You did what?

Got a good "goof up" you'd like to share? Tell us what you did wrong, what happened, and what you learned. We'll send a PCB clipboard to contributors whose stories we publish. E-mail us at goofs@tmworld.com.

One of my most exciting EMC lab moments came when an electrician decided to cut the connector from a 100-kVA extension cable. But the electrician didn't unplug the cable from its power source. He used a wire cutter the size of a tree trimmer. He placed the cutter around the cable, held the cable in place with his foot, and took a mighty slice.

The resulting metal discharge produced a flash of blue light. It melted several vinyl floor tiles, vaporized one of the wire cutter's jaws, and welded the cutter at its hinge point before the circuit breakers could cut power. Fortunately, nobody was injured. The electrician proved that AC mains circuit breakers really work.

I used to spend a lot of time on the test bench. In one job, I worked on a 10-kV power supply used with an x-ray source. The power supply had an AC mains transformer that was about the size of a shoebox. The transformer was powered by a variac, which lets you slowly increase voltage so you avoid blowing up your equipment or hurting yourself. Variacs work well, but only if you properly connect them to their load. I didn't.

When I wired the variac, I reversed one of the primary leads with the wiper lead, turning the variac into a step-up transformer. I turned on the variac and started increasing its output voltage with the knob. Almost immediately, I heard buzzes and crackles, then I saw arcs across the circuits. From that experience, I learned not to connect a variac to its load without checking the connections.