so an electrician could troubleshoot the operating mechanism of the starter without the main power on (thus not exposing the equipment and electrician to potential serious damage/injury). Power isolation switches would have prevented the electrician’s injuries.

I was not called to testify, because the matter settled out of court.The engineering lesson to be learned from this experience is simple: A defective design, installation, or maintenance can cause injury as well as damage equipment. Even the best design cannot overcome an improperly installed, modified, or maintained electrical system. And quite frequently, all four factors interact with disastrous consequences.

Shefchick, P.E., is a consulting forensic electrical engineer in Sunnyvale, Calif.

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