Understanding Stray Pool Voltage from an Engineer

Q. I don't understand how the water in a pool can have voltage. What is this 'stray voltage' stuff?

A. The first issue, I suppose, is the fact that there is voltage on the utility distribution neutral. This comes from the fact that there is current flow in this neutral. Whenever there is current flow in a conductor, a voltage will be developed along the conductor. This is simply due to Ohm's law. Let's consider some numbers. If a conductor has an impedance of 1 Ohm, and there are 5 Amps flowing through it, there will be a potential gradient along the conductor of 5 Volts.

As long as distribution systems utilize the 'grounding' conductor as a neutral for their transformers, this elevated voltage will exist. The solution is for the utility to use two-bushing transformers for all their applications. Connecting the primary of the transformer across two phases, instead of from phase to 'neutral' will keep all the primary current within the phases and will remove it from the grounding system. Of course, this also means that the utility companies would have to run an additional phase conductor to their rural customers.

The second issue is that the earth is really not a very good conductor. The meaning of this isn't intuitively obvious. Let me try to explain. Philosophically, a perfect conductor is defined as an equipotential conductor. The voltage at any point on the conductor is the same. This would be what you would have with a super-conductor. The only way to bring this about is if there is zero Ohms resistance in the conductor. Any current flow, then, flowing through zero Ohms, produces zero volts from one end of the conductor to the other.

This is not the case in the real world. In the case of a lightning storm, the current from the lightning strike strikes the earth and then flows in sheets on (near) the surface of the earth. Since the soil has resistance, current flowing through this resistance produces a voltage, according to Ohm's law. Since the earth is not a very good conductor, voltage gradients exist across soil.

We use the earth as a reference for your electrical systems. This is convenient, since our bodies are almost always in contact with the earth in one form or fashion. For convenience, we call the earth 'zero' volts. By way of example, we also say that a parked car is 'standing still.' Is it? Isn't it really sitting on the surface of the Earth that is hurtling through space at thousands of miles per hour? The car isn't really standing still, but for the sake of our mathematical calculations, we say that it is. If a car passes our parked car, it is easy for us to ascertain the speed of the moving car because we consider our parked car to have a speed of 'Zero.'

So, with the swimming pool, there are multiple references. There is the earth, which we have conveniently labeled as zero volts. There is the utility neutral, which will always have voltage on it due to the use of single-bushing transformers. If the earth were a perfect conductor, there would be no elevated voltage on a utility neutral because the earth would be a super-conducting path for all the neutral currents. If the pool itself had no connection to the electrical system, this would not be a problem. But the pool has pumps and filters which are powered by the local electrical company. The water runs through pipes and the enclosures of the pumps. The water becomes the same potential as the electrical enclosures (some call this stray voltage). The electrical enclosures are bonded to the electrical system. No matter how you do it, the pool is eventually connected to the utility primary neutral and its elevated voltage.

Grounding the pool doesn't get rid of this voltage (It's already grounded) because the earth is not a super-conductor. All grounding will do will be to create a potential gradient on the soil around the ground rod. The thing to be aware of here is that the pool will be a certain number of volts above the earth.

Will this potential gradient ever be in a location where barefoot swimmers could access it? For example, with one foot on the soil outside the deck and one foot on the deck? Something to consider when you build your next pool.

Oh, and by the way, make sure you use standard rebar in the shell of the pool. Epoxy coated rebar does not provide the conductive structure necessary to bond the shell.

I hope I haven't confused the issue.

Eric Stromberg, P. E
Houston, Texas