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Lightning protection for your PSTN line?

Devaka Cooray
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Joined: Jul 29, 2008
Posts: 3053

OK, this became a sorta headache when I heard the story that my friend's DSL modem caught fire by some lightning surge that reached the modem over the PSTN copper cable. The ordinary strategy that I always use to protect my home network when there is lightning outside is, disconnecting the modem from the internet by removing the cable, so I can assure the fact that no lightning surge can reach my network through the copper cable. This might be the best solution unless you want to keep always connected to the internet. The thing I can't understand is how some big internet-dependent companies/factories keep the networks and computers at their workstations unaffected with any significance of lightning surge. For sure they use copper cables to connect with their ISPs, and they can't simply cut their internet off to be protected from surge. What sort of mechanism these people really use?

There are some "surge protectors" I can buy at cheap prices. Oh, and some APC UPS's come with a surge protectors for phone lines. Can these gadgets really protect you from lightning surge? What I'm not really sure about these gadgets is whether they can avoid a high voltage to be sparked and jumped to the output terminal and reach my network, even after the connection is electronically disconnected or whatever thingy happens inside. I know there might not be a perfect solution, but what would be the best solution that you can you use to protect your expensive machines from lightning surge that comes over 'internet'?

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wes thom

Joined: Dec 29, 2012
Posts: 2
Devaka Cooray wrote: The thing I can't understand is how some big internet-dependent companies/factories keep the networks and computers at their workstations unaffected with any significance of lightning surge. For sure they use copper cables to connect with their ISPs, and they can't simply cut their internet off to be protected from surge. What sort of mechanism these people really use?

A telco CO, connected to wires all over town, may suffer about 100 surges with each storm. So how often is your town without phone service for four days after a direct lightning strike? Never? Exactly. Because protection was never about a protector. Protection was always about something completely different that does the protection.

A direct lightning strike means hundreds of thousands of joules must dissipate somewhere. Protection means you know where energy dissipates. A protector (hundreds of joules) will somehow block or absorb a surge? Only when profits create myths.

Some protection systems do not even have protectors. But every protection system always has what does all protection - single point earth ground. Hundreds of thousands of joules dissipate harmlessly outside the building when a direct lightning strike is connected to and is absorb by earth. Either that current goes to earth before entering a building. Or that current goes hunting for earth via appliances, destructively. Your choice.

Some incoming wires (cable TV, satellite dish) connect directly (low impedance) to earth. Other wires cannot be earthed directly. So a protector makes the same low impedance (ie less than 10 foot) connection to earth. That distance and other parameters are critical. And that explains why every telco CO suffers 100 surges per storm without damage.

The most common source of surges are utility wires most exposed - AC electric. A direct lightning strike to AC wires far down the street is a direct strike to every appliance. The incoming path is obvious. But to have damage, an outgoing path must exist.

All telco and cable wires are required to have effective protection. AC electric is not. How does that surge, incoming on AC mains, also go outgoing to earth? A common path is via a telephone or cable modem. Incoming on AC mains. Outgoing via signal wires. Damage is most often on a modem's outgoing path.

The naive (using observation) see damage on the outgoing path. Then assume a surge was incoming on the cable or phone wires. Also forget that both an incoming current path and an outgoing current path must exist.

The next post is from an AT&T forum that describes how surges do damage and how DSL modems must be protected. Nothing new here. These concepts were well understood over 100 years ago. Since most are educated by advertising, then a majority will only be learning this for the first time. Meanwhile, this AT&T solution is why telco COs, mobile phone towers, electronics atop the Empire State Building, and munitions dumps routinely suffer direct lightning strikes without damage. Again, what provides that protection? Simple. Where do hundreds of thousands of joules dissipate? Single point earth ground. Advertising will not discuss what they cannot sell for a profit. But earth ground (even to every protector) defines protection.

A protector is only as effective as its earth ground. Protection is not about a protector. Protection is always about where hundreds of thousands of joules harmlessly dissipate. And the electric current path from cloud to that earth ground.

wes thom

Joined: Dec 29, 2012
Posts: 2
From an AT&T forum:
"How can I protect my DSL/dialup equipment from surges?":
Surge protection takes on many forms, but always involves the following components: Grounding bonding and surge protectors. ...

Grounding is required to provide the surge protector with a path to dump the excess energy to earth. A proper ground system is a mandatory requirement of surge protection. Without a proper ground, a surge protector has no way to disburse the excess energy and will fail to protect downstream equipment.

Bonding is required to electrically connect together the various grounds of the services entering the premises. Without bonding, a surge may still enter a premise after firing over a surge protector, which will attempt to pass the excess energy to its ground with any additional energy that the services surge protector ground cannot instantly handle, traveling into and through protected equipment, damaging that equipment in the process. ...

Now, if all the various service entrance grounds are bonded together there are no additional paths to ground through the premise. Even if all of the grounds cannot instantly absorb the energy, the lack of additional paths to ground through the premise prevents the excess energy from seeking out any additional grounds through that premise and the electronic equipment within. As such, the excess energy remains in the ground system until dissipated, sparing the protected equipment from damage. ...

By far, the whole house hardwired surge protectors provide the best protection. When a whole house primary surge protector is installed at the service entrance, it will provide a solid first line of defense against surges which enter from the power company's service entrance feed. These types of protectors can absorb/pass considerably more energy than any other type of protector, and if one does catastrophically fail, it will not typically be in a living space. ...

Plug in strip protectors are, at best, a compromise. At worst, they may cause more damage than they prevent. While they may do an acceptable job of handling hot to neutral surges, they do a poor job of handling any surge that must be passed to ground. ...

Then, to add insult to injury, some strip protectors add Telco and/or LAN surge protection within the same device, trying to be an all-in-one sale. Remember bonding? When Telco or LAN protection is added to a strip protector, if the premise ground, which is not designed to handle surges, cannot handle all of the energy, guess where that excess energy seeks out the additional grounds? You got it! The Telco and LAN connections now becomes the path, with disastrous results to those devices. ...
I agree. Here's the link:
subject: Lightning protection for your PSTN line?