larry@kitty.UUCP.UUCP (06/13/86)
In article <3607@reed.UUCP>, kamath@reed.UUCP (Sean Kamath) writes: > > Well, there has been a lot of talk lately about line voltages, and I > don't know if this has come up. I know it is possible to buy surge > suppressors for AC & phone lines. I know there are such thing as phone > line spikes (I hear them at 1 am on my ultra-cheap phone from time-life > books, or whatever.), so the question is, is it as easy as installing a > MOV in the line? I'm sort of worried what might be getting rammed into > my internal modem. Anyone got any ideas? Most telephone line transients result from lightning strikes causing current to be dissipated through the sheath of the telephone cable to ground. This INCLUDES cable which is buried underground. A voltage is induced in the individual telephone cable conductors due to the HUGE currents and magnetic fields which accompany a lightning strike (to give you an idea how large the currents are, lightning strike currents are measure in KILOamperes, with a typical lightning strike being 100 kA). Except in extreme cases, the lightning voltage itself therefore never hits the individual conductors (which would cause destruction of dielectric), but its propagated through capacitance and inductance between the cable sheath and its pairs. The resulting transients are "longitudinal" in nature, being from each wire of a pair to ground; these transients are NOT generally from conductor-to-conductor. What does the above mean in practical terms? For one, placing a MOV (Metal Oxide Varistor) just across tip and ring will do almost zip for providing transient protection. You need TWO MOV's, one from tip to ground, and one from ring to ground. And I mean GROUND, like a #12 AWG wire to a water pipe - NOT the third wire of an AC power outlet. Because the MOV is not going to be very effective unless a low impedance path to earth ground exists. So, it is possible to build your own telephone line surge protector using two MOV's as indicated above. However, it is not quite that simple since the MOV's have to be carefully selected. First, the MOV should exhibit a leakage current flow at 50 volts DC of LESS THAN 500 microamperes, which is equivalent to a resistance of at LEAST 100,000 ohms. At 130 volts DC, the leakage current should still be less than 1,000 microamperes. Exceeding these values can CAUSE TROUBLE with your telephone line, such as: (1) false ring tripping; (2) hum and noise due to longitudinal unbalance; and (3) trouble indications at the telephone company central office due to automatic line insulation testing equipment thinking your line is in trouble. The effective breakdown potential of the MOV should be around 180 volts DC to preclude any false trouble indications. In my opinion, any product which claims to be a telephone line surge protector and consists of a single MOV across tip and ring is ALMOST WORTHLESS. You need to go between the conductors and ground, for the reasons mentioned above. I don't want to get off on a tangent here, but there is something which is better than a a MOV: a gaseous discharge tube, especially what is called a "three element" gaseous discharge tube. These devices exhibit an infinite impedance in the "off" state, and conduct much faster than a MOV. In telephone parlence, these are referred to as "rare gas protectors". However, the cost of such a component is several times the cost of a MOV. ==> Larry Lippman @ Recognition Research Corp., Clarence, New York ==> UUCP {bbncca|decvax|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry ==> VOICE 716/688-1231 {rice|shell}!baylor!/ ==> FAX 716/741-9635 {G1, G2, G3 modes} seismo!/ ==> "Have you hugged your cat today?" ihnp4!/
wtm@neoucom.UUCP.UUCP (06/13/86)
The author of the original article mentions hearing chips from his cheap phone at about 1:00 am. Sounds like an automated test procedure being run on his loop. Where I live is what used to be Bell territory. I think we have ESS-IV (but I'm not sure, so please no flames, I'm ignorant). I've noticed that regularly at 2:30 a.m. my circa 1930 Stromberg-Carlson crank phone emits a single "ding-dong", while all the other phones are silent. I think that it's bell coil and coupling capacitor have a low enough frequency response to pick up the polarity reversal of some kind of automated subscriber loop test. Now for the real question. Ma Bell has a protector block where the line comes in the house. Everybody has these, as this is regulated by law as well as internal telephone edict. Basically, the protector is a spark gap(s) and silicon resistors that blow when the gap sparks, thus isolating the network from you. Sorry if the descirption isn't 100% correct--this is the kind I know, but there are several types of protectors around. I'm describing mine, which was installed in 1929. Point is, the protector block is really more meant to protect the network from you rather than you from it. You might want to consider some additional protection. A gas discharge tube is the best way to go, as it responds quickly enough to present a reasonable amount of protection. Several places sell phone spike protectors, which are actually gas tubes in little plastic boxes sold at considerable mark- up. Since modems mix outside power & electronics with Ma Bell electronics, they are quite closely regulated and scrutinized. Of course, in these days of deregulation and Hong Kong imports, there may be some laxity in quality control of off-brands. As long as you have a major brand modem, you can probably be reasonably assured that it already has mov and/or gas tube protection inside. (I've torn quite a few apart, and so far all the ones I've seen have it!) Note that a gas tube often looks like an overgrown ceramic diode with glass/metal seals on the ends. Check your modem. Most of all beware about what you connect to your phone line. It sure isn't cool the screw up the network by attaching a weird device. You might possibly screw up your neighbor's line too if you goof up--this is because many lines are spilt by using duplexing transformers. (This practice is common in my neighborhood which is fairly old, and has fews lines.) By the way, 2 years ago my house had a direct lightning hit. It blew up part of the chimney and shattered a big mirror (due to current flow in the silver heating the glass). Several of the light switches were vaporized. At the time, I had three Bell phones and a cordless. The three Bell phones and protector block were o.k., but the cordless was fried. (There were just two wires sticking out of the pc board where the MOV used to be!) Needless to say, everytime there is a thunderstorm, I still get a little nervous now! (I was home at the time.) ~~Bill Bill Mayhew -Elec. Engr. Div. Basic Medical Sciences N. E. Ohio Coll. of Med. Rootstown, OH 44272 USA (216) 325-2511 ...!allegra!neoucom!wtm (wtm@neoucom)