Technical Considerations Regarding Static Potentials

Someone once asked the question, "If an aquarium exhibits a static potential above ground, wouldn't a ground probe dissipate this static potential differential with essentially "zero" current flow, after the initial voltage was dissipated?" To answer this, we must have an understanding of what these static potentials are.

Voltage is a potential difference. The generator of the voltage doesn't go away unless the static voltage is truly static (as in a capacitor). If the "static voltage" is one created by induction, it will still be present to generate some kind of current even if shorted through a ground probe. You may measure essentially no voltage, but you could still have current. How this affects fish and coral is an open question. People stick a volt meter probe in the water and touch something that they think is "ground". If they read a voltage, then they may be measuring an induced voltage (from lights or other things), and providing a current path isn't the answer if none already exists. True static voltages are "dissipatable" such as those generated by walking across a carpet (though these are probably not much of an issue with marine aquaria since there is usually so much conductive surface film (salt) that resistance will eventually dissipate such a charge).

Now, most people consider water to be impervious to electromagnetic waves, especially when its salt water because salt water is a conductor and would "short out" the radio waves, right? Well it is true the sea water has a conductivity of 4 Siemens/meter (as opposed to distilled water with a conductivity of only 0.0002 S/m, which is still high compared to something like glass at 0.00000000000001 S/m). In the presence of low frequency energy like 60 Hz, sea water is not a low-loss dielectric because of the mobility of the chlorine and hydrogen ions. Therefore induced charges (electrons) are able to diffuse throughout the volume of the aquarium water. This prevents a potential difference to be created across the resistance of the water or from any stored charge as in a capacitor.

Normally one would expect fluorescent bulbs to produce the most induced voltage in the tank. This is because of their relatively large surface area (compared to something like a power head), and the fact that the entire bulb length has an ionized gas within it that is radiating at 60 Hz. Since the bulb is short relative to the wavelength of 60 Hz (5,000 km or 3,107 miles in air) and it is close to the aquarium (typically only a few feet at most from the water surface), we consider the bulbs to be in the "near field" (well within 3 to 5 wavelengths), so transmission is by induction. Since the Intrinsic Impedance of Free Space (air) is 377 ohms and the Intrinsic Impedance of sea water at 60 Hz is (1+j)(0.0077) ohms we see that there is a large impedance mismatch. Were the bulbs to be in the far field (> 15,000 km distant) most of the energy would be reflected at the air-water interface, but since we are in the extreme near field, induction will occur and we will observe impressed voltages in our aquarium.

The higher the frequency, the less energy penetrates into a conductor. The amount to which penetration occurs is called "skin depth". At very high frequencies, electricity actually flows on the surface of copper conductors and not in the interior of the wire. Skin depth is related to frequency and conductivity and is different for different materials. The skin depth of sea water for 60 Hz energy is about 32.5 meters. In other words, 60 Hz current induced from outside the aquarium will exist on the outside 32.5 meters of the tank. Except for the largest tanks, this means that induced current densities from 60 Hz sources will permeate the aquarium uniformly.

A reference on the web for empirical data on aquarium voltage and current is http://www.aquarium.net/1298/1298_3.shtml

Ground Fault Interrupters (GFI) and Aquarist Safety

Many folks drop a grounding probe in their tanks and connect it to a water pipe or the neutral in their wiring thereby creating a current path where one might otherwise NOT have existed. This is worse for the fish than a very localized current (short within a pump housing) or a static voltage. The Safety to the aquarist is a different issue. What about the use of Ground Fault Interrupters (GFI) to protect the aquarist?

Fuses and circuit breakers are too slow to protect one from shock . They protect equipment, not people. The GFI is electronic and faster (GFI = 10 ms). A grounding probe has to be used as a GFI reference before it will work properly. I use GFIs all over my system. It must be understood however that using GFIs on pumps and lights isn't necessarily going to stop detrimental currents in the aquarium.

For example, if one power head's hot lead (black wire) is exposed to the water and another power head's neutral lead (white wire) is exposed to the water, then there will be a current flow between them as well as any sea life that gets in the way. Lets say that both power heads were plugged into the same GFI. Since all the current coming in the black wire is going out the white wire (some through the power heads, some through the water and fish)... there is a balance, but an unhealthy one for the tank. The GFI would not trip. Adding a ground probe would cause the GFI to trip in this example.

Second example: If a GFI is used on the pumps and lights and there is a grounding probe in place, any "induced" voltage will cause a current through the water and out the ground probe. Light, heat, motion, and induced electrical currents all represent "consumed energy". The current into the "emitter" of these energies will be the same as the current going out of the emitter. The circuit is balanced and the GFI will not trip, but energy is still created in various forms which can be transmitted to the aquarium. Consider an improbable case that will illustrate this point.

Suppose I had a motor (like we'd find in a power head) and I supply it power from a GFI-protected source. Then I properly install a ground probe. Now lets say that I connected an electrical generator to the motor through an insulated mechanical coupling. When the motor is turned on, it turns the generator which in turn creates a voltage. I will then connect one side of the generator to ground and the other side to my aquarium water. What will happen? Current will flow through the aquarium and out the ground probe. Will the GFI detect this and trip? No, because even through its current results from the motor-driven generator's energy, the motor's lines remain balanced.

So now we have a system that can electrocute both the aquarist and the fish even though there is a properly installed GFI and ground probe. Were I to remove the ground probe, there would no longer be a current path affecting the fish (they are now the "birds on a wire"). The aquarist however, is still at risk if he touches the water and a ground.

In reality, induced voltages and currents will be small... so small that they are probably NOT an issue for the safety of the aquarist, but with a grounding probe present, they could be unhealthy for the fish and coral (and I suspect they probably are, since sea life has little insulation to mitigate the current flow through their bodies. I can't believe that a continuous current flowing through one's body would not cause havoc with cellular ion transport, not to mention the "jamming" of sensory organs).

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