Pager Phones: Circuit Protection.

摘要

Wires leading to or from underground facilities can develop hazardous voltages because of lightning exposure. The locations of many mine portals are particularly susceptible to lightning effects. The application of protective devices to limit this potential danger is necessary. Requirement: The Code of Federal Regulations, Title 30, Mineral Resources, Part 75, paragraph 75.52 1 identifies the requirement for lightning arrestors to be used on ungrounded, exposed power conductors and telephone wires. 'Each ungrounded, exposed power conductor and telephone wire that leads underground shall be equipped with suitable lightning arrestors of approved type within 100 feet of the point where the circuit enters the mine.' For telephone circuits, a wide range of protective devices are available. The public service telephone companies and Rural Electrification Authority have considerable experience with the problem. The current recommendation by these organizations for such installations is to use gas-tube protectors, operating with a well-designed and constructed grounding circuit. Although pager phones are d-c systems, with signals in the audio range, many installations have coupled RF (Radio Frequency) signals on the same cable that is used for the pager system. The telephones themselves are constructed with isolating circuits, so as not to disturb RF transmission. Lightning arrestors and protective circuits must also provide this same isolation and circuit integrity for the RF signals. Capacitive protective elements cannot be used for lightning protection of such circuits, as they would short out the RF signals. Lightning strokes, either cloud-to-cloud or cloud-to-ground, release tremendous energy. The voltage associated with cloud-to-earth lightning stokes is about 5 to 20 million Volts, but the danger is in the large amounts of energy developed by the fields induced in exterior cabling. Arrestor Connection: In early telephone installations, carbon blocks of 3 mil (0.003 inch) carbon were used as a protective breakdown device for phone circuits, but in the past ten years gas-tube devices have become more acceptable. When sufficient energy is developed between two circuits, such as a telephone line and ground reference, the protective device ionizes and provides a low-impedance path to ground, thus Arthur I3 Little, Inc keeping the hazardous voltage from reaching the remainder of the system.