Telephone line surge protection

First version written by Tomi Engdahl at 1996, revised at June 1998

Why surge protection is needed ?

Telephone lines are long copper wires from central office to your apartment. Usually those wires are between few hundred meters to few kilometers. Those wires are connected in the central office to the telephone center/exchange. Normally there is only the 48V DC voltage from central office battery in the line, and when phone ring there is 70-130V AC voltage in the line. That's what in the line in normal operation.

But the situation in the thunderstorm is different. When the lightning hits the ground with it's millions of volts and thousands of ampreres strike every wire in the ground or hanging in the air will notice it. Near lightning strike gan generate a surge pulse of thousands of volts to telephone lines. If this can go to the electronics inside the telephone or modem, the electronics will destroy at the same moment.

What telecom operator has done to it ?

The surge can destroy electronics in both subscriber's and central office end, so you must think that telecom operator must have done something to this problem. Yes they have.

First they have installed surge suppression devices to their central office to make sure that small surges dos not damage it. They have usually also installed some sort of protection to subscriber's side of the line to the place where telephone cable enters the house or to the the telephone pole just next to the house. That type of protection devices are very common on rurals installations, where there are very long lines and they are usually hanging in the air from telecom poles so they are much more prone to lightning strike surges.

In bigger cities lightning protectors are not always used because different buildings are usually electrically so tightly connected to each other (though electric feed, plumbing etc.) and cables are in the ground so there is not possible to generate so big surge voltages because of potential differences between different points (central office and subscriber).

What types of voltage surges there are ?

There are two types of surges: differential and common mode. Differential surge is a voltage surge which is generated between the telephone wires. That is usually not great, because telephone wires are twisted pairs and is easy to stop by palacing suitable protection component between telphone line wires.

Other type of surge is common mode surge, in which the potential of the telephone wires raises thousands of volts from your ground potential. Usually the telephone wires are at potential quite near to the ground of your building. But if the lightning strike hits the central office (or some else place in telecom cabling near you), the potential of both of the telephone line wires can raise quite high (even thousands of volts). The common mode surges are usually bigger and more problematic than differential surges.

How telephone can handle surges ?

Telephones are devices sitting in the end of the telecommunication line electrically isolated from rest of the building. They are not very prone to common mode surges, unless they are special phones which are connected also to mains (for example those which have answering machine in them).

The thing which breaks ordinary telephone is differential surge, because it is the surge which can get to the electronics of the telephone. Modern telephones usually damage easier, because electronics will break down more easily than the transformers, carbon microphones and mechanical rotary dials used in older telephones. In cheap modern telephones there is usally not much done to protect them from surges, because good protection could easily cost more than the telephone itself. It is economically wise. If cheap telephone breaks down people go to shop and buy new one. If telephones are damaged often, then it migh be a good idea to install a good surge suppressor.

How about modems ?

Modems are more prone to lighting surges than telephones. There are two reasons for that: modems have more dedicate microelectronics in them than simple telephones and modems are connected to computers, which are connected to mains electricity. For those reasons modem electronics are prone to be damaged by both differential and common mode surges.

Differential surge easily can go through the modem line transfomer in the same way the normal modem signals go. To protect modem electronics, a good practice is to add protection to both sides of the line transformer: gas arrestor or VDR in the primary side of the transformer to take most of the surge and zener diodes to the secondary to take what goes through the primary protection and transformer. Quite typical circuit for modem line input protection is to put smal resistors (10 ohms or less) in resies with both lines coming to the modem. Then after those resistors there is the overvoltage protection device (gas arrestor and/or VDR, usually clamping at 130V). The resistors which are put in series with the telephone line work as sort of fuses for large surges and they are nonflammable type for fire safety.

The harder is the protection against common mode surges. The line transformer in the modem will keep the common mode voltage out of modem electronics, but it has limited isolation strength (few kilovolts). The telecommunication regulations demand that modem line transformer must withstand 1-2 kV of common mode voltage. This is enough for normal overvoltages, but large surges can easily have higher voltages. Overvoltage suppressors can't be generally used between phone line and modem case unless the modem is guaranteed to be properly grounded, because of user safety. So that's why they re not used in normal computer modems.

Why external surge suppressors are sometimes needed ?

The protection provided by telecom operator and the devices connected to telephone line are not always enough. The protection usually is designed so that it minimizes the costs. It is not worth to protect cheap telephones with expensive protectors, because strong lightning surge is quite rare and telephones are inexpensive to replace if such thing happens. Usually surge protection can be pretty sloppy is the electronics that are being protected are designed with surges in mind.

The situation migh be different, when there is something more expensive than ordinary telephone connected to line. For example expensive computer systems are usually worth to protect, because the damage caused by the lightning strike can cause very expensive damage. For example in PC case, lightning strike can not only destroy the modem (which is not usually very expensive), but also something else inside of the PC. That can become very expensive if valuable information is lost and the PC is very important at your business.

So sometimes extra protection is needed. The people which operate large computer systems know that sooner or later lightning strike will hit and break something. If there is enough protection, the damage is avoided or at least minimized.

Operation of the surge suppressors

To be able to get rid of surges the surge suppressors have to protect the equipments against two kinds of surge voltages:

To be able to do this the surge suppressors have to have good ground connection. If the circuit does not have a good ground connnection is can only protect against differential surges. The protector must also act quicly to be effective. A lightning surge has a typical rise time of 10 microseconds and a duration of about 1 millisecond.

When surge protectors are installed to a telephone/data communications room they are typically connected a common grounding bar which is connected to a good ground through a heavy grounding wire. The quality of the grounding (resistance and inductance) effect very much on how well the surge protector will protect against common mode surges. Even the best surge protector can't work well unless it is properly grounded. So it is recommended that you install the surge protector near the main grounding bar.

Some telephone line surge protectors sold in USA use the mains connector ground for grounding (those surge protectors which have mains surge and telehone line surge protectors in some case). The ground in the mains connector is not very good grounding point and using it as ground for surge protector can induce par of the surge also to the mains wiring.

The basic surge protectors use metal oxide varistor (MOV) to do the protection. This circuit below used one MOV between line wires for protection against differential surges and two MOVs from line wires to ground connection to protect against common mode surges.

      ----------o---------o--------- to equipment
                |         |
                |        MOV
TELCO           |         |
               MOV        o--- TO GROUND
LINE            |         |
                |        MOV
                |         |
      ----------o---------o-------- to equipment
The MOV between the line wire must be selected to have such voltage that it does not start to conduct at the normal telephone line voltages but stops the harmful higher voltages. Typica telephone line DC voltage is 48V and ring voltage is typically 90 vrms or 130V peak but can be upt to 130V RMS. Some modems use 130V RMS rated VDR which starts to conduct at about about 190V peak. Bell system Technical Reference #61100 mentions that a worst case telephone line voltage can be 105 VDC + 130 VAC= 289 V peak. If the MOV voltage is set too low the circuit will not pass "on-hook" requirments because it leaks too much current (for example FCC part 68 requirments demand 10 Mohm DC impedance in "on-hook" state).

The MOVs form line to ground connection to line wires have typically somewhat higher voltage rating to make sure that they do not start to conduct at normal ground potential differences seen in the situation where the surge protector is used. For example one AT&T unit has 130 V RMS MOV's in SERIES to ground, so they won't clip until about 380V peak and one PATTON says it clips at 310V in 500 Ns.

Because MOVs have limited surge handling capacity some surge protectors use resistors between the line and the surge protector to somewhat limit the surge current and dissipate the surge energy. Because the surges can have high energy the resistors must be able to handle high power surges safely. By having enough powerful non-flammable resistors (0.5-2W) usually is a safe choice. The fuses in the circuit are ment to cut the connection if something hazardous happens. If big surge happens it will quite propably burn the fuse. The fuse itself does not usually help much in fighting against surge but it will make sure that if for some reason the ground potential is no longer ground, no damaging currents will flow to telephone line (for example mains current accidentally entering to thin telephone cable wire can burn it and start fire). So the fuses melting the phone wire in catastrophic failure.

TELCO  >----o/\o------/\/\/\----------o------------> OUT TO EQUIPMENT
LINE                                 MOV
                                      |---MOV--> TO GROUND
            FUSE      20 OHMS 2W      |
The MOV arrangemen in this circuit is somewhat special. In this arrangement the rurge voltage in which the MOVs start to conduct ia approximately double the voltage ratings of the MOVs used in this circuit. In any surge case the surge energy will flow through MOVs (so the energy is divided between them). This arrangement is shown here because it is used also on some commercial surge protectors. The MOV arrangement in this circuii equivalent to the first circuit if the first MOVs have half the voltage rating and twice the capacitance of the second MOVs.

Always just VDRs no not provide enough protoection. DVRs are quite fast, but have limited surge handling capacity. If nore capacity is needed it is quite common to use higher power but slower gas tube arrestors in front of the VDR surge protection circuit. This arrangement (when derigned correctly) gives high surge handling capacity and quitre fast operation. If even faster opration is needed then is possible to add some fast special semiconductor surge protection devices after MOVs (zener diodes, avalanche diodes, surgectors, TISPs etc.).

Non-commercial designs

The following non-commercial surge suppressor circuits are collected from various sources (BBSs, FTP-sites etc). I am only including those schematics here to make a good collection of surge suppression circuits. I have not tried those circuits myself, so I can't say if they are effective or not.

Circuit one

This circuit is designed by Reijo Salminen, who posted it to MITS BBS at spring 1991. The circuit is designed for protecting modems and telephones connected to telephone line. The circuit is designed for telephone lines used in rural areas. The protector is connected between modem and incoming telephone line. The ground connector is connected to main electricity ground of the building through good grounding wire.

 Line                 A    I                     V       ---> to modem
                           B   B

A = 230V surge gas tube protector
B = 600V surge gas tube protector
RRRR = 10 ohm 1 W resistor
V = varistor 250V

The resistors in the circuit limit the surge energy passing through the circuit and they work as fuses in case of large surge. For safety, those resistors should be non flammable type.

Over voltage protectors B are rated so that in case of electricity grounding fault where ground pin becomes accidentally hot (220V AC), those surge protectors does not pass current to telephone line. If the surge protectors pass current to telephone line when electricity ground potential is raised against telephone line, there is apparent danger of fire.

In Finland modems are tested at 2000V voltage pulse and in USA with 1000V pulse, so the protection should be enough in both cases. It shold be mentioned that the circuit is not 100% proof, so the best protection is disconnect modem from telephone line connector when thunderstrom is coming.

Circuit two

This circuit was designed by Tim Jackson at 1990. It was presented in his article "TELEPHONE LINE SURGE ARRESTOR" found in telecommunication archives.

The trick is to install the unit in the line between the telephone jack and your modem (ie: not too far from the modem, like in another room) and connect the earth lead from the circuit to the earth pin on the SAME PLUG that feeds your PC.

                     10 ohm 5 Watt
Phone Line A     |                              |
                <=>300V Gas Arrester        ____|_____      To Modem
                 |                          |   A    |
                 |                          |TISP2290|
        |--------+--------------------------|C       |
      Earth      |                          |   B    |
                 |                          ----+-----
                <=>300V Gas Arrester            |
                 |                              |
                 |                              |           To Modem
Phone Line B     |                              |
                     10 ohm 5 Watt
Circuit drawn as best as possible with ASCII by Pat Verner.

The phone line has a gas arrestor from each leg to earth. In other words, two gas arrestors. One from A to earth and one from B to earth.

The line then has a resistor in series with each leg (A and B) before being connected to the TISP2290 (the Texas Instruments chip mentioned earlier). This chip has three pins. The outer two (A and B in the diagram) are connected to the resistors while the centre one (C) is connected to earth. The metal tag of this component is internally connected to the earth pin (C), just for the record. The modem is fed from the outer two pins of the TISP2290.

The bulk of the energy involved in a surge is dissipated by trusted (and slow as treacle) gas arrestors. The TISP2290 absorbs the high speed spike that the gas arrestors miss and is itself protected by the two resistors which provide a little current limiting. The modem, being fed from the same point as the TISP2290 is protected by the whole circuit.

For those who have to know, the TISP2290 works in a manner similar to a zener array connected between the A and B wire and earth so as to limit the voltage between any of three points to about 200 Volts. As you know this is not entirely effective and so if the voltage rises to 290 Volts (hence TISP2*290*) then this crafty critter cuts in triacs to crowbar the offending points to earth until the surge has passed.

Commercial designs

The following circuit are the circuit used in two commercial units. The circuit diagram is drawn by looking inside those commercial units and drawing the circuit diagram. Those circuit diagrams are scanned from my memo I made when I examined those surge suppressors. Those circuit diagrams should be readable at most parts and the text should clear out the detais not easily readable from picture.

Furse ESP-TN

This circuit is a circuit diagram of commercial surge protector sold under name Furse ESP-TN for protecting normal telephone lines. The circuit is designed for protecting normal telephone lines and is packaged to metal box where there are connectors for telephone line and thick ground wire (the connector in the bottom of the circuit). The components were installed to circuit board where there is lots of copper thickened by lots of tin.

Furse ESP-TN circuit diagram

The circuit uses on 260 V doube gas arrestor as first protection. After that there is 2.2 ohm series resistors followed by surgesuppression network built from 180V TISP and three DVRs. After that there are two more serial resistors and network built from three 180V zener diodes. Wuite complicated circuit and no wonder why it costs quite much (about 100 US dollars).

Telematic Lighting Arrestor SAPN (Telematic Surge Barrier)

This is another telephone line surge arrestor sold under name Telmatic SAPN Line Barrier by Black Box. The protector has following specs:

This circuit is built using small circuit board fitted inside metal enclosure. The unit has 200V voltage rating, it has BAPT approval for connecting to public telephone network and it is NEMP tested. This protector model is also sold for a little less than 100 US dollars by Farnell Electronic Components.

Telematic SAPN circuit diagram

The circuit is quite straighforward circuit made of gas arrestors, resistors and zener diodes. Firs protection for large surges are one gas arrestor connected between line leads and two other gas arrestors connected between line leads and ground connection. All gas arrestors are type JES 0394 2029-23-BY. After that there is 4.7 ohm resistors followed by surge suppression network built from zener diodes (type BWX50-180) which handle the surge which has passed those gas arrestors. There was a place and markings in the circuit board for capacitors C1 and C2, but those were not installed in the circuit.

Tomi Engdahl <>