Stray Voltages

Stray Voltages


Stray Voltages: Explained!
By David Kessner

Ok. I finally did it. I finally figured out the whole mystery of stray voltages in a tank.

I'll explain it, but first a disclaimer: All tanks should have a ground probe and a GFI socket. Parts of this document talk about doing unorthodox things to outlets and ground probes--which fall under the category of "Kids, don't do this at home". Other parts imply that ground probes are not all that good, while it is arguably true for your fish, it is NOT true for YOU. Don't risk death. Use a ground probe and GFI socket. Your tank is not worth your life. We now return to our regularly scheduled program...

It happens all the time. Someone takes a multimeter and measures the voltage between their water and earth ground. They get 30 to 60 volts and then panic, thinking that this is somehow bad and that they should do something to get rid of it. They Then post a question on the net and they get dozens of answers saying the same thing. "The stray voltage is caused by electrical induction with your pumps. Use a ground probe to get rid of it". Sometimes, this stray voltage is fingered as the cause of fish stress, Head and Lateral Line Erosion (HLLE), and other mysterious fish deaths.

I'll cut to the chase here and say that both of these are incorrect. Induction is not the cause of stray voltages. Ground probes are not going to remove it. And it cannot possibly be the cause of fish deaths. I can't imagine how these myths got started, especially since it is very easy for someone to find the true cause of stray voltages.

The true source of the stray voltages is capacitive coupling, not inductive coupling and not resistive. Ground probes ( or a multimeter ) actually complete the circuit for electricity to flow. Since electricity doesn't flow unless the circuit is completed, electricity won't be flowing in a tank without a ground probe. Therefore, mysterious fish deaths in tanks without a ground probe cannot be attributed to stray voltages since current is not flowing in the tank to begin with.

I'm about to go into a long, technical explanation for the cause of stray voltages. If you don't want to read it, you can stop here. I've said / typed all the important practical information already. I realize that what I'm saying goes against decades of common misbelief so some proof is required. What follow is the proof.

For the proof, I assume that you know basic electricity. Ohms law, resistor dividers and the general properties of transformers, capacitors and inductors. You don't need a great deal of this knowledge, but enough so that you can picture what's going on since there are no pictures in this post.

To find the source of this stray voltage, we must eliminate sources that don't fit the evidence. Possible sources are induction, resistive coupling and capacitive coupling. There are other possible sources, but the odds of these being the cause are so remote that we can ignore them.

Here is the evidence that can be easily measured:

  • The "stray voltage" changes ( and is repeatable ) depending on what is turned on -- including the lights.
  • If you put a resistor between the ground probe and earth ground then measure the current through the resistor, you will measure up to about 40 uA. The voltage across the resistor will depend on the value of the resistor, according to ohms law.
  • The stray voltage / current does not change with the position of the ground probe..
  • The voltage measured between two ground probes is zero, regardless of their position or orientation.
  • If you disconnect the NEUTRAL line to the pumps / lights / etc. while leaving HOT connected, the stray voltage almost DOUBLES!
  • The resistance, as measured by my multimeter, between the ground probe and neutral / hot of the pumps / lights / etc. is infinite ( I.E. open ).  

Let's start with induction, since that is the common myth. If this were happening, then electrically the tank would look like a transformer. The wiring in the pumps / heaters / lights would work like the primary coil of the transformer. The water itself would be the secondary coil.

Unfortunately, the evidence does not support the induction theory. In a transformer, if current is not flowing in the primary then no current will flow in the secondary. If induction were the cause then disconnecting HOT would cause the stray voltage to go to zero, not double. If induction were the cause, then almost zero current would flow through the ground probe but we have 40uA. If induction were the cause, then moving the ground probe around the tank or using two ground probes would give us different results. Conclusion: It's not induction!

So that leaves resistive coupling or capacitive coupling.
Electrically, resistive coupling would look like a voltage divider. One resistor of the divider would be the insulation ( or lack thereof ) surrounding the pumps. The other resistor would be the multimeter itself, the ground probe or a real resistor that we put in.

You can actually calculate the "resistance" of the insulation using ohms law. Let's assume that we have 25 uA of current flowing. Using ohms law the X ohms = 120 volts / 0.000025 Amps, or 4.8 Meg ohms. My multimeter is calibrated for up to 40.0 ohms, so it should be able to measure this without problems. But since it measured as an open circuit, then resistance theory can't be correct.

Two notes about resistance...You'll notice that I didn't take into account the resistance of the water or the resistor that I used to make the current measurements. That's because those things have much less resistance than the estimated 4.8 meg ohms. It was a short cut for sure, but the resistance of the water and resistor was less than 100 k ohms or about 2% of the total resistance.

Also... The way most people measure the stray voltage is by placing the multimeter between the tank and earth ground. In this case, the multimeter itself acts as one resistor in the voltage divider. The multimeter has a high but finite resistance. That's why I used a separate resistor between the ground probe and earth ground and then measured the voltage across this resistor. The resistor was anywhere from 100 to 100k ohms. When you have two resistors in parallel, one of low value and the other of high value, the low value resistor will be dominant. The high value resistor ( the multimeter ) will not play a huge electrical role and mess up the measured values. In this way, I could be assured that my measurements were correct.

With capacitive coupling, you must know that a capacitor can behave like a resistor when an AC current is applied to it. The resistance is proportional to and increases with frequency. The formula for calculating the resistance at a given frequency is "Resistance = 1/ ( 2 * Pi * Freq * Cap ), where resistance is in ohms, freq is in Hz and Cap is in Farads.

So, with capacitive coupling set up exactly like the resistive coupling above, but with a cap as the electrical link between the AC Mains and the water, if we expect to see a 4.8 meg ohm resistor, then we can use the above formula to find the expected capacitance. So.... 4.8 meg Ohms = 1/ ( 2 * Pi * 60 * Cap ). solving for Cap give us 552 pF.

My multimeter will measure capacitance, but it isn't calibrated into the <1000 pF range. So I expected some "not quite right number: I measured the capacitance between the ground probe and hot or neutral at around 1100 pF. This was surprising since I didn't expect the number to be that far off. That's when I did the "disconnect neutral and leave hot connected" experiment. In this experiment the stray voltage doubled! The numbers made sense: At 1100 pF, 60 Hz, and 24 uA, the resistance of the Cap should be about 2.4 meg ohms. This should cause double the current to flow through our voltage divider, which causes our measured voltages to be double. The evidence matches this perfectly when neutral is disconnected.

With neutral conned, the AVERAGE VOLTAGE inside the pumps, lights, etc. is exactly half of the 120 volts applied to it. That's because of the voltage drop from the current flowing through it. When we disconnect neutral, there is no current flowing so there is no voltage drop. When there is no current running through the pumps, then the average voltage is 120 volts. This explains why the voltage doubles / halves depending on the neutral connection.

So, now all the evidence points to capacitive coupling!

for capacitive coupling to work, a circuit must be formed. This is normally done by a ground probe ( or the multimeter ). If there is not a circuit, then there cannot possibly be any current flowing!

People claim that once they add the ground probe then they cannot measure any more stray voltage--thus the ground probe must have removed it! This is, of course, not true. Essentially, what's happening is they are measuring the voltage drop across a zero Ohm resistor ( the ground probe ). And of course there will be no voltage drop across such a resistor! That's why they claim that it went away.

Naturally, life is more complex than that. It's not a zero Ohm resistor, since you have to take into account the resistance of the water and the proximity of everything. But generally speaking, adding the ground probe will bring the measured "stray voltage" very close to zero.


  • Very Important! use a ground probe and GFI for your safety.
  • "Stray Voltage" is cause by capacitive coupling, not induction.
  • There is nothing you can do about it, unless you don't use electricity in you tanks ( including lights ).
  • A ground probe will cause, not prevent, current from flowing in a tank.

The lack of current flowing has never been attributed to the death of anything ( except the lack of the normal electrical impulses in the brain and nervous system ). So, ground probes cannot possibly solve things like HLLE, fish stress, etc.

I hope that this removes a lot of the popular myths that have been circulating for decades about stray voltages in our tanks! A probe ( and GFI outlet ) is required to keep us from electrocuting ourselves if there is an accident. This is most important and goes beyond the health and well being of our fish.

When used ( and it should be used ), the probe will allow these small amounts of current to flow through the water. Some people will say that this small current could pose a health hazard to the fish. I remain skeptical on these hazards, but point out that the safety benefits of the probe / GFI far outweighs any questionable health risks.

This is completely opposite from the popular myth that a ground probe will remove any electrical currents from the tank and protect the fish. It's also interesting that many people have reported "health improvements" in their fish after installing a ground probe. The results of my tests show that this should be exactly opposite, assuming that the "questionable health risks" of stray voltage are true.

What this all implies is that the health risks are a non-issue, and that the hype surrounding it is due mostly to conjecture, misinformed opinion, and not-so-scientific observations. But this is just my opinion. Either that, or plant / animal cells actually grow better in the presence of an electrical AC field.

David Kessner

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