Swimming Pool Shocking Due to Poor Bonding of Concrete Deck

Stanely, North Carolina, USA

Background

The owner of a home was concerned about a shocking sensation that he and his family felt as they got out of their swimming pool. They experienced the sensation only on certain sides of the swimming pool. The swimming pool has a concrete deck.

Possible Causes

The initial investigation considered several possibilities:

• Faulty wiring on the owner's side.
• Overhead system neutral was open or damaged.

Action Taken

The possible causes were ruled out by:

• Temporarily de-energizing the owner's wiring by pulling the power meter.
• Inspecting the system neutral because the area where the owner's home was located is subjected to ice storms and tree-related damage.

The next step in the investigation was to measure the shocking voltage and to check for proper bonding at the swimming pool. The shocking voltage was measured at 1.6 VAC from the pool's water to the wet concrete deck in the area of interest. The deck and coping stone were not bonded in this area. The customer was informed of the lack of bonding at the swimming pool's concrete deck and coping stone.

The next step in the investigation was to measure the neutral-to-earth voltage (NEV) along the underground line at the pad-mounted transformers. The next setup was to take current readings and NEV measurements on the circuit. Table 1 shows the results. Location A was the single-phase line feeding the owner, and Location B was at the three-phase to single-phase transformer upstream.

Table 1: Measurements Taken at Two Locations Relevant to the NEV Investigation

The current readings at Location A indicated that the shocking voltage was a function of voltage unbalance on the three-phase connection at Location B and the heavily loaded single-phase line.

The next step was to run a temporary jumper from the termination pole system neutral at Location A to the transmission tower leg. Table 2 shows the results of this action. The shocking voltage at the pool was reduced by 60%, from 1.15 VAC to 0.68 VAC.

Figure 1: Results of Running a Temporary Jumper from the Termination Pole System Neutral at Location A to the Transmission Tower Leg

Solutions

The solution to the owner's shocking voltage is to bond the concrete deck and the coping stone. Also, the #2 copper underground cable should be installed from the system neutral to the transmission tower leg. The #2 copper underground cable will provide another path for the neutral current. An alternate solution to the #2 copper cable is to open-wye the owner's tap to balance the three-phase load and to reduce the current on the neutral.