I'm into salt water fish tanks and I'm trying to get a better understanding of when a GFCI would (or wouldn't) trip under different situations.

Suppose I had 1 submersible pump in the tank and for some reason it cracked. (For example, if I were hammering a nail into the wall above the tank and I accidentally dropped the hammer into the tank and it unluckily crashed into the underwater pump and smashed it open). For the sake of argument, lets suppose it completely smashed the pump, exposing both the hot and neutral leads. If the pump were plugged into a GFCI outlet, should the GFCI trip? I'm not sure whether it matters or not, but the pumps in this hobby have only 2-prong male receptacles (i.e. there is no ground prong on most of them).

I would guess that the current in the hot lead would arc through the salt water to get to the neutral lead and since salt water has higher resistance than the metal contacts and wires within the pump, the higher resistance would decrease current flow and this would trip the GFCI. But, I really don't know.

Anyone?

N a GFCI circuit, anytime the current in the hot wire exceeds the current in the neutral, it shuts off because that means current is leaking through to ground perhaps through somebody. If there is no other path to ground from the water in the aquarium than the neutral, the GFCI should not trip. With a low resistance connection from the hot wire to the neutral through the water, It should draw enough current to trip the breaker. Circuit breakers only allow so much current to flow out regardless of where it goes. If the pump had a ground wire as well as the neutral, the GFCI might beat the circuit breaker to shutting off the circuit.

Thanks Labman!  Something about this still doesn't make sense to me.

Since V=IR, wouldn't the high R that arose because the current suddenly had to travel through the salt water (salt water having a higher resistance than metal) cause a decrease in current in the neutral lead as long as V remained at 120V? In that case why woudn't the current in the hot lead exceed that of the neutral?

Thanks again!

In the pump motor not only do you have the resistance of the wire, but you also have back EMF. This is caused by the interaction of the magnetic fields that are running the motor. A running motor, coil, transformer, etc. acts to a current like a much higher resistance that that of the wires mAking up the circuit. The other thing is that what the salt water may lack in conductivity, it makes up in volume. Just as larger wires offer less resistance to current, so does a whole tank of water. Exposing both wires to the saltwater, will cause a large increase of current. Ordinary tap water will also conduct dangerous amounts of electricity.

As long as there is no other path to ground than the neutral, the current in both wires must be the same. The GFIC will detect any leakages too small to trip the breaker, and disconnect the power before the stray current can injure anybody.