The Case of the Hot Desk

July 28, 2011

Early in my career, I had a job servicing office machines for a large national laboratory. These were largely mechanical calculators and adding machines. (Yes, it was before electronics took over those functions.)

One day, we got a call that an employee was experiencing mild shocks when he operated his adding machine. This wasn't an uncommon complaint, since these machines had aggressive line filters with considerable capacitance between line and ground. A failure in the third wire ground (usually from a bad power cord) would allow sufficient capacitive leakage to produce minor shocks. Additionally, well-grounded equipment and a dry climate made for some severe static discharges in carpeted offices. So I grabbed a can of anti-static spray, a new power cord, my multimeter, and a line plug tester and headed out.

Imagine my surprise when I found a concrete floor, an outlet with a working ground, and a good power cord. Yet there was a definite tingle when I touched the adding machine. Connecting my meter between ground and the machine showed no voltage, but I could feel it. Finally, it clicked, and I measured between ground and the steel desk. The meter read around 50VDC, which just happened to be the on-hook voltage of the multiline phone that sat on the desk with the adding machine. Sure enough, a little investigation found the phone connected to a terminal block screwed to the side of the desk, and a stray wire from the phone cable was touching the desk. With a bit of tape and a call to the phone company, problem solved!

This entry was submitted by Michael Wolf and edited by Rob Spiegel.

Michael Wolf is a retired engineer who worked for Los Alamos National Laboratory, Amtech Corp., and Cell Robotics Inc. He holds 13 patents, and equipment in which he had a major design role has won numerous awards, including 4 R&D 100 awards, Photonics Circle of Excellence, and Medical Design Excellence.

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