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Phone line protection devices

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Full Member level 2
Jun 26, 2001
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Does anybody have some infos about this devices, I mean low cost types metal-oxid varistors or other types of fuses; with manufacturers and distribuitors for Europe?

I'm interested too. I lost 4 modems due to lightening through phone line!

On 2002-04-11 03:43, arebee wrote:
I'm interested too. I lost 4 modems due to lightening through phone line!

There exist gas discharge devices in Farnell
catalog. You can also make a special PCB
setup, where you make an almost-connection
between the incoming tracks:

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track-1 track-2



The best way to protect the phone line or ethernet is using of TVS and Sidactors.
The MOV are primary protection devices ~220V AC.
Try Future Electronics - they have offices in europe.

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For lightning the best way to protect is with two gas arrestors across the lines down to earth
------ line 1
I gas arrestor
------ earth
I gas arrestor
------ line 2
The earth must be connected to mains earth or preferably to a earth spike in the ground. Gas arrestors are only guaranteed for 10 strikes and then have to be replaced. A direct lightning strike can still destroy the equipment if it is large enough.

Hi xirix,

Part 1:
This is really a difficult task. I once had my modem destroyed by a lightening stroke and, while repairing it I noticed the manufacturer uses a 60V switching transistor in series with the line. The differential mode protecting varistor was a 275V. So, the transistor would always break first, and thus "protect the varistor"! And the manufacturer was 3Com/USRobotics, what to tell about the others...

Part 2:
Since the thelephone line is DC powered at 48V to 60V and the ring voltage superimposed is 70 to 90Vrms (128Vpeak) the differential protection should be dimmensioned no lower than 200V.

Part 3:
So for the differential protection you could use a 200V (S10K130) Varistor. But if the switching transistor on your board is a low voltage type, you have to replace it for a high voltage type (Bipolar or FET, NPN/PNP N or P channel depends on your modem card) or the like or there will be no effective protection.

Part 4:
Then there is the common mode protection.
During lightning, high voltages are induced between both line wires and ground. So the discharge path is from the telephone line through the electronics to ground - normally the power mains neutral wire. To protect against common mode, you have to connect the protection from each of the line wires to ground. I would suggest also two 200V Varistor type (S10K130), but away from the computer. Best at the point where the telephone line enters your house.

Part 5:
Varistors are fast and cheap, but they can't stand much energy, so you can also employ a gas arrester between each line wires and ground. I would suggest one Three Electrodes 2026-23 Balanced Trigard from Joslyn -
Also far away from computer!

Part 6:
The connection to ground should have a low impedance, so I recommend using a braid, not a wire for that connection.

Have a nice surge protection...



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