The main goal of phone and cable/satellite filtering is preventing that abnormal high voltages reach your modem, computer, TV, decoder, phone set, etc coming from your phone line, cable TV or satellite cable. This kind of situation is more common than you think. If a lightning strikes a pole where your phone line and/or your cable TV line are located, the lightning will reach your equipment and literally fry it. In fact, the lightning doesn’t need to reach the pole: striking nearby is enough to induct high voltage on your phone or cable TV line.

This circuit can be based on MOV’s or on a component called gas arrestor, where the circuit discharges the lightning, throwing it to ground. Surge suppressors using this component are not so common. Gas arrestors have life expectancy lower than MOV’s but on the other hand can handle higher voltages without self-destructing.

On Figures 9 and 10 you can see the circuitry used on our APC surge suppressor. As you can see, there is no communication between the phone/cable/satellite filter and the AC line filter, except for grounding. The same goes for the phone and cable/satellite sections: they don’t share any components. The phone filter is based on four MOV’s and four capacitors (see Figure 9) while the cable/satellite filter, which is located inside the metallic protection surrounding the two coax connectors, is based on a transient voltage suppression diode (transorb) – a semiconductor that acts similarly to a MOV but is faster –, also having one fusistor and one small coil.

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Figure 9: Phone filtering section.

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Figure 10: Cable/satellite filtering section.

Both phone line and cable/satellite filters work by throwing the excess voltage to ground. Thus, once again, if you don’t have proper grounding at your home or office these filters won’t work correctly and you won’t be protected.