<a href=showthread.php?s=&postid=15292072#post15292072 target=_blank>Originally posted</a> by MeReefBeef
Gabba, I'm sorry but that is abuse of Ohm's law. A voltage potential can be taken from any two points, regardless of whether there's actual current between them. If you use a voltmeter to measure, say where current is going into your lights and a point in the water of your tank you would measure a voltage, and create a short to the water in your tank. You have thus created a parallel resistance (the voltmeter) which is where some current will run. There is a voltage difference between those two points, but not necessarily current running between them (w/o the voltmeter probing).
Another example would be the voltage potential between the sky and the earth. There is usually a voltage difference between the two, however, until the potential reaches high enough for it to counter the resistivity between the two (the air), we will not see current (lightning).
This is precisely why I'm bringing this up in the first place. A presence of voltage does not entail a presence of current at all, especially since all your electric devices should have insulation. If you had some kind of a wire that was not insulated, the high conductivity would quickly short out all your electronics.
I don't disagree with your presumptions. You are correct in your example using the sky and earth as having a different voltage potential. The best answer I could find as to the resistance of dry air is 4 x 10^13 ohm meters at sea level, a near infinite resistance, or rather an insulator.
Because air is an insulator and not a resistor, there is not enough voltage or current to ionize the air molecules and create any flow of electrons. With no flow of electrons, there is no current, just like your example of the hood over the tank water.
The fact is, these two planes are electrically isolated unless there is a contact point between the two.
If there is never any interaction between the two points, then ANY voltage measurement is arbitrary and meaningless.
Lightning forms when the voltage potential becomes so great that the air molecules ionize to conduct the voltage to ground (earth). Then, there IS current flow! Up until that point, there are really two 0V references, or two isolated ground planes. One in the sky and one on earth. When air molecules ionize and form a path to ground, there becomes a current flow of this voltage conducting across its surface. Up until that point occurs, all voltage differences are arbitrary.
To put it another way, in an electrically conductive circuit, voltage is normally measured with reference to ground.
If there is a voltage difference between a light hood and ground and there is a voltage difference between the tank water and ground, then voltage potentials DO exist. For this voltage to exist, there MUST be some measurable current flowing TO the hood or water itself. The fact is, if your lighting hood has voltage on it, there is current flowing to that hood from some source with respect to its own ground.
You can be assured that you will have detectable current flow when these two planes become electrically connected, like when you stick your arm in the tank water and touch the hood at the same time.
What we want is to remove these voltage potentials.
When I was using ohm's law before, I was speaking of electricity within a circuit. This statement: "If I=0, then no matter what R equals, you will have 0 Volts. In order to have a number greater than Zero Volts, you must have BOTH current and resistance equal to any value above Zero." is true mathematically for a given electrically connected circuit, where voltage is measured with reference to 0V or ground within that circuit. All voltage potentials outside of that are arbitrary until they become connected to the circuit. I do not believe this is an "abuse of ohm's law", because it breaks it down to the basics, and that is what this law is there for, and everything electrically speaking is based on.
Properly grounding your devices AND the tank water completes the path to earth ground, leaving you with zero stray voltage and zero voltage potentials between all electrically connected equipment and the water (or near zero if you want to discuss chemical interactions of metals, but for our purposes, it's zero).
What you mention about electromagnetic induction is possible, however, grounding the system won't help in that case, as your electronics are a SEPARATE system from the water in your tank. In other words, the water is being affected by the electronics from induced magnetic fields, but once again it is NOT dissipating any power hence, there should be no current in there, EVEN if you measure a voltage between the gfci and water.
How will it NOT help? If there is a voltage potential from induced voltage, what happens when you stick your grounded arm in the tank, or to another device like the light hood that IS grounded while touching saltwater, or you have salt creep that completes an electrically conductive path? Like I said before, voltage measurements are arbitrary until there is a path for it to flow. Grounding everything removes these potential paths for current flow, and helps in all of these cases, because it removes the potential for voltage to exist with reference to itself.
What we are talking about here is creating a system ground that everything should live on. Earth is the ground that everything should be referenced off of. Not the sky or some arbitrary isolated 0V. That is the potential for danger in all cases!
