Many years ago, I worked at an engineering university that did its annual Spring Musical at a local supper club. Powering the lighting system required a temporary tie-in to the kitchen power. The electrician opened up the big panel at the service entrance, and clearly there were three hot lines, a neutral, and a protective grounding terminal. Just to be sure, he measured the voltage between one of the hot lines and the neutral, and found it was 120 volts, so everything was as it should be. He hooked up our power-distro box, said we were all set, and we hung the show.
When we did the dimmer check, we seemed to have a lot of bad lamps. Dimmer one was fine, and so was dimmer two, but bringing up dimmer three gave us a brief flash of light on stage and then darkness. Dimmer four was okay, too, and so was dimmer five, but dimmer six also gave us a brief flash of light on stage and then darkness. While the lighting crew was replacing the blown lamps, smoke started coming out of our control desk. We’d fried its power input transformer.
The electrician had assumed that the kitchen power was three-phase Wye: 120 volts from each hot, ungrounded power conductor to the neutral and 208 volts between any two hot conductors. In fact, it was three-phase delta, with a grounding center-tap between two of the phases. So, it had 240 volts between any two hot conductors, 120 volts between two of them and the grounded neutral, and 208 volts between the third hot conductor and neutral. Every third dimmer and the control console were running on 208 volts.
All was not lost. Reconfiguring our system to run on 120/240-volt single-phase was not hard; we hadn't destroyed more lamps than we had spares, and the control desk’s power transformer was replaced by one from the local Radio Shack. Plus, the students got a good lesson in practical engineering: never assume anything, always check all voltages, and verify phase relationships.
“Electrical Power Measurement Techniques” is a double-length session scheduled for Sunday, October 26, from 9:30 to 12:30pm during LDI that will help attendees avoid blowing lamps, burning control desks, and worse. Roger Lattin of IATSE Local 728 leads a team of industry professionals in showing session attendees how to use meters safely to measure voltage, load, phase relationships, and more in temporary power distribution systems.