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Lightning Strikes (Updated 4-Jul-02)

THE BEST PROTECTION FOR YOUR COMPUTER AND MODEM is to unplug the phone line from the modem and unplug the power cords going to your computer and everything that is attached to your computer (monitor, printers, etc.) when not in use, and to not use your computer during thunderstorms or when power disturbances (brownouts and blackouts) are likely.

If you don't unplug everything - the next best protection for your modem is to unplug the phone line to the modem. [You can also use a switchbox to switch your modem off the phone line without having to unplug the phone line.]

Some modems are more likely to be damaged than others: All dial-up modems have a DAA (data access arrangement). The DAA is responsible for the on/off-hook control of the phone line. Many newer modems are made with a solid-state DAA which is much more likely to be damaged during a thunderstorm. Modems using an electro-mechanical DAA generally are less likely to suffer damage during a thunderstorm.

An electro-mechanical DAA typically uses a relay to control on/off hook. If you hear a click when your modem picks up or hangs up the line, your modem probably uses an electro-mechanical DAA. When the modem isn't on a call, your modem is isolated from the phone line by the air gap of the relay.  The isolation isn't total - there's a separate circuit in the modem to detect when the line is ringing.

A solid-state DAA typically uses an "electronic switch" - a silicon device that provides either very high or very low resistance between the phone line and the rest of your modem. The circuit that detects when the line is ringing may be integrated on the electronic switch.

If a lightning-induced surge occurs on your phone line and you have a solid-state DAA, the surge can damage the electronic switch, and your modem will no longer go off-hook. It may respond to AT commands just fine, but it will never be able to make a call as the switch is no longer able to complete the connection to the phone line. With an electro-mechanical DAA, the surge may enter the ring-detection circuit - often designed to provide effective surge protection - and your modem may survive undamaged. If damage occurs, it's possible that only ring-detection capabilities will be lost, and the modem will be able to place calls. But, with a big enough surge, it's still possible for any modem to be damaged beyond use.

It is also possible for the modem to be damaged such that it is always off-hook. In this case, it may still be possible to use the modem if you keep it disconnected from the phone line - plugging it in only when you are ready to dial out. (In some cases, even though the modem 'works', it will not achieve the performance/speeds prior to the strike.)

Testing for damage: Using Hyperterminal, give the modem commands to go off-hook (ATH1), blind dial (ATx1D####), etc. If the modem doesn't go off-hook or dial, it will need repair or replacement.  (If the modem goes off hook and dials, but is unable to get carrier, or unable to achieve connect rates before the storm, the modem may be damaged, or something else on your line may now be interfering: check your portion of the line, and have the phone company do a check of your line.)

Repair: Most people will not be able to repair a modem themselves. Often, a better choice is to replace the modem. Even if your modem is under warranty, you'll probably have to pay shipping and be without a modem during the repair/exchange process.

In some cases, the damage might be repairable: Bob Wake from Ontario writes:

In cases of non-severe lightning, where the strike is actually miles away hitting only the phone line, the usual fault is not with the DAA circuit. In modern modems, there are usually low value resistors (around 100 ohms) right at the tip and ring inputs. These are what usually fail. They're usually small surface mount and have their value printed right on them. You can tell if they've failed by looking for a small burnt or melted circle in the middle.

Anyone who is fairly handy with soldering can remove these easily and a regular 1/8W resistor can be substituted by installing it standing up. The pads for the resistors are jsut big enough to solder. The values aren't critical as long as they're between 50 and 150 ohms.

Preventing damage:

  • Unplug the phone line from the modem.
  • Unplug the computer (and everything attached to the computer) from power outlets.
  • External modems: unplug the modem from the computer, power, and phone line.
  • Use a modem with electro-mechanical DAA
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