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.