jhs@mitre-bedford.ARPA (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
edens@ALMSA-1.ARPA (Crede Edens) (02/25/85)
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
binder%dosadi.dec@brl-tgr.ARPA (Dick Binder) (02/26/85)
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.ARPAhes%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.
jrv@mitre-bedford.ARPA (03/06/85)
I can't see why more than one MOV would be necessary to protect the AC power (hot to neutral), because I can't see anything that would drive a transient from hot to ground or ground to neutral. Big, inductive loads switching on and off (motors, relays) create transients from hot to neutral. Any sources outside the building can only drive transients from hot to both ground and neutral, because ground and neutral are connected at the circuit breaker box. Assuming the building is wired with grounded outlets, all three wires run in parallel from the breaker box to the outlet, so even VHF radiation can't drive a big differential mode signal. The only source I could think of for a big signal from ground to something else is a lightning strike to just one wire - and I wouldn't expect any MOV to handle that! - Jim Van Zandt