The isokeraunic level is the number of thunderstorm days per year. This isokeraunic number varies from over 100 along the Gulf coast of Florida to less than 5 in the Pacific Northwest. Nevertheless, virtually all areas in North America are subject to lightning strikes to some degree. Exterior equipment, rooftop equipment, and equipment connected to aerial and buried copper cables are subject to possible damage. In contrast, in areas of heavy lightning activity, it is quite common to use protected-type line repeaters regardless of whether the cable is aerial or buried.
Aerial cables are especially susceptible to lightning strikes.
The huge energy pulse from lightning momentarily raises the potential of ground at that strike point and then travels along the copper pairs to the central office where it finds a lower ground potential. In some central offices, gas tubes are installed at the top of the relay rack with office repeaters. Primary protectors must be present and grounded properly. Many items of transmission equipment, such as office repeaters, have secondary protection in the form of solid state surge limiters, but they are not effective if primary protection is bad. When the lightning voltage causes the primary gas tube to conduct, it represents a short circuit to the span powering regulator.