commgrp@silver.bacs.indiana.edu (09/18/87)
>I am wondering what kind of steps I can take to minimize the >damaging effects of lightning... > > Ralph M. Hightower, Jr. The following was posted by Howard Hull on rec.ham-radio, 5 Aug. 1985: LIGHTNING PROTECTION 1. First order protection - On the tallest object associated with your structure, mount an extended umbrella-like fixture a few meters in diameter, with numerous sharp points along the periphery and across the crown, spaced about 1 meter apart. (You can make the thing from re-bar and heavy duty chicken wire unless you have high winds like we have around here.) Use a large diameter conductor (1 to 2 cm.) to connect the umbrella points together at the center and thence down to a suitable ground stake located at a place where soil moisture is prevalent, but more import- antly, try to make the conductor run in a straight line with *no* sharp corners; use a minimum radius of 1.5 meters on any bends in the ground wire. Keep this wire at least 2 meters from any power or communications conduit at all places along its route. Theory: The multitude of points will emit a trickle corona continuously, resulting in a space charge of ionized air within 20 meters of the umbrella. The space charge will terminate the cloud-to-ground elec- tric field across a broad hemisphere and will reduce the local field gradient to a value below that needed to form "leaders". The umbrella will likely not ever be hit by lightning; however, the conductor gauge is set to minimize the damage inherent in such a strike. (A strike, if it occurs, will likely be a secondary, (resulting from the shift in electrostatic field just after a strike) to another object within a fraction of a km.) This approach, you should note, puts additional stress on your neighbors (they will see a slight rise in their hit statistics) as it only postpones the discharge until the cloud has moved past your installation. The ground conductor is spaced from other conduits so that the Electromagnetic Pulse (EMP) associated with the 10000 Ampere surge will not be able to develop equivalent currents in parallel conductors adjacent to the ground wire. Using a large diameter and avoiding bends reduces the per length inductance discon- tinuities. This discourages the abandonment of your ground conductor in favor of nearby metal objects such as power conduits (resulting in hazardous elevation of the system ground potential to thousands of volts above the mains). 2. Second order protection - Protect your primary power entry by use of a surge protector having four main elements a.) Line fuses for each hot main NO FUSE FOR THE WHITE NEUTRAL. No circuit breakers (too slow). b.) Self extinguishing gas discharge tubes or arc chutes routed to a primary ground stake *separated* by 3 or more meters from the umbrella ground mentioned above, *not* using the same stake, even, and using the same linear routing algorithm mentioned above. c.) Heavy gauge inductors, 1 microhenry or thereabouts for typical 30 to 50 Ampere per phase service levels, to choke the surge out of the consumer side of the system. NONE IN THE WHITE NEUTRAL. d.) Post choke line clamping to WHITE NEUTRAL. This is where the witchcraft comes in. One candidate is the Metal Oxide Varistor (MOV). They have two disadvantages: They age, gradually reducing their threshold over time until one day they evaporate in a ball of fire during a line surge. They have a rather remote threshold character- istic compared to, say, a Silicon TransZorb. They have several advan- tages: They are cheap. They come in packaging that is familiar to professional electricians. They are generally more robust than Selen- ium or Silicon protectors. They have a smaller geometry than a Selenium protector. Another candidate is a combination protector made up from a ground referenced 50 Ampere triac in series with either a lower rated voltage MOV or TransZorb element, with the triac gate wired back to (an artfully positioned) tap on the gas tube/arc chute ground. From here (this stuff belongs in a fire-rated NEMA box) the WHITE NEUTRAL and GREEN NEUTRAL are tied together at this one point only, and passed through a medium size conductor to the primary ground stake by a route that is separated by 1.5 meters from the gas tube/arc chute ground. Theory: If your power line gets hit, the gas tube fires and conducts the surge current to ground. The 20 kilovolts experienced by your service entry (for about 10 microseconds) will go through the chokes and will cause the MOV or complex protector shunt to break down and draw a steadily rising current (to many tens of Amperes), but immediately choked to a reduced voltage. The fuses will, after a while, be blown away. Until then, the MOVs will clamp the WHITE NEUTRAL to the mains (perhaps resulting in noticeable rise of the common-mode voltage). It is this common-mode elevation which destroys your out-of-building communications interfaces. With everything in the building coming to 2000 volts above your neighbors (including your local telephone operating company), any common-mode paths will be severely stressed. However, especially withing the building, they will be less stressed than they would have been if the mains were allowed to diverge from the WHITE NEUTRAL. 3. Third level protection The most effective common-mode protection is an Ultra-Isolator Trans- former. It is also rather expensive compared to differential line protectors and secondary Silicon TransZorb protectors. Although many Ultra-Isolator Transformers were utilized during the 1970's by sensi- tive computer installations, it was realized eventually that the most damage to main-frame equipment was done by differential surges (main to main on three-phase systems). The common-mode threat was seen as too little to justify the cost and complexity of installation of an ultra-isolator, which, by the way, can also be done ineffectively, resulting in no net improvement in the level of protection. The companies that make ultra-isolators issue complete and effective instructions concerning their installation. The difficulty is in getting industrial electricians to follow the directions. Thus for the benefit of the main-frame and peripheral power supplies, for cost effective purposes, a good differential surge eliminator inside the enclosure of each system power supply is recommended. However, remem- ber that the common mode is the most destructive to your distributed data communications peripherals; unfortunately, to protect them you must provide the entire computer room and distributed CRT terminal load with an ultra-isolator transformer, or see that each unit is designed to withstand momentary local and global differences of thous- ands of volts on the signal returns. Even then, on occasion, only one violator located in a critical location and tied to a non-isolated power system elsewhere in the building can blow the whole scheme. Theory: Not much theory here. The entire primary winding of the transformer may get lifted to 2000 volts, but the secondary remains referenced to the computer room ground stake. The box shields around the the wind- ings are tied to the stake, and short out the electric field that might otherwise couple to the secondary. Saturation of the trans- former core protects the differential mode. The differential protect- ors installed in each power supply dissipate the surges locally and since each takes a small part of the surge energy, no concentration of damage will likely occur. 4. Fourth order protection You may get surge protectors for all communication lines leaving the building. Each will need a reliable path to a stout ground. (DEC usually specifies that the computer frame GREEN WIRE ground be done with a heavy gauge wire, and all surge protector grounds be separately returned to the distribution transformer secondary neutral grounding point.) You may add Silicon TransZorbs to power supply rails in data communications equipment. Theory: If one of your comm lines gets hit, or gets involved in an induced surge, the elevation in voltage not dissipated by the protector is conducted through the internal diode clamps included in most IC line drivers and receivers to a ground or supply rail, and thence to a TransZorb (a back-to-back zener with a heavy silver anode and thermal- ly conductive silver leads). If enough protectors are in place, the common-mode surge is clubbed to death by the collective capability of all peripheral surge protectors operating together. And that about does it. Needless to say, if you do a good job of protecting your site, and one of your neighbors gets hit, you may be damaged anyhow by currents resulting from the elevation of your neigh- bor's electrical ground. This is especially true in Hawaii (and even more so on their mountain tops) where the ground is made of lava rock. If you get hit by lightning, your entire site goes to 25000 volts with respect to the surrounding neighborhood. This bleeds down to appx 2000 volts over the next 100 microseconds or so. If you have several buildings to worry about, such as may be the case for a university campus, putting an umbrella protector on every build- ing will only cause the cloud to ground potential to develop to the point that when you finally do get a strike, it will be a *real killer*. It has been pointed out elsewhere that most lightning strikes are from the ground up to the cloud. Thus, More Theory (speculation): I suspect that the mechanism is something like this: Collisions of air molecules with each other and the things that make up the surface tend to knock electrons off the air molecules. There are other charge pair generation mechanisms as well, such as natural radioactive decay of Radon 222 and its decay products. (This specific mechanism is not my theory - see JGR Vol 90 No D4 Pgs 5909-5916 June 30, 1985, Edward A Martell, NCAR.) The electrons, because of their charge, are sticky. They cling to the surfaces of various semi-insulators (rocks and dry dirt) and near the surface of conductors until enough of them are implanted to provide a counter electrical field gradient to repel later arrivals. The positive air ions are separated by thermal energy, and molecular screening prevents the immediate recombination. The charge separation is effected by the rising of the warmed posit- ively ionized air. Once the charge is separated, mutual repulsion drives the electrons into the conductive ground layers. Later, as the air rises and water condenses, positively charged droplets accumulate in descending air columns at the front of the storm just ahead of the rising column. A field gradient is thus established with respect to the ground, where all the electrons are. As the ground is conductive, the electrons follow the cloud until, with the aid of conductive moisture and the turbulence of the rising and descending air column interface, leaders are established and a strike path is ionized and carried into the descending air. The electrons travel up the path in a flash (parts of which will have oscillations at radio frequency) and then distribute themselves (at a more leisurely pace, accompanied with local flashes and secondary flashes) in accordance with upper level gradients until there is nolonger sufficient gradient to ionize the cloud-to-cloud paths. Time scales: Main strike and individual secondary strikes each about 10 microseconds. Duration of ionized path, reversals and secondaries about 100 microseconds. Duration of high altitude electrical coronae readjustment about 1 millisecond. Localized differences in the final potential may result in some reverse strikes from a few overcharged negative clouds to the ground, or subsequently more numerously (after air motion), cloud to cloud "readjustments". Howard Hull -- Frank Reid reid@gold.bacs.indiana.edu
smh@mhuxu.UUCP (09/22/87)
In article <14917@topaz.rutgers.edu>, ron@topaz.rutgers.edu.UUCP writes: > Telephone lightening arresters are commonly used on all circuits these > days. Surge protectors for phone lines are also available, but their > use is less common. Yes, the phone company installs primary protectors where the wires enter your home. These are designed to keep the voltage on the phone line from rising above 600 to 1000 volts depending upon the model, and the number of times that it has been zapped. I found out the hard way that this is not good enough to protect some modems. Now I use a secondary lightning protector on all lines with modems. These are available from such places as Inmac for $59. I have had two of the Inmac protectors smoked (burned up) by lightning strikes but they saved the modem in each case. If some one knows of a better protector I would be interested. I am on the end of a long telephone line that runs through a valley. Whenever a storm hits the valley, our phone rings on every strike. However, only when we get a strike in our yard do we get the protectors smoked.
henry@utzoo.UUCP (Henry Spencer) (09/25/87)
> > Telephone lightening arresters are commonly used on all circuits these > > days... > > Yes, the phone company installs primary protectors where the wires enter > your home. These are designed to keep the voltage on the phone line from > rising above 600 to 1000 volts depending upon the model, and the number > of times that it has been zapped. I found out the hard way that this is > not good enough to protect some modems... It is worth knowing that the phone company does not give two hoots about what happens to *your* equipment. They are only interested in protecting their own; that is what those protectors are for. And by the way, they care a lot more about the lines and the exchanges than about the phone in your house, even if you are renting it from them. Keeping your equipment intact is strictly your problem. -- "There's a lot more to do in space | Henry Spencer @ U of Toronto Zoology than sending people to Mars." --Bova | {allegra,ihnp4,decvax,utai}!utzoo!henry
henry@utzoo.UUCP (Henry Spencer) (09/30/87)
> It is worth knowing that the phone company does not give two hoots about > what happens to *your* equipment. They are only interested in protecting > their own; that is what those protectors are for... I've been taken to task for this in private mail, and should amend it slightly. The main purpose of the phone company's protectors on lines is prevention of shock and fire hazards. Protection of subscriber-end equipment actually did play some role in setting their protection standards, but the equipment they were thinking of was the phones and modems that were current at the time... generally rather more robust than much of today's equipment. -- "There's a lot more to do in space | Henry Spencer @ U of Toronto Zoology than sending people to Mars." --Bova | {allegra,ihnp4,decvax,utai}!utzoo!henry