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Potential pitfall: you are connecting a power supply to metal objects, and then putting those metal objects into the same water you are soaking in - while increasing the conductivity of the water by adding salt. So far I have never heard of an incident, but I can't understand why.
Isn't the voltage so low that it couldn't possibly cause a problem?
Yes, you are correct - assuming that everything works as it is supposed to. If a wire gets crossed, all bets are off. I have seen several of these with the line voltage going into the box - it could happen that a wire melts down and ties the output to the line input. I have also seen new versions with a wall adapter plugged right into the recepticle: no high voltage ever enters the unit at all. Seems much less likely to ever have a problem.Now, I know that most spas also have a high-voltage heating element in contact or close proximity to the water. Those units are ETL or UL listed and have been tested as a whole unit to keep them safe. GFI protection, grounding and bonding, current collectors - it's all there and tested. These units are almost always added on after the fact.
Chas, Have you ever heard of this happening or are you just playing on everyone fears?