Lightning Protection of Equipment with Remotely Mounted Lightning Arresters

1. The curves apply to any type of modern lightning arrester. Only the sparkover voltage has to be known; this should be obtained from the manufacturer. 2. When the arrester is between the equipment and the surge, the maximum voltage at the equipment approaches 200% of the arrester sparkover voltage. For short distances where the voltage at the equipment is less than two, Figs. 3 and 5 can be employed to determine the voltage at the equipment. 3. When the equipment to be protected is between two arresters, and if the surge is applied externally, the maximum voltage at the equipment approaches 150% of the arrester sparkover voltage. For short distances where the voltage at the equipment is less than 150%, Fig. 11 can be employed to determine the voltage at the mid-point 4. Figs. 3 through 5 were constructed on the basis of 1,000-kv/μsec surge on the OH line. However, since simple inverse proportionality exists between permissible feeder length and slope of surge front, any slope can be considered. If 100 feet is the answer from graphs (1,000-kv/μsec wave), then 200 feet is the answer for a 500-kv/μsec wave. 5. If system parameters are unknown, Figs. 3 and 5 should be used directly with no concern for all the variables involved.