Engineering Encyclopedia
Saudi Aramco DeskTop Standards
ELECTRICAL GROUNDING
OF BUILDINGS AND STRUCTURES
Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees.
Any material contained in this document which is not already in the public
domain may not be copied, reproduced, sold, given, or disclosed to third
parties, or otherwise used in whole, or in part, without the written permission
of the Vice President, Engineering Services, Saudi Aramco.
Chapter : Electrical
File Reference: EEX-205.03
For additional information on this subject, contact
PEDD Coordinator on 874-6556
Engineering Encyclopedia
Power System Grounding
Electrical Grounding
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TABLE OF CONTENTS
SECTION
PAGE
INTRODUCTION............................................................................................................. 4
APPLICABLE STANDARDS AND CODES ..................................................................... 4
STATIC ELECTRICITY GROUNDING ............................................................................ 5
Causes of Static Electricity ........................................................................................ 5
Conditions for Buildup of Static Electricity.................................................................. 6
Material Characteristics ........................................................................................ 7
Speed of Separation............................................................................................. 7
Area in Contact..................................................................................................... 7
Effect of Motion Between Substances .................................................................. 7
Atmosphere Conditions ........................................................................................ 8
Hazards of Static Electricity and Control in Various Areas......................................... 8
Agitators, Stills and Similar Equipment................................................................. 8
Belts-Pulleys......................................................................................................... 8
Pipelines-Manifolds .............................................................................................. 9
Steel Equipment and Process Units ..................................................................... 9
Tank Cars-Loading Racks-Spur Tracks................................................................ 9
Tanks-Atmospheric............................................................................................. 10
Tanks-Floating Roof ........................................................................................... 10
Tankers and Barges - Marine Facilities .............................................................. 11
LIGHTNING PROTECTION GROUNDING ................................................................... 13
Nature of Lightning................................................................................................... 13
Lightning Rod ..................................................................................................... 16
House Protection ................................................................................................ 17
Why does lightning hit trees?.............................................................................. 18
Current Discharge .............................................................................................. 18
Equipment and Structures to be Considered ........................................................... 19
Risk Index ................................................................................................................ 19
Equipment/Structure Classifications ........................................................................ 19
First Class........................................................................................................... 20
Second Class ..................................................................................................... 20
Third Class ......................................................................................................... 20
Fourth Class ....................................................................................................... 21
Fifth Class .......................................................................................................... 21
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Requirements for Good Lightning Protection ........................................................... 22
Component Parts of a Lightning Protection System................................................. 23
Lightning Protection Practices ................................................................................. 24
General............................................................................................................... 25
Tanks and Tank Farms....................................................................................... 25
Non-Conduction Heavy-Duty Stacks .................................................................. 27
Steeples/Minarets............................................................................................... 27
High Masts.......................................................................................................... 28
Power Stations and Substations......................................................................... 28
Communication Towers...................................................................................... 29
Supplemental Reading............................................................................................. 30
BUILDING AND STRUCTURE GROUNDING REQUIREMENTS................................. 31
Residential Building ................................................................................................. 32
Industrial Building..................................................................................................... 32
Manned Structures................................................................................................... 32
Unmanned Structures .............................................................................................. 32
Distribution and Utilization Equipment ..................................................................... 33
Fences ..................................................................................................................... 34
OFFSHORE PLATFORMS ........................................................................................... 35
WORK AID 1: SAUDI ARAMCO AND INDUSTRY STANDARDS,
DRAWINGS AND CODES APPLICABLE TO STRUCTURAL GROUNDING..... 36
WORK AID 2: CALCULATING THE LIGHTNING RISK INDEX ................................... 38
GLOSSARY ................................................................................................................. 39
BIBLIOGRAPHY ........................................................................................................... 43
ADDENDUM A: SAUDI ARAMCO DRAWING AB-036387 .......................................... 44
ADDENDUM B: NFPA 780........................................................................................... 45
ADDENDUM C: API RP 2003 ...................................................................................... 46
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LIST OF FIGURES
Figure 1. Lightning Bolt ................................................................................................ 15
Figure 2. Lightning Rods/Air Terminals ........................................................................ 17
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INTRODUCTION
This module examines the affects of lightning and static
electricity, how to protect buildings and structures and is divided
into the following sections:
•
Standards and codes applicable to grounding buildings
and structures
•
Static Electricity Protection
•
Lightning Protection
•
Building and Structure Grounding Requirements
•
Offshore Platform Grounding Requirements
APPLICABLE STANDARDS AND CODES
You should consult the following Saudi Aramco Standards and
industry Standards for answers to questions on building and
structural grounding, and lightning and static electricity
protection information:
•
SAES-P-111
•
National Electrical Code (NEC) (ANSI/NFPA 70)
•
NFPA 780
•
IEEE Std. 142 (Green Book)
•
API RP 2003
Refer to Work Aid 1 for a comprehensive list of applicable
equipment and structural grounding standards, drawings and
codes.
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STATIC ELECTRICITY GROUNDING
This section will discuss the hazards that static electricity can
create and the methods that are available to eliminate these
hazards. Specifically, the following topics will be discussed:
•
Causes of Static Electricity
•
Conditions for Buildup of Static Electricity
•
Hazards of Static Electricity and Control in Various Areas
Causes of Static Electricity
Static electricity is generated by the movement of electrons that
occur when unlike materials are in contact with each other and
are then separated. When two unlike materials are in intimate
contact, electrons from one material move across the interface
to the surface of the other material. The protons remain on the
original material. When the materials are separated, electrical
charges are produced on the materials.
If the two materials are good conductors, the excess electrons
will easily flow back to the material with the positive charge, and
there will not be a static electricity discharge. But, if either or
both of the materials are insulators and are not grounded, some
of the excess electrons will be entrapped when the separation
occurs, and the materials will be charged with static electricity.
The potential of the static electrical charge is related directly to
the amount of charge that is deposited on the material and is
inversely proportional to the capacitance of this material. The
relationship is expressed by the following equation:
V=
Q
C
where:
V = potential, in Volts
Q = charge
C = capacitance in farads
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The developed potential can continue to grow if there is
continuous charge generation. At some voltage level, the
leakage current will equal the rate at which the charge is being
generated, and a stabilized condition will be reached. If the
sparking potential is reached, sparking will occur.
The first person to experiment with static electricity was Thales
of Miletus, in 600 B.C. He found that amber, when rubbed on a
piece of cloth, would attract small, light objects (pieces of leaves
or dust).
Static electricity can be generated in the following situations:
•
Pulverized materials passing through chutes or
pneumatic conveyors.
•
Belt drives that use belts of non-conductive material.
•
Gas, steam, or air flowing through an opening.
•
Motion that involves changes in the relative position of
contacting surfaces.
•
The human body in a low-humidity area. This generation
can occur as a result of the contact of shoes with floor
coverings or by personnel working near machinery that
generates static electricity.
Conditions for Buildup of Static Electricity
The possibility that static electricity will be produced and the rate
at which static electricity will be produced depends on the
following:
•
Material Characteristics
•
Speed of Separation
•
Area in Contact
•
Effect of Motion Between Substances
•
Atmospheric Conditions
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Material
Characteristics
One of the materials or substances must have a higher
insulating property than the other material or substance to
generate static electricity. The amount of static electricity that
exists between two materials will be proportional to the
difference between the dielectric constants of the materials.
Speed of
Separation
As the speed of separation of two substances is increased, the
potential of the static electricity is increased.
Area in Contact
The area of the contact between the substances has a direct
bearing on the amount of static electricity. A larger contact area
allows a greater charge to be transferred from one substance to
the other. As the area in contact increases, the potential of the
static electricity increases.
Effect of Motion
Between
Substances
Static electricity is often thought to be a property of friction. This
misunderstanding occurs because ‘rubbing’ two materials
together will cause static electricity. This static electricity occurs
because the seemingly smooth items actually have peaks.
When the items are rubbed together, the area of contact is
increased. Increased motion will increase the amount of static
electricity.
Liquids that are sprayed from a nozzle can generate static
electricity, and liquids in a tank that are agitated (stirred) can
generate static electricity. These static electrical charges are
caused by the motion of the liquid against the stationary
components. Another very good example of static electricity
that is increasing due to motion is a belt and pulley. As the
speed of the belt increases, the amount of static electricity
increases.
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Atmosphere
Conditions
It is well known that humidity conditions are related to the
production of static electricity. As humidity increases, the
potential for static electricity decreases; therefore, the hazard of
static electricity increases in an operation that requires
controlled low-humidity conditions.
Hazards of Static Electricity and Control in Various Areas
The accumulation of static electricity on equipment, on materials
being handled or processed, and on operating personnel
introduces a potentially serious hazard in any area where
flammable liquids, gases, dusts, or fibers are present. The
discharge of the static electricity from an object to ground or to
another object can be the cause of a fire or an explosion if the
discharge takes place in the presence of readily flammable
materials or combustible vapor and air mixtures.
SAES-P-111 requires that tanks, tank cars, tank trucks, other
large storage containers, associated filling equipment, and other
equipment that can accumulate sufficient static electricity to
ignite hydrocarbon vapors shall be bonded and grounded in
accordance with API RP 2003.
Agitators, Stills
and Similar
Equipment
Vessels resting on earth, rock and oil, concrete, or brick
foundations, or on concrete or steel supports are adequately
grounded to prevent the accumulation of static electricity; no
special grounding devices are required. However, where
insulation exists between the vessel and the supports,
grounding must be provided.
Belts-Pulleys
Belts that are made of rubber, leather, or other insulating
materials, that are running at moderate or high speeds,
generate considerable quantities of static electricity. Generation
occurs when the belt separates from the pulley. The charges
will exist on the pulley (regardless of whether the pulley is
conducting or nonconducting) as well as on the belt.
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•
If the pulley is made of a conducting material, such as
metal, the charge will be dissipated through the shaft and
bearing to ground and offer no ignition hazard. Where
the machinery frame is insulated, or, the bearings are
composed of insulating materials such as nylon,
provisions for bonding and grounding are required.
•
A conductive belt or a belt made conductive through use
of belt dressings must be used to prevent the
accumulation of static charge. The belt dressings must
be renewed frequently to be considered reliable and
effective.
•
The use of flat belts in hazardous areas must be avoided.
The risk of static ignition from V-belts is negligible.
•
Static combs are ineffective in draining off the static
electrical charge and should not be used.
PipelinesManifolds
Permanent bonds between the separate lines in the piping
system or between piping systems must be provided at tank car
racks and in buildings where volatile materials are handled.
Steel Equipment
and Process Units
The requirements for preventing a static electrical charge from
accumulating on steel equipment and process units and the
subsequent discharge are as follows:
•
Process equipment, (mainly steel vessels resting on steel
or concrete structures) is required to be adequately
grounded to prevent the accumulation of static charges.
•
Where electrical devices are installed on process
equipment, grounding must be provided in accordance
with NFPA 70 (NEC).
Tank CarsLoading RacksSpur Tracks
The requirements for preventing a static electrical charge from
accumulating on tank cars, loading racks and spur tracks and
the subsequent discharge are found as follows:
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•
Tank cars are considered to be adequately grounded
through the rails to prevent any hazardous accumulation
of static charges on the tank body.
•
Where a tank car is unloaded or where rack installations
are unloaded, a bond wire must be provided between the
nearest rail and fill line or to the rack structure. A number
of fill pipes can be electrically connected, and a single
bond wire from the group can be attached to the rail.
•
No additional bonding of the tank car is required because
the car is adequately bonded to rails.
•
To prevent arcing where stray currents are likely to occur,
the rails must be bonded to each other. This bond must
be a conductor not smaller than 25 mm2 (No. 4 AWG),
and it must be an adequate ground.
•
Additional protection against stray currents must be
provided through installation of insulated pipe joints
between the loading and unloading facilities and the
connecting yard piping.
•
If the spur track is connected to a railroad equipped with
rail-circuit signal systems, the spur must be isolated by
rail joints that are insulated.
TanksAtmospheric
The shells of petroleum product storage tanks must be
grounded at a minimum of four points that are spaced
equidistantly around the base of the tank. Each point must be
bonded to the area ground grid or to a ground rod. The
resistance between the tank shell and remote earth must not
exceed 10 ohms.
Tanks-Floating
Roof
The requirements for prevention of the accumulation of a static
electrical charge on tanks with a floating roof and for prevention
of the subsequent discharge are as follows:
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The roof seal must be maintained to provide a tight closure that
reduces the chance of a vapor ignition at the seal. The
possibility of vapor ignition at the seal must be further reduced
by the installation of metallic shunt strips at each pantagraph
hangar in the sealing mechanism from the roof to the tank shell.
These metallic shunt strips must be spaced a maximum of 3m
(10 ft.) apart and must be bolted to the sealing ring and to the
roof per Standard Drawing AB-036387, which is located in the
Addendum. The metallic shunt strips and the roof also must be
bonded to the tank shell.
Tankers and
Barges - Marine
Facilities
The requirements for prevention of accumulation of a static
electrical charge on tanks and barges and for prevention of the
subsequent discharge are as follows:
•
Insulated flanges and insulation for gangplanks must be
provided at marine terminals where stray currents can
enter a tanker or a barge via gangplanks or piping as
follows:
- Leakage from power systems where return circuits
through the earth can cause currents to flow through
nearby piping in contact with the earth.
- Potentials generated by galvanic action through
contact between piping and certain types of soil.
- From cathodic protection systems.
•
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Separated bodies and insulated flanges can become
electrostatically charged when the product flows through
the loading arm or hose. Such flanges must be bonded
to the pier and/or ship piping. All metal on the shore side
of the insulating flange must be electrically continuous
and grounded via the dock piping, and all metal on the
ship side must be electrically continuous and grounded
via the ship piping.
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•
Where cathodic protection is not provided, and where
conductive hoses or metallic loading arms are used,
insulating flanges must be permanently installed between
the loading hose and pier piping. These insulating
flanges will electrically insulate the ship from the pier
piping.
•
Where cathodic protection is provided on submarine
loading lines, insulating flanges must be provided on the
shore end of the submarine lines. At least one joint of
the loading hose must be certified by the manufacturer to
be electrically nonconductive. Submarine lines used for
crude or fuel oil cannot accumulate static charges on
isolated flanges due to high electrical conductivity of
these oils. Two grounding connections on separate
platform legs must be provided for grounding barges.
•
An insulated flange must have an insulated material
between the standard flange faces. Each flange bolt
must be encompassed by an insulating sleeve and must
have insulating washers at both ends of the bolt.
•
Insulating gaskets, bushings and washers must be of a
material that is electrically nonconductive and
nonhygroscopic on all surfaces. Flange edges must be
sealed with a 50 mm (2") wide polyethylene pipe wrap
tape.
•
Gangplanks must be insulated at the pier end or at the
ship end or at both ends. As an alternative, the
gangplank can have an insulating joint between pier end
and ship end. The insulation must be provided through
use of rubber tires, rubber rollers, rubber mat or
insulating joints similar to the method for use with
insulating pipe flanges.
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LIGHTNING PROTECTION GROUNDING
This section will familiarize you with the hazards to life and
property that are created by lightning, a form of static discharge,
as well as the grounding methods used to reduce the dangers.
This section includes the following information:
•
Nature of Lightning
•
Equipment and Structures to be Considered
•
Requirements for Good Protection
•
Practices for Lightning Protection
Nature of Lightning
Lightning is the discharge of high-potential static charges
(usually negative) within clouds or from a cloud to the earth
(Figure 1). These static charges normally attract the charges of
opposite polarity on the surface of the earth or on tall objects.
When the static charge reaches a critical level (when the air
insulation between the cloud and the earth breaks down), the
static charge develops a stepped ionized path, resulting in a
high current discharge (stroke) that neutralizes the cloud charge
and earth charge.
Lightning occurs in thunderstorms. Updrafts carry water
particles and raindrops up. The motion of the drops through the
air causes a static charge to build up.
Initially, the atmosphere and the ground are neutral, but then
they build up a charge potential between them, this happens
because the air is an insulator. Eventually the charge potential
becomes so great that you get conductance -- the air insulator
breaks down.
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The thunderstorm continues to build up a charge potential
between the ground and the clouds -- if it's greater than what
the atmosphere can resist, then there is a breakdown and an
electric current flows. This varies with factors that make the
atmosphere more or less conductive – e.g. humidity -- more
moisture in the air column makes the air more conductive less
resistive -- the breakdown potential is lower for this air than for
dry air.
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Figure 1. Lightning Bolt
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Lightning is lazy -- it wants to take the path of least resistance, it
doesn't want to do a lot of work. A typical lightning flash is 7 to
10 miles long. It does its job in 50 meter steps -- it's solving little
bits of the equation to decide which way to go for each step of
the way in millionths of a second for each step.
The last 50 meters of the pathway is special -- the ground
actually comes up to meet it. It's like the cloud has a long arm - it's reaching out for a handshake -- the ground is reaching up
with a short arm -- and then they shake hands. Once they do,
it's like a wire for the electrons -- they now have a path. The last
bit, the handshake from the ground -- is called a "dart leader" -whereas the "step leader" comes down from the clouds.
After the path is established, the current then moves up the
channel from the ground, as electrons move down the channel
from the clouds. The current moving up is the high luminosity
that you see from a lightning flash, although you perceive it as
light traveling downwards. The pathway is established
downward, but the bright light comes from Earth's surface and
travels up towards the clouds. 90 percent or more of the flashes
travel in this way.
Now, let's look at what happens when you add lightning rods (air
terminals) to the equation. In the case of a house, it'll be easier
for the lightning to find a path if it finds something that is less
resistant or more conductive than air -- so if its near a house,
then conduits or wires help out with that. But with a lightning
rod, perhaps it will only travel along that path to the ground. If
you're the tallest thing around, then you're more likely to get hit
than other things.
Lightning Rod
In 1752, Benjamin Franklin invented the first lightning protection
system, the lightning rod.
A lightning rod or air terminal is a metal rod sticking up into the
sky, with a conductive wire copper wire running from the rod
down the structure and into a ground rod or plate (Figure 2).
What makes lightning rods good, among other things, are very
sharp, pointed metal rods and good copper wiring downleads
and solid copper rods ground rods driven into the ground.
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Figure 2. Lightning Rods/Air Terminals
In terms of lightning protection, there are two philosophies:
1. With a lightning rod, you're providing an easier path for the
lightning to follow. It provides a pathway preferential to bricks.
2. The other theory is that you're keeping the electric potential
from building up. One of the things the lighting rod can do is
bleed off some of the charge that might build up at the ground
instead.
So, either the lightning rod dissipates the charge so you don't
get a strike, or actually channels the lightning safely.
But, charge builds up quickly in a storm -- so if your rod is not
big enough, it won't bleed off enough in time and you'll get a
strike, or you'll get hit and not have the capacity to handle the
flash -- then the lightning can still cause some damage.
No lightning rod will guarantee that you won't be hit by lightning.
House Protection
If you're the tallest house in a neighborhood, you'll get hit more
often than the shortest house in the neighborhood. So, should a
person in an isolated house on a ridge invest in a lightning rod?
Yes, the lightning rod would attract lightning. So it makes some
sense because the lightning is trying to find the quickest route to
the ground
What if his house were in a crowded subdivision?
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If your house were in a neighborhood crowded with houses,
then you're still taking your chances without a lightning rod, but
then you've got more to worry about if you do get a lightning rod.
In an area with many houses potential targets, it is unlikely that
your house will get hit by lightning. If you put a lightning rod up,
then you are attracting lightning which might not otherwise strike
the area. This is the case anytime you put a conductor in an
electric field. So, now other houses around you are in more
danger from being hit or damaged and would have to get
lightning rods too.
Lightning is a strange phenomenon. It doesn't always listen to
logic. It doesn't always go for the tallest object or best conductor
in an area. It can do whatever it wants.
Why does
lightning hit trees?
Sometimes lighting will go straight through a tree, other times it
goes along the surface just under the bark. Why it hits a tree -if the storm is overhead, then the positive charges that build up
at the ground level are attracted to the negative charges
building up at the base of the storm clouds. The positive
charges collect on whatever is below the cloud. The negative
charges from the cloud base try to get to the ground by the
shortest route. I f the tree collects enough positive charges, then
it will be made part of this channel.
Trees are basically lightning rods -- good ones, actually. So, the
homeowner is surrounded by lightning rods up to the clearing by
the house. Lightning is "attracted" to sharp, pointed, conductive
objects like wet tree leaves and pine tree needles.
Current Discharge
The discharge current increases from zero to its maximum in 1
to 10µs, then declines to half the peak value in 20 to 1000 µs.
This discharge can be repeated one or more times over the
same path, in rapid succession, because of the recharging in
the cloud. The average peak stroke current is about 40,000 A,
although some peak stroke currents are as great as 270,000 A.
[IEEE Std. 142, para. 3.3.1.1]
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Equipment and Structures to be Considered
Per SAES-P-111 the following buildings and structures shall
always be provided with a satisfactory lightning protective
system:
•
Buildings and structures over 30 m (100 ft) in height.
•
Schools
•
Hospitals
Lightning protection for other buildings/structures depends on
their “Risk Index”.
Risk Index
NFPA 780 and BS 6651 provide information on how to
determine the ‘Risk Index.’
The Risk Index Tables are in NFPA 780. Each table has a list
of conditions. The Electrical Engineer selects the condition that
is correct for the building or structure that is being considered
for lightning protection and then records the risk figure for that
condition. Once the risk figure for all seven tables has been
determined, the Electrical Engineer sums the seven risk figures.
The total is known as the Risk Index.
SAES-P-111 requires a building/structure be protected if the
calculated Risk Index is four (4) or greater, when calculated
using NFPA 780, or the overall calculated risk is greater than 1
in 100,000 per year, if calculated using BS 6651, Appendix A.
Equipment/Structure Classifications
Equipment and structures can be separated into five
classifications according to the need for lightning protection.
These classifications are listed in IEEE Std. 142 and are as
follows:
•
First Class
•
Second Class
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•
Third Class
•
Fourth Class
•
Fifth Class
First Class
First class equipment and structures need very little or no
additional protection. This class includes the following:
•
All metal structures except tanks or other enclosures of
flammable materials.
•
Water tanks, silos, and similar structures that are largely
constructed of metal.
•
Flagpoles made of conductive material.
The only real requirement for this class is to connect the
equipment or structure to a suitable grounding electrode.
Second Class
Second class equipment and structures consist of buildings with
conducting surfaces and non-conducting framework, such as
metal-roofed and metal-clad buildings. This class requires the
addition of down conductors to connect the exterior roof and
cladding to suitable grounding electrodes.
Third Class
Third class equipment and structures consist of metal-framed
buildings with non-conducting facings. These buildings need
the addition of conducting air terminals that are suitably located
and connected to the frame. The conducting air terminals must
project beyond and above the facing in order to act as the
lightning terminal points and to thus eliminate the potential of a
puncture of the facing.
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Fourth Class
Fourth class equipment and structures consist of non-metallic
structures, either framing or facing. These structures require
extensive protection treatment. The following are examples of
fourth class structures:
•
Buildings that are constructed of wood, stone, brick, tile,
or other nonconducting materials and that are without
metal reinforcing members.
•
High stacks and chimneys. Even with reinforcing
members, these stacks and chimneys should have full
lightning protection treatment of air terminals, down
conductors, and grounding electrodes.
An example of this class at Saudi Aramco includes the stacks
for boilers.
Fifth Class
Fifth class equipment and structures consist of items of high risk
or loss consequences that normally receive full lightning
protection treatment, including air terminals or diverters, down
conductors, and grounding electrodes. This class includes the
following:
•
Buildings of great aesthetic, historical, or intrinsic value.
•
Buildings containing readily combustible or explosive
materials.
•
Structures containing substances that would be
dangerous if released by the effects of a lightning stroke.
•
Tanks and tank farms.
•
Power plants and water pumping stations.
•
Transmission lines.
•
Power stations and substations.
There are many examples of this class at Saudi Aramco.
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Requirements for Good Lightning Protection
Direct lightning protection (lightning protection systems) consists
of placement of air terminals (lightning rods) at the top perimeter
of the structure to be protected, and connection of the air
terminals by adequate down conductors to the grounding
electrodes (earth). The down conductor should not include any
high-resistance or high-reactance portions or connections and
should present the least possible impedance to earth without
sharp bends or loops. Steel-framed structures, which are
adequately grounded, meet these requirements with only the
provision for terminating the stroke on a metallic air terminal.
The metallic air terminal is connected to the frame structure, to
avoid the possibility of puncturing any roofing or siding to reach
the frame. In the absence of a steel framework, a down
conductor providing at least two paths to earth for a lightning
strike to any air terminal is generally adequate.
Air terminals that are attached to the structure itself are pointed
solid rods or pipes at least 10 inches (0.25m) long to possibly 2
feet (0.61m) long. On building edges, 10 inches (0.25m)
terminals should not be separated by more than 20 feet (6.1m),
and 2 ft. (0.61m) terminals should not be separated by more
than 25 feet. (7.6m). Fifty feet (15.2m) of spacing will suffice
within the periphery.
At least two down conductors should be provided on all
structures; only one down conductor is needed for masts,
spires, and flagpoles. The greater the number of down
conductors and grounding electrodes, the lower the voltage that
will be developed within the protection system, and the better
the protection. Every down conductor must be connected, at its
base, to an earthing or grounding electrode. This grounding
electrode should be within 2 feet (0.61m) of the base of the
building and should extend below the building foundation, if
possible.
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Interior metal parts of a non-metal-framed building within 6 feet
(1.83m) of a down conductor should be connected to the down
conductor. Exterior emergency ladders should also be bonded
to the nearest down conductor. On a flat-top building protected
by air terminals, all metallic parts and equipment that are
projecting higher than the air terminals (such as air-conditioning
equipment) should be bonded to the lightning protection system.
For high-rise buildings and towers, an equalizing horizontal
bonding loop should be installed approximately every 100 feet
(30m).
Component Parts of a Lightning Protection System
NFPA 780 provides detailed information on the components of a
lightning protection system. The principle components of the
lightning protection system are as follows:
•
Air Terminals
•
Down Conductors
•
Joints and Bonds
•
Ground Terminations
Air Terminals – or lightning rod is a metal rod typically made of
copper or aluminum. No part of a flat or gently sloping roof on
structures is to be more than 7.5 m (25 ft) from the nearest
horizontal conductor.
Down Conductors - Two or more down conductors must be
provided on most kinds of structures. One down conductor is
permitted for flag poles, masts, spires or similar structures. The
total number of down conductors on structures having a flat or
gently sloping roof, and on irregular shaped structures are to be
such that the average distance between the down conductors
does not exceed 30 m (100 ft). The bend in a conductor that
embraces a portion of a building, such as an eave, must have a
radius that is greater than 200 mm (8 in). The angle of any turn
must not exceed 90o, and the conductors must preserve a
downward or horizontal course.
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Enforcing rods that are butt-welded together are acceptable as
down conductors, but reinforcing rods that are overlapped and
bound with tye-wire are not acceptable as down conductors.
Down conductors should be installed within the building or the
structure to avoid the potential ‘removal for gain’ that can occur
with external copper conductors. In order to prevent lightning
from ‘jumping’ off the down conductor and to the conduit, down
conductors must not be installed inside a metallic conduit.
Joints and Bonds - Joints and bonds must be made to the
same standard as required for electrical installations.
Grounding Terminations - The earth resistance of all lightning
protection grounding terminations must be tested through use of
an earth tester that is to be clamped to any convenient part of
the lightning protective system. The combined resistance to
earth of the whole of the lightning protection system must be as
low as economically possible but must not exceed 25 ohms.
Other grounds, such as substation grids or consumer
grounding, must be bonded to the lightning protection grounds.
The intent of the grounding is to minimize the risk that is due to
differential voltages that could cause hazards to personnel or
‘sideflash’ possibilities. Reinforcing rods in reinforced concrete
foundations are not required to be bonded to the ground
termination.
Lightning Protection Practices
IEEE Std. 142, Section 3.3.4 provides the practices for lightning
protection. This information is divided into the following seven
(7) sections:
•
General
•
Tanks and Tank Farms
•
Non-Conducting Heavy-Duty Stacks
•
Steeples (Minarets)
•
High Masts
•
Power Stations and Substations
•
Communication Towers
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General
Buildings and structures involving hazardous liquids, gases, or
explosives require additional protection. In these buildings and
structures, the object of the additional protection is to keep the
current away from the structure without use of the building’s
metal skin or the framework as a down conductor. A separate
diverter protection system is employed for these buildings and
structures (e.g., tanks, tank farms, and explosive manufacture
and storage).
The diverter element consists of one or more masts, or one or
more elevated wires (between masts or poles), that meet the
requirements of lightning protection. The masts or poles are
normally at least 10 feet (3m) from any part of the structure to
be protected. Similarly, elevated wires that are above the
structure must remain not less than 10 feet (3m) above the
structure. Metal masts can act as grounding conductors. Wood
poles should have an air terminal securely mounted to the top of
the pole. Copper or copper-weld conductor should be provided
along the pole as a grounding conductor. The guy wires for an
elevated wire span can be designed to serve as grounding
conductors. Suitable earthing electrodes are necessary, as with
all other types of grounding conductors.
Tanks and Tank
Farms
Provided that the base of the tank is adequately grounded, a
tank that contains flammable liquids or gases does not always
need to be protected against lightning. Direct lightning strikes to
the tank top or walls are permitted as long as the steel is thicker
than 3/16 inches (0.476cm). These strikes are allowed because
there is little danger of the lightning strikes puncturing the tank.
Steel tanks with steel roofs and floating metal roofs are
generally considered to be self-protecting. Tanks with
nonmetallic roofs are not self-protecting and should be
protected with air terminals, conducting masts, or elevated
ground wires. In all cases, joints and piping connections should
be electrically continuous. All vapor or gas openings should be
closed or flame-proof. The possibility of a direct strike to the
vicinity of a vent or leak is eliminated by an air terminal of
suitable height.
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Refer to Saudi Aramco Drawing AB-036387 for grounding of
floating tanks, and API RP 2003 for additional information about
protection of storage tanks from lightning.
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Non-Conduction
Heavy-Duty
Stacks
Heavy-duty stacks (including stacks in petroleum and in
chemical plants) require air terminals that are connected to a
loop conductor around the top of the stack and at least two
down conductors that are connected to grounding electrodes at
the base of the stack. Air terminals should be made of solid
copper or stainless steel and should be uniformly distributed
around the top of cylindrical stacks, at intervals not exceeding 8
feet (2.44m). On square or rectangular stacks, air terminals
should be located not more than 2 feet (0.61m) from the corners
and should be spaced not more than 8 feet (2.44m) apart
around the perimeter.
The height of the air terminals for nonflammable stack gas may
be as little as 18 inches (0.46m). The height of the air terminals
for ventilating stacks that emit explosive gas or dust should be
not less than 5 feet (1.52m). The height of the air terminal
where the gas or dust is explosive and under forced draft should
be not less than 15 feet (4.57m). Also, the air terminals should
be tilted outward at 30o from the vertical. When the effluent is
corrosive, as in flue gas, a 1/16 inch (1.6mm) thick lead coating
on the air terminal is required. The loop is also kept below the
top of the stack.
Steeples/Minarets
Minarets (Manarah) are not directly covered in any SAES or any
standard drawing; however, minarets are tall structures and
shall be protected against lightening strikes.
Minarets are prone to lightening strikes due to their height and
usually they have metallic topping that attracts lightening. If the
minaret is made of conducting metals or house electrical
equipment , then equipment grounding is definitely required.
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High Masts
Equipment on the sides of very high masts, such as television or
FM antennas, can be protected from direct stroke damage
through the addition of lateral spikes or thorns projecting
outward from the sides of the mast. At heights above the critical
radius of 100 or 200 feet (30 or 60m), spikes in a horizontal or
near horizontal position with suitable spacing will cause strokes
coming from the side to terminate on the spikes rather than on
the mast itself. This practice will greatly reduce the possibility of
damage to electrically fragile components by the termination of
the lightning stroke arc. The number of spikes around the mast
(three, four, five, or six), the length of the spikes, the vertical
spacing along the mast need to be determined for optimum
economics, and in accordance with the principles of lightning
protection. When masts are installed on top of a building, the
bottom of the mast structure must be bonded to the building
grounding network at a minimum of two points.
Power Stations
and Substations
While transmission-line protection against lightning is an
inherent part of the design and is well documented, the
protection of power stations and substations has received little
attention. Lower power stations and substations require
protection from direct strokes. Masts or overhead static wires
(or both) can be used to ground lower power stations and
substations to the grounding network of the higher power station
or substation.
Protection of the attached overhead lines by means of an
overhead grounded conductor or diverter (static wire) for 2000
feet (610m) away from the power station or substation is
recommended to preclude direct strokes on this section of the
line and to reduce the duty on the power station surge arresters.
The spacing of this overhead grounded conductor or diverter
and the associated down conductors from the phase conductors
must not be less than the basic impulse insulation level of the
lightning protection system. Otherwise, side flashes to the
phase conductors will occur and cause unnecessary outages.
The installation of overhead grounded conductors is not
practical unless the attached overhead lines are 69 kV or above
(at Saudi Aramco only).
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Communication
Towers
The following is general requirements on lightning protection for
communication towers:
•
Communication towers must be grounded by two 35 mm2
(No. 2 AWG) conductors from points on diagonally
opposite tower legs. These conductors are to run as
directly as possible, but preferably by separate routes, to
the ground grid or other grounding electrode.
•
Towers at transmission substations or industrial
complexes are to located within the resistance area of
the installation either by proximity or by suitable
configuration of the buried grounding conductors.
•
Towers in remote locations will require a grounding
electrode. This electrode is not to exceed 2 ohms of
ground resistance. Ground rods will suffice in areas of
low soil resistivity; otherwise, a ground grid is to be
installed. Unless a power system that utilizes the ground
grid requires a larger conductor, a 35 mm2 (No. 2 AWG)
conductor is to be used for the grid.
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Supplemental Reading
Read the supplemental text Grounding and Shielding in
Facilities, pages 29, 46, and 85. These pages provide general
information on lightning protection for buildings, including the
following:
•
Grounding for lightning protection
•
Where lightning terminals are required.
•
Installation of lightning terminals.
•
Installation of ground cables.
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BUILDING AND STRUCTURE GROUNDING REQUIREMENTS
The following is a list of applicable standards, drawings and
codes that apply to Saudi Aramco buildings and structures:
•
SAES-P-111, Grounding
•
Standard Drawing AA-036572, Grounding Arrangement
for 115 kV Disconnect Switch Structure
•
Standard Drawing AB-036562, Standard Switch
Operating Platform
•
SAES-O-101, Standard Security Fence
•
SAES-P-100, Basic Criteria
•
SAES-T Series, Communications Standards
•
IEEE STD. 142, Recommended Practice for Grounding
of Industrial and Commercial Power Systems
•
IEEE 367, Guide for Determining the Maximum Electric
Power Station Ground Potential Rise and Induced
Voltage from a Power Fault
•
NFPA 76A, Essential Electrical Systems for Health Care
Facilities
•
NFPA 76B, Electricity in Patient Care Areas of Hospitals
•
NFPA 780, Lightning Protection Code
This section will familiarize you with the application of the basic
safety codes to different facilities. This section includes the
more important aspects of safety grounding for the following
types of buildings and structures:
•
Residential Building
•
Industrial Building
•
Manned Structures
•
Unmanned Structures
•
Industrial Plant Areas
•
Distribution and Utilization Equipment
•
Fences
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Residential Building
Grounding for residential buildings starts with the system
grounding at the service disconnect(s). Ground wires are then
run from the main service panel, with the power conductors, to
the equipment or the electrical outlets. System grounding is
normally accomplished through connection of the grounded
conductor and the grounding conductor to the grounding
electrode system. In the case of small residential buildings, the
grounding electrode system often consists of underground metal
piping and ground rods. Ground loops for grids are seldom
required for residential buildings.
Industrial Building
An industrial building is a facility in which products are
manufactured or stored.
Industrial buildings are usually part of a complex with large
power requirements. Substation grid grounding, building ground
loops or rings, and an extensive system of ground conductors
that tie all necessary items back to their respective power
source are used.
Manned Structures
Manned structures are facilities that are occupied during normal
business hours or on a 24- hour basis.
Manned structures are grounded in the same manner as
industrial buildings.
Unmanned Structures
An unmanned structure is a building that is not occupied during
normal business hours. Examples of unmanned structures are
pumping stations and water treatment plants. The same
grounding rules apply to unmanned structures and to manned
structures.
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Distribution and Utilization Equipment
The following are grounding requirements for distribution and
utilization equipment:
•
A multiple grounded neutral system (whether cabled or
overhead) is to be used in residential and non-industrial
locations. The neutral conductor is taken to all locations
on the system and is grounded at points of utilization. In
overhead systems, the neutral conductor is also
grounded at each pole. Grounding at points of utilization
must be in accordance with the National Electrical Code,
Article 250-H, Grounding Electrodes, except that the
water piping must not constitute the grounding electrode,
but it is to be bonded to the grounding electrode.
Equipment at points of utilization is to be grounded by
connection to the multiple grounded neutral and the local
electrode.
•
A single-point grounded neutral system is to be used in
industrial plants where cabling will normally predominate.
The neutral is to be grounded at the transformer only and
cannot be brought out for load use.
•
Utilization equipment is to be grounded by a metallic
connection to the system neutral and to the ground grid
or ground bus. The grounding connection is to conform
to Articles 250-57 and 250-91 (b) of the National
Electrical Code.
The ground loop impedance of the circuit that is formed by the
line conductor from the power source to the equipment and by
the grounding path from the equipment back to the power
source neutral must be low enough to allow sufficient fault
current to pass to operate the protection device. The following
equation should be satisfied:
Zgl ≤
E
IK
where:
Zgl
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E
=
line to neutral voltage, V
I
=
overcurrent device, A
current rating of fuse or trip setting of
K
=
overcurrent devices
constant, 3 for fuses, 1.5 for other
Difficulties in attaining an adequately low value of Zgl are
unlikely to arise but can occur at low voltages/high current
ratings.
Residential occupancies must have ground-fault circuit
protection for all 115V, 15A, and 20A receptacle outlets or
circuits supplying the outlets that are installed outside or in
bathrooms.
Construction sites are to have ground-fault circuit protection for
all 115V, 15A, and 20A receptacle outlets or feeders supplying
the outlets that are not part of the permanent wiring.
Conduit is not to be the sole means of grounding equipment,
except for overhead lighting within buildings that are installed
with rigid conduit.
A bonding jumper is to be installed at flush-mounted, groundingtype receptacles to connect the receptacle grounding terminal
and the box. Reliance is not to be placed upon contact devices
or yokes to provide the connection.
Lighting fixture outlet boxes are to be grounded and a bonding
jumper is to be installed to connect the fixture to the box.
Fences
Refer to SAES-P-111, paragraph 11 for Fence Grounding
requirements. Metal fences that are near/around substations,
ground grids, or passing under high voltage (69kV or above)
transmission lines must be grounded.
Except for fences around substations, fences constructed with
concrete posts and PVC coated fencing material are not
required to be grounded. (See SAES-M-006 for additional
information on fences).
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OFFSHORE PLATFORMS
An offshore platform is a large structure with a deck-like
construction on which the drilling rig of an oil or gas well is
erected or other production equipment (such as a GOSP). This
platform is supported by a number of steel jacket legs.
The mandatory requirements for offshore platform grounding
are in Saudi Aramco Engineering Standard SAES-P-111,
Chapter 10.
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WORK AID 1: SAUDI ARAMCO AND INDUSTRY STANDARDS,
DRAWINGS AND CODES APPLICABLE TO STRUCTURAL
GROUNDING
Saudi Aramco Engineering Standards
SAES-M-006, Saudi Aramco Security and General Purpose Fencing,
SAES-O-101, Standard Security Fence
SAES-P-100, Basic Criteria
SAES-P-104, Wiring Methods and Materials,
SAES-P-107, Overhead Distribution Systems,
SAES-P-111, Grounding, 30 December 1998
SAES-P-119, Substations,
SAES-T series, Communications Engineering Standards
Saudi Aramco Drawings
Standard Drawing AB-036387, Tank Grounding
Industry Standards and Codes
ANSI/NFPA 70 (NEC), National Electric Code, Article 250
NFPA 76A, Essential Electrical Systems for Health Care Facilities
NFPA 76B, Electricity in Patient Care Areas of Hospitals
NFPA 99, Health Care Facilities
NFPA 780, Lightning Protection Code
API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and
Stray Currents
BS 6651, The protection of Structures Against Lightning,
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IEEE
IEEE Std. 142, Recommended Practice for Grounding Industrial and Commercial
Power Systems, 1992 w/correction 1993
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WORK AID 2: CALCULATING THE LIGHTNING RISK INDEX
Refer to NFPA 780, Appendix H for procedure and tables for
calculating the Risk Assessment.
Note: per SAES-P-111, for Saudi Arabia, the number of days
with thunderstorms is assumed to be 10 (isoceraunic level), that
provides F = 8 (see Table H-2f).
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GLOSSARY
bonding
1 The permanent joining metallic parts to form an
electrically conductive path that will ensure electrical
continuity and the capacity to conduct safely any
current likely to be imposed.
2 The electrical interconnecting of conductive parts,
designed to maintain a common electrical potential.
bonding jumper
A reliable conductor to assure the required electrical
conductivity between metal parts required to be
electrically connected.
dielectric constants
The property that determines the electrostatic energy
that is stored per unit volume for unit potential gradient
(given relative to a vacuum).
effectively grounded
Intentionally connected to earth through a ground
connection or connections of sufficiently low
impedance and having sufficient current-carrying
capacity to prevent the buildup of voltages that may
result in hazards to connected equipment or to
persons.
equipment ground
A ground connection to non-current carrying metal
parts of a wiring installation, electric equipment, or
both.
equipotential
An identical state of electrical potential for two or more
items or areas.
ground
A conducting connection, whether intentional or
accidental, by which an electric circuit or equipment is
connected to the earth or to some conducting body, of
relatively large extent, which serves in place of the
earth.
ground bus
A bus to which the grounds from individual pieces of
equipment are connected and that, in turn, is
connected to ground at one or more points.
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ground clamp
A clamp that is used in connecting a grounding
conductor to a grounding electrode or to something
that is grounded.
ground conduit
A conduit that is used solely to contain one or more
grounding conductors.
ground current
Current flowing into or out of the earth or its equivalent
serving as a ground.
ground detector
An instrument or equipment that is used for indicating
the presence of a ground on an ungrounded system.
ground fault
an insulation failure between a conductor and ground
or equipment frame.
ground grid
A system of horizontal grounding electrodes that
consists of interconnected bare cables buried in the
earth to provide a common ground for electrical
devices and metallic structures, usually in one specific
location.
ground lug
A lug that is used to connect a grounding conductor to
a grounding electrode or to something that is
grounded.
ground mat
A solid metallic plat or a system of closely spaced bare
conductors that are connected to and often placed in
shallow depths above a ground grid or elsewhere at
the earth surface in order to obtain an extra protective
measure minimizing the danger of exposure to high
step or touch voltages in a critical operating area or
places that are frequently used by people. Grounded
metal grating, placed on or above the soil surface or
wire mesh placed directly under the crushed rock, are
common forms of a ground mat.
ground-return circuit
A circuit in which the earth is utilized to complete the
circuit.
grounded
Connected to earth or to some extended conducting
body that serves instead of the earth, whether the
connection is intentional or accidental.
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grounded circuit
A circuit in which one conductor or point (usually the
neutral conductor or neutral point of transformer or
generator windings) is intentionally grounded, either
solidly or through a grounding device.
grounded concentric
A grounded system in which the external (outer)
conductor is solidly grounded and that completely
surrounds the internal (inner) conductor throughout its
length. The external conductor must be jacketed.
grounded conductor
A conductor that is intentionally grounded, either solidly
or through a current limiting device.
conductor grounded
A conductor used to establish a ground: for instance,
ground grids, ground rods, ground wells, etc.
grounded conductor
A system or circuit conductor that is intentionally
grounded.
electrode
A system of conductors in which at least one conductor
or point (usually the middle wire or neutral point of
transformer or generator windings) is intentionally
grounded, either solidly or through a current limiting
device.
grounding conductor
The conductor that is used to establish a ground and
that connects an equipment, device, wiring system, or
another conductor (usually the neutral conductor) with
the grounding electrode or electrodes.
grounding connection
A connection that is used in establishing a ground and
that consists of a grounding conductor, a grounding
electrode and the earth (soil) that surrounds the
electrode or some conductive body which serves
instead of the earth.
grounding outlet
An outlet that is equipped with a receptacle of the
polarity type and that has, in addition to the currentcarrying contacts, one grounded contact that can be
used for the connection of an equipment grounding
conductor.
grounding transformer
A transformer that is intended primarily to provide a
neutral point for grounding purposes.
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guard wire
A grounded wire that is erected near a lower-voltage
circuit or public crossing in such a position that a high
(or higher) voltage overhead conductor cannot come
into accidental contact with the lower-voltage circuit, or
with persons or objects on the crossing without first
becoming grounded by contact with the guard wire.
reactance grounded
Grounded through impedance, the principal element of
which is reactance.
resistance grounded
Grounded through impedance, the principal element of
which is resistance.
solidly grounded
Grounded through an adequately grounded connection
in which no impedance has been inserted intentionally.
system grounding
An auxiliary solidly grounded conductor that connects
the individual grounding conductors in a given area.
ungrounded
A system, circuit, or apparatus without an intentional
connection to ground.
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BIBLIOGRAPHY
SAES-M-006, Saudi Aramco Security and General Purpose Fencing,
SAES-O-101, Standard Security Fence
SAES-P-100, Basic Criteria
SAES-P-104, Wiring Methods and Materials,
SAES-P-107, Overhead Distribution Systems,
SAES-P-111, Grounding, 30 December 1998
SAES-T series, Communications Engineering Standards
Standard Drawing AB-036387, Tank Grounding
ANSI/NFPA 70 (NEC), National Electric Code, 1996
NFPA 76A, Essential Electrical Systems for Health Care Facilities
NFPA 76B, Electricity in Patient Care Areas of Hospitals
NFPA 99, Health Care Facilities
NFPA 780, Lightning Protection Code
ANSI C2, National Electric Safety Code
API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray
Currents
BS 6651, The protection of Structures Against Lightning,
IEEE Std. 142, Recommended Practice for Grounding Industrial and Commercial Power
Systems, 1992 w/correction 1993
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ADDENDUM A: SAUDI ARAMCO DRAWING AB-036387
Course handout - Saudi Aramco Drawing AB-036387, Tank
Grounding
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ADDENDUM B: NFPA 780
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ADDENDUM C: API RP 2003
API RP 2003 Chapters 4 and 5
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