Lightning and surge protection

Means of protecting electrical systems, buildings, and other property from lightning and other high-voltage surges. From studies of lightning, two conclusions emerge: (1) Lightning will not strike an object if it is placed in a grounded metal cage. (2) Lightning tends to strike, in general, the highest objects on the horizon.

One practical approximation of the grounded metal cage is the well-known lightning rod or mast. The effectiveness of this device is evaluated on the cone-of-protection principle. The protected area is the space enclosed by a cone having the mast top as the apex of the cone and tapering out to the base. If the radius of the base of the cone is equal to the height X of the mast, equipment inside this cone will rarely be struck. A radius equal to twice the height of the mast (2X) gives a cone of shielding within which an object will be struck occasionally. The cone-of-protection principle is shown in the illustration.

The probability that an object will be struck by lightning is considerably less if it is located in a valley. Therefore, electric transmission lines which must cross mountain ranges often will be routed through the gaps to avoid the direct exposure of the ridges.

There are a number of protective devices to limit or prevent lightning damage to electric power systems and equipment. The word protective is used to connote either one of two functions: the prevention of trouble, or its elimination after it occurs. Various protective means have been devised either to prevent lightning from entering the system or to dissipate it harmlessly if it does. Overhead ground wires and lightning rods are used to prevent lightning from striking the electrical system. Lightning arresters are protective devices for reducing the transient system overvoltages to levels compatible with the terminal-apparatus insulation. See Surge suppressor

Immediate reclosure is a practice for restoring service after the trouble occurs by immediately reclosing automatically the line power circuit breakers after they have been tripped by a short circuit. The protective devices involved are the power circuit breaker and the fault-detecting and reclosing relays. See Circuit breaker, Electric protective devices

单词	lightning and surge protection
释义	Lightning and surge protection Lightning and surge protection Means of protecting electrical systems, buildings, and other property from lightning and other high-voltage surges. From studies of lightning, two conclusions emerge: (1) Lightning will not strike an object if it is placed in a grounded metal cage. (2) Lightning tends to strike, in general, the highest objects on the horizon. One practical approximation of the grounded metal cage is the well-known lightning rod or mast. The effectiveness of this device is evaluated on the cone-of-protection principle. The protected area is the space enclosed by a cone having the mast top as the apex of the cone and tapering out to the base. If the radius of the base of the cone is equal to the height X of the mast, equipment inside this cone will rarely be struck. A radius equal to twice the height of the mast (2X) gives a cone of shielding within which an object will be struck occasionally. The cone-of-protection principle is shown in the illustration. The probability that an object will be struck by lightning is considerably less if it is located in a valley. Therefore, electric transmission lines which must cross mountain ranges often will be routed through the gaps to avoid the direct exposure of the ridges. There are a number of protective devices to limit or prevent lightning damage to electric power systems and equipment. The word protective is used to connote either one of two functions: the prevention of trouble, or its elimination after it occurs. Various protective means have been devised either to prevent lightning from entering the system or to dissipate it harmlessly if it does. Overhead ground wires and lightning rods are used to prevent lightning from striking the electrical system. Lightning arresters are protective devices for reducing the transient system overvoltages to levels compatible with the terminal-apparatus insulation. See Surge suppressor Immediate reclosure is a practice for restoring service after the trouble occurs by immediately reclosing automatically the line power circuit breakers after they have been tripped by a short circuit. The protective devices involved are the power circuit breaker and the fault-detecting and reclosing relays. See Circuit breaker, Electric protective devices
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