Pools, Shocks and Stray Voltage
Summer heat brings swimming pool use, increased energy use and the drying of the soil. This combination, in conjunction with the non-conductive nature of modern swimming pools, can lead to shock problems around the pool.
This situation can be very complex and difficult to resolve, particularly with regard to existing pools.
It helps to understand that an electric potential (voltage) may exist between the "grounded" conductors of an electric system and local "earth" ground. This potential is most frequently referred to as "stray voltage," but is also called tingle voltage and neutral-to-earth voltage (NEV). It is generally thought that grounded conductors are at the same potential as "earth" ground, but due to many complex issues, this is rarely the case. Adding grounding electrodes seldom has a beneficial effect on this voltage: there are numerous grounding electrodes already on the electric system at every service drop (meter), transformer and most electric poles.
Neutral-to-earth voltage is brought into the pool vicinity by the equipment grounding conductor that is connected to the pool circulation pump and pool equipotential bonding grid required by the 2008 National Electrical Code(NEC), Article 680-26(B). Non-conductive materials used in the construction of modern pools generally prevent the pool water from coming into electrical contact with the grounded conductors of the electric system through such obvious means as metallic plumbing or steel reinforced concrete sides.
Consequently, pool water becomes bonded to the local "earth" potential by water seepage through the liner. Paradoxically, the concrete deck, metal coping, stanchions, and other metallic fittings are brought to the potential of the electric grounding system via the connection of the equipotential bonding grid to an equipment grounding conductor either directly or indirectly.
The most adequate solution to "stray voltage" is to ensure that all conductive items included in or on the pool structure are bonded together in a common equipotential bonding grid. The pool water must also be effectively bonded to the equipotential bonding grid to eliminate a voltage differential between it and the bonded surroundings. The 2008 edition of the NEC now requires an intentional bond (electrode) in contact with the pool water (Art. 680.26(C)). The equipotential bonding grid needs to include the decking or soil in some manner for several feet away from the edge of the pool coping.
Some common problems encountered involve the degradation or incompleteness of the equipotential bonding grid around the pool. Most noteworthy of these is the lack of electrical bonding to each individual coping section around the pool deck edge. Robust, redundant mechanical connections or exothermic welding are necessary.
The pool environment is fairly corrosive to metals, particularly aluminum and steel, due to moisture, galvanic action, chloride salts and the alkalinity of concrete. Additionally, concrete settlement creates breaks in the equipotential bonding grid allowing differing potentials to develop around the pool. The importance of an adequately designed, electrically and mechanically sound equipotential bonding grid cannot be overemphasized in NEV mitigation.
The NEC lists the "minimum" requirements needed for the practical safeguarding of persons and property. It is not intended as a design specification. Experience has shown the "minimum" requirements do not adequately resolve NEV. Redundancy and adequacy of bonding points is important. Equipotential bonding is not an area to cut costs. Once the pool is built, it is difficult and expensive to retrofit, replace, or repair shortcomings. Other perceived remedies such as neutral isolators mask the fact that a safety issue exists: namely, inadequate bonding between contact points allowing electric potential differences to manifest, regardless of the source.