Common use of Lightning Clause in Contracts

Lightning. When lightning induced surges appear at the point of connection to a building (the service entrance), a high common mode potential is generated between the current carrying conductors and ground. This potential produces a flow of current that seeks a path to earth to complete the circuit. Lightning can easily induce a 3000-ampere transient into a power line. When this transient reaches a building, the building ground at the service entrance can rise to 60,000 volts (assuming a building earth resistance of 20 ohms). The reference potential for ground in the rest of the building would rise proportionately. In order to protect the building against these high voltage surges, it is important to establish a low resistance earth ground at the service entrance. The National Electrical Code (Article 250, Part 4) specifies that the grounding at a building's service entrance should have a resistance to ground of 25 ohms or less. The IEEE Green Book (Recommended Practice for Grounding, ANSI/IEEE Standard 142-1982) recommends that the ground resistance be less than 5 ohms. If the building contains highly sensitive electronic communications equipment, a ground resistance of 5 ohms or less is recommended if this value can be practically achieved with the given site conditions.