~ National Lightning Safety Institute ~

Section 4.7.3

The adoption by many water safety groups of our indoor pool suspension rules when lightning is observed has created questions among responsible officials and individuals. Some additional information may be helpful.

1. We at NLSI could find no reports of deaths or injuries in indoor pools related to lightning causes. “We could find no reports” means just that. There may be such instances, but they were unreported if they happened. This is not surprising since the overall annual U.S. government lightning deaths and injuries statistics are known to be underreported by some 30%. Furthermore, as they say in the legal profession, “Absence of evidence is not evidence of absence.”
2. There are many reported and verifiable incidents of lightning incidents in home bathtubs. The exact mechanism varies. Could be lightning attachment to copper water pipes. Could be contact with nearby electrical circuit such as a light switch or outlet. If it can happen in a tub, it can happen in a pool, no?
3. The physics of the event are well understood: when a body is free-floating in water, it is not a part of any circuit path (this does not account for “direct lightning strikes”) and cannot receive shocks. When a body in water is in contact with a circuit path, it will conduct current. “In contact” can mean:

• Feet on the pool floor
• Touching sides of the pool
• In contact with ladders, underwater lights, railings, etc.

Persons not in the water but inside a pool building who are a part of the circuit path also can become victims, just as they may be at risk in any dwelling. Wet floors at pool facilities are very good conductors.

NLSI operates under the principle that safety has priority over any other issues. “Safety is the prevailing directive” says the Institute of Electrical and Electronic Engineers. We have adopted that slogan. In almost any situation where lightning could threaten, moving from a high-risk situation to a low-risk location is recommended. A super-conservative attitude? Yes, we are guilty as charged. But, as an aquatics professional, there is no way I am going to make a telephone call to a next-of-kin to say an unfortunate accident has occurred on my watch.

We summarize our views: “At the first signs of thunder or lightning, all pool activities should be suspended (showers, too) until 30 minutes after the last observed thunder or lightning.” Hear thunder after 27 minutes of activity suspension? Start your 30-minute clock all over again.

One final thought: Lightning safety for outdoor pools deserves some consideration, too. In short:

• Places outside simply have degrees of “more safe” and “less safe” with respect to lightning — no place is absolutely safe.
• Indirect effects of lightning need to be considered. Lightning can travel considerable distances horizontally on the surface of the earth. “Radial arcing” on conductive (wet from rain?) ground has been observed up to 40m. Lightning striking a metal conductor (for example, fence wires) and then traveling for many kilometers has been observed. Lightning striking trees and other tall objects with consequential “flashover” to people has also been observed. Two dangerous mechanism for people may be present:
• Touch voltages — People touching or resting against metal conductors. Sitting on metal bleachers or leaning against conductive light poles are but two examples.
• Step voltage — When lightning strikes nearby, voltages and currents diminish with distance: about 50% per meter depending on the conductor. Remember, lightning contains hundreds of millions of volts. Say that voltage V1 intercepts a grounded foot closest to the ground strike. Say that a lesser voltage, V2, intercepts the more distant foot.  The two voltages must find equalization via a common circuit path: up one leg — across the chest/abdomen — and down the other leg.
• Small shelters intended for sun and rain protection cannot be made 100% safe for people. Evacuation to a large permanent structure by large crowds in a short time may not be possible.
• Early warning of the hazard is essential. Disruption of activities with all of its unhappy consequences is inevitable. “Safety is the prevailing directive.”