EH-13 Effective Grounding
                        ENVIRONMENT, SAFETY & HEALTH

                                   BULLETIN

Assistant Secretary for                              U.S. Department of Energy
Environment, Safety, & Health                        Washington, D.C. 20585
Office of the Deputy Assistant Secretary
for Environment, Safety, & Health

DOE/EH-0001 (Rev.)                Issue No. 13                     August 1985

Since 1975, 41 reportable losses from lightning (totaling almost $445,000 in
property damage) have occurred at DOE facilities.  While few have occurred in
buildings, the magnitude of those few illustrates that there is more to a
lightning protection system than is apparent to the uninitiated.  The
following article stresses that proper design, installation, and maintenance
are essential.

Effective Grounding - The Goal And Crux Of A Lightning Protection Scheme

The vital aspects of a grounding system's five key subsystems must be
analyzed and attended to.

  Not long ago an electrical consultant, on behalf of an insurance
underwriter, was looking for recourse in the loss of a large lightning-
ignited wood frame building.  Nothing remained but charred remnants and ashes,
but the consultant had two leads:

  o The building's electrical circuit-breaker box had been found at a
spot 30 ft from the point where it had been fastened to an inside wall.

  o A neighbor, who had heard lightning's deafening crash of thunder and had
gone outside to watch, reported that the entire building seemed to burst into
flame almost at once.

  Which of the two leads led to the accepted conclusion?

  Had the circuit-breaker box been literally blown off the wall by a
tremendous surge of lightning current, a surge which, due to improper
installation of the electric power system, had not been grounded?

  Or did the eyewitness report indicate flaws in the lightning-rod system and
therefore affirmation of the common assertion that "an improper lightning-
protection system can under certain conditions do more harm than good"?

  The answers to both questions lay underground.  The electrical consultant
was advised to comb through the rubble until he was assured he had found all
grounding devices associated with both the lightning protection and the
electrical power systems.

  Conditions for creating a mechanical force sufficient to hurl a breaker box
off a wall and 30 ft away appeared to be absent, and as expected, the power
company's ground rod was found to have been properly installed and connected.
The insurance company's chances for recourse from those associated with the
design and the construction of the electric power system were reduced to a
glimmer.

  The lightning protection system's ground, on the other hand, was found to
be woefully substandard.  There were only two ground rods, their tops
protruding about 18 inches out of the ground.

  The soil on the site is laced with sand, and rainfall in the area is less
than the U.S. average.  Proper grounding for the building's lightning
protection system required a minimum of four ground locations in addition to
a tie-in to the electrical ground because of the presence of high-resistance
sand.  Each ground should have been enhanced beyond minimum standards by
sinking multiple ground rods, buried copper plates, or other copper material.
An ideal installation would have consisted of a buried copper counterpoise
cable encircling the building, tying in all down conductors, thus assuring
uniform ground potential.  Ground contact could have been enhanced with copper
ground plates or ground rods at each corner of the structure or at each
downlead location.

  Since fire ensued, the lightning flash that struck the building was a "hot
bolt" with a long duration current that provided enough heat transfer to
ignite the flammable wood framing and composition roofing.

  Igniting the building was not the lightning's intent; it simply wanted to
use the lightning protection system as a low-resistance route toward its
predetermined destination - ground, where its heavy flow of negative polarity
current could dissipate in positively charged earth.  What the bolt found at
ground level, however, was dry soil of poor conductivity.  The two 8-ft long,
1/2-inch diameter ground rods offered only 6-1/2 ft each of underground
contact -- not nearly enough contact even in moist clay soil.

  Therefore high heat was transferred to flammable material all along the
roof conductors and the down conductors of the lightning-protection system.
Dissipation of the lightning current took place above ground (along the
conductors) as well as below ground.

  The incident dramatically illustrates the importance of having a grounding
system that provides (1) low electrical resistance, (2) adequate dissipation
area according to the amount of current expected in relation to the total
amount of soil resistance, (3) a degree of balance among the resistances
encountered at different ground locations on the site, and (4) a building
lightning protection system that will intercept the lightning flash, divide
the current, and conduct it safely past the building's structure, contents
and occupants.

The Lightning Flash

  During a thunderstorm, air turbulence, produced when a cold front uplifts
warmer moist air, causes a separation of electrical charges in the
atmosphere.  Negative-charge centers form in the lower portions of
thunderclouds and postive charges accumulate in the upper portions.  The
earth, which carries a slight negative polarity in normal weather, takes on
an increasingly strong positive charge as turbulence continues.

  Lightning may begin to flash first between oppositely charged cloud areas.
Such cloud flashes outnumber cloud-to-ground flashes by three or four to one.

  A flash to earth may be initiated when a local discharge between a
thundercloud's negative region and a small positive area at the cloud's
leading edge knocks loose electrons which have been attached to water drops
or ice crystals.

  The free electrons are attracted by the strong positive field directly
below, and avalanche downward in discrete steps averaging about 150 ft in
length.  As the electron stream zigzags downward, positive-point discharge
streamers strain upward from sharp earthbound objects below - the tops of
trees, ends and corners of buildings, chimneys, poles and even blades of
grass.

  When the negative stepped leader stroke nears earth, a dominant positive
streamer shoots up to meet it, completing an ionized path between cloud and
ground.  Immediately, there is a massive, highly luminous return stroke.

  The return stroke consists of an extremely fast downward flow from the
cloud of the negative charge that was laid along the ionized channel during
the stepped leader stroke.  The massive current flow is initiated when the
upward positive streamer connects with the downward negative stepped leader
stroke using the ionized path as the conductor.  The downward flow of current
starts at the lower end of the ionized column.  The current is flowing
downward, but the luminosity proceeds upward and is caused by the tremendous
heat and the extremely rapid movement of the air lying in the path of the
ionized column.

  Very few lightning flashes consist of a single downward leader stroke, an
upward streamer, followed by the downward return stroke (this complete
sequence is called a pulse).  There are usually subsequent pulses.  A typical
destructive lightning flash consists of three pulses, all occurring in about
0.4 seconds. The peak current flow of a typical flash is 25,000 A at 30
million V (Fig. 1).

  Current and voltage magnitudes vary widely, from a few hundred amperes and
a few million volts in weak lightning flashes to one reported flash of
340,000 A and a voltage estimated to reach 100 million V.  A "cold bolt" is a
quick zap, while a "hot bolt" may contain 20 pulses or more and endure for as
long as a second or more.  Recently, the National Aeronautics and Space
Administration monitored a flash consisting of 48 pulses.

Lightning's Destructive Effects

  Lightning flashes have three potentially destructive effects:

  o Thermal effect.  Fire is a common result of heat up to 50,000 degrees F or
more generated in the lightning channel.  Thunder is caused when superheated
air around the channel expands at supersonic speed.  Lightning's heat converts
moisture in building materials to high-pressure steam, sometimes causing
concrete and mortar to spall, crack or disintegrate, and lumber, fiberglass
and similar materials to split, splinter or shatter.  This high heat also
vaporizes, melts or welds electrical and electronic components.

  o Electrical effect.  Lightning side-flashes may occur.  These are
electrical sparks between conductors and nearby unconnected conductive bodies.
Sometimes these may cause problems.

  o Mechanical effect.  Lightning's powerful current flow attempts to
straighten out a conductor in the same way that water under pressure tries to
straighten a garden hose.  Magnetic or electrical induction may also create
and impose physical force under certain conditions.

Design Factors

  Each lightning-protection system must be custom designed to the structure
it is to protect.  Vital design factors are a building's height, configuration
and construction materials.

  Height.  The tallest building in the immediate area is the likeliest target,
and its height determines the degree of shielding a lightning-protected roof
might provide to a lower roof or to an adjacent lower structure.

  The protected zone of a building up to 25 ft high is contained within a
line struck at a 1 to 2 ratio (vertical to horizontal).  A building 25 ft to
50 ft high offers a 1 to 1 protected zone.  Buildings taller than 50 ft
provide protection to ordinary structures within an arc of 150-ft radius
(Fig. 2).

  Height also affects system component size and strength requirements.
Structures up to 75 ft high may use Class I materials (as defined by LPI, the
Lightning Protection Institute, in its publication LPI-176), including
crimp-type connectors for example.  Class I copper cable must weigh at least
187.5 lb/1000 ft according to the material standard, LPI-176.

  Configuration.  A gable roof with a span of less than 40 ft and rise of at
least 1 ft to a run of 8 ft only needs air terminals atop the ridge.  So does
a gable roof with a span of more than 40 ft and rise of 1:4.  Objects that
interrupt a roof's or wall's silhouette require special design scrutiny to
assure that the lightning-protection scheme does not deprive any of the main
structure's air terminals of two paths to ground.  By requiring a minimum of
two paths to ground, there is assurance that the lightning current will be
divided and that it will be led to at least two ground terminal locations.

  Construction Materials.  The materials used in framing and/or cladding a
building affect lightning-protection system design in a number of ways, such
as:

  o Copper lightning-protection systems cannot be installed on aluminum-clad
structures because of potentially harmful electrolytic action between
dissimilar metals.

  o For the same reason, a bimetal connector must be used when an aluminum
conductive body is bonded to a copper conductor system or vice versa.

  o Electrically continuous steel columns may be substituted for
lightning-protection system downconductors.

  o When steel columns serve as downconductors, allowable ground rod spacing
is decreased from the standard 100 ft to every other perimeter column
or 60 ft.

  Buildings housing explosives are not included in the Lightning Protection
Code (ANSI/NFPA-78) nor in the Installation Standard LPI-175.  Such structures
are usually protected with grounded overhead cable systems that shield the
structures from sideflash current and from any contact with the main stream
of lightning current.

  Buildings housing flammable liquids and gases require a 1 to 1 ratio for a
zone of protection for any structure up to 50 ft high.  For structures more
than 50 ft high, the protected zone is determined by striking a 100-ft arc --
as opposed to the more liberal 150-ft arc permitted for buildings without
such critical contents (Fig. 3).

The Five Key Subsystems

  Lightning-protection design can be simplified by dividing the system into
five segments.  These are the key subsystems and are listed in the preferred
order for developing a lightning-protection scheme.

1. Roof System. First, air terminals extending a minimum of 10 inches above
protected objects should be placed at a maximum distance of 20-ft intervals
(or 25 ft if rods exceed 2 ft in height) on high points of tile roof within 2
ft of flat roof edges, within 2 ft of corners and within 2 ft of gable ends
and other sharp roof features.  On large roofs, additional air terminals
should be installed at 50-ft grid spacings in mid-roof areas (Fig. 4).  On
very large roofs, cross-run conductors should be connected to mid-roof
conductors at 150-ft intervals to provide shortened paths to ground from
mid-roof terminals.  Care should be taken to provide two horizontal or
downward paths from each air terminal (except "dead ends," an air terminal
that has just one path to ground, such as those located on a lower roof that
is small enough to need only three or less terminals).  When there are
four or more air terminals an additional path to ground is required.

2. Conductor System.  Conductors are sized by weight according to height of
the protected structure and whether steel building columns are used as
conductors in lieu of downconductors.  For Class I structures (75 ft high or
less) copper conductors must weigh at least 187.5 lb per 1000 ft and
aluminum conductors 95 lb per 1000 ft.  For Class II structures (structures
over 75 ft high or those whose steel framing is used in lieu of
downconductors), conductors must weigh at least 375 lb per 1000 ft for
copper and 190 lb per 1000 ft for aluminum.  Conductors that tie in metal
bodies on the roof that are within the protective zone of an air terminal may
be of smaller size.  If copper cable is used as a secondary conductor, minimum
strand size is 17 AWG and cross section area is 26,240 sq cm, or a No. 6 solid
copper wire may be used.  If aluminum cable is used an increase in size will
be necessary to maintain required conductivity and strength.

  Two downconductors at diagonally opposite corners are required on any
building regardless of size, up to perimeters of 250 ft.  An additional
downconductor is required for each additional 100 ft of perimeter.
Preferred downconductor locations are at roof corners, which are most often
struck.

  When steel Columns are used as downconductors, spacing is reduced to
every other perimeter column or 60 ft, whichever is less.

3. Grounding System.  A grounding system connection must be located at each
downconductor, and the lightning protection grounding system must be
interconnected with utility grounds.  The type and configuration of a
grounding system depends on the resistivity, moisture content and
penetrability of the soil, and the degree of tolerance of sensitive contents
and/or equipment to heightened electric fields during a lightning discharge
to the building.

  For example, extremely low ground resistance may be needed for a structure
housing volatile substances or fragile electronic gear.  Aside from those
special considerations, these are minimum grounding requirements:

  o In moist, easily penetrated soil, such as clay loam, individual 8-ft
copper, copper-clad or stainless steel ground rods driven to 10-ft depths are
required (Fig. 5).  Class I buildings require 1/2-inch diameter rods; Class II
structures call for 5/8-inch diameter rods.

  o In sandy or gravelly soil, additional ground rods driven to 10-ft minimum
spacings in-line or triangularly may be required (Fig. 6).

  o Where bedrock near the surface prohibits driven rods, copper conductors
may be laid in 12-ft long, 1-ft minimum depth trenches extending out from the
wall in clay soil and in 24-ft long trenches in sandy or gravelly soil
(Fig. 7).

  o In shallow soil, a copper Class I or larger counterpoise cable should be
laid in trenches, rock crevices or directly on the bedrock to encircle the
building (Fig. 8).

  o In extreme cases, extra metal, preferably copper or, alternatively,
stainless steel, can be placed in hollows or depressions to enhance
grounding (Fig. 9).

  A number of areas in the U.S. present extremely difficult grounding
conditions such as at Orlando, FL, where ground rods can be driven up to 100
ft deep without success in achieving acceptably low resistance.  At Pensacola,
ground rods have been driven 150 ft deep or more without achieving resistance
readings less than 150 or 300 ohms.

  Where deep-driven rods can be expected to reach moist soil at some
reasonable depth, the costs may be acceptable, but in extreme cases,
alternatives should be considered.  For example, it may be better and
considerably less expensive to trench extensively for a buried counterpoise
cable as an alternate to the required number of single ground rods in deep,
hard dry soil.  The counterpoise cable, encircling the building, provides
great length for dissipating the current into the ground and serves to
equalize the electric potential as well.

  Examples of difficult grounding conditions include:

  o The Visitors Center at Estes Park, CO, sits on bald bedrock.  The
lightning protection system's downconductors were brought around to one side
then run down a slope for about 800 ft to an available pocket of silty soil.
The soil was ponded to hold water, and an acceptable soil resistance reading
was achieved.

  o The Grand Hotel at Grand Canyon has a counterpoise grounding system with
cables fastened to rock and run down to where they could be covered with
trucked-in soil.

  o A cliff-edge residence at Los Alamos, NM, could only get ground at one
corner; so all downconductors were tied together, and a cable was run about
50 ft to a very large buried ground plate.

  o An 80,000-sq ft building at Austin, TX, has a grounding system consisting
of a counterpoise cable with 0.032-inch thick, 3-ft square copper ground
plates connected to the cable at 50-ft intervals.

  The principle objective is to achieve ground-testing instrument readings of
less than 50 ohms.  Fortunately, lightning is cooperative.  According to
grounding experts, ionization of soil around a driven rod or buried cable
during a lightning discharge results in less actual resistance than readings
obtained by low frequency testing equipment would indicate.

  4. Bonding System.  Certain rooftop vents, stacks and air handling units are
examples of metal bodies that contribute to lightning hazards because they
are grounded or assist in providing a path to ground for lightning currents
(Fig. 10).  Such metal bodies need to be bonded to the lightning protection
system.

  Metal bodies that are objects located at or above roof level, and not
within the protective zone of an air terminal, may be struck directly and
must therefore be bonded with a main-size conductor.  If such an object has a
metal skin less than 3/16-inch thick, it can be punctured by lightning's
tremendous heat and must be fitted with an air terminal to prevent
burn-through.  There are air terminals with special mounting bases available
for this purpose.

  Ungrounded metal objects located below the roof and safe from a direct
lightning strike may be hazards in extreme cases.  A decision on bonding of
such metal bodies is based on ground resistance, induction, which is related
to building height, and on the amount of electrical interconnection between
grounded conductors present along that height and the effectiveness of the
secondary metal body serving as part of an alternative path for the current
to reach ground.

  Lightning-protection design usually excludes bonding of metal bodies
located between roof and ground.  If such bonding is deemed desirable or
necessary, it should be clearly defined, and the extra cost should be
understood and accepted by the client.

  5. Surge Suppression System.  After electrical power was introduced as a
commodity in the 19th century, gas or air gap type arresters were introduced
to intercept and shunt to ground damaging waves of current induced into power
lines by distant lightning strikes.  Such arresters are recommended as
integral parts of modern lightning protection systems.

  With today's proliferation of electronic devices and systems that may be
sensitive even to airborne transient voltages from distant lightning flashes,
extremely fast-response current wave shaping and suppresion devices have
become parts of a total lightning protection scheme.

  Surge suppression is not included in ANSI/NFPA Lightning Protection Code,
NFPA-78 nor in the Installation Standard, LPI-175.  It is recommended that
the selection and placement of protective devices for electronic equipment be
carefully studied to make sure the right type of surge suppression device is
used based upon the equipment being protected.

Ground Resistance Measurement

  Low ground resistance is essential to good lightning protection, and
resistance readings should be taken, particularly where large buildings are
located on excavated soil and in cases where the soil is sandy, gravelly, or
shallow due to high bedrock.

  To determine ground resistance, the lightning protection system should, if
possible, be isolated from any other ground connection.

  If the building is small and the lightning protection system can be totally
disconnected from any other ground connections, such as electrical,
resistance can be measured by a three-point technique.  In this technique,
test probe B is located at a convenient distance from the ground rod or
ground rod system (rod A) to be tested, and an auxiliary current probe C is
located at a greater distance from A.  Distance AB should be about 62% as
great as distance AC.  The latter distance (AC) should be no less than 50 ft.

  An alternating current (I) is passed through the electrode or electrodes
to be tested through test probe B and through auxiliary test probe C.  The
voltage (V) between points A and B is measured by test equipment that also
monitors auxiliary probe C and calculates the ground resistance as V / I.

  If the building's grounding system is complex, the resistance of single
ground rods may be measured.  The average single ground rod resistance (Rm)
must be multiplied by a factor depending on the number of ground rods (N)
spaced at least 35 ft apart.  The total ground rod resistance, R, can be
calculated from the formula:
       R = (1.1)(Rm) / N.

Summary

  Lightning protection is frequently a standard design feature for a new
project.  Actually, each project has its own peculiar characteristics that
require special study.  Soil conditions, building height and configuration,
treatment of the grounding conductor system connection with the electric
power grounding system and other considerations must all be evaluated.  A
good lightning protection system protects your client's project and avoids
potential problems for you.



------------------------------------------------------------------------------
Bulletin is published to provide DOE managers and contractors with
information relevant to DOE programs and operations.  For more information,
contact Nona Gilbert Shepard, Editor, Office of the Deputy Assistant
Secretary for Environment, Safety & Health, U.S. Department of Energy,
Washington, D.C. 20545; telephone FTS 233-2958; Comm. (301) 353-2958.


By Marvin M. Frydenlund, Electrical Consultant Magazine, May/June 1985.
  Reprinted by permission.
Mr. Frydenlund is Managing Director of the Lightning Protection Institute.
.
.