jhs@mitre-bedford.ARPA (02/22/85)
*** Correction to text - discard previous version. " What is the proper configuration for hooking up an MOV to a power line? (i.e. in series or in parallel?). If it hooks up in parallel to the power line, why wouldn't one MOV work for your entire house? It seems that it would, because all the lines in the house are connected to together in parallel. Joe " -------- The MOV is a shunt device: it has a high impedance until a transient causes a large voltage to appear across it, at which point its resistance becomes small, or more precisely, it prevents the voltage from getting much bigger by shunting a large current through the MOV. To protect equipment with MOVs connected to the AC line, you would want to choose an MOV rated for a voltage which would not NORMALLY appear across the line. Be sure to find out (does anybody know?) whether the MOV is rated in terms of instantaneous peak voltage or RMS line voltage. The peak voltage is sqrt(2) times the RMS value of 115 or 120, thus about 170 volts. To allow for manufacturing tolerances, an MOV which limits peaks to about 190 or 200 volts would probably be about right. Another parameter you need to choose is the maximum shunt current you need to handle in the MOV. The right choice depends on the application. For EMP protection, FEMA recommends a rule of thumb of 25 to 120 Amps per FOOT of exposed conductor if the conductor is vertical and 5 to 10 Amps per foot if it is horizontal. In either case, a practical limit would occur at about 2000 Amps because more than that will arc over around even your MOV. So probably a 2000 Amp peak current rating would handle most transients in the real world. I'm not sure just how LIGHTNING transients would differ from EMP, but it is generally claimed that the risetime is longer (slower) for lightning. I suspect that the above rules of thumb for EMP would result in a conservative design that would handle lightning nicely also. **** correction to following paragraph: **** The MOV would be connected across the line (in parallel, in your terms). A single MOV at the power entrance point would be very useful if all wiring beyond that point is in metal conduit. MOVs have a reputation of being fairly reliable, provided that they are adequately rated to handle the shunt currents they experience. However, the use of two or more MOVs in parallel might be a good idea for added protection in case a failure should occur. Bear in mind that both EMP and lightning are fairly wideband phenomena, EMP particularly extending well into the VHF region. Because they are so wideband, they act like (they in fact ARE) radio waves and can get picked up by even a few FEET of conductor betwee the "central" MOV and the device in question. MOVs at various other points in the system would provide better protection to specific (expensive) equipment especially if the entrance-point MOV(s) failed or if the distribution wiring in the building is not shielded. The LEAD LENGTH and inductance in series with the MOV will greatly reduce its effectiveness in limiting voltage peaks. It is best to use techniques similar to those used in building VHF/UHF amplifiers: very short leads, wide "straps" rather than ordinary wire, or else large diameter conductor, avoid bending the conductor (because bends introduce inductance) etc. Additional protection can be provided more locally by using Zener diodes on low-voltage lines and gas gap devices on antenna feedlines. NOTE THAT AIR GAPS ARE NOT EFFECTIVE against EMP transients because the risetime is much faster than the time required to ionize air. Buy a gas discharge device specifically designed for EMP or lightning protection, and then check it periodically (after every major electrical storm, for example). I hope the above is helpful. Please note that I am only an "instant" expert on this subject rather than a "real" expert, so I may not be able to give you much more detail than the above. However, I would be glad to try to answer any further questions or point you at someone who can do so. 73, John Sangster, W3IKG jhs at mitre-bedford.arpa
harrison@ucsfcgl.UUCP (Peter Harrison%MIS) (02/28/85)
There was a great article in Byte about a year ago by Steve Ciarcia that had diagrams, parts lists etc. for bulilding your own protection for c. $15.