Protection For Telephone Line

Posted on Feb 6, 2014

A long time ago when telephones were so simple almost nothing could go amiss from an electrical point of view, Telecom operators installed surge protection on all telephone lines exposed to storm risks. Paradoxically, now that we are hooking up delicate and expensive equipment such as telephones filled with electronics, fax machines, (A)DSL modems

Protection For Telephone Line
Click here to download the full size of the above Circuit.

, etc. , this protection has disappeared. However, if you have the good fortune to live in the countryside in a building served by overhead telephone lines, there`s an obvious risk of very high voltages being induced on the lines during thunderstorms. While we have lost count today of all of the modems, fax machines and other telephones that have been destroyed by a bolt of lightning`, surprisingly you only have to invest a few pounds to get a remarkably efficient protection device like the one we are proposing here. During a storm, often with lightning striking near a telephone line, the line carries transient voltages up to several thousands of volts. Contrary to the HV section of television sets or electrical fences, on which practically no current is running, in the case of lighting striking current surges of thousand of amps are not uncommon. To protect oneself from such destructive pulses, traditional components are not powerful or fast enough. As you can see on our drawing, a (gas-filled) spark gap should be used. Such a component contains three electrodes, insulated from each other, in an airtight cylinder filled with rare gas. As long as the voltage present between the electrodes is below a certain threshold, the spark gap remains perfectly passive and presents an impedance of several hundreds of MW. On the other hand, when the voltage rises above this threshold, the gas is very rapidly ionized and the spark-gap...

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