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From [e-mail] (Howard Hull) Thu Aug  1 10:22:25 1985
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From: [e-mail] (Howard Hull)
Newsgroups: net.dcom
Subject: Lightning Protection (long)
Message-ID: <[e-mail]>
Date: 31 Jul 85 23:22:25 GMT
Date-Received: 13 Aug 85 02:27:15 GMT
Distribution: net
Organization: High Altitude Obs./NCAR, Boulder CO
Lines: 199

Protecting one building or tower from lightning is fairly straightforward:

    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 importantly,
       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
       electric 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 discontinuities.  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
       characteristic compared to, say, a Silicon TransZorb.  They have
       several advantages: They are cheap.  They come in packaging that
       is familiar to professional electricians.  They are generally
       more robust than Selenium 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
       Transformer.  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 sensitive 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, remember 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 thousands
       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
       windings are tied to the stake, and short out the electric field
       that might otherwise couple to the secondary.  Saturation of the
       transformer core protects the differential mode.  The differential
       protectors 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
       thermally 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 neighbor'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 building 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 positively 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".

Well, I've done it again.  Darn.  If this is too long, I suppose you should
flame me for it, or if I am guilty of mis-representing known (un)truths, that
would qualify as well.  But I wanted to at least try to clear up the nature
of lightning and its hazards a little.
                                     Howard Hull
[If yet unproven concepts are outlawed in the range of discussion...
                   ...Then only the deranged will discuss yet unproven concepts]
        {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull


--
Dave Horsfall (VK2KFU), Alcatel-STC Australia, [e-mail]
[e-mail], ...munnari!stcns3.stc.OZ.AU!dave
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