Following the tragic tale from @telectrix about lightning damage to phones and modem leading to a serious loss of beer money I decided to take a look at an el-Chepo SPD from Amazon to see if it looks worth considering. This one:
When it arrived the box looked somewhat aged, but that is not so important (might explain the low cost as excess stock?) seen here:
Somewhat bizarrely considering its purpose is is wrapped in an anti-static bag
Otherwise looked like the sales photo and easy enough to get apart:
The insides show a pair of 3-terminal gas discharge tubes on the 'In' side (connected to external phone line) to take the brunt of any surges, followed by 1 ohm resistors and some transient suppression diodes to clip the peaks of those firing in to conduction on the 'Out' side of the device (connected to your modem):
PCB is cheap but of reasonable quality, but not sure quite how much of the GND area would remain after a 10kA surge:
Overall it looks quite well made for the surprisingly low cost. Earth arrangement would need consideration though, as you want some short path to true Earth for the surge to go, but also you want your phone/modem not to have its CPC at many kV difference due to inductance and dI/dt of the surge being diverted. Maybe a 0.1uF capacitor of decent size / working voltage (class X style) between the earth for this SPD unit going to a nearby rod and the supply socket CPC of the modem/phone.
TL;DR Conclusion is it might save your phone for a couple of beers.
When it arrived the box looked somewhat aged, but that is not so important (might explain the low cost as excess stock?) seen here:
Somewhat bizarrely considering its purpose is is wrapped in an anti-static bag
Otherwise looked like the sales photo and easy enough to get apart:
The insides show a pair of 3-terminal gas discharge tubes on the 'In' side (connected to external phone line) to take the brunt of any surges, followed by 1 ohm resistors and some transient suppression diodes to clip the peaks of those firing in to conduction on the 'Out' side of the device (connected to your modem):
PCB is cheap but of reasonable quality, but not sure quite how much of the GND area would remain after a 10kA surge:
Overall it looks quite well made for the surprisingly low cost. Earth arrangement would need consideration though, as you want some short path to true Earth for the surge to go, but also you want your phone/modem not to have its CPC at many kV difference due to inductance and dI/dt of the surge being diverted. Maybe a 0.1uF capacitor of decent size / working voltage (class X style) between the earth for this SPD unit going to a nearby rod and the supply socket CPC of the modem/phone.
TL;DR Conclusion is it might save your phone for a couple of beers.
