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Protection for Telephone Line Schematic Circuit Diagram

Historical Surge Protection: Ensuring Telephone Line Safety

In the past, when telephones were relatively simple and robust, telecom operators installed surge protection on all telephone line vulnerable to storm-related risks. Strangely, in today’s era where we connect delicate and expensive electronic devices like phones, fax machines, and (A)DSL modems, this protection has vanished. However, if you reside in a rural area with overhead telephone lines, the risk of extremely high voltages induced during thunderstorms is significant.

Need for Effective Surge Protection: A Small Investment for Safety

Countless modems, fax machines, and telephones have succumbed to lightning strikes, emphasizing the pressing need for effective surge protection. Surprisingly, investing a modest sum can secure your devices with highly efficient protection, as demonstrated by the device proposed here. During storms, lightning strikes near a telephone line induce transient voltages reaching several thousand volts. Unlike high-voltage sections in TVs or electrical fences, where minimal current flows, lightning strikes can cause current surges reaching thousands of amps.

Choosing the Right Protection: Introducing the Gas-Filled Spark Gap

To safeguard against these destructive pulses, conventional components lack the necessary power and speed. As illustrated in our diagram, a gas-filled spark gap proves indispensable. This component comprises three insulated electrodes within an airtight cylinder filled with rare gas, ensuring a robust and swift response to lightning-induced surges.

Protection for Telephone Line Schematic Circuit Diagram

Understanding Spark Gap Impedance: Passive State and Conduction

In its passive state, the spark gap remains inert as long as the voltage between its electrodes stays below a specific threshold, presenting an impedance in the several hundreds of megaohms range. However, once the voltage surpasses this threshold, the gas within the gap rapidly ionizes, transforming it into a full conductor. In this state, the spark gap can handle immense currents without getting destroyed.

Impressive Pulse Absorption: Robust Protection

The spark gap used here, comparable in size to a standard one-watt resistor, can absorb a standardized 5,000-amp pulse lasting 8/20 ms. Due to its three-electrode design. The voltage between the line wires or any wire and the ground cannot exceed the sparking voltage, approximately 250 volts. While this level might seem high. It aligns with the requirements for authorized telephone equipment bearing the CE mark. To enhance safety, an additional security device is added using a VDR (GeMOV or SiOV, based on the manufacturer). Limiting the voltage between line wires to a maximum of 250 volts.

Optimizing Ground Connection: Minimizing Inductance

Considering the brief nature of lightning-generated pulses. The ground connection’s inductance must be minimized. A short, heavy-duty wire (1.5 mm² c.s.a. minimum) is essential to avoid impeding the high-frequency signals constituting the pulse. A well-established ground connection ensures the assembly’s effectiveness. It’s vital to recognize that this device does not impact the low-frequency signals of telephones and fax machines, nor does it disturb (A)DSL signals, preserving the integrity of communication systems.

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