While this post may very well fall in the category of: “don’t try this at home,” it is meant to help inform the reader as to the actual function of GFCI devices–breakers and receptacles–what they can do and cannot do.
Most people are under the mistaken impression that if they throw their hair dryer into the bathtub, the GFCI device it is plugged into will trip–thus preventing electrocution of the occupant of the tub.
In reality a LOT of things have to go wrong for electrocution to happen–especially in modern homes that have plastic plumbing drains and supply lines.
All of a GFCI’s useful life, it simply sits wherever it is–doing its job.
Its job is to monitor, 24/7, for any change in amperage between the hot conductor and the neutral conductor. As little a difference as .005 amps (5 milliamps) will be seen by the GFCI device as a “leak” and the device and everything connected to it will be shut down.
If you throw your hair dryer into the tub, under most scenarios, it will NOT result in even 5 milliamps of leakage. While this may seem counter-intuitive, this is indeed the case. There has to be a path to ground, for the hot conductor contacting the water, for the change in current between the hot and neutral wires to be enough to trip the GFCI. Most modern tools, including your hair dryer, has no ground wire present, because all the exterior shell is plastic. There are only the hot and neutral conductors. If there was a ground wire present, then indeed the current would have two paths back to ground and there might be sufficient difference to trip the GFCI if the water is conductive enough.
As you can see in the following video neither the GFCI device on the end of the dryer cord, nor the GFCI receptacle it is plugged into, tripped while the dryer was motoring around in the tub.
Even with metal pipes connected directly to metal sinks and tubs the connections are typically isolated by gaskets and porcelain and plastic parts. Direct human contact with the metal piping (assuming the piping is connected to the house grounding system) by someone playing with the hair dryer running around in the tub water, should result in tripping the GFCI.
Without a GFCI the potential for shock is dependent on how direct a contact the person touching the pipe makes with the hot conductors in the water and/or the conductivity of the water itself. Ordinary tap water is a poor conductor but can become more conductive with the addition of salts and other minerals etc.
All of this is why GFCI protection is necessary and essential for personal safety around water use areas.
Appliances with metal exteriors, like a toaster, will typically have a ground wire and would represent a much more dangerous thing to be throwing into the tub, and should instantly trip the GFCI assuming the water is conductive enough.
The toaster, sitting dead on the bottom of the tub, is not going to look nearly as cool as your hair dryer motoring around the tub, as it heats the water either.
At least this is the theory–assuming the GFCI is indeed functional.
Makes you want to go test your GFCI’s doesn’t it?
The proper test for a GFCI is to plug something (with a light) into the device and push the test button. If the light goes out the device is functional. Of course it does not have to be a light–it could be something sound related, like a motor or a radio. You just want to be able to tell visually or audibly that the receptacle has been turned off.
By Charles Buell, Real Estate Inspections in Seattle
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There are some problems with this approach. All touching it with your hand does is tell you that YOU are not grounded. You are isolated from the ground (rubber sole shoes, floor coverings, location etc). The idea is, if you were to get a shock from it, your muscles would contract pulling your hand away from the cover instead of toward it. 
