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Does a coaxial lightning surge protector also protect against power surges?

I don't know if there is a difference between lightning surges and power surges, thus I ask this question.

I have a broadband modem that is connected by cable and I heard that these cables are resistant to lightnings. But if they are, why are coaxial cable surge protectors even for sale?

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In many countries, cable broadband is carried in underground conduits that are less likely to be hit by lightning. Some cable broadband is provided (at least partly) over optical cable which is not conductive and does therefore not transmit an electrical surge. Phone-line broadband can be supplied on phone lines that are suspended from telegraph poles above the ground, and are much more likely to be hit by lightning.

In general, every conductive cable that enters your computer is a potential path for a destructive surge and might benefit from surge protection. This could include Ethernet cables, serial cables or perhaps USB cables that link to a device that has a separate connection to an unprotected electricity supply, etc.

an average bolt of positive lightning (from the top of a thunderstorm) may carry a current of 300 to 500 kiloamperes, transfer a charge of up to 300 coulombs, have a potential difference up to 1 gigavolt (a billion volts),

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They are two different problems. The modem has a coaxial input for the cable carrying the data stream. The coaxial surge protector looks after this should something happen on the cable or associated components in the suppliers network. If the modem also has a mains power supply this will make it susceptible to mains surges following lightning strikes. A mains surge limiter is used to protect that from the excessive voltages that can kill components. I had to replace a wi-fi router because of a lightning induced mains surge just a couple of months ago (blush) so they do happen.

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