Modem lightning protection

Having been born and raised in a large city, I would not even consider living in a city again. I like the farm life and that's where I live now. But the rural life has it's problems, beyond getting snowed in during the winter. One of the main problems is the lack of high speed internet. I have only one option, and that is dialup internet via my phone line. And yea, I mean a real phone wire, because that is the only reliable phone service in case of an emergency. My prepaid cellphone works fine in town, but out here on the farm, I'm lucky to be able to send and receive a text message, but it's far too unreliable to make a call. So, I just keep the old wired phone line, which comes with basic dialup internet, which is slow, but works most of the time.

However, every year I lose at least one modem from lightning. Usually during the summer months. Well, I just lost another one on Sunday. The storm was coming in, but still at a distance. I was getting ready to shut off the computer and unplug the phone line from the external modem, when some distant lightning flashed, and killed my modem. This has been going on for years. Every year I buy two three or more modems to replace those fried by lightning. I've even had modems fried from justr being plugged into the phone line, and not turned on. So, I do my best to unplug the phone line whenever I turn the computer off.

To complicate matters, I am a weather spotter, and need to view radar maps when bad weather is in the area, and that means being online before a storm approaches, and risking my modem as the storm gets close.

Is there anything that can be used to protect a modem that REALLY WORKS? Years ago, I tried surge protectors and found them useless. I have not wasted my money on them in years. I know they are nothing more than a MOV, and the modems have those built in anyhow.

Is there anything that REALLY WORKS?

Dont waste your time suggesting I get satellite internet. I'm retired, and can not afford the $100 a month they want for their packaged deal, which includes tv. And I have no interest in the tv crap even if it was affordable.

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Good for you. I can't stand the city either. Tried it once more for 6 months and that was over 30 years ago. 'nuff for me.

As long as there is enough firewood, beer and food I'd be just fine.

If the cell deal is reasonably priced you could get a cell modem stick for you computer and somehow lash that to a good directional antenna. Because it seems you need only a few more dB's to get you over the hump. In a true redneck sense that ain't no real rural place :-)

First things first. Since we don't know what fries things (differential? common mode?) you need to do a post mortem on as many fried modems as possible. Hoping you haven't chucked them. Figure out which parts in there died, can be more than one at a time. That makes it much easier to come up with ideas. Signal line protection ain't rocket science but in modems they will only spend limited amounts of money on that. So yeah, you can do better. Some hints:

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Personally I am not a fan of resettable fuses. I'd use the longest meltable fuses available and then keep a stash on hand for when the thunderstorm has passed.

Yeah, satellite is expensive unless you can share it with people around you. But chances are, your nextdoor neighbor is 30 miles over yonder.

Regards, Joerg
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3-terminal gas tube arresters are very good, this is what the phone co. uses to protect THEIR OWN equipment. I have a 10 Ohm metal film resistor (doubles as a fuse) in series with both wires of the phone line, and then connected to the gas tube. The center terminal is connected to a solid ground with heavy wire (like 12 Gauge). I have had the same modem (and PBX) connected for 20 years, and never had any damage. This is past where I used to have the DSL modem, and that did get blown out, but the PBX and regular modem have not been harmed. I have occasionally had to replace the 10 Ohm resistors when they blew. When the gas tube pops, it can be heard in the next room. I did have to replace the gas tube some years ago when it had absorbed too many discharges.


Reply to
Jon Elson

I prefer living in small cities close to larger cities. I live a little further out in the country (though in a development) than I care for, now, but perfectly positioned WRT a large city (~40mi from center city). Best of both worlds, though the Internet sucks (because I am in a low density area).

The fact that you lose so much equipment is an indication that your house grounding probably has an issue. Is the phone line entrance right next to the power entrance? Are they bonded together? Are you using water pipes for the ground and are they separated from the power entrance by some distance? These are common situations in older homes and will make your equipment a target for lighting. You can probably fix the problem, though it might not be easy.

The other suggestion is to use a whole house surge suppressor in addition to suppressors at the devices. Change them relatively often since they tend to get "eaten up" by each subsequent strike. They're cheap.

Make sure everything is grounded *properly* and that there is a spark gap suppressor on the phone entrance. Again, MOVs help on the phone and power inputs to your electronics.

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I agree with all of this. Driving a new copper ground rod near the phone entrance might help, but then it needs to be bonded to the phone, power, and any other existing grounds.

There should be a protector of some kind (probably MOV) installed in the "phone company only" half of the box where your phone lines come in to the house. It usually looks like a little grey block, with two stud terminals on it for the phone wires, and a central metal tab that connects to a ground wire or ground bar. If it has been there for a while, it's probably not doing much for you anymore. It's a toss-up whether it's simpler to ask the phone company to come fix it (it's their baby, but they won't want to roll a truck unless you can't make calls) or to buy a handful from somebody like Graybar and change it yourself on occasion.

Also, having an external surge protector on the phone line might mean that the MOV in the $5 protector pops, instead of the one in the $30 modem.

The phone company uses various Polyphaser products on the RF lines at their cell towers. The same company also makes protectors for regular phone lines, such as

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. Not cheap, but possibly cheaper than replacing multiple modems.

Random thought: if your phone wire comes in from a pole (not underground), and you can get to the poles near your house, inspect each one to see if its ground wire is still there. The pole factory put a fairly heavy bare wire along the entire length of the pole, and coiled it up on the end that goes in the ground. Everything on the pole that should be grounded (like the suspension strand for the phone cable) was connected to this wire at each pole when the wires/cables were put in. Sometimes the wire is protected by a U-shaped piece of wood, metal, or plastic for the first few feet above the ground.

In these latter days, often times the bottom part of the ground wire is missing. It will have been cut as high as a person can reach with a pair of cutters, and again at the ground, and then removed from the pole and sold for scrap. Whoever owns the pole might fix it if you ask. (If it has power lines on it, usually the power company owns it. If it has

*only* phone lines, usually the phone company owns it.) Getting this fixed won't *directly* help the problem of stuff coming in on the phone lines, but it's a step in the right direction.

Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.

Matt Roberds

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The surge protector in your demarcation point is probably fried, rusted, or not installed correctly. It looks like a ceramic wiring block and there should be a ground wire on it. Ask the phone company for a new one if needed. It's theirs to maintain.

Reply to
Kevin McMurtrie

The problem is that your house and the telephone exchange is at differential ground potential during a strike. Assuming the thunderstorm is approaching from the direction of the telephone exchange, any strike into the exchange and potentially also in poles in the opposite side of the exchange, will create a ground potential rise at the exchange grounding electrode. This will also increase the common mode voltage of your telephone line, potentially flashing over to your mains ground.

Having high differential voltages on twisted pair telephone lines is very unlikely and the same should apply to ABC (Aerial Bundled Cable) mains cables. Simple POTS without any mains or ground connections survive quite well, compared to modems, faxes etc. with mains or ground connection. This clearly indicates, this is a common mode problem, not so much a differential mode issue.

For this reason, the natural approach would be to use an audio transformer with sufficient (several kilovolts) isolation between the telephone line and your equipment. Unfortunately POTS requires DC continuity, so you would need a relay to connect the transformer on-line. Now there is a risk of flashover from the relay contacts to the relay coil and into the rest of the electronics.

Take a look at

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You should have a ground bar close to your mains entry, into which _every_ conducting material is connected, including pipes, mains, antenna and telephone line suppressor grounds as well as any real ground electrodes. The wires from the various suppressing devices to the main ground bar should not be longer than about 10-50 cm, in order to avoid too much inductance. Thus, all those external cabling should be routed close to this ground bar in order to achieve such short ground wire distances.

A real spark gap protector is a good first line of defense against overvoltages coming through mains wiring, thus, individual mains outlet specific suppressors have a better chance of surviving a much longer time.

It should be noted that with a lightning hitting your lightning rod or you get a high voltage from mains activating the spark gaps, the internal potential of the house will rise, no matter what grounding electrode is used. It is safe inside the house, but since external cables grounded elsewhere can have highly different (common mode) potential.

So a modem flashover can be due to a lightning strike near your house and some current flowing into the telephone system ground or in the opposite direction a lightning strike at the exchange or cabling can flashover in your modem to your own local grounding electrode.

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Reply to
Jan Panteltje

Does acoustic modems work with anything more advanced that 110-300 baud FSK modems ? I very much doubt that something like 33-48 kbit/s would be out of the question.

Anyway, the important thing is that the line side of the barrier is passive or at least very low powered, so that it can use battery powered or get power from the telephone line itself.

Having a mains powered power supply for the line side will just move the flashover point from one place to an other :-).

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On a sunny day (Wed, 02 Jul 2014 11:25:47 +0300) it happened wrote in :

Well, better slow than no signal at all, not even counting the cost of all those modems the OP bought over the years.

I have done phone line interfacing with optos, been some years, on the driver side there was a special Motorola? opamp for that, drive with current source, then those are linear. Bridge rectifier with opto in the DC side on phone line for Rx.

Lighting is tricky, I once had a desklamp, one evening big lightning strike, and the copper on the bulb socket had a hole in it. Nothing else damaged, no idea how that could happen.

I have had a TV for repair where all tracks on the tuner PCB were evaporated, nothing else looked damaged.

I have also seen St Elmo's fire walk all over the place, in a lightning storm. Now that looked scary..

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Reply to
Jan Panteltje

I assume you also have telephones attached to the copper. Have any of them failed?

Reply to
John S

Fuses are usually far too slow to protect semiconductors. Gas based ones at least stand a chance of doing the job on a good day.

My only experience of losing a modem was way back from a more or less direct hit. It was an early model fast for the time Boca modem in an extruded aluminium case. The transistor responsible for taking it off hook was essentially toasted but nothing else was affected.

Once I replaced the failed device the modem functioned OK until it was out evolved by newer cheaper models and replaced.

Storm surges from distant thunderstorms shouldn't damage them unless there is some other local earthing problem. Surge arrestors even the big industrial ones on mainframes seem quite happy to save themselves by letting more delicate electronics further down the line fry.

Martin Brown
Reply to
Martin Brown

build a motor-generator set with the drive link being a 1m or so glass rod

on the generator end put your ADSL modem, router and a wifi-access point

should be good for 1MV or so :)

umop apisdn 

--- news:// - complaints: ---
Reply to
Jasen Betts

On a sunny day (2 Jul 2014 11:54:18 GMT) it happened Jasen Betts wrote in :

Yea, but slow bitrate, shine a LED in the glass rod and you are done.

2 glass rods, or 2 diferent color LEDs in one with filters.

mm come to think of it, those optical interfaces you can just buy, glassfiber.

100 meters would give you many MVs.
Reply to
Jan Panteltje

I second that.

The current might be going in through the phone line and out through the power line or computer connection. Obviously it would be nice if the current doesn't pass through the modem, regardless whether the power or phone line is struck. One solution may be to put the modem in an earthed metal box, and right where the wires pass through the walls of the box, connect gas discharge tube (GDT) arrestors from each incoming phone wire to the box, and MOVs from the power lines to the box. Get the data into and out of the box over wifi through a C-band waveguide-sized (or larger) hole in side of the box, or plastic fibre optically isolated RS232 if it's that kind of modem.

GDT arrestors have low capacitance, so you could run the phone line to several of them in turn, on the way into your metal box. In series with the wire between the GDT arrestors you could put 10 Ohm resistors, or thin resistance wire. Ideally these would be rated for high voltage before they flash over. I use very fine sacrificial manganin enamelled wire, going through a tiny hole in an earthed metal partition, in the hope that any arc that briefly replaces the thin wire will tend to stop at the partition and not bother going any further.

I have not had a nearby lightning strike so I have no idea whether my scheme works yet.


Reply to
Chris Jones

You "disagree with all of this", then suggest the solution is to do exactly what I say. Since you're a newb, I'll just say that you're weird.

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That's one problem, yes. However, if everything is right with the system, this has been taken care of by the phone company and the modem itself.

No, it indicates a ground problem (or the protection is missing). There should only be one ground on the house. Think of a cork on the ocean.

Not necessary. Modems (and phones) survive by the millions without doing anything to the phone line. Fix what's wrong.

*EXACTLY*. That's what I was saying.

But that has to go to the same ground as everything else in the house, or it's not doing anything.

Cork on the ocean. If everything is at the same potential, no damage.

Ground problem.

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They do work quite well IME. The trick is to provide a big enough TVS to hold down the voltage until the fuse has let go. Since you normally don't have much in terms of bandwidth issues on a dial-up line, bigger is better.

AFAIK those are limiter device, not fuses. There has to be something that physically opens the line during a really bad thunderstorm.

What I found in fried telco equipment was toasted overvoltage protectors. Often they use cheap MOV and those are like a bank account. They can take so many Joules in their lifetime and then ... phut ... gone. Semiconductor-based TVS are often the better option.

Yeah, I think the OP should look at his grounding situation first like Keith suggested. When that's taken care of it is time to look at the limiter and fuse situation.

Regards, Joerg
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Well I prefer the country to the city... but it's the suburbs that I don't like. (the worst of both worlds.)

I do feel your pain, I lost several modems also. We bought one of those expensive power line surge suppressors that also had a phone jack. That helped some... It's a hard thing to measure. And like you, most of the time we tried to keep the modem jack unplugged.

Well we now use Hugh's net satellite. It's just for the internet. Sans the cost of the dish and installation (~$400?) We pay $80/month for a premium plan... 1 gig a day. I think they start at $50/ mo. with less download allowance, but that may be plenty if you've been living with dial up. George H.

Reply to
George Herold

Well, as others have said, good low-impedance grounding is the basic prerequisite for any kind of surge protection. That's where all the various surge suppressors from power and communication lines are bonded to a central equipotential "point". Given that a modem connetcs both to a phone line and a power line, the surge can come in from either direction. This means that any surge-protective components on the phone line and on the power line have to "meet" at one low-impedance ground connection, preferably right at the point of entry into your house. This ground connection has to be bonded as short and thick as possible to the ground rod(s) and also to any other significant metal structures your house has.

If that's not possible for you for some reason (old house with undocumented wiring, various ad hoc grounds over whatever metal part was handy at the time, damaged or loose connections, rust and corrosion, etc.) then your quest becomes more difficult. Obvously, bringing the house grounding system in order would be the preferred safe approach. If that's not an option, financially or otherwise, your only other way out would be to attempt a local equipotential bond to the power line ground right at the place where your phone and power lines meet, that is, right before the modem.

A way to do this would be to use the local earth ground from the mains socket where your computer is plugged in (it has to be local right at the point of use, and the modem and computer with all connected accessories must be powered from this central point only, not from any other socket in the house). This would essentially put the whole modem-computer-system onto an "island", isolated from any other discharge paths. To this local earth ground you will need to connect all the surge suppressors ("whole house" type for the power line and "telecommunications" type for the phone line) that protect the lines leading to this "island". Such an "island ground" could be established with electrical boxes containing the surge protectors located right besides the mains socket in question. The phone wire should be "feed-through" connected, passing first to the surge protective components and from there to the modem. Same with the power line and the socket. For safety reasons the power line and the phone line must each have its own box (so that there is no danger of short circuit between power and phone lines inside) and the boxes' metal cases bonded together and to Protective Earth with a low-impedance connection.

Such a solution is obviously sub-optimal compared to a proper whole-house grounding and comes with a lot of caveats: Never connect anything that has a conductive path to the modem (any computer peripherial, stereo system connected to the sound card, TV connected to the graphics card, etc.) to any other socket in the house except the one where the island ground is established, bond any antenna connection to the computer (if with a TV or radio card) to the same grounding point, etc ... That could become more difficult than it seems (extension cords get connected carelessly, things get tangled up in a mess of wires, ...) over time, making any island ground solution a nightmare to maintain.

My advice would be: By all means, try to bring the whole house grounding system in order. Only if you really cannot do this, and only as a last ressort, use an "island" ground solution with local surge suppressors on both phone and power lines. If you do use an island, separate the SPDs for phone and power lines into 2 boxes and make really sure that Protective Earth connection is reliable since that's the only ground connection that the "island" has. Your life may depend on it. Be aware that island grounds come with their own sets of problems, safety and otherwise. Be aware that the location of connection of the various pieces of equipment is important: you cannot just plug in "something" "somewhere". Be aware that ground loops are not your friends and that breaks in a Protective Earth connection can become deadly enemies. Taken all together, a proper whole house grounding should be the way to go. Also note that surge protective components do wear out, and in an area where lightning is common, they need to be replaced regularly.

Last but not least, everything you do with an electrical installation is at your own risk. If you don't feel confident, don't try. And even if you do feel confident, make really sure that you understand what you are attempting before opening anything.

Regards Dimitrij

Reply to
Dimitrij Klingbeil

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