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Twisted pair ethernet thunderstorm protection

The idea is taken from Vladimi Inyin's article Thunderstorm protection (In Russian only). The circuit is modified a little. Also, I have mentioned analogues of used elements. Is applicable to 100 Mbit ethernet (FastEthernet).

thunderstorm protection circuit
pseudo-graphic form


D1...D4, D6...D91N4007
D5, D101,5KE7.5, 1N6267, 1N4007
D111,5KE7.5, 1N6267

How does it work

Lets protect ports of our ethernet cards, hubs and switches from induced high voltage, which appears in UTP/FTP/SFTP/SSTP cabled during thunderstorms. Usually there is no hing voltage between signal lines used pairs of Fast Ethernet UTP cable (contacts 1-2 and 3-6 of RJ45 jack). Also there should be no high voltage between rack ground and signal lines. Due to inverse polarity of suppressor D11 relatively to diods D1...D10 there is no circuit between signal lines through this scheme. When high voltage appears between any pair of signal lines or signal line and rack ground, the D11 suppressor opens. When extra charge is suppressed, D11 closes system returns to initial state.


  • "Grounds" of all protectors, connected to RJ45 ports of single computer or/and switch (in general, of all closely mounted hardware) must be interconnected. If you switch or hub has "Rack ground" contact, also connect it there.
  • Most computer cases (except notebooks) has a bolt where ground can be connected to. But! If the rack is not grounded properly, it is possible to find there up to half of input power voltage (about 110V/AC). In this case it is not allowed to connect rack and protection circuit. Such a nice voltage would not cause hard damage of swith, but shall prevent it from working properly. Possible solutions are:
    • ground your rack properly
    • connect "ground" of thunderstorm protector to central heating radiator or cold water pipe. Computer rack keep intact. Do not forget to measure AC and DC voltage between protector's "ground" and your pipes.
    • just live with worse protection level
  • Unused contacts of RJ45 ports (4-5 and 7-8) must be aired. This means that corresponding wires between RJ45 jack and protector must me also aired.
  • It is reccommended to ground unused wires of main part of cable (that lies outdoor/on the roof/etc). So, they should be connected to "grounds" of protectors and real ground (if any). But it is allowed to do so only if the other end of cable is also grounded.
  • Shield of the cable (if any) must be wired together with unused lines or linked to ground via 400V discharger (spark-gap).


  • Properly mounted circuit with alive elements almost doesn't affect resistance between signal wires. Even "long" links (those are non-standard, of above 150m length) work fine.
  • It is not necessary to change suppressor for varistor or use varistors as threshold element for connecting to ground. They have significant leakage current, at least for AC compenent. It is checked: works awfully, in hard conditions doesn't work at all.

1 GE (Gigabit ethernet)

In order to support 1 GE (GigabitEthernet) you must:
  • use 8 shoulders diod bridge instead of 4 (for all 8 conductors).
  • use pairs of sequentially linked 1N4148 instead of single 1N6267 or 1N4007.
  • since 1 GE ports are more sensitive to surges, it is mandatory to link cable shield and thunderstorm protection scheme GND pin to ground via 2 different 400V dischargers.
  • all hardware cases must be linked ground.
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