Augenbra%udel-eecis3.delaware@UDEL-LOUIE (02/20/85)
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
jhs%Mitre-Bedford@d3unix.UUCP (02/22/85)
" 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. 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. However, note that MOVs may become ineffective after being zapped a couple of times, so it might be well to install several of them in parallel at the entrance point. It is also a good idea to inspect or test them periodically. 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
jshaver@APG-3 (John Shaver STEEP-TM-AC 879-7602) (02/22/85)
That is not true. You have three wires in most power receptacles today. At the frequencies represented by the rise-time of spikes, the two ground-wires (or neutral) may not be zero impedance from one another. Thus three MOVs is a good plan. You need also to have indicating fuses in series with each of the MOVs to know when they have been wiped out and need replacing by a particular heavy lightning stroke.
peterb@pbear.UUCP (02/23/85)
Acutally one MOV would not work at all. You would need three, MOV from : Hot to Neutral Neutral to Ground Ground to Hot This protects the circuit from transients on any or any two of the three legs of the feed. The real trouble is that the entire load of a house can tax the MOVs unless you get the more expensive rated MOVs, and besides most of the devices in the house are insensitive to transients. Second, the MOVs on the feed box would protect sensitive equipment from the outside transients, but would not filter out the transients that originate within a house. Yes the MOVs would trip and suppress the transient, but also the power suppies of TV's, computers, etc would also see the transient since they are on the short side of the transient, so these devices would not have adequate protection. The best advice is to install the MOVs as close to sensitive equipment as possible and if you put MOVs in a outlet box, only plug in compatible equipment into that box so that you can reduce possibility of transients defeating the MOVs. In my parents house, every spring the thundershowers knock out the rectifiers in the TV. after two years and progressing through voltage ratings up to the highest (~4Kv), I installed MOVs across the line AND across the transformer output AND across the rectifier AND across the output side of the logic supply. (Not across the tube supply of course!) The cost was only about 15$, but after having the rectifier replaced twice, and an entire main board replaced because the transient took out most if not ALL of the IC's, I think the overkill will save the boards. And for the last four years, there has not been a single problem due to transients in the TV. (But transients did blow the radio that was plugged into the SAME outlet). This is why it is recommended that MOVs be placed in EACH item that you wish to protect. Peter Barada ima!pbear!peterb 20$ of MOVs are worth more than the agrevation of blowing delicate electrical equipment.
peterb@pbear.UUCP (02/23/85)
Most MOVs are designated both by peak current and NOMINAL voltage These devices work for both AC and DC. (AC devices will last longer though). If you hooked up MOVs at the feed box for the house, it would protect from the outside disturbances, but not inside transients. If you want to protect logic, I recommend tranzorb devices by GS (general semiconductor), specifically the 1N5908 This devices charecteristics are: 1.5Kw Peak Pulse Power dissipation @ 25 deg c Tclamp (0 V to BVmin) < 1 pecoseconds! (theoretical) (can not measure it if < 3 picoseconds) Forward Surge rating (1/2 cycle): 200amps, 1/120 second @ 25 deg c Steady State power dissipation: 5W @ 75 deg c, lead length 3/8 inch Repitition rate (duty cycle): .05% It comes in a molded case, (9.4mm X 5.08mm) Clamping voltage is 7.2 volts, and rises only slightly to ~ 8 volts @ 100v and rises slightly beyond. This is unlike MOVs that may rise to quite higher than the nominal voltage indicated. 10Kpf capacitance, reducable to 200pf by schottky diode in series, 1KA @ 7.2 V This is only one of the diveces GS makes, and this is intended for logic protection. Originally designed for MIL work to protect from nuke EMP, these devices work quite well. for mor info, write GS for a catalog: General Semiconductor Industries, Inc. 2001 West Tenth Place Tempe, Arizona 85281 (602) 968-3101 This is not an endorsment of mine, however, a friend does endorse them. Peter Barada ima!pbear!peterb PS cost of that little gem is about 75 cents!
wmb@sun.uucp (Mitch Bradley) (02/24/85)
> What is the proper configuration for hooking up an MOV to a power > line? (i.e. in series or in parallel?). 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. That would work if you expect all the transients to be developed on the wiring outside of your house. This may be a reasonable expectation in some instances. However, if you are interested in protecting againt lightning strikes nearby, there could be significant spikes induced in the wiring inside the house. The other problem with using one MOV for the whole house is protecting the MOV's. MOV's are sort of like big zener diodes, in that above a certain voltage, they start conducting heavily, but below that voltage they don't conduct very much. Notice I said "sort of", because they don't work on the same principle as zener diodes, nor are their breakdown characteristics as sharp or as well-defined as for zeners. Anyway, the problem is this: when the voltage across the MOV reaches the breakdown voltage and it starts conducting, something has to limit the current flow or the MOV will be itself destroyed. Typically, the current is limited by the resistance and inductance in the wiring. Since the wiring coming into a house is typically much heavier than the internal wiring, it is not as effective at limiting the current into the MOV as the smaller gauge internal wiring. This means you need a bigger MOV. Mitch Bradley
binder@dosadi.DEC (Wherever you go, there you are.) (02/25/85)
To use MOVs, you need three of them. Inside your outlet box, connect one from
green to white, one from black to white, and one from black to green. There
is good reason why a single set of MOVs won't protect a whole house - wires
don't have zero resistance to DC, and their impedance to AC is often rather
surprising. Spikes at one point in the house may or may not exist elsewhere.
Cheers,
Dick Binder
UUCP: { decwrl, allegra, ucbvax... }!decwrl!dec-rhea!dec-dosadi!binder
ARPA: binder%dosadi.DEC@decwrl.ARPAedens@almsa-1.ARPA (Crede Edens) (02/26/85)
There is a very good article in the December 1983 BYTE magazine on MOVs plus pictures and diagrams showing how to connect them. I found it very helpful. Crede Edens EDENS@ALMSA-1.ARPA
hes%ecsvax.uucp@brl-tgr.ARPA (Henry Schaffer) (02/28/85)
Re: protecting your house from lightning carried on the incoming power lines. 1) Most homes around here have 3 line, single phase 120/240 service. All three lines should be protected- taking 3 MOVs or ONE three- electrode gas tube. When the gas tube is fired (due to voltage on any pair of lines) it "shorts" all three lines together. 2) At the entrance to the house wiring the power lines can indeed supply lots of power and so the protection device must be able to pass lots of current and dissipate lots of power. Once again a gas tube fits the description. A company I've dealt with (through a distributor) which makes a variety of gas tube surge protectors is TII (I don't remember what it stands for, and I can't find my catalog.) An example of a home lightening surge protector (from a 6 year old spec sheet): Three wire, maximum discharge voltage: 450V@500AMP, 800V@1,000AMP 2 1/2" x 2 1/2" x 1 3/8" + 1/2" threaded nipple $21.54.