National Weather Service Taunton MA
950 AM EDT Fri Jun 24 2005

National lightning safety preparedness week

Part five - indoor lightning safety

A house or other substantial building offers the best protection from lightning. In assessing the safety provided by a particular structure it is more important to consider what will happen if the structure is struck, rather than whether the structure will be struck by lightning. The shelter must provide a mechanism for conducting the electrical current from the point of contact to the ground. These mechanisms May be the outside of the structure, May be contained within the walls of the structure, or May be a combination of the two. On the outside, lightning can travel along the outer shell of the building or follow metal downspouts and gutters to the ground. Inside, lightning can follow electrical wiring, plumbing, and telephone lines to the ground.

Many small open shelters on athletic fields, golf courses, parks, roadside picnic areas, schoolyards, etc. Are designed to protect people from rain and Sun but not lightning. A shelter that does not contain plumbing or wiring throughout or some other mechanism for grounding from the roof to ground is not safe. These should be avoided during thunderstorms. Similarly, stay off porches.

There are three main ways that lightning enters homes or buildings, 1) a direct strike, 2) through wires and pipes that extend outside the structure, 3) through the ground. Once in the structure, the lightning can travel through electrical, phone, plumbing, and radio/television reception systems. It can also travel through any metal wires or bars in concrete walls or flooring.

When indoors during a storm, stay off corded phones. Lightning can travel long distances in both phone and electrical wires. Stay away from windows and doors since these provide a direct path for lightning to enter. Do not lie on the concrete floor of a garage as it likely contains a wire mesh. In general, basements are safe but avoid contact with concrete walls which May contain metal reinforcing bars. Do not do laundry since washers and dryers not only have contacts with plumbing and electrical systems but they also contain an electrical path to the outside through the dryer vent. Do not wash your hands, do not wash dishes, do not take showers, since they connect to plumbing.

Typical surge protectors do not protect equipment from a lightning strike. Do not unplug electronic equipment during a storm. That should be done well before the storm arrives. If you plan to be away from your home when thunderstorms are possible, be sure to unplug unneeded equipment before you leave.

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