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.
I guess, Alex. I would think metal trim on a bathroom counter could produce a host of problems, including corrosion over the years. I was a bit surprised the solution in this case wasn't the removal of the trim.
My solution, as it is for so many things, would've been to put duct tape over it. That doesn't go over too well at home, though. However, my local store recently started stocking white duct tape. Heretical though that might be -- to me, duct tape should come in any color as long as it's grey -- it is much more asthetically pleasing to significant others.
I wonder if the purpose of the metal trim was to prevent damage to the edge of the countertop--something like the metal strip on drywall corners. Possibly the countertop was made of something less durable than the materials used today.
Funny that the charge didn't happen immediately after using the electric toothbrush, or did it? I guess you can count me in with the others who would have opted to replace the counter top because of many reasons, including the charge. But the fix definitely would not have been done with duct tape--white color or otherwise!
1) I have seen these metal edges on older counters in kitchens or bathrooms to provide a dressed-up trim on either formica or tile counters. This trim was primarily eliminated when granite, CorianTM, and laminated edges were developed and esthetically preferred.
2) I was corrected years ago, by a former Navy Officer, that "Duct" tape is properly called "Duck" tape as it was developed in WW II for the Navy primarily to use in Ships, and Submarines over asbestos pipe insulation. Hence, the reason for the Navy Gray color. It obviously found many other uses including sealing of sheetmetal ductwork.
I'm having a hard time understanding how the metal band on your countertop could act like a secondary winding driven by the primary in the base of your toothbrush holder/charger. I have such a toothbrush and the magnetic flux in the base is so feeble that moving the tooth brush a fraction of an inch from the base stops the charging effect in the toothbrush. I just can't see any way this weak field could induce a current flow in the counter trim strip. Furthermore, the trim strip is essentially a one-turn secondary. if it's a continuous strip, it's a shorted one-turn secondary with zero volts output, or if not a closed loop, and being one turn only, I believe its induced voltage would be so low as to be imperceptable.
There are other possible causes of the shock that was felt. I would look to see if the whole bathroom lighting and accessory outlets have been wired correctly, and that the ground and hot supply leads weren't mixed up resulting in electrically hot (supposedly) grounded surfaces.
I'm sure there are other cases like this, but I clearly recall a situation in London, England where an electrician mixed the hot and ground leads to a bathroom outlet that resulted in fatally shocking the user of an electric shaver.
@Myron: We were in London in 2008 (just watching the economy collapse around us) and there were no electrical outlets in the bathroom, just as we found in Paris the previous week. The Paris hotel provided an extension cord so my wife could dry her hair using the bathroom mirror but the London hotel refused to provide an extension, said it would violate Code. So, I'm surprised to read your comment about switched leads in a bathroom outlet.
The toothbrush charger would have to be emitting a major field to induce that much current in the trim, probably does not have enough copper in the base to create that kind of field. You suggestion to look elsewhere is a good starting point. (Both of our induction toothbrush chargers perform just as you mentioned: when the brush is ~>5 mm from its dock it stops charging.)
I'm a retired Electrical Engineer. It is not possible for the coil in the toothbrush charger to induce 30-50 volts into the metal trim on the countertop. That kind of voltage can only come from leakage. If removing the toothbrush charger fixed the problem, then insulation leakage in the charger base is the problem and water has conducted the current to the metal trim. The coupling between the primary coil in the charger (60Hz to maybe 75KHz) is far too small to induce such voltage into the trim on the countertop.
AR, you are right on. The voltage was probably measured between the metal trim and an earth ground point. I suspect that water has gotten into the charger and caused a leakage path to the trim. A GFI would trip if the leakage was in the 5ma range but a person can feel a shock at less current with wet hands. Assuming a 10meg input meter, the leakage was about 1/4 ma.
I also detected a misunderstanding of the GFI operation. A GFI cannot detect a fault on the secondary of a transformer. If the voltage was indeed coupled from the charger winding (it was not), then the GFI would not detect it as leakage current.
I think you guys need a new editor, someone that knows at least a little something about electrical theory. If the toothbrush charger had that much magnetic flux then all kinds of strange things would be happening near the charger. The toothbrush could be charged while sitting a number of feet from the charger. Any old cassettes would be erased of their recordings and strange movements of nearby ferrous metals!
I totally agree with 'armorris' on this - the problem could not (well, let me rephrase that a bit - it is highly improbable) be due to the toothbrush charger coupling to the metal rim. There is a leakage path someplace and I would seriously suggest some more aggressive work to figure out what it is. Having something put 30 to 50 V out there is NOT a good thing.
I had a somewhat similar situation a good many years ago. I had added a workbench in a bedroom which was located in the basement (the bedroom and the interior wiring had been added by the previous owner) and, when the workbench's outlets were plugged into the wall outlet, I was getting a 'tingle' when I held a grounded tool. Not good.
I tore into the electrical system downstairs and finally found the culprit - on one of the metal boxes on a homemade extension cord, the screw on the cord clamp was too long and was contacting the hot wire in the cord. This essentially put 120VAC on the ground - the connection was not solid enough for a 'solid' short but did manage to drag the grounds up to about 50 V above earth ground. A very solid 'tingle' indeed.
I think your story would make a good Sherlock Ohms posting. If you;re interested, please flesh it out with a tad more detail and send it along to: rob.spiegel@ubm.com
So I'm showering in a campground shower house and the lights keep getting dim, and then bright again. I'm thinking "boy they sure have a piss poor electrical system here".
I begin to realize that the lights seem to dim every time I stand up straight, and that a couple of times the room went completely black.
I FINALLY realize that they are getting dimmer as my head approaches the shower head, and that the room is black when my head touches the shower heat.
My friend in the next stall confirms my fears. The room isn't getting darker...MY VISSION IS FAILING.
Apparently there was some recent plumbing work done, and the building ground wire which was connected to the copper plumging didn't appreciate the new plastic main that had replaced the original steel pipe from the well.
I was basically semi-electrocuting myself as I became a conduit for the high ground potential making it from the shower head to the drain via my body and the high mineral water "rheostat" I was creating with the gap between my scalp and the shower head.
So was the countertop energized all the time or only when the toothbrush was being charged. Also, was the change generated from the energy going from the base to the toothbrush? How did you rule out the possibility that the base was directly influencing the trim?
I was in London in 1984 on a business matter, and the electrocution was a big news item in the papers at the time. It has been some time, but as I recall, a contractor had replaced the outlet and wiring near the mirrored medicine cabinet and switched the hot and ground leads making the metallic frame of the victim's razor electrically "hot." While shaving, the victim touched his razor and the water spigot which was indeed at ground potential, and was electrocuted. I believe the power supply (the "mains" as they say over there) was 50 Hz, 220 Vac.
Between what two points are you measuring 30 - 50V? You can't get a shock on a closed loop with a circulating current, (see works of Kirchhoff & Lenz), so I assume you measured between the trim and ground. If so, simply patch an incandescent bulb -any wattage- between the same two points and measure again. I wager that the bulb will not light, and the voltage will drop to a few dozen mV, because you have a small capacitive coupling from some energized conductor to the floating trim. Maybe the toothbrush charger or even a hair-dryer power cord. If so, just ground the trim and fuggedaboutit. You can feel currents too small to trip a GFCI, and it's not necessarily dangerous - just startling.
When the primary in the base is efficiently coupled to the seconday in toothbrush, you probably get 3 volts and 20 milliamps to charge the battery.
But the primary in base can not be efficiently coupled to the loop of metal trim. Think about what the magnetic field pattern around the base looks like. With the toothbrush in charging position, the field should be well confined. With the toothbrush off, I would expect the flux pattern to be the size and shape of grocery store donut. Where can you place the base so that the loop cuts through any significant part of the donut.
Take your voltmeter, short the leads together to get a two-foot loop. Lay it around the charger. What do you see? Nothing like 50 volts.
Change the meter over to milliamps and repeat the experiment.
Even better is now apply this technology to cordless phone charging for kiosks in airports but not bars. Not enuf security.
make a million denaros.
I don't think the problem was reported accurately.
"I'm getting a shock off the countertop in the bathroom"
Between the metal edge and what is my 1st question.
a) the plumbing?
b) wet feet?
My second question is …Was there a grounded outlet or transformer outlet or a GFI outlet?
My 3rd question is.. was the 40~50Vac measured on a DMM with 1MΩ probe impedance or what?
My 4th question is.. Would you get the same voltage if the charger was moved away from the counter surface? Plug reversed? or brush removed from cradle?
Answers to these questions will give a more accurate conclusion.
I often measured 50Vac just with my body and stray hum in any room on a 10MΩ probe and floating gnd clip. This is just common mode stray E field that becomes differential relative to the probe ground floating.
If you don't have a grounded outlet with GFI installed yet. I suggest you do so promptly. I showed my nephew how to install one without any experience when he moved into my place at 15yrs of age. He had one of the fancy Braun toothbrushes and I had no outlet in my bathroom.
I suspect your leakage was promoted by high humidity in the wood studs and conducted to the metal rim.
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