While testing the emergency lighting system during a renovation at a theater in Knoxville, Tenn., I noticed that certain emergency fixtures weren't operating correctly. I also noticed that general lights and numerous receptacles were still operating during the test. Since the generator was operating correctly, I assumed the electrical contractor connected the general light fixtures and receptacles to the emergency system by mistake. We started our search for the cause by shutting down the generator. The emergency lighting went out, but some of the standard light fixtures and receptacles still had power. I then noticed the "B" Phase power indicator light was still lit on the TVSS device. At that point the electrical contractor opened the cover on the main switchgear where we found a misplaced truck mirror shunting the "B" phase of the main breaker. It was touching the "A" Phase, but the small amount of rubber around its edge provided some insulation. But finding the mirror was only the beginning of the problem.
The electrical contractor could not safely remove the mirror without turning off the main power coming into the switchgear, so we called the electric utility. After sending three different crews to the site, utility personnel determined they couldn't do anything until the next morning. The main service was connected to the utility transformer inside a transformer vault within the building. Turning off (removing the primary fuses) power to the transformer doesn't turn the power off to the switchgear. The secondary of the utility transformer was connected to a downtown city loop. Turning the loop off meant turning off downtown Knoxville.
The next morning, the electric utility sent their best line crew to evaluate the problem. After they turned off the utility transformer and cut 11 sets of 4-#500 kcmil feeders, the power was off to the building.
The electrical contractor then removed the mirror, the utility turned the transformer back on, and the building regained power. Total outage time was about five minutes. Utility linemen spent the rest of the morning splicing the feeders back together while they were still hot.
To this day, no one will say how the mirror found its way into the switchgear. The electrical contractor installed the CTs while they were installing the switchgear. My guess is that the utility personnel used the mirror to help them wire the CTs to their meter and forgot to remove it when they finished their work. Imagine what would have happened if the mirror was wedged between any of the phases in the switchgear.
David Woytek, Jr.
Cleveland