The Case of the Shocking Bathroom Countertop

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September 02, 2011

My wife called me one day with an odd problem: "I'm getting a shock off the countertop in the bathroom." I promised to investigate that evening, but since she has a scientific background, she was determined to isolate the problem herself. Less than an hour later, I received a second call. She was able to locate the source of the shocks, our electric toothbrush.

I assumed, of course, that something was wrong with the charging base of the toothbrush. Upon further investigation, that was not the case. It turns out the toothbrush and charging base were behaving as designed.

The electric toothbrush comes with a rechargeable battery inside a sealed handle. The base contains a primary transformer winding, and the handle of the toothbrush contains a secondary winding. We'll come back to this in a bit, but first, a bit about our home: It was built in the 1930s and has some lovely features like lots of hardwood trim.

One of the more unusual features of our home is the bathroom countertop. There is a metal trim strip that runs around the entire perimeter of the bathroom counters. The trim became a secondary winding to the charging base on the toothbrush, and a current flow was being induced into the metal trim. I later measured the voltage: It was ranging between 30 and 50 volts. Ouch!

There were two possible solutions to the problem. The simplest was to discontinue using the electric toothbrush on that countertop. The next was to cut a slit in the metal trim around the countertop to disrupt the current flow. We chose the former solution.

This entry was submitted by Steve Maves and edited by Rob Spiegel.

Tell us your experience in solving a knotty engineering problem. Send your stories to Rob Spiegel for Sherlock Ohms.

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