lightning and computers

[Wiz]

a friend of mine's house was hit by lightning while his computer was on.

The machine survived, but the modem got fried. I was wondering why the rest of the computer did not go the way of the internal modem?

[Szech]

I'm guessing the computer was plugged into a surge protector, so then the computer was safe from any damage. But the telephone wires probably got a jolt, and that's probably what toasted the modem (telephone wires have electricity too!). Some surge protectors have a plug so that you can protect your telephone wires from surges too.

[Wiz]

actually, it was not, that is why i'm shocked. I have convinced him to get a UPC though.

[psyklone]

most systems that i have worked on that suffered damage due to a lightning strike or surge during a storm sustained damage to the modem and then not much else. it's very common actually. your system can be taken out by a lightning strike not in the immediate area because the surge can travel through phone lines just as well as it can travel through power lines.

and about this comment "the computer was plugged into a surge protector, so then the computer was safe from any damage" ... while it is always obviously a good idea to have a surge protector installed, in the case of a very close strike they are not that effective. a piece of plastic/metal that is about a foot long and two inches by two inches wide is not going to stop a bolt of lightning that's travelled a mile plus to hit .

[SDT]

A power supply id meant to take a much larger voltage and amperage and then rectify & reduce it down to a level that the mother board can deal with. A modem uses a much lower voltage (48 volts during a ring and 24 volts for operation) and is more sensitive to voltage & amplitude modulation (thats how they work right?). The modem doesn't have big capacitors and a transformer to absorb the jolt the way a power supply does. Thus it is the first thing to go in an electrical storm. Your friend is lucky though.... al little more power and the spark would have probably fried the mother board.
A "Good" quality UPS will offer some protection. Surge protectors are not much help unless they are meant to protect a computer ($$$). If you think of the investment you have in a pc then it makes some sense to protect it.... unfortunately it costs some cents to do so.

[Red Rage]

This happens all the time In my area. We've concluded that the Phone Lines are not shielded as well against such a thing.

Psyklone is right about the surgeprotector thingy. The only thing those are good for is multiple outlets. If you read the package or call the manufacturer they will not protect you from lightning, In fact it is very hard to find any thing that will (with out pay'n a left nut for it). Lightning is just too powerful and unpredictable to say, "Yes your safe".

~Joe

[tonym]

The modem died doing it's job!

Most modems have a POTS interface that can withstand a surge voltage of 2kV. Some, but not all achieve this with gas tubes or MOVs across the line, or they use the isloation of the transformer on the card to achieve surge resistance on the secondary side.

HOWEVER, a lot of modem folks don't have any per se protection on their cards (it isn't a requirement in the US, interestingly enough), so if the card gets a "hit" much above 1kV, it will die. But since there is galvanic isolation (the transformer) on the card, the failure won't propagate further than the card.

The PS didn't die because they all are required by UL/CSA/TUV standards to have MOVs and some form of line filtering included on the AC front end. The MOVs clip the transient voltage (lightning hit) and the line filter attenuates the clipped transient to an acceptable level. This works for hits that don't have a tremendous energy (Joules) associated with them. The integral MOVs can take the hit and survive. But if the hit exceeds the energy rating of the MOVs, they fail shorted and blow the PS fuse or trip your AC outlet circuit breaker.

This is why it's great to have a power strip that has surge protection to the highest Joule level (Watts-seconds) that can be obtained, as a $30, 1500 Joule surge protector can save your $2000 PC. And the added telecom surge suppression feature is worth it's weight in gold if you get annoyed opening your case and replacing your modem after each thunderstorm!

Tony