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When lightning strikes over the ocean how far away do you have to be to not be affected by the electrical current?
Answer 1:

Here is an answer I got from an expert (see below). He also said that tens to hundreds of feet is a good bet for being safe. Unfortunately, it's almost impossible to predict where lightning will strike so it is also difficult to guarantee being a certain distance away from a strike!

Each flash is unique as to amplitude/polarity/rise time/duration/return strokes. Ocean water varies according to salinity/temperature/polutants/etc.
So we end up with a complex model: "It depends" is the right answer, though an unsatisfactory one.
Science believes that lightning follows "skin effect"behavior with current not penetrating to any depthand magnitude falling off rapidly with distance.
National Lightning Safety Institute (NLSI)

Answer 2:

Lightning can strike as much as 10 miles away from the rain area in a thunderstorm; that's about the distance that you are able to hear the thunder from the storm. In some instances, it may even be difficult to tell that a storm is nearby. However, if you can hear the thunder from a storm,chances are that you are within striking distance of that storm. The most effective safety measure you can take to protect you and your family from lightning is to remember the following- If you can hear the thunder, you are within striking distance of the lightning, take shelter immediately!!

Here is some stuff on protection...
Houses and large buildings provide the best protection. But, you can still be injured or even killed by lightning inside!! There are three main ways lightning enters homes and buildings: (1) a direct strike, (2) through wires or pipes that extend outside the structure, and (3) through the ground. Regardless of the method of entrance, once in a structure, the lightning can travel through the electrical, phone, plumbing, and radio/television reception systems. It can also travel through any metal wires or bars in concrete walls or flooring as well as windows and doors. This makes it important to avoid contact with these conductors. Phone use is the leading cause of indoor lightning injuries in the United States. Lightning can travel long distances in both phone and electrical wires, particularly in rural areas where other conductors are limited. A shelter that does not contain plumbing, wiring, a metal shell, or some other mechanism to provide grounding is not safe!


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