ADSL overvoltages and protection

Every summer we get thunderstorms, with the accompanying lightning. Last summer one ADSL modem and computer Ethernet interface I was using was destroyed by lighting. So I decided that an article on ADSL overvoltage protection would be a good idea. It took some time to get this article ready, but here it is.

ADSL overvoltage protection has many things that are same as in normal phone line protection (they both use the same telephone wires), but there are some differences that cause that not all telephone line protection devices work well with ADSL signals. This article tries do describe those differences and give you some general information on surge protection in general.

Lightning can cause damage in the telephone-line-interface circuitry even if it does not hit the wired directly. It is possible to protect the telephone-line-interface form indirect lightning hits with right overvoltage protection measures. A proper protection is a combination of right grounding practices on the whole house and suitable protection devices connected to telephone line. ELEKTRONISTEN LAITTEIDEN YLIJÄNNITESUOJAUS is a good article written in Finnish on protecting electronics devices against lightning. Here is a picture from that document that shows a proper house grounding:


You don’t install surge protectors to defend against a direct lightning strike — there is no real defense against a direct lightning strike – you can make it less likely with a well-engineered lightning rod protection system. In a direct hit situation, lightning hitting a surged protected circuit can easily arc through any surge protector you can get.

You can protect against the effects of indirect hits (like energy induced to telephone and power lines). You install surge protectors to defend against currents induced by nearby lightning in your wiring. This is protection against damage lightning can cause without actually striking your wiring, or your building. If lightning strikes a mile away from you, and hits the ground, or a tree: this lightning can still induce currents in unshielded underground and overhead power and data cables.

Lightning strikes article tells that lightning can induce surges on the ac-power line as well. If there is no surge protection, the induced currents may destroy sensitive electronics such as communications equipment computer power supplies. Typical protection inside a somewhat protected home could include surge protectors for sensitive/important electronics, and even a UPS for a couple of items like computers. The surge protectors protect against the most cases. And in case they fail, there is a second option is to have a recovery plan that covers this situation (usually involves insurance and backup of your data). Whatever you do consider expenses, risks, benefits.

Ask Slashdot: Best Option For Heavy-Duty, Full-Home Surge Protection? article discusses on full-home surge protectors that install next to the fuse box. It’s not a trivial task. There isn’t a simple “plug ‘n play” solution. There is no commercially available surge protector apparatus able to be fitted to a home electrical system and other utility lines entering a building with a price that is remotely affordable to the average homeowner is capable of providing remotely robust protection against a direct strike. But there are are systems that can handle higher surges than consumer grade plug-in surge protectors. If you are in high lightning risk area, consider the option of using whole house protection and burying the electrical lines from property line to the house. But be warned that the the cost of a sophisticated whole home UPS/surge protectors can easily more than the equipment it protects! Expect to spend some time and money to do it right or risk not only a false sense of security but the chance of making things worse. In high risk environment also install some lightning protection air terminals on the roof of your house, and run some down conductors to ground rods.

And remember that whole house surge protection does not replace local surge protection. It stops most of the spike but not all of it. You still have to have surge protection strips locally for sensitive equipment. Effective lightning protection is layered. The socket surge protectors are actually meant to be used in combination with the other layers, not standalone. A close enough lightning strike will induce strong currents in the wiring between the fuse box and your appliances. The surge protectors are designed to protect against the resulting voltage and not much more, and obviously a central surge protector can not protect your appliances if it’s not between the surge and the appliance. Stronger surges from lightning strikes into the power lines outside your house on the other hand will not be stopped by the small surge protectors alone. You need both.

Right grounding is essential. Without it over-voltage protection devices can not work properly. My picture describing the grounding of a typical house in Finland (where grounding is done well):


Back to the main topic which was ADSL telephone line protection.

COMMON MISTAKES IN LIGHTNING PROTECTION OF PHONE LINE INTERFACE CIRCUITS By Joe Randol article give some good information high lightning effect phone line equipment:

Lightning almost never strikes phone lines directly. When it does, there is usually extensive damage including melted copper and scorched circuit boards. When a phone line gets a direct strike, there is pretty much no protection scheme that will protect the wiring and the equipment.

The vast majority of lightning surges on phone lines are induced on the phone line by lightning striking something nearby, such as a tree, a building, or the ground itself. The huge currents associated with the nearby strike generate intense electromagnetic fields that couple into the tip/ring leads by electromagnetic induction. Another mechanism by which lightning appears on phone line interfaces is called “ground potential rise.” Either way, a large common mode surge is induced on the phone line.

Each end of the phone line is typically equipped with a “primary protector” at the point where the phone line enters the building. The job of the primary protector is to divert most of the lightning energy on the phone line to earth ground. Primary protectors have some form of crowbar overvoltage protection devices connected tip-to-ground and ring-to-ground. The devices used in primary protectors can be gapped carbon blocks, gas tubes, or SCR-type silicon devices.
If only one side of the primary protector triggers in response to a common mode surge, the surge will be converted from common mode to differential. This is why most telecom standards specify immunity to both common mode and differential surges.

By industry convention, the maximum “let-through” of a carbon block primary protector is roughly 1000 volts differential and 1500 volts common mode. Most industry standards for lightning immunity of phone line TE assume that a primary protector is installed on the phone line. Thus, most industry standards focus on the “let-through” energy that gets past the presumed primary protector and can damage the TE.

Lightning protection installed behind the primary protection is often referred to as “secondary protection” and there can also be tertiary protection. Secondary protection is typically included inside the CPE equipment such as a router near the point where the phone line enters the equipment. Sometimes an external primary protector is installed near the equipment. Tertiary Protection is that protection that may be in the router circuitry but not on the inside tip and ring portion of that circuitry.

I have written a document Telephone line surge protection on telephone line surge protection. It shows circuit diagrams of several telephone line surge protection devices. Many of them rely on several different surge protection components like this Furse ESP-TN that uses a combination of gas arrestors, VDRs and TISPs:

It is a quite typical construction. This kind of protector works well for normal telephone line use (up to 4 kHz audio), but is not suitable for ADSL. When designing the protection circuits you need to take into account the required bandwidth of the circuits. The necessary bandwidth for ADSL is 1.1MHz, ADSL2+ is 16MHz and VDSL2 is 30MHz. Employing the incorrect protection may greatly reduce the available bandwidth.

Usually the biggest problem on the normal telephone line surge protectors is their high capacitance that attenuates high frequencies. Typically the VDR component in the surge protector is the one that has highest capacitance and it causes the biggest problems. I have measured capacitances on several VDRs and found out that they are typically few hundred picofarads but can be over one nanofarads. This high capacitance will practically kill most of the ADSL signal entering the surge protector. The 1 nF capacitance has impedance of 160 ohms at 1 MHz! Have this on your ADSL line and you will see lots of signal attenuation and signal reflection due impedance mismatching (both not good for ADSL performance). Gas arrestors and zener diodes have typically very much lower capacitances (picofarads to tens of picofarads typically) and are not normally big problems.

So I would think the design similar to one used in Telematic Lighting Arrestor SAPN (Telematic Surge Barrier) (uses just gas arrestors, resistors and zener diodes) could work better for ADSL signals because all components have pretty low capacitance:

The capacitance is the main thing that makes the surge protector suitable or unsuitable for ADSL use (there can be other things than can disturb the signal also but usually on surge protectors the capacitance is the main issue). So if you want to evaluate the suitability of your existing telephone surge protector for ADSL protection and you have a multimeter that can measure capacitance (or other instrument than can do that) then here is one test you can try: Keep the surge protector not connected to anything and measure the capacitance between the line wires. If you get capacitance that is hundreds of picofarads or more, you know that the protector is unsuitable for ADSL. If the capacitance is somewhere in few tens of picofarads it is quite probable that it will quite probable pass ADSL signal without too much problem, so it could be worth to try it in real application. For real life test on how much the protector affect the signal, you can use the signal strength information available on the management console of many ADSL modems (do test with and without protector, and if results are close to each other then all should be good).

Here are some articles pointers on ADSL/VDSL protection design:

Primary Protection: Design and Vendors article gives a description of primary protection for DSL ports and the components used in the design. The article includes a list of current vendors and examples of the available Primary Protection devices sold by those vendors.

GDT Protection Solutions for ADSL and ADSL2+ Solution application note gives a good overview how to protect ADSL lines and what components to use for it.

Littelfuse Reference Designs offers specific examples of how SIDACtor® devices can be used to ensure long-term operability of protected equipment and uninterrupted service during transient electrical activity.

SY-S-ADSL Surge protector device for signal product info page has technical data and circuit diagram of some protectors.

Attain optimal VDSL circuit protection, signal integrity, and bandwidth article tells that designers are finding that it is more challenging to implement effective circuit protection for VDSL drivers in telecommunication applications while still maintaining signal integrity and bandwidth. VDSL protection is much more difficult to achieve than Asymmetric Digital Subscriber Line (ADSL) protection due to several factors arising from the higher bandwidth requirements, such as lower loss, tightly coupled transformers with lower leakage inductance, and faster driver ICs that may use less robust semiconductor technologies. Protection devices in wide-bandwidth VDSL designs require low capacitance, typically less than 20 pF.

LAN/WAN Ethernet Overcurrent And Overvoltage Protection article is also worth to read because there are also cases where you need to protect the Ethernet side of your communications equipment (not just the ADSL/VDSL side). Overvoltage protection network technology Ethernet, Twisted Pair, Cat 5 page give some details of some commercial Ethernet protectors.

Innovative IC can’t trap lightning but can let you know it’s coming is an interesting product announcement on an IC that can let you know when lightning is approaching and how far away it is. Maybe this can be helpful in protecting your system.


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  2. Tomi Engdahl says:

    TVS arrays provide protection for Ethernet, USB and Smartphones–USB-and-Smartphones?cid=EDNToday

    Multiple individual and families of transient voltage suppressor (TVS) arrays from ProTek Devices are now available with configurations designed to protect Ethernet, USB, Smartphone, and other portable applications.

    The GBLCSC08CLC is a replacement for multilayered varistors (MLV) an ultra low capacitance – 0.4pF (typical – junction capacitance) TVS array, available only in a bidirectional configuration and rated for 125 watts peak pulse power (8/20 μs surge waveform).

    At higher operating frequencies or faster edge rates, the TVS arrays resolve typical insertion loss and signal integrity challenges.

  3. Tomi Engdahl says:

    Surge protection—Stop fried electronics

    Surges and spikes on data lines can fry your communications boards and garble data. This article describes the operation, installation and selection of what is probably the most common method of data line protection. Surge suppressors divert excess energy away from the port being protected into a ground connection. The operation of these devices relies on a high quality ground connection in order to safely shunt away unwanted energy.

    Surge Suppressor Operation

    Shunting harmful currents to ground before they reach the data port is the job of components such as Transient Voltage Suppressors (TVS, often referred to by the trade name Tranzorbs), Metal Oxide Varistors (MOV) or gas discharge tubes. These devices all work by turning on at a set voltage. Once the clamp voltage has been exceeded, the devices provide a low impedance connection between terminals. These shunting devices are most often installed from each data line to the local earth ground, and should be selected to begin conducting current at a voltage as close as possible above the system’s normal communications level.

    While transients may not always conform to industry specifications, both the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC) have developed transient models for use in evaluating electrical and electronic equipment for immunity to surges. These models can offer some insight into the types of energy that must be controlled to prevent system damage.

    Surge suppressors must be installed as close to the port to be protected as possible, and must have an extremely low impedance connection to the local earth ground of the unit being protected.

    the system designer should consider the clamping voltage of the unit as well as its physical attributes, such as connector type and method of making the grounding connection.

    Single Stage Devices

    The most common device uses a single TVS or MOV for each protected line. This type of unit is usually small and inexpensive. If a proper ground connection is made, they should offer protection against most transients.

    Three Stage Devices

    More advanced units use three components on each protected line to handle much larger surge currents and to provide internal self protection, reducing the risk of undetected failures. The first stage is a gas discharge tube; this stage can shunt very large currents, but is slow reacting and requires a relatively large voltage before conduction begins. The second stage is series impedance; this stage limits the current flowing into the final stage of the circuit. Finally, a TVS device clamps at a voltage acceptable for the data port and maintains the clamp until the gas discharge tube begins conduction.

  4. Tomi Engdahl says:

    A bolt from the blue

    The phrase “A bolt from the blue” is sometimes used to refer to a sudden, brilliant idea. Just such a thing happened when ams (formerly know as Austria Microsystems; URL: came up with the AS3935 sensor IC. It was created so that people could be protected from litteral bolts from the blue: lightning strikes.

    A programmable lightning sensor, the AS3935 detects when potentially hazardous lightning activity is approaching and in the vicinity. It detects cloud-to-cloud and cloud-to-ground flashes, so users or automated machinery can evaluate the risk presented by approaching storms and take appropriate action.

    thunderstorms can travel at ground speeds up to 113 km/hr
    The AS3935 can give you better warning; it can detect lightning activity up to 40 km away

  5. Tomi Engdahl says:

    What Happens When You Get Struck By Lightning

    While there is still debate within the scientific community as to the exact nature of the process, most believe that cloud-to-ground lightning originates when conditions within a thunder head strip electrons from rising water vapor to create an electrical field. Free electrons gather at the bottom of the cloud while positive ions move to the top.

    Humans are good conductors.

    Lightning strikes do, however, differ from the industrial shocks you’d receive from high-voltage equipment. First, the level of voltage is greater in lightning—most industrial shocks generate 20-60kV, but a bolt of lightning generates 300kV. Second, the duration of a lightning strike is much shorter. Man-made shocks last about a half second (500 miliseconds), on average, until the worker is either blown clear or the circuit breaker trips. A lightning strike courses through you in in just 3 milliseconds.

    When a bolt of lightning does strike a human frame, very bad things happen.

    Burns aren’t the only way lightning will hurt you. A lightning strike can act as a massive fibrillator, upsetting the heart’s electrical rhythm and causing cardiac arrest. That’s in addition to bursting blood vessels and damaging the cardiac muscles.

  6. James says:

    Would it also be possible to build an ‘active’ surge protection circuit for adsl lines? i’ve read that there are thyristor based systems: but they don’t give any details there

    • tomi says:

      Check out this device which uses active crowbar circuitry:


      Transients are initially clipped by zener action
      until the voltage rises to the breakover level,
      which causes the device to crowbar. The high
      crowbar holding current prevents d.c. latchup as
      the transient subsides.

      I am not sure if this specific device is suitable for ADSL, but the typical 40 pF Off-state capacitance would indicate it could be a good candidate.
      There are similar devices with different specifications from other manufacturers.

  7. Tomi Engdahl says:

    Designing truly portable lightning detectors

    Lightning detectors can play a crucial role in protecting people as well as equipment in locations prone to violent lightning storms

    On a golf course, baseball field, or many other outdoor venues, the extra time that a lightning detector can provide could mean the difference between life and death.

    At best, our senses can detect thunder and lightning up to about 6 miles (10 km) away, but that distance can be reduced by terrain and/or noise sources. While the speed of isolated storms is typically about 12 miles (20 km) per hour, some storms move much faster. In extreme circumstances, a supercell storm may move about 40 to 50 miles (65 to 80 km) per hour.

    In fact, lightning is a complex combination of several different events — breakdown, return strike, in-cloud activity, and subsequent strike which give it a unique electrical signature.

  8. gymnastikmatte says:

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  9. How to protect RS-232 serial connections « Tomi Engdahl’s ePanorama blog says:

    [...] RS-232 port originate from a variety of sources. These sources include inter-system ground noise, telephone line surges, and wiring faults. The Protection of RS-232 Serial Connections document explains in detail the [...]

  10. brazilian body wave says:

    Good article , thanks and we want more

  11. Tomi Engdahl says:

    6 common mistakes to avoid in telecom technology grounding

    To minimize costly downtime, keep clear of these common errors that can undermine investment in surge-protection technology.

    Grounding mistake 1: Not understanding impedance guidelines
    Grounding mistake 2: Wrapping ground wire
    Grounding mistake 3: Size of wire
    Grounding mistake 4: Wire connections
    Grounding mistake 5: Technology selection
    Grounding mistake 6: Lack of single grounding point

  12. Tomi Engdahl says:

    Use two-stage protection in GbE applications

    Adequate circuit protection is crucial to maintaining and increasing the lifespan of electronics, and especially for high-speed gigabit Ethernet (GbE)-based applications that in many deployments face additional challenges of higher temperatures, continuous operation and compact design. It often is assumed that a low voltage transient voltage suppression (TVS) diode solution will provide sufficient protection; however, even the most complex and expensive diodes have limited effectiveness as a single-stage solution. Regardless of their voltage rating, diodes can expose electronics to excessive voltage levels at the time of the surge.

    A cost-effective solution is now available that complements a TVS diode with new transient current suppressor (TCS) technology. TCS devices placed in series with GbE electronics will limit the current before a dangerous level is reached. Once the voltage protection device activates, surge voltage is diverted to ground to ensure that the voltage and current remain within safe levels and the electronics are protected.

  13. BCAA Matrix Side Effects says:

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  14. Lightning protection « Tomi Engdahl’s ePanorama blog says:

    [...] You can protect against the effects of indirect hits (like energy induced to telephone and power lines). If there is no surge protection, the induced currents may destroy sensitive electronics such as communications equipment computer power supplies. You install surge protectors to defend against currents induced by nearby lightning in your wiring. This is protection against damage lightning can cause without actually striking your wiring, or your building. If lightning strikes a mile away from you, and hits the ground, or a tree: this lightning can still induce currents in unshielded underground and overhead power and data cables. [...]

  15. Valutazione oro Napoli says:

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  16. Tomi Engdahl says:

    APC ProtectNet 2-linjan puhelimen/faxin/modeemin ylijännitesuoja (rj11/rj45 tyyppi)

  17. Tomi Engdahl says:

    Telephone line surge protection

    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.

    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.

    Read more:

  18. Tomi Engdahl says:

    GDTs on GR-1089 PSTN

    TE Circuit Protection has designed a Gas Discharge Tubes (GDT) that has a DC sparkover voltage 420 V. This device has capacitance at 1 MHz of =10000 MΩ. It is commonly used in GR-1089 : Public Switched Telephone Network Equipment.

  19. Tomi Engdahl says:

    xDSL lightning defeat device

    Make a device that automatically properly disconnects my line from the DSL modem when a thunderstorm approaches.

    Living in the countryside with a 4+ km DSL line thunderstorms kill about 1 modem, 1 firewall and some of my network every year. From the damage I have seen the spikes are at least 1kV.

    I am going to ask you guys to help me out here because most of the hardware and the software i can cover myself, but I am struggling with finding the right kind of (cheap) relay that preferably would handle 5-10kV and still be friendly to the DSL signals.

    The basic workings would be using a radio receiver and a microcontroller to detect near by ligthningstrikes, then disconnect the line from the DSL modem for a preset amount of time and possibly connect it to a sparkgap/exchangable varistor or maybe shorting it to ground even (input welcome).

    First I did the CPU/Motordriverboard and a small board for motor resistors and the home microswitch.

  20. Stephen says:

    Dueltek Distribution Presented: Electronic products are brightened to harm due to surges and also spikes on your energy distribution. This six method surge & overload safety mount is built to guard your products from this kind of damage.


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