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
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
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.
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
- The voltage measured
between two ground probes is zero, regardless of their position or
- If you disconnect the
NEUTRAL line to the pumps / lights / etc. while leaving
connected, the stray voltage almost
- 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
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.
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
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
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
- "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
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
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
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
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
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.