larry%kitty.UUCP@seismo.css.gov (Larry Lippman) (06/30/86)
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!/