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Major regulation authorities have set specific standards or test recommendations for qualifying telecom equipment, including central-office and customer-premises ADSL modems. Such standards depend on the type of equipment, its location in the network, and the network's geographical region. Yet a parallel element, particularly one with "crowbar" characteristics, supplies the necessary protection against lightning strikes. A crowbar device can short-circuit the twin-pair telephone lines to dissipate the overvoltage with minimal heat.
Protection standards also address overvoltages, which occur when an ac power line makes electrical contact with the telephone lines. To block such overcurrents, the protection circuit typically combines the parallel crowbar device with a voltage-clamping device, and possibly a positive-temperature-coefficient (PTC) resistor, in series with each telephone line.
This article details the various standards with which designers must become familiar. The more familiar they are, the better armed they are to protect an ADSL modem's telephone and data lines from overvoltage surges. Four tables list the main protection standards associated with specific telecom equipment, both in the U.S. and in Europe and Asia.
Also described are the various devices used in protection schemes, such as the aforementioned crowbar and clamping devices that help suppress damaging overvoltages.
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