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Page 218

| 12 Lightning or Surge Arresters |

| Function.. |

| Lightning or surge arresters serve the same purpose on an electric circuit or line as a safety valve on a boiler. A safety valve on a boiler relieves a high pressure by allowing steam to blow off until the high pressure is reduced to normal, at which time the safety valve closes again and is ready for the next abnormal condition. So also is the operation of a lightning or surge arrester. In an electric system, this abnormal pressure takes the form of high-voltage surges in the conductors that may be brought about by the static electricity created by lightning (hence the name, lightning arrester). High-voltage surges may also result from improper switching and other abnormal operations in which the circuit may be involved. The function of the surge arrester is to allow the discharge of any dangerous overvoltage before it can do damage and then to restore the line to normal operation after the discharge. |

| To accomplish this protective function satisfactorily, arresters must: |

| 1. Not allow current to flow to ground so long as the circuit voltage remains normal |

| 2. Provide a path to ground, when the voltage rises to a predetermined valve above normal, to dissipate the energy from the surge without raising the voltage at which the circuit is operating |

| 3. Stop the flow of current to ground, as soon as the voltage drops below the predetermined value, and restore the insulating qualities between the conductor and ground |

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