Protect Your Line: Building a Lightning Surge Shield

Circuit diagram:

Description

In the past, when telephones were exceptionally basic, very little could go amiss from an electrical perspective. Telecommunication operators implemented surge protection across all telephone lines vulnerable to storm risks. Ironically, today, with the integration of sensitive and costly equipment like telephones, fax machines, (A)DSL modems, and more, this protective measure has largely disappeared.

However, should you reside in a rural area with a building supplied by overhead telephone lines, a significant risk exists of extremely high voltages being induced on those lines during thunderstorms. Despite the numerous modems, fax machines, and telephones that have been damaged by lightning strikes, it’s surprisingly affordable to obtain a remarkably efficient safeguarding device, such as the one we are proposing here.

During a storm, frequently with lightning striking nearby telephone lines, the lines carry transient voltages reaching several thousand volts. Unlike the high-voltage sections of television sets or electrical fences, where practically no current flows, in the case of a lightning strike, currents of thousands of amps are not uncommon. To protect oneself from these destructive surges, traditional components are generally insufficient in terms of power and speed.

As illustrated in our drawing, a (gas-filled) spark gap should be utilized. Such a component contains three electrodes, insulated from one another, within an airtight cylinder filled with a rare gas. As long as the voltage between the electrodes remains below a certain threshold, the spark gap remains entirely passive, presenting an impedance of several hundred megohms. Conversely, when the voltage surpasses this threshold, the gas is rapidly ionized, and the spark gap abruptly transforms into a full conductor, capable of absorbing immense currents without being destroyed.

The one we are employing here, which possesses a size comparable to a standard one-watt resistor, can absorb a standardized 5,000-amp pulse lasting 8/20 milliseconds. Given that 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 approximately 250 volts in this instance. Such protection could theoretically suffice, but we have elected to incorporate a secondary security device constructed with a VDR (GeMOV or SiOV depending on the manufacturer), which also restricts the voltage between line wires to a maximum of 250 volts.

Even if this value seems high to you, we should remember that all authorized telephone equipment, bearing the CE mark, must be able to withstand it without damage. This is not always the case, however, with some low-end devices manufactured in China, but that presents a completely different challenge. Since pulses generated by lightning are very brief, the ground connection of our assembly must be as low-inductance as possible.

Therefore, it must be short and composed of heavy-duty wire (1.5 mm2 c.s.a. is the minimum). If not, the coil, comprising 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 has no effect on the low-frequency signals of telephones and fax machines and it does not disrupt (A)DSL signals.