On Monday, September 1, 2003, at 12:19 PM, Henri Yandell wrote: > However, someone has pointed out that a lightning strike will go > through a > surge protector. > > So I have the facts that: > > * Surge protectors do not protect against lightning. > * RJ-11/RJ-45/Cable do not 'surge' without lightning (?)
I come from a family of electrical engineers, and both my brothers are power system engineers, so I've talked to experts about this quite a lot. Most of the cheap surge protectors contain cheap components called MOVs (metal oxide varisters). They are resistors that vary their resistance depending on the voltage across them. In a power strip, they are connected across the hot and ground wires, and when the voltage is increased beyond a certain threshold they decrease their resistance and send the current harmlessly off to the household ground. MOVs are sort of like a pressure valve in a plumbing system. There are a couple of problems with MOVs. First, it takes a little time for them to change their resistance. Power line surges are usually relatively slow, and they can react in time to do some good. With a lightning strike, the damage may be done before they can react. More expensive MOVs react faster, so you get what you pay for. Second, it's not too hard to burn them out when too much current passes through. I've opened up power strips to find the MOVs toasted to a crisp. They stopped that one big surge, but they weren't around for the next one. The more expensive surge protection power strips will have an LED on the outside that either lights up or fails to light when the MOV is cooked. Again, you get what you pay for. Better (=more expensive) surge protectors often use gas discharge arrestors instead of MOVs. These work under the same theory as MOVs, except that they consist of a glass tube containing a gas that ionizes when the voltage gets above a certain point. When it's ionized, it can conduct the current to ground. These gas discharge arrestors can react more quickly than an MOV, and don't burn out as easily, but they cost more. I guess, you get what you pay for. The surest way to protect yourself from power line surges, whether caused by lightning or man-made switching glitches, is with a good uninterruptible power supply. Many of the better UPSs have the computer running off the battery all the time, and the battery voltage is clean and constant. You plug it into the wall only to keep the battery charged. The charge and discharge circuits are usually isolated from each other. > so it seems that the RJ-45 etc surge protectors are a scam. I wouldn't go that far. Most of the ones I've torn apart are MOV-based and will work well, within the limitations of their MOVs. > Does it have to do with how close the lightning is? How does lightning > hit > an underground cable line? Lightning doesn't actually have to hit a line to generate a dangerous pulse. A close lightning strike will generate an electromagnetic field that will induce a voltage in a nearby conductor. (The conductor is essentially an antenna getting a very strong radio signal.) This is unlikely to affect a buried wire because it's surrounded by a somewhat conductive medium. Long above-ground runs of networking or telephone wires in your house can be in this EM field. A few years ago lightning hit a tree near the north end of our house. All the telephones were zapped and two out of the three modems were fried. The only one that survived was connected through the protected telephone jacks in a TrippLite UPS. (It also took out the radio-controlled garage door opener, a couple of radios and a television; the EM pulse probably fried their tuners through their antennas or the cable connection.) I keep all my computers on smart UPSs from APC, TrippLite and Belkin. The UPSs talk to the computers through USB or serial connections so the computers know to shut down when the UPS batteries are drained from a prolonged power outage. The power can be out for 10 or 15 minutes before the computer shuts itself off. (Belkin has no MAc OS X software for its inexpensive UPSs, but APC does.) I leave all my stuff plugged in and turned on all the time. That's what sleep mode is for. | The next meeting of the Louisville Computer Society will | be September 23. The LCS Web page is <http://www.kymac.org>. | This list's page is <http://erdos.math.louisville.edu/macgroup>.
