Well I have some issues with some of the info here. It is not that it is misinformation but it is no exactly accurate in many ways either. I must say I was quite surprised by some of the responses by people who are electricians.
I read this entire thread and I do have GFI's on my equipment these days and I do agree that it is a simple cheap protection that should be used not only on the aquarium but throughout many parts of the home.
I've had many experiences with broken heaters over the 35 years I've kept aquariums and I've never gotten more then a mild shock from one.
In order to get a significant shock I would have have an exposed hot wire in the water and then contact some ground such as standing on a concrete floor with no shoes on or reaching out and grabbing a water pipe AND that circuit I just created using my body has to provide less resistance then any other existing path for the current. The degree of shock I will get is directly related to the difference in resistance of the two available paths which is the path created by my body and the path between the exposed leads in the broken heater.
Someone claimed a shock though the glass? Seriously? I'd like to hear the explanation for how that occurred because I can assure that it will take a lot more then 15 amps at 120volt to bridge a piece of glass much thinner then would ever be used in even the smallest aquarium.
Electrocute my fish? An exposed hot wire in water fully contained by an insulating material (like glass) will not energize the the box of water. Even if both the common and hot were exposed then the flow will be 95% in the straightest possible line between the hot and common. In the case of a heater both are less then an inch from each other. As you get closer to the to the exposed circuit you might feel a mild shock (if you are grounded) but the vast majority of the energy will continue flowing from one lead to the other.
Damage to my heart from a typical 15 amp 120 volt circuit generally requires I take the shock across my chest meaning hot lead in one hand and ground in the other. Sticking both hands in the water will not create a viable circuit when the hot wire is exposed. It will not result in taking a significant shock across the chest as there is no completed circuit.
Even if I place the load in one end of the tank and common in the other and then placed both hands in the tank but am not touching any other source of ground I will only get a mild shock because electricilty always follows the path of least resistance and that path is in the aquarium, not in me. The reason humans are conductive to the degree we are is because we are mostly water but the aquarium is all water so there is no question that is the path of least resistance. That would be what is called an ungrounded short.
A GFCI does not protect against ungrounded shorts. That is, if a person contacts the hot and common conductors, the current will flow through the body but still show as balanced to the GFCI. In this scenario, the body has become part of the load. In other words even though the person is getting a shock the GFI will see them no differently then any common appliance plugged into the GFI and will not trip. It is only when the hot wire completes a circuit though something other then its paired common that a GFI trips.
While I agree people should be more careful then they are and a GFI is one of the simplest precautions a person can and should take the fact is if it was as easy to get a serious shock as this thread implies then 10's of 1000's, if not 100's of 1000's, of people would have been seriously shocked or electrocuted in this hobby. Especially 30 or 40 years ago when broken heaters were much more common then they are today and very few models had any type of fail safe on them.
Anyway ... thats my first post.... will that irritating you have not posted message go away now?
