Protection For Telephone Line

A long time ago, when telephones were simple and reliable from an electrical standpoint, telecom operators installed surge protection on all telephone lines exposed to storm risks. Ironically, this protection has diminished as more delicate and expensive equipment, such as electronic telephones, fax machines, and DSL modems, has become prevalent. For those residing in rural areas with overhead telephone lines, there is a significant risk of high voltages being induced during thunderstorms. Many modems, fax machines, and telephones have been destroyed by lightning strikes. Fortunately, a modest investment can yield a highly effective protection device. During storms, lightning can induce transient voltages that reach several thousand volts on telephone lines. Unlike the high-voltage sections of televisions or electric fences, which carry minimal current, lightning can cause current surges of thousands of amps. Traditional components are insufficient to protect against such destructive pulses. A gas-filled spark gap is recommended for this purpose. This component consists of three electrodes insulated from each other in an airtight cylinder filled with rare gas. When the voltage between the electrodes remains below a certain threshold, the spark gap is passive, presenting an impedance of several hundred megohms. However, when the voltage exceeds this threshold, the gas ionizes rapidly, turning the spark gap into a conductor capable of handling colossal currents without damage. The specific spark gap used can absorb a standardized 5,000 amp pulse lasting 8/20 ms. By utilizing a three-electrode spark gap, the voltage between the line wires or between any wire and ground is limited to approximately 250 volts. While this level of protection is theoretically adequate, an additional security device, such as a Voltage Dependent Resistor (VDR), is recommended. This component, either a GeMOV or SiOV, depending on the manufacturer, also limits the voltage between line wires to a maximum of 250 volts. Although this voltage may seem high, all authorized telephone equipment bearing the CE mark is designed to withstand it. However, some low-end devices manufactured in China may not meet this standard. Since lightning-generated pulses are very brief, the ground connection of this assembly must be low-inductance, requiring a short, heavy-duty wire (minimum 1.5 mm² cross-sectional area). If the ground connection is not appropriately designed, the coil formed by the ground connection can obstruct the high-frequency signals of the pulse, rendering the assembly ineffective. It is important to note that this device does not affect the low-frequency signals of telephones and fax machines, nor does it interfere with DSL signals.

The described surge protection device utilizes a gas-filled spark gap to safeguard sensitive electronic equipment against high-voltage transients caused by lightning strikes. The spark gap is designed to remain passive under normal operating conditions, effectively insulating the connected equipment from potential damage. The rapid ionization of the gas within the spark gap allows it to transition from a high-impedance state to a low-impedance state when the induced voltage exceeds the specified threshold. This capability enables the spark gap to handle large current surges, providing an effective barrier against lightning-induced surges.

In addition to the spark gap, the inclusion of a VDR provides an extra layer of protection by clamping the voltage to a safe level. This dual protection mechanism ensures that even in the event of a lightning strike, the equipment connected to the telephone lines remains safe from damage due to excessive voltage. The careful selection of materials, such as heavy-duty wire for the ground connection, further enhances the device's performance by minimizing inductance and ensuring efficient grounding.

Overall, the implementation of this surge protection device is essential for safeguarding modern telecommunication equipment in areas prone to thunderstorms. The integration of advanced components, such as the gas-filled spark gap and VDR, reflects a comprehensive approach to protecting sensitive electronics from the unpredictable nature of lightning strikes.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

, 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 suddenly becomes a full conductor to the point of being able to absorb colossal currents without being destroyed.

The one we are using here, whose size is of the same magnitude as an ordinary one watt resistor, can absorb a standardized 5, 000 amps pulse lasting 8/20 ms! Since we are utilizing a three-electrode spark gap, the voltage between the two wires of the line or between any wire and ground, cannot exceed the sparking voltage, which is about 250 volts here.

Such protection could theoretically suffice but we preferred to add a second security device made with a VDR (GeMOV or SiOV depending on the manufacturer), which also limits the voltage between line wires to a maximum of 250 volts. Even if this value seems high to you, we should remember that all of the authorized telephone equipment, carrying the CE mark must be able to withstand it without damage.

This is not always the case however with some low-end devices made in China, but that`s an entirely different problem. Since pulses generated by lightning are very brief, the ground connection of our assembly must be as low-inductance as possible.

It must therefore be short, and composed of heavy-duty wire (1. 5 mm2 c. s. a. is the minimum). If not, the coil, composed of the ground connection, blocks the high frequency signal that constitutes the pulse and reduces the assembly`s effectiveness to nothing. Finally, please note that this device obviously has no effect on the low frequency signals of telephones and fax machines and it does not disturb (A)DSL signals either.

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