and following your reasoning about a ground path, I will concede that this could not possibly be an inductive effect.
That sends me back to finding the root cause. I have taken as careful a look as possible at the environment based on your knowledge about old houses, and here's a list of answers to some of your questions...
The countertop is wood, and I know it was wet (conductive).
Our lath is the dry wood type.
Our wiring is in metal conduit, and that does provide the third-prong ground.
The ground for the electrical system is tied to a ground rod, and probably to the cold water pipes at some point, too.
I made all my voltage measurements between neutral and the point of interest, since I'm not sure about the grounding. That is how I measured the relatively high voltage of 50 volts.
The process of elimination has lead us to the toothbrush, and since the cause of the shocks is not solved, I decided to take it apart. The device is indeed fully encapsulated in a potting compound, as Dennis suggests. However, the design of the toothbrush did not include adequate strain relief where the cord exits the housing, and one side of the cord has frayed with copper exposed. (See image). Murphy's law being in effect, it was of course the line side of the supply cord that was frayed, and the root cause of the case of the shocking countertop seems to be a simple conductive path from the line side of the supply cord, through a damp counter top and the metal edge trim on said countertop, through us, and to ground.
Needless to say, I won't be attempting to fix the toothbrush! Thanks again Dennis for the careful reading, and helping me get to the bottom of this.
A GFCI (Ground Fault Circuit Interrupter) does NOT need a ground of any kind to provide its protection. In fact, NEC (National Electrical Code) allows a GFCI to be installed in locations (such as very old homes) where no safety ground is available. Although it must then be marked "No Equipment Ground", it will do its job just as well as it would if grounded. However, Code requires that, if a safety ground is available, it must be connected to the ground terminal of the GFCI. Think of the equipment/safety ground as yet another layer of protection should the GFCI itself fail.
A GFCI works by comparing the current flow on the line ("hot") conductor to the flow on the neutral conductor. If the difference reaches about 5 mA or more, a relay is tripped - removing power from the circuit. The assumption is that the missing current is flowing through someone's body - and 5 mA is about 50% of "let go" current for sensitive people. At higher currents, involuntary muscle contractions can make it impossible to let go of something that's shocking you. A 5 mA shock feels very unpleasant but, with a GFCI in place, it flows only for a small fraction of a second. The "TEST" button on a GFCI causes a small current to flow from GFCI output line ("hot") to the incoming neutral (so it doesn't return on the outgoing neutral - which causes the unit to trip if operating properly. At no time is a "ground" connection necessary for the GFCI to function.
As long as I have the attention of some non-electrical engineers, I'd like to dispel another very pervasive and dangerous MYTH that the EARTH ground of the 3rd prong of a 3-prong outlet is what renders it safe. An earth ground rod has such a high impedance (up to 25 ohms allowed per Code) that it would never trip a circuit breaker should a piece of equipment plugged into it develop a short between line ("hot") and its touchable conductive parts (chassis, connectors, etc.). What DOES make the 3rd prong a "safety" ground is its connection to NEUTRAL at the main breaker panel. This low-impedance path will allow a very high fault current to flow, which will trip the circuit breaker quickly. It is also a Code requirement that the safety ground conductor be inside the same conduit as the current-carrying conductors (line and neutral) of that branch circuit. Read more in my generic seminar handout at www.jensen-transformers.com.
Not 100% correct. A GFCI device has to have a return path, neutral for its internal electronics to power up. That's automatically the case for a GFCI outlet but not for a panel mounted GFCI circuit breaker. It has to be wired to the neutral bar and then branch circuit neutral has to run through the GFCI. Doing neither will turn the GFCI into a conventional circuit breaker. So, while a safety ground may be optional, a return path (ground) for the GFCI device is not.
I don't understand your argument bdcst. Operating power for GFCI internal circuitry always comes from line and neutral. The only difference between a GFCI outlet and a panel-mounted GFCI is that the latter must have the N conductor looped through the magnetic pickup loop (that senses the imbalance) of the device. Contrary to what you state, NOT doing that will make the breaker trip on the slightest load current (because the neutralizing magnetic field around the N conductor is not sensed by the loop). In a GFCI outlet, both L and N run through the sensing coil internally. But NEITHER device requires connection to the ground, whether safety ground or earth ground. Neutral is, by definition, a load-current-carrying conductor. Although it is grounded (hopefully only at one place), it should never be referred to as "ground".
You missed the point, minor though it might be. If you do not connect the extra lead from the circuit breaker to the neutral bar, the leakage current sensing electronics will not power up and the breaker GFCI will operate only as a conventional circuit breaker.
Standard circuit breakers do not have a flying neutral lead. So it is possible for a DIY novice to forget to connect it to the neutral bar as well as not rerouting the branch neutral through the breaker. One would hope the DIY installer would make good use of the test button to insure their handiwork was working properly.
I've seen a sufficient number of instances where folks really mess things up in spite of clear instructions written by a good technical writer. I've also seen poorly written instructions that could cause even an astute engineer to scratch his head, at least for a moment.
OK, I get it now ... you're talking about incorrectly installed devices. But that same situation can exist when an ordinary GFCI outlet is installed by someone who doesn't understand or notice the difference between the input and output (line extension) terminals. In fact, a pandora's box of everyday things can become lethal hazards if installed or used by the ignorant (especially those who are too lazy or "too smart" to read instructions). Look at how often the ubiquitous 3-to-2 prong adapter is used without a thought to the safety ground connection!
I totally discredit the prior assumption about induction to the metal strip around the counter top. That was not reasonable by any stretch. The discovery of the frayed cord was a much more reasonable, it is a good suspect. The fact that the internal components are encapsulated does make a repair more difficult, but if the conductors are not damaged, the repair can be fairly simple. One easy method that would be very effective would be to seal the damaged area of the cord with a small amount of a waterproof flexible epoxy sealer, or even a non-epoxy sealer froma company such as MasterBond. The hard part would be matching the color. But a durable flexible good adhesive seal would be a long lasting and safe method of repairing the toothbrush charger.
Of course, my second suggestion is to find a reliable but inconspicuous means to attach an electrical ground to the metal surrounding the countertop, and grounding the faucets, drain, outlet box, and any other metallic fixturing in that area.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.