🚀 go-pugleaf

>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