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. Paradoxically, with the advent of delicate and expensive equipment such as electronic telephones, fax machines, and (A)DSL modems, this protection has diminished. In rural areas served by overhead telephone lines, there is a significant risk of high voltages being induced on these lines during thunderstorms. Many modems, fax machines, and telephones have been destroyed by lightning strikes. Fortunately, investing a small amount can provide effective protection. During storms, when lightning strikes near a telephone line, transient voltages can reach several thousand volts. Unlike the high-voltage sections of television sets or electric fences, which carry minimal current, lightning can induce current surges of thousands of amps. Traditional components are not fast or robust enough to handle such surges. A gas-filled spark gap is recommended for protection. This component consists of three electrodes insulated from one another within an airtight cylinder filled with rare gas. As long as the voltage between the electrodes is below a certain threshold, the spark gap remains passive and exhibits an impedance of several hundred megohms. When the voltage exceeds this threshold, the gas ionizes rapidly, transforming the spark gap into a conductor capable of handling immense currents without damage. The spark gap in this design can absorb a standardized pulse of 5,000 amps lasting 8/20 ms. With a three-electrode spark gap, the voltage between the line wires or between any wire and ground is limited to a sparking voltage of approximately 250 volts. Although this protection may seem adequate, an additional safety device using a voltage-dependent resistor (VDR), such as GeMOV or SiOV, is included to further limit the voltage between line wires to a maximum of 250 volts. While this voltage level may appear high, all authorized telephone equipment bearing the CE mark should withstand it without damage. However, some low-end devices manufactured in China may not meet these standards. Given that lightning pulses are very brief, the ground connection of the assembly must be low-inductance, requiring a short, heavy-duty wire (minimum 1.5 mm² cross-sectional area). If the ground connection is not optimized, the coil formed by the ground wire can block the high-frequency signals of the pulse, rendering the protection ineffective. It is important to note that this device does not affect low-frequency signals of telephones and fax machines, nor does it interfere with (A)DSL signals.

The circuit design incorporates a gas-filled spark gap as the primary surge protection component. This device is strategically positioned in parallel with the telephone line to divert high-voltage transients away from sensitive equipment. The spark gap's design allows it to remain inactive under normal operating conditions, maintaining high impedance to avoid interference with standard telephone signals. When the voltage exceeds the specified threshold due to a lightning strike or other surge events, the gas within the spark gap ionizes, allowing current to flow and effectively shunting the excess voltage to ground.

The inclusion of a voltage-dependent resistor (VDR) enhances the system's reliability by providing an additional layer of protection. The VDR functions by clamping the voltage to a safe level, ensuring that even if the spark gap fails to activate, the sensitive equipment remains protected from damaging transients.

The assembly's ground connection is critical for effective surge protection. A low-inductance ground connection minimizes the potential for voltage spikes due to inductive effects, which could compromise the protection strategy. The use of heavy-duty wire is essential to handle the high currents that may occur during a surge event, ensuring that the ground path is robust and effective.

Overall, the circuit is designed to protect against transient overvoltages while allowing normal operation of telephone and data communication equipment. Its implementation is particularly vital for installations in areas prone to thunderstorms, safeguarding investments in electronic communication technology.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.

🔗 External reference