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(I know nothing about electronics, so excuse my ignorance.)

I understand surge protection for power outlets are necessary, or at least recommended, for expensive electronics. But are telephone lines susceptible to the same voltage spikes as power lines?

I am guessing the answer to this question is "No," since I've never actually seen anyone using surge protectors for phone/fax/modem lines. But why are phone lines less susceptible than power lines? What about them makes them more resistant to these spikes?

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  • \$\begingroup\$ I was just about to answer before Nick closed it! That's OK. The reason I think these are used is to protect phone lines from lightning strikes. \$\endgroup\$ – htownclyde Mar 5 '16 at 5:40
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    \$\begingroup\$ @htownclyde Good thing you didn't post that as as answer, because that's incorrect (and perhaps even dangerously misleading). Such consumer-level things will not protect against a lightning strike, they are not designed for it. \$\endgroup\$ – Nick Alexeev Mar 5 '16 at 5:44
  • \$\begingroup\$ @NickAlexeev Are you sure? link This description for a similar item says it protects modems/etc from lightning. \$\endgroup\$ – htownclyde Mar 5 '16 at 5:48
  • \$\begingroup\$ @NickAlexeev Oh, I see where my comment was wrong. It isn't phone lines, it's the appliance using the lines. Sorry for the mistake \$\endgroup\$ – htownclyde Mar 5 '16 at 5:51
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    \$\begingroup\$ @htownclyde A direct lightning strike is a lot of energy. While it's possible to make a surge arrester that can deal with lightning, most of the consumer-level devices are based on MOV and TVS. Consumer devices can deal with things like transient spikes created by motors. \$\endgroup\$ – Nick Alexeev Mar 5 '16 at 5:58
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Telephone lines are just as (if not more) susceptible to induced surges as power lines. So much so, in fact, that the phone company installs their own surge suppressors on every line. They're built into the network interface box that's installed on/in your home.

Therefore, any aftermarket devices installed by consumers are mostly redundant, and that's why they are not frequently used, unless the equipment being used is particularly sensitive.

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  • \$\begingroup\$ Can you explain why telephone lines may be more susceptible to induced surges than power lines? \$\endgroup\$ – Kyle Mar 5 '16 at 15:15
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    \$\begingroup\$ @Kyle: Because they operate at higher impedances. A phone line is generally characterized as a 600-ohm impedance, while the power grid has an impedance that's just a few ohms. Higher impedance means that a given amount of induced energy results in higher voltages. Also, the equipment that's connected directly to the phone line is more delicate (compared to power supplies, motors, etc.) -- and that's what's really being protected. \$\endgroup\$ – Dave Tweed Mar 5 '16 at 15:30
  • \$\begingroup\$ Why do phone lines operate at higher impedances than power grids? \$\endgroup\$ – Kyle Mar 5 '16 at 21:35
  • \$\begingroup\$ @Kyle: Each kind of network is optimized for its purpose (communications vs. power distribution). The characteristic impedance of each comes about as a result of the optimization of other parameters. \$\endgroup\$ – Dave Tweed Mar 5 '16 at 22:10
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They are used because an ounce of prevention is worth a pound of cure, but if your area isn't prove to lightning storms or faulty electrical power grids, it's not a common occurrence. Anecdotal evidence suggests it is not common, so you have to deal with some unknown amount of chance. You may have a better chance to be hit by lightning than lightning inducing a surge in the Telco wiring to your house that leads to your phone or modem/pc frying itself.

Frankly, the smaller gauge of Telco vs mains wiring means it's more important to have a power outlet surge protection. Smaller gauge wiring has a higher resistance leading to a lower voltage/current. And internal esd protection handles the rest.

You also have to factor in commercial/consumer product marketing being the major driving force behind these products. It's an infomercial (read gullible/mis or under informed) economy. Many products are aimed at the ignorant, using fear and doubt (fud) to produce sales.

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    \$\begingroup\$ I see that this answer was posted after the question was put on hold, how is that? I was in the middle of writing an answer, and it removed the option to post the second that Nick closed the question. Is there a certain level of rep that allows you to post answers after a question has been closed? \$\endgroup\$ – htownclyde Mar 5 '16 at 5:45
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    \$\begingroup\$ @htownclyde web or mobile users have a longer grace period. Like five minutes. I started answering and submitted under five minutes. Regular Web users only get like 30 seconds. Maybe less if their connection supports push updates \$\endgroup\$ – Passerby Mar 5 '16 at 5:48
  • \$\begingroup\$ Grace period, not have period. Damm auto correct. \$\endgroup\$ – Passerby Mar 5 '16 at 5:51

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