Oisd Paper

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Principles of Lightning Protection

For Oil and Gas Installations

A.J. Surtees, BSc, PhD, MBA, FICD, CEng, MIEE, SMIE (Aust)

Technical Manager

ERICO Inc. USA

MECHANISMS OF LIGHTNING DAMAGE

There are two basic mechanisms by which lightning may

enter and cause damage at a site. The first is a direct strike

where the building or surrounding structures receive a direct

lightning discharge. The effect is a large increase in the local

earth potential with subsequent damage to equipment

connected to outside services via such means as power

feeders, telephone subscriber lines, data or control cables and

physical pipelines.

The second mechanism is due to magnetic or capacitive

induction where a distant strike may induce large voltage

transients into power lines, communications lines and the

pipeline. The US IEEE standard 587 describes typical peak

amplitudes of voltage and current in power and

communications lines.

The mechanism of coupling to a buried pipeline is different.

The pipe, being insulated presents a high resistance to

ground and over its length a large capacitance to ground.

The earth potential rise associated with a ground strike in thevicinity of the pipe, is capacitively coupled to the pipe. This

potential is transferred in both directions from the point of

the strike, resulting in local earth potential rises at metering

stations and other facilities housing such equipment as

monitoring and telemetry electronics. It is this mechanism

which is responsible for damage to the sensitive metering

and electronic equipment. Thus, a strike some hundreds of

kilometres from a particular site may be responsible for local

damage.

The waveform of the transient induced into the pipe is

modified by the pipe capacitance. This means that protection

methods which may be effective against the standard 8/20spulse must be reviewed. Our experience is that the pulse rise

time is slowed, consequently protection networks involving

series inductive elements are largely ineffective.

A GENERIC APPROACH TO LIGHTNING

PROTECTION

Global Lightning Technologies has developed a generic Six

Point Plan for the protection of structures or facilities. The

concept behind the plan is that it prompts the user into

considering a holistic approach to lightning protection, one

embracing all aspects of potential damage, from the more

obvious direct strike to the more subtle mechanisms of

differential earth potential rises and voltage induction at

service entry points.

1. Capture the li ghtni ng stri ke at a preferred and know

point. This involves the use of an effective air terminal(s) on

the structure or vessel to be protected. In the design of area

protection, it is important to realise that many points on the

structure will be competing for the downward lightning

leader by launching upward interception streamers. Theeffectiveness of a lightning terminal is the measure of its

response time in launching such a streamer. The earlier the

streamer launch with respect to extraneous emission points

on the surrounding structure, the better the terminal will be

in ensuring it will take the strike (at a known point) and

prevent random striking or bypasses to adjacent uncontrolled

points.

2. Convey the li ghtning energy to ground in a safe manner

via a known route. This involves the use of a dedicated

down conductor, capable of withstanding the full energy of

the lightning discharge and conveying this to the grounding

system with minimal danger of side flashing to adjacentearth points. The ability of the down conductor to screen

adjacent equipment from the large electro magnetic impulse

associated with the discharge current, which may reach

energy levels as high as 250kA 8/20s, is also a measure of

its effectiveness in reducing damage by induction.

3. Ensure a low impedance earthi ng system to dissipate the

lightni ng discharge. The need to understand the

characteristics of an earthing system under impulse

conditions (associated with the higher Fourier spectral

components of the lightning discharge), is crucial if an

effective earth system is to be designed. An effect earth

system is one in which the potential rise of the surroundingearth is minimised and the rate of potential fall off from the

injection point is maximised.

4. Elimi nate earth l oops. Ensuring that a single point

earthing policy is adopted and that equipotential earth

bonding is used throughout the installation will help

eliminate common damage caused by differential earth

potentials.

5. Protect service entr y points of power f eeders. This

involves the installation of voltage clamping devices capable

of handling the large energy content (kA rating) of the over


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voltage surge, as well as reducing the extremely fast rising

edge (dv/dt and di/dt) of this transient.

6. Protect service entr y points of data or control li nes. This

involves the installation of high speed protective barriers. For

effective voltage limiting, a hybrid circuit is usually

employed where both speed of activation and energy

handling capacity are optimised in a multi-stage protective

module.

POINT 1. CAPTURE THE LIGHTNING STRIKE

AT A PREFERRED POINT

The potential of a direct strike to a communications tower

servicing a remote telemetry link, an offshore exploration

platform or even an elevated fuel storage tank may be high

with the resultant danger of equipment damage or even fire

as witnessed recently at the Cilacap refinery in Indonesia.

The design of an effective air terminal to protect such

structures requires some understanding of the mechanism of

the lightning discharge.

THE THUNDERCLOUD ORCUMULO-NIMBUS

Lightning is a natural phenomenon which develops when the

upper atmosphere becomes unstable due to the convergence of a

warm, solar heated, vertical air column on the cooler upper air

mass. These rising air currents carry water vapour which on

meeting the cooler air usually condense giving rise to convective

storm activity. Pressure and temperature are such that the

vertical air movement becomes self sustaining, forming the basis

of a Cumulo-nimbus cloud formation with its centre core capable

of rising to more than 15,000 metres.

To be capable of generating lightning, the cloud needs to be 3-4

km deep. The taller the cloud the more frequent the lightning.The centre column of the Cumulo-nimbus can have drafts

exceeding 120 km/hr creating intense turbulence with violent

wind shears and consequential danger to aircraft. This same up

draught gives rise to an electric charge separation which

ultimately leads to the lightning discharge.

The surface of the earth is initially negatively charged to the

order of 5 x 105 C, giving rise to an electric field intensity of

approximately 0.13 kVm-1. The lower atmosphere takes on an

opposing positive space charge. As rain droplets carry charge

away from the cloud, from the earth the storm cloud takes on the

characteristics of a dipole with the bottom of the cloud negatively

charged and the top of the cloud positively. It is known from

waterfall studies that fine precipitation acquires a positive

electrical charge. Larger particles acquire a negative charge. The

up draught of the Cumulo-nimbus separates these charges by

carrying the finer or positive charges to high altitudes. The

heavier negative charges remain at the base of the cloud and the

surface of the earth starts to accumulate positive charge. This

gives rise to the observed phenomenon where more than 90% of

cloud-to-ground discharges occur between a negatively charged

cloud and positively charged earth (negative lightning).

THE LIGHTNING DISCHARGE

The separation of electrical charge within a cloud allows electric

potential to increase to a point where a neutralising discharge

must occurs. The method by which this discharge takes place can

take on one of five different mechanisms:-

Cloud-to-cloud

Cloud-to-air

Intra-cloud discharges

Ground-to-cloud discharges and

Cloud-to-ground strikes.

Approximately 50% of all lightning discharges are cloud-to-

ground strikes. Ground-to-cloud discharges are extremely rare

and generally only occur from high mountain tops or tall man

made structures.

Cloud-to-ground discharges are further subdivided into positive

and negative leader discharges, of which about 90% are of the

negative category.

MECHANICS OF THE LIGHTNING STRIKE

The development of a cloud-to-ground discharge is a two staged

sequence, with one process being initiated from the cloud while a

second process is simultaneously being initiated from the groundor earth bound structures. Both mechanisms rely on an excessive

electron build up with subsequent ionisation and avalanche into

an electric current flow.

The Cloud Initiated Discharge - Leader

As a cloud accumulates charge, the electric field builds up to

the point where the air starts to breakdown forming an

ionised discharge called apilot streamer. This initial

discharge rapidly traverses about 30-50 meters towards the

ground. The presence of wind shear tends to blow away the

ionised air, halting the progression momentarily until

additional negative charge accumulates at the tip of the

column and air breakdown again occurs, allowing theionisation process to advance a further 30-50 metres. This

more intense discharge is generally known as thestepped

leader. The process repeats itself in a series of discrete steps

with a time interval of roughly 50 s. The course taken by

each of the steps in the leaders propagation towards the

ground is determined by the path through the air which

ionises more easily. This gives rise to the characteristically

zigzag nature of the cloud-to-ground discharge and the

branching of the lightning into many "fingers" in an attempt

to reach ground.

Being a highly ionised column, the tip of the leader is at

essentially the same potential as the charged cell from which ithas originated. As this tip approaches the ground, the potential

gradient further increases accelerating local ground ionisation.

At this point the potential difference between the leader and the

earth may be as high as 107 V, resulting in local air breakdown.

A ground originating discharge then begins to move up towards

the leader, intercepting at some tens of meters above ground

level.

The Earth Bound Discharge - Streamer

At the ground level, a point discharge such as a sharp metal

protrusion serves to enhance the electric field intensity as the

leader tip approaches, to the point where electrons are


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accelerated sufficiently to cause ionisation as they collide with

gas molecules. As the kinetic energy of the electrons is less than

this ionising potential, additional electrons are released and an

avalanche discharge results. To start the process, an initial

liberated electron is required. This can come from the natural

field intensification due to the presence of a charged cloud or

from deliberated introduced means such as a radioactive source

or spark gap. Such techniques of enhancing the emission of free

ions is the basis of early streamer emission, enhanced-ionisation

air terminals.

Once the electric field strength exceeds about 2 kVm-1 the

number of liberated free ions becomes adequate to cause a

current to flow which weakens the electric field. This current is

known as the upwardstreamer currentand can reach

magnitudes of several tens of amperes and can take the form of a

faintly luminescent discharge emanating from sharp protrusions.

Time captured photograph has shown that this upwards steamer

channel can reach several hundreds of meters as it propagates to

meet the descending leader.

The Main Discharge Or Return Stroke

Once the ionised channel has been completed by the junction of

the streamer and the leader, the build up of positive charge in the

earth flows upwards along the ionised discharge channel to

neutralise the large negative charge in the cloud giving rise to

what is known as the return stroke. Alternatively, the process

can be described using conventional electron flow as, electrons

migrating from the negative cloud to the positive earth. This is

characterised by a rapidly increasing electric current whose rate

of rise is typically 1010 amperes/sec.

Peak currents averaging around 30 kA appear typical with

minimum currents being about 3 kA. Maximum discharges

exceeding 200kA have been recorded.

It is also possible to have consecutive discharges down the same

channel. This occurs when the initial discharge neutralises the

localised charge cell in the cloud that initiated the stroke. Nearby

charge cells then flash across to the ionised channel and use it to

discharge to ground. In this manner up to 16 discharges have

been observed using the one channel.

The average energy released in a discharge is 55 kWhr, a

significant amount of energy by modern generation standards.

The danger of the discharge lies in the fact that all the energy is

expended in only 100-300 microseconds and that the peak

discharge current is reached in only 1 to 2 microseconds.

The following parameters are typical of the return stroke:-

Upward speed of return stroke is typically one-third to one-

half the speed of light near the ground and decreases as it

approaches the cloud.

Total time between ground and cloud < 100s

Peak current in first return stroke about 30kA

Time to peak < 10ms

Leader channel is heated to 30 000K

All charge contained in leader and branches is deposited to

ground down same channel.

Subsequent restrikes take on the following parameters:

Peak currents from 20 to 400kA

Time between return strokes 3 to 100ms

Number of return strokes 1 to 15, average of 4

Rising times even faster, typically a few nanoseconds.

These are some of the parameters which make lightning difficult

to control. The need to ensure that the lightning discharge is

effectively captured using a well designed early streamer

emission terminal is the key to such control.

THE DYNASPHERE EARLY STREAMER EMISSION AIR

TERMINAL

The result of many years of theoretical and ongoing field

research is the DYNASPHEREEarly Streamer Emission

Terminal. This product provides the design engineer with an air

termination relatively free of space charges which is capable of

creating photo-ionisation and which concentrates electric field to

release free electrons on the approach of a lightning leader.

The Dynasphere is a passive air terminal which requires no

external power source, relying solely on the energy contained in

the approaching leader for its dynamic operation. This

remarkable terminal has the ability to concentrate only thatelectric field which occurs in the millisecond time slots as the

leader charge approaches the ground.

The principle of operation relies on the capacitive coupling of the

outer sphere of the terminal to the approaching leader charge,

which in turn raises the voltage of the spherical surface. This rise

in voltage produces a field concentration across the insulated air

gap between the outer sphere and central grounded finial. As the

leader continues to approach the voltage on the sphere rises until

a point is reached where the air gap breaks down. This

breakdown creates local photo-ionisation and the release of

excess free electrons. These then accelerate under the intensified

field to initiate an avalanche condition and the formation of astreamer current begins.

Unlike the Dynasphere, pointed rods and other types of

enhancement terminals tend to create a corona space charge

above the emission point which serves to reduce the electric field

there by inhibiting streamer initiation. Also, unlike other air

terminals using battery or corona generated discharges, the

Dynasphere is radio-quiet only producing a spark discharge as

the leader approaches.

CONVENTIONAL APPROACH TO CALCULATION OF THE

PROTECTION RADIUS

In 1901, the British Lightning Committee formed to address theprotection which a Faraday rod would afford. After much debate,

it was resolved that this would be the area falling under a 450

cone drawn from the tip of the rod. Experience over the years has

highlighted many deficiencies associated with this method,

where lightning has bypassed air terminals and struck within the

safe area.

The deficiencies associated with this approach are depicted in

Figure 1. It was for this reason that the rolling sphere concept ofprotection for taller structures was introduced in the late 1970s.


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Figure 1. Lightning enters the "cone of protection" (Franklin

Rod method of protection) due to the inability of the

structure to launch a streamer.

COLLECTION VOLUME METHOD

The over simplification of the cone of protection approach

has prompted research into a more scientific approach to the

calculation of the effective protection radius afforded by a

lightning terminal. One such model is that of the Collection

Volume.

The derivation of the collection volume design concept can be

understood by considering the approach of the downward

leader. The charge Q distributed along this leader causes rapid

increase in electric field between it and ground points. When a

critical field value is reached, the ground point launches an

upward intercepting leader. The distance at which this occurs is

called the Striking Distance. The critical electrical field is

dependent on both leader charge and distance from the ground

point. Figure 2 shows how it is possible to form striking

distance hemispheres around an isolated ground point. The

greater the leader charge, the greater the striking distance.

Unfortunately, this simplistic approach of creating striking

distance hemispheres is not infallible in practice. Regard must be

taken of the relative velocities of the approaching leaders.

Figure 2 shows how it is possible to reach critical electric field

and launch an upward leader. If the downward leader is near the

periphery of the sphere, its velocity may carry it onward to

intercept another upward leader. Therefore, it is possible for the

downward leader to enter a striking distance hemisphere without

interception.

To cater for this the model requires that a limiting parabola be

placed on the hemisphere. This parabola is derived from velocity

factors and completes our collection volume. It can now be

stated that a downward leader entering such a volume is

theoretically assured of interception by the ground point

concerned. Figure 5 also shows how collection volumes become

larger with increased leader charge. That is, the larger the

magnitude of the current stroke, the larger the collection volume.

The collection volume model assumes that all points on the

structure are potential strike points and as such exhibit their

own natural collection volumes or attractive radius.

Figure 2. Collection volume and hemisphere bounded by

limiting parabola defined by charge on approaching leader.

THE BENJI CAD PROGRAM

A proprietary computer program has been developed by

Global Lightning Technologies Pty. Ltd. which evaluates the

protection radius afforded by an air terminal under different

conditions of leader intensity. Known as BENJI after thefounder of lightning research, Benjamin Franklin, the

program compares the protection radius produced by the air

terminal to the attractive radii produced by the electric field

intensification of competing points on the structure (corners

and edges, antennae, equipment, masts etc). The program

then optimises the placement, and number of air terminals,

to ensure that all these competing points lie within the

protective radius afforded by the ESE terminals.

The technology upon which the ESE air terminal and

computer model are based, follows that which is included in

the Australian / New Zealand Standard NZS/AS1768-1991

Lightning Protection, Appendix A.

POINT 2. SAFELY CONVEY THE LIGHTNING

ENERGY TO GROUND

Once the lightning discharge has been captured it is necessary to

covey this energy to the ground in a safe and controlled way.

Complications such as side flashing to adjacent conductors,inductive coupling of the large electromagnetic pulse onto nearby

signal lines and control of the excess energy content, need to be

considered. To this end ERICO has developed a special insluated

down conductor which comprises carefully selected dielectric

materials to create a capacitive balance and ensure insulation

integrity under high impulse conditions. In addition, a

conductive outer sheath allows electrostatic bonding of the

building through cable securing saddles. This ERICORE down

conductor evolved after extensive studies of potential voltage rise

in structures due to lightning injection.


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Figure 3. ERICORE insulated downconductor

The construction of the ERICORE also serves to reduce the

mutual inductance. A value of inductance of 1.6H/m is

normally regarded as quite small. However when a current is

impressed which is rising at the rate of 100kA/s, the effect of the

voltage developed due to this inductance (Ldi/dt) becomes

dominant. As an example, a single 60 metre down conductor

will rise to a value in excess of 1MV with an average discharge.

It is for this reason that a number of conductors are frequently

specified with standard protection methods.

Some of the practical benefits of ERICORE:-

It provides the design engineer with the ability to select

the most convenient lightning route to ground. The down

conductor can utilise air ducts etc. and be located remote

from electrical and sensitive electronic equipment.

The lightning is contained in the 50mm copper core

conductor and is oblivious to impedance irregularities in

the structure. The risk of side-flashing is reduced.

The structure carries only that minimal current which is

due to capacitive coupling to the main conductor.

Accordingly, voltages across concrete and reinforcing

members remain small. This leads to the conclusion that

no special bonding techniques are required.

By constraining the lightning injection energy to the core

of the cable, the amount of radiated field is reduced and

induction to adjacent cables, such as RF feeders on

telecommunications towers or data lines, is reduced.

POINT 3. ENSURE A LOW IMPEDANCE

GROUNDING SYSTEM

The importance of ensuring that the grounding system affords a

low earth impedance and not simply a low resistance must be

understood. A spectral study of the energy content associated

with the lightning impulse reveals both a high frequency and low

frequency component. The high frequency is associated with the

extremely fast rising front (typically < 10 s to peak current) of

the lightning impulse while the lower frequency component

resides in the long, high energy, tail or follow-on current in the

impulse. The grounding system appears to the lightning impulse

as a transmission line where wave propagation theory with the

normal rules of reflection and group velocity, apply.

Measurement of earth resistance with conventional low

frequency instruments may not provide results which are

indicative of the earth systems true effectiveness under

lightning discharge conditions. ERICO has developed an

Earth System Analyser in which a fast pulse is injected into

the earth test point to simulate the performance under

lightning impulse conditions. The peak current and voltage

amplitude within an effective measurement interval of

approximately 500ns is measured and used to calculate the

effective impedance.

The magnitude of the current pulse is programmable

according to the local conditions. The measurement window

lies with current pulses of 1 to 5A, 10 to 250V (peak) and

impedance range of 1.5 to 250 ohms. By effectively gating

off any pulses returning to the instrument after about 500ns,the instrument can be used to isolate distant grounds in a

complex system (greater than 75m away), allowing only the

earth-under-test to be measured without the need for

disconnection.

The instrument is also capable of providing repetitive pulses

at 30 second intervals to allow remote tracing of pulse

currents, their magnitude and flow direction.

POINT 4. ELIMINATE EARTH LOOPS

The current associated with a direct strike is typically 30kA

but may be as high as 270kA and exhibits a rise time ofmany thousands of amperes per second. When this current is

discharged through the lightning protection system, the

potential of the local earth system, with respect to the general

mass of the earth, will rise to a high level. The actual

calculation of this earth potential rise depends not only upon

the resistance of the earth grid but because of the high rates

of rise involved, also upon the inductance of the discharge

path.

Ignoring inductive effects, a simple calculation shows that

for a good earth resistance of 1 ohm and a discharge

current of 30kA, the earth potential rise will be 30kV. Given

that this rise can never be entirely eliminated, the aim in anywell designed lightning protection system is to equalise the

potential gradient to ensure that all equipment rises

uniformly in potential. This process is known as Earth

Potential Equalisation or EPE, and is achieved by bonding all

separate grounds points into a common ground system.

In practice, bonding involves connecting together all metallic

masses at a site with suitable conductors to ensure that they

are at the same electrical potential. Once bonded, this

common electrical potential must be matched to that of the

earth mass itself via a suitable connection to the grounding

system. This approach ensures that during a voltage

transient, all equipment within the site will rise and fall


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together as the surge current flows and potentially hazardous

voltages will not develop across the equipment.

Not only does this offer protection to the equipment housed

within, but it also ensures that personnel do not come into

contact with hazardous voltages when touching two pieces of

separate equipment - the touch potential.

Effective bonding design usually entails the adoption of a

single point earthing approach, in which all equipment

within the shelter is connected to a master bus bar which is

in turn bonded to the external grounding system at one point

only. In addition, the respective ground points for all

services, be they AC mains, telephone, data, coax feeders,

control signals or RF cables, should enter the shelter at a

common point - usually via an aluminium gland plate which

is itself also securely bonded to the external grounding

system.

POINT 5. PROTECT SERVICE ENTRY POINTS

OF POWER FEEDERS

This involves the installation of voltage clamping devicescapable of handling the large energy content (kA rating) of

the over voltage surge, as well as reducing the extremely fast

rising edge (dv/dt and di/dt) of this transient.

PRINCIPLES OF POWERLINE PROTECTION

The need for transient protection on power supplies is

becoming more apparent to the general community as failure

of equipment due to power lines transients becomes more

prevalent. This need is recognised by inclusion of new

requirements in modern surge protection standards such as

IEEE/ANSI C62.41, AS1768-1991, IEC 61643, IEC1024 and

BS6651. The question that requires further thought is how

bestto provide point of entry protection against transientswhich are conveyed along power circuits.

Experience has shown that failure of equipment due to

lightning induced surges can be attributed to two basic

mechanisms - over voltages with their excess energy content

and the extremely fast rise times associated with the

lightning impulse. Primary over voltage protection is usually

provided by Metal Oxide Varistors (MOV) or Spark Gap

devices (SG). Secondary protection is provided by low pass

power filtering which serves to reduce the peak let-through

voltage and the dv/dt of the impressed surge.

A number of devices and technologies are available for theprotection of the mains power entering a facility, all of which

require a decision as to the effectiveness offered. Typically

the devices available fall into two broad categories -shunt

protection or series hybrid protection.

SERIES HYBRID (FILTER) VS. SHUNT PROTECTION

Shuntprotection is the most basic form of protection

comprising over-voltage clamping devices which act to divert

the energy from a transient surge down to earth. Series

hybrid protection combines the energy diverting

characteristics of a shunt protector with a low pass filter.

This serves to further reduce the extremely fast rate of rise of

the voltage transient, further reducing the potential for

damage to the equipment. Such devices are usually known as

Power Filters or Surge Reduction Filters (SRF).

Field experience over the last fifteen years has shown that

simple shunt protection is generally adequate for the more

robust types of equipment such as lighting, air conditioning

and motor plants (pumps), but is inadequate at providing a

safe level of protection for equipment using semiconductor

electronics. Where such equipment is connected to the

mains, SRFs must be used to limit both the magnitude and

rate of rise of the voltage transient.

SHUNT PROTECTION

Shunt protection devices are referred to under a variety of

names including Transient Voltage Surge Suppressors

(TVSS), Surge Protective Devices (SPD) or sometimes

simply as Arrestors. They usually employ Metal Oxide

Varistors (MOV), air gaps, Silicon Avalanche Diodes (SAD)

or a combination of these.

The parameters which specify the performance of surge

suppressors are the voltage level to which they will clamp a

typical transient (let-through or residual voltage) and the

maximum peak current that can be diverted (peak kA rating).

The let-through voltage of a device is the maximum clamped

voltage appearing across the device when a surge is diverted.

The most common test waveform used to specify the

transient performance of surge suppressors is the 8/20s

current pulse - Figure 4 and Figure 5. This is usually

generated from a 6kV 1/50s charged capacitor source.

Larger voltages are generally used in the generation of very

large 8/20s pulses. The peak current of the 8/20s

waveform which should be considered for the testing of point

of entry protection devices, is specified in ANSI C62.41 as10kA (Category C3). For critical locations or exposed sites

this figure is often increased.

Figure 4. ANSI C62.41 Category A test pulse - 0.5s 100kHz

open circuit voltage ring wave.

Figure 5. ANSI C62.41 Category B test pulse - 1.2/50s

unidirectional open circuit voltage waveform, and resultant

8/20s current waveform


7/12

In addition to thesingle shotcapability of a surge suppressor,

consideration needs to be given to the effects of multiple

strike lightning. As mentioned earlier, statistical results from

various lightning detection and tracking systems indicate that

over half of all lightning flashes consist of more than one

strike. The subsequent re-strikes follow the same lightning

channel and generally exhibit a faster rate of rise of current

than the initial strike. These multiple strikes are separated by

some tens to hundreds of milliseconds. The effect of such

multi-pulses on surge suppressors is to cause a cumulative

heating effect which is suspected, rather than the energy in

the strike itself, as being the cause of many of the recorded

suppressor failures. At present there is no standard for testing

under multi-pulse effects, but research carried indicates that

Metal Oxide Varistors actually fail under applied multipulse

currents which may only be 75% of the single shot rating of

the device.

ARC GAP DIVERTERS

Air gap arrestors are designed to arc over when transient

over-voltages occur and then extinguish when the transient

has passed. They are generally capable of diverting large

surge currents. The voltage across these devices when an arc

has formed is very low (typically tens of volts). However, the

voltage required to cause the arc to form is high, typically

>3kV The let through voltage seen across such devices is

characterised by a voltage spike reaching several thousands

of volts with a steep leading edge (high dv/dt).

The main problems with the use of such devices stems from

the large voltage required to force them into conduction. For

example, co-ordination problems with secondary protection

can arise where the secondary protection operates at a lower

voltage level than the arc gap arrestors. Also if a transient

occurs which is below the strike voltage of the arc gap, then

no protection is provided.

Extensive research by ERICO Inc. has revealed novel

techniques of pre-triggering spark gaps at voltages well

below their inherent 2kV striking voltage. This research is

on-going with the view to producing shunt protectors with

extremely low let-through voltages (


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T

100kA

30kA

10kA

3kA

40mmMOV

60mmMOV

120kAMOVTEC

1kA

10 100

Number of impulses

Impulse

magnitude

1 1000 10000

Figure 7. Estimate of MOV life - typical MOV specifications

showing the relationship between the magnitude and number

of surges which can be safely diverted

MOV devices are generally rated for the maximum energy

absorption in one impulse, however as illustrated in Figure 7,

a large number of smaller impulses will also cause failure of

the MOV.Increasing the single shot rating of the protective

devices will significantly increase the lifetime for smaller

impulses.

POINT 6. PROTECT SERVICE ENTRY POINTS

OF DATA OR CONTROL LINES

The protection of data or signal line circuits generally

requires a protection circuit capable of extremely fast

activation (to ensure clamping at the typical tens of volts

used in most signalling protocols), whilst at the same time

being able to handle significant surge energies.

For such purposes, a hybrid type circuit employing a number

of different components is usually used. Certain of thesecomponents are able to with stand large amounts of current

but are slow to activate (there by allowing the transient

voltage to rise to levels significantly higher than their

clamping voltage), whilst others are extremely fast (allowing

no overshoot and clamping at the precise voltage required)

but are only able to withstand tiny amounts of energy before

self-destructing.

The following are some of the most commonly encountered

components used in various signal line protection barriers:-

Gas Arresters - These devices are available in either a 2

or 3 leg configuration. They are made of a ceramic tubewhich is filled with an inert gas. When a certain potential

difference exists between any two of the legs, the arrester

fires. The breakdown voltages vary from 70V up to 15kV.

The three leg version enables the surge to be clamped to

ground irrespective of which line the surge was present

on. Gas arresters are comparatively slow to activate (often

several microseconds) so should be used in combination

with faster devices for optimum protection.

Metal Oxide Varistors - MOVs are voltage limiting

devices which clamp the voltage rise of an impressed

surge once the clamping threshold is exceeded. MOVs

are faster than gas arrestors at conducting, however on

their own they are still not fast enough to limit a voltage

transients to a safe level for typical data circuits operating

somewhere in the 5-30V region.

Solid State Devices such as Silicon Avalanche Diodes -

These are special diodes with extremely fast turn on

characteristics, typically in the picosecond domain. This

means that their clamping threshold is extremely well

defined with very little overshoot. They are available in a

range of clamping voltages - 7.5, 12, 15, 30, 36 ... 200V

making them an ideal protection component for data line

protection.

GA are able to handle significantly more energy than SADs

however, as they are slow to turn on, the let-through voltage

rises to well above the turn on threshold and easily exceeds

the safe operating limit of most signal/control circuits. It is

for this reason that a hybrid circuit comprising the above

three components (designated a level 3 protector) is typically

used in the design of data protection units.

With any signal or power transmission system employingtwo lines and a separate earth, two types of transient can

occur:-

ADifferential Mode transient where the voltage surge

appears across the two lines independent of their potential

with respect to earth, and

A Common Mode transient where the voltage surge is

common between each line and earth.

The selection of any data line protector should ensure that

both Common and Differential modes are eliminated.

PROTECTION PRACTICES SPECIFIC TO OIL

AND GAS INSTALLATIONS

Modern pipeline systems incorporate a range of sophisticated

computing, instrumentation and communications equipment.

Unfortunately, pipelines act as very efficient collectors of

lightning energy, exposing this equipment to a high level of

risk of damage due to lightning activity. It is essential,

therefore, that these systems be equipped with adequate

transient protection to ensure the correct operation of the

pipeline system.

Lightning activity and metallurgic effects are particularhazards for long, buried pipelines, producing dangerously

high voltages along the pipe. For the safety of personnel and

for the protection of equipment, it is essential to ensure that

these hazards are safely dissipated to earth.

Experience has shownthat the most extensive damage is

generally sustained by equipment electrically connected to

the pipe. This is to be expected with the pipe acting as a most

efficient collector of lightning energy. The range of pipeline

surge protection devices marketed by ERICO Lightning

Technologies is shown in Error! Reference source not

found.


9/12

Figure 8. Part of ERICO's range of Cathodic

Protection systems

It is common practice to sectionalise pipelines by the

inclusion of insulated joints of the flange or monolithic type.Cathodic Protection (CP) voltage is applied to these pipeline

sections.

At metering and scrubber stations, an earthing system is

generally installed and the out of ground pipe work between

the insulated joints connected to it. Without protection,

induced transient pipeline voltages can easily breakdown the

insulated joint resulting in a permanent low resistance path

to earth. Flange type insulated joints are particularly

susceptible.

There are two common methods of protecting insulated

joints. Special cell can be used as an electrolytic switchblocking voltages in the cathodic protection range while

shunting hazardous voltages to ground.

Such cells require regular maintenance to check electrolyte

levels as well as careful installation to minimise lead

inductance for effective lightning protection.

A second method involves the use of gas arresters electrically

connected across the insulated joints. When a transient

voltage exceeds the breakdown voltage of the arrester, the

gas within the arrester ionises and creates a low impedance

path to shunt the surge energy to ground. The arrester is self

restoring reverting to a virtual open circuit. Providing the

surge rating of the arrester is not exceeded, it will exhibit

almost unlimited life. Figure 9 illustrates an explosion proof

100kA 8/20s insulated joint protector.

Figure 9. IJP unit with surge rating of 100kA and

housed in an explosion proof enclosure.

CATHODIC PROTECTION

Modern switched mode power supplies used for cathodic

protection are electronically controlled and regulated for

maximum efficiency and operational accuracy. The CP

power supply is connected between the pipe and a buriedearth system located perhaps 50-100 metres from the pipe

and known as a ground bed. Input sensing comes from the

pipe and a reference ground electrode.

In an unprotected system surge currents will flow from the

pipe to points of different potentials such as the ground bed

and reference cell. There may also be flash overs within

equipment cubicles caused by the cubicle being connected to

yet another earth, for example the station earth system.

Protection of electronic circuitry requires multistage

protectors with the primary protector being a high energy

absorption device such as a gas arrester.

As mentioned previously, gas arresters are relatively slow to

operate and can allow dangerously high voltages to pass

before fully conducting. For this reason secondary protectors

such as MOVs and pulse rated clamping diodes are typically

used.

All cabling inputs and outputs from electronic CP power

supplies require protection with hybrid devices designed to

divert surge energy to a common ground. This should be the

station earth mat, to which all metalwork should be bonded.

As with all transient protection, the philosophy is to create

an equi-potential plane which will uniformly rise in potential

with respect to true earth.


10/12

Figure 10. Block diagram of a hybrid lightning protection

unit for a dual CP system

Under normal operating conditions, the protector is

transparent to the CPU and electrical isolation between the

ground bed, the reference earth and the station earth mat, is

maintained. In the event of a surge on the pipe, the protector

acts to clamp the potential difference between the anode

ground bed, the two pipe terminals and the reference earth to

levels that will not cause damage to the CPU. A heavy duty

gas arrester is used to divert the transient energy to ground

via the station earth mat. The protector has been designed for

use with CP power supplies rated at 20V/5A.

Installation Considerations

CPU protectors of this kind should be installed with a

minimum of lead length between the unit and the CPU.

Under fast rise time transient conditions, cable inductance

becomes significant and high transient voltages can be

developed across long leads. It is essential that the protector

be grounded to the station earth mat.

Figure 11. Wiring arrangement for the CPU Protector

provided by ERICO.

PIPELINE POTENTIAL CLAMPING

Traditionally, large electrolytic capacitors have been used to

protect pipelines from these hazards. However this method is

not ideal, particularly in areas of high ambient temperature

where the de-ratings applied to capacitors make them

virtually useless.

The Pipeline Potential Clamp (PPC) is connected between

the pipeline and ground. A high energy gas discharge tube

at the front end of the unit protects the pipe and associated

equipment from lightning and other high energy transients,

diverting the energy to ground. Inductive filtering and Metal

Oxide Varistors act as a secondary protection stage,

dampening the fast rise time and keeping voltages to safe

levels. A series of diodes are used to protect the pipe from

AC voltages and telluric effects, clamping the pipe-to-ground

potential to within safe limits.

INTRINSIC SAFETY

The use of lightning protection units in certified hazardous

areas requires some consideration. Providing LPUs or

transient barriers contain no energy storage component and

may be certified as simple apparatus, then they can be

connected in intrinsically safe circuits on the hazardous side

without upsetting the integrity of the IS circuit. Intrinsically

safe barriers incorporating lightning protection on their input

circuitry are now becoming available.

Figure 12. A typical gas pipeline metering station showing

the location of lightning protection units for insulated joints,

cathodic protection and pipe connected transducers.


11/12

GENERIC APPROACH TO THE PROTECTION OF EQUIPMENT

IN HAZARDOUS AREAS

All equipment located within hazardous areas which is

connected to support equipment lying outside the area (ie.

located within the safe area) should be protected using

Intrinsically Safe Barriers (ISB). It is preferable that the

brand of barriers used also incorporate lightning transient

protection. The barriers must be installed outside the

confines of the hazardous area ie. in the safe area - Figure 13

LP E

Figure 13. Typical installation layout of an intrinsically safe

area

Typical equipment requiring protection includes:

Pressure regulators, digital links

Pressure release valves, analogue links

Pressure transducers, 4-20mA links

Flow computers - RS485 links

Insulated flanges

Two options exist for protection of non-certified equipment

(RTU, modem, charger, batteries, telecom lines etc.) needed

to support the instrumentation in the zone 1 area. These

options depend on where the additional equipment is

located:-

Support equipment not certified as intrinsically safe and

located within the zone 1 area, should be installed inside

intrinsically safe enclosures. Feeders from this equipment to

the safe area should be protected with ISB devices.

Support equipment installed outside of the hazardous area,

should also be protected with Universal Transient Barriers

(UTB) to ensure that any impressed currents are maintained

to safe levels.

When installing lightning protection barriers, it is important

to ensure that the total loop resistance of the ISB, UTB and

RTU does not exceed the maximum specified loop resistance.

Equipment installed within the safe area should be

referenced to a single point Safe area Earth System (SES).

All ISBs installed should be referenced to the SES. The SES

should be bonded to the IES where distances permit (


12/12

Appendix

PARTIAL REFERENCE LIST OF VOLATILE STORAGE FACILITIES

PROTECTED BY ERICO INC.

TASMANIA

Berriedale Sewage Plant, Ellis Point,Berriedale 1

Comalco Aluminium Powder Plant,Bell Bay 1

Prince of Wales Sewage Plant, DerwentPark Road 1

Sewerage Treatment Plant, Burnie 1

WESTERN AUSTRALIA

Industrial Plant Kwinana 1

NEW SOUTH WALES

Blue Circle Southern Cement - Berrima Plant 1Clarence Colliery Waste Water

Treatment Tank 1Coal Storage - Hunter Valley 1Griffith City Council -

Sewage Treatment Plant 2Hunter Valley Open Cut Mine 1Hunter Valley Colliery 2000T CoalStorage Bin 1

Hunter Valley Open Cut No.1 1Newvale Colliery 1Sydney Water - North Head Sewage

Treatment Plant 1Tower Colliery - Wilton 1Waterboard -

Rouse Hill Sewerage Treatment Plant 3Westcliff Colliery Borehole No 1 1Westcliff Colliery - Methane Drainage 6Wingeecarribee Shire Council -

Moss Vale Sewage Treatment Plant 3

QUEENSLAND

Airport Treatment Plant 1OK Tedi Mining, PNG 1QCL Cement Silos, Gladstone &Townsville 2Qld. Cement & Lime, Townsville 1Qld. Cement & Lime Silo, Gladstone 1Racecourse Mill-Sugar Refinery, Mackay 1Surge Bins - Abbot Point Coal 2Thallanga Mine Site 1White Mining, North Goonyella Mine 1

INDONESIA

Kaltim Prima Coal 5Kawasan Industri Gresik (Jawa Timur) 3Rumah Sakit Semen Gresik 2Semen Gresik (Persero) Proyek Tuban 2Treatment Centre of Industrial Bureau 1

TAIWAN

Chung Chou Sewerage Treatment Bldg 1

THAILAND

Egat Mah Moh, Lampang 1Rayong Wire, Rayong 3Sane Chemical, Chonburi. 1TGCI Factory, Saraburi 5The Siam Cement Office, Nakornratchasima 1

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