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Content

Lightning risk 2A few gures 2

Storm formation 2

Lightning strike phenomenon 4

Different voltage surge types 6What is a voltage surge ? 6

The four voltage surge types 6

Different propagation modes 10Common mode 10

Differential mode 10

Overvoltage protection devices 11Primary protection devices 11

Secondary protection devices 13

Serial protection device 13

Parallel protection device 13

The technologies used in surge arresters 15

Surge arrester layouts 16

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Voltage surges

and their protection devicesLightning risk

A few guresBetween 2,000 and 5,000 storms are constantly forming around the earth.

These storms are accompanied by lightning which constitutes a serious risk for both

people and equipment. Strokes of lightning hit the ground at a rate of 30 to 100

strokes per second. Every year, the earth is struck by about 3 billion strokes

of lightning.

Throughout the world, every year, thousands of people are struck by lightning

and countless animals are killed.

Lightning also causes a large number of res, most of which break out on farms

(destroying buildings or putting them out of use).

Lightning also affects transformers, electricity meters, household appliances, and all

electrical and electronic installations in the residential sector and in industry.

Tall buildings are the ones most often struck by l ightning.

The cost of repairing damage caused by lightning is very high.

It is difcult to evaluate the consequences of disturbance caused to computer

or telecommunications networks, faults in PLC cycles and faults in regulation

systems. Furthermore, the losses caused by a machine being put out of use can

have nancial consequences rising above the cost of the equipment destroyed by

the lightning.

Storm formationThe storm cloud is generally of the cumulo-nimbus type. It is characterised by

its anvil shape and the dark colour of its base (g. 1). It constitutes a gigantic heat

machine with a base at an altitude of roughly 2 km and an apex at an altitude

of 14 km.

Fig. 1 - Cumulo-nimbus.

Electrical development of a storm cloudDuring summer storms, the process starts by hot air rising from the ground.

As it rises, it collects water droplets until it becomes a cloud (g. 2).

Fig. 2 - Cloud formation.

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Voltage surges

and their protection devicesLightning risk

Beginning of the electrication mechanismThese water droplets are then separated by violent rising and falling air currents.

As they rise, the droplets are transformed into ice crystals. The water and ice

particles then collide with each other, thus creating positive and negative electrical

charges (g. 3).

Fig. 3 - Beginning of the electrication mechanisms.

Beginning of the active phaseNext, the charges of opposite signs separate. The positive charges made up of ice

crystals stay in the higher part of the cloud while the negative charges made up

of water droplets remain in the base. A small quantity of positive charges remain

in the base of the cloud. Lightning begins to develop inside the storm cloud.This is the development phase (g. 4).

Fig. 4 - Development: beginning of the active phase, lightning inside the cloud, strong anabatic winds.

Maturity of the active phaseThis cloud forms an enormous capacitor with the ground. In the half hour following

the rst lightning formed within the cloud, ashes of lightning begin to form betweenthe cloud and the ground. They are called strokes of lightning. The rst rain appears.

This is the mature phase (g. 5).

Fig. 5 - Maturity: intense activity within the cloud, maximumvertical development, strong convective activity.

End of the active phaseNext, the cloud gradually becomes less active while the ground lightning increases.

It is accompanied by heavy rains, sleet and strong gusts of wind: this is the phase

where the cloud, which contains several hundreds of thousands of tonnes of water,

bursts (g. 6).

Fig. 6 - Cloud burst: decrease in activity inside the cloud,occurrence of violent phenomena on the ground: strokes of lightning, heavy rains, sleet, strong gusts of wind.

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Voltage surges

and their protection devicesLightning strike phenomenon

Characteristics of discharge of lightning

Beyond peak

probability

Current peak Gradient Total duration Number of

dischargesP (%) I (kA) S (kA/µs) T (s) n

95 7 9.1 0.001 1

50 33 2 0.01 2

5 85 65 1.1 6

This table shows the values given by the lighting protection committee (technical

committee 81 of the I.E.C.). As can be seen, 50 % of lightning strokes are of a force

greater than 33 kA and 5 % are greater than 85 kA. The energy forces involved are

thus very high.

It is important to dene the probability of adequate protection when protecting a site.

Furthermore, a lightning current is a high frequency (HF) impulse current reaching

roughly a megahertz.

Summary

Lightning comes from the discharge of electrical

charges accumulated in the cumulo-nimbus clouds

which form a capacitor with the ground.

Storm phenomena cause serious damage.

Lightning is a high frequency electrical phenomenon

which produces voltage surges on all conductive

elements, and especially on electrical loads and wires.

The effects of lightning

A lightning current is therefore a high frequency electrical current. As well asconsiderable induction and voltage surge effects, it causes the same effects as any

other low frequency current on a conductor:

thermal effects: fusion at the lightning impact points and joule effect, due to the

circulation of the current, causing res

electrodynamic effects: when the lightning currents circulate in parallel

conductors, they provoke attraction or repulsion forces between the wires, causing

breaks or mechanical deformations (crushed or attened wires)

combustion effects: lightning can cause the air to expand and create overpressure

which stretches over a distance of a dozen or so metres. A blast effect breaks

windows or partitions and can project animals or people several metres away from

their original position. This shock wave is at the same time transformed into a sound

wave: thunder.

voltage surges conducted after an impact on overhead electrical or telephone

power lines.

voltage surges induced by the electromagnetic radiation effect of the lightning

channel which acts as an antenna over several kilometres and is crossed by aconsiderable impulse current.

the elevation of the earth potential by the circulation of the lightning current in the

ground. This explains indirect strokes of lightning by pace voltage and the

breakdown of equipment.

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Voltage surges

and their protection devicesDifferent voltage surge types

What is a voltage surge ?A voltage surge is a voltage impulse or wave which is superposed on the rated

network voltage (g. 1).

Fig. 1 - Voltage surge examples.

This type of voltage surge is characterised by (g. 2):

the rise time (tf) measured in µs

the gradient S measured in kA/µs.

These two parameters disturb equipment and cause electromagnetic radiation.

Furthermore, the duration of the voltage surge (T) causes a surge of energy in the

electrical circuits which is l ikely to destroy the equipment.

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Fig. 2 - Main overvoltage characteristics.

The four voltage surge typesThere are four types of voltage surge which may disturb electrical installations andloads:

atmospheric voltage surges

operating voltage surges

transient industrial frequency voltage surges

voltage surges caused by electrostatic discharge.

Atmospheric voltage surgesConducted voltage surges are caused by a stroke of lightning falling on or near an

overhead power line (electricity or telephone). The current impulses generated are

propagated right up to the house (g. 3).

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Fig. 3 - Conducted voltage surges.

They are gradually damped as they pass through the lines and the MV 75 or 22 kV

protective spark-gaps or surge arresters, the transformers that they meet as they

travel. One part of the wave, however, travels as far as sensitive loads.

Induced or radiated voltage surges

An indirect stroke of lightning which falls anywhere on the ground is equivalent to avery long antenna which radiates an electromagnetic eld.

The steeper the current rise front (50 to 100 kA/µs), the greater the radiation.

The effects are felt several hundred metres, if not kilometres away.

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Voltage surges


ConsequencesField to cable coupling: the electromagnetic eld will couple with any cable

encountered and generate common mode and/or differential mode voltage surges.

These voltage surges are then propagated by conduction (g. 4).

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Fig. 4 - Field to cable coupling.

Field to cable coupling:

inductive crosstalk: in the same way, the voltage surge current circulating in a

cable generates in its turn an electromagnetic eld whose electromagnetic H

component induces a voltage surge in any cable which forms a loop

This is called inductive crosstalk.

capacitive crosstalk.

In the same way, the electromagnetic eld which is formed when a voltage surge

occurs induces a voltage surge on neighbouring cables owing to interference

capacitance between the cables.

This phenomenon is especially encountered in cable paths or chutes. It may produce

harmful effects when a high power cable is placed near low current cables

induction in the frame loops (g. 5).

A signal cable galvanically links a microcomputer to its printer. Each device is

earthed by a feeder which uses a different path from that of the signal cable.

The resulting overvoltage is proportional to the surface thus formed by the two

cables. For example, for a surface area of 300 m2 and a stroke of lightning of

100 kA/µs falling 400 metres away, the voltage surge induced in common mode on

the signal link will be roughly 15 kV !...

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Fig. 5 - Frame loop.

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Voltage surges


Rise in earthing connector potential (g. 6)

A stroke of lightning which hits the ground causes a lightning current which is

propagated in the ground according to a law depending on the type of ground and

earthing connector. A voltage surge occurs between 2 points on the ground, causinga potential difference of 500 V between the legs of an animal 1 metre apart, over

100 m away from the impact. Similarly, for an average current of 30 kA and an

excellent earthing connector of 2W, the rise in frame potential will be 60 kV in

relation to the network according to the law of Ohm. The rise in equipment potential

occurs independently of the network which may be overhead or underground.

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Fig. 6 - Rise in earth potential.

Operating voltage surgesA sudden change in the established operating conditions in an electrical network

causes transient phenomena to occur. These are generally high frequency or

damped oscillation voltage surge waves (g. 1 page 6).

They are said to have a slow front: their frequency varies from several dozen to

several hundred kilohertz.

Operating voltage surges may be created by:

voltage surges from disconnection devices due to the opening of protection

devices (fuse, circuit-breaker), and the opening or closing of control devices (relays,

contactors, etc.)

voltage surges from inductive circuits due to motors starting and stopping,

or the opening of transformers such as MV/LV substations

voltage surges from capacitive circuits due to the connection of capacitor banks

to the networkall devices that contain a coil, a capacitor or a transformer at the power supply

inlet: relays, contactors, television sets, printers, computers, electric ovens, lters,

etc.

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Voltage surges


Transient industrial frequency voltage surges (g. 7)These voltage surges have the same frequencies as the network (50, 60 or 400 Hz):

voltage surges caused by phase/frame or phase/earth insulating faults on a

network with an insulated or impedant neutral, or by the breakdown of the neutral

conductor. When this happens, single phase devices will be supplied in 400 V

instead of 230 V, or in a medium voltage: Us x e = Us x 1.7

voltage surges due to a cable breakdown. For example, a medium voltage cable

which falls on a low voltage line

the arcing of a high or medium voltage protective spark-gap causes a rise in earth

potential during the action of the protection devices. These protection devices follow

automatic switching cycles which will recreate a fault if it persists.

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Fig. 7 - Transient industrial frequency voltage surge.

Voltage surges caused by electrical dischargeIn a dry environment, electrical charges accumulate and create a very strong

electrostatic eld. For example, a person walking on carpet with insulating soles will

become electrically charged to a voltage of several kilovolts. If the person walks

close to a conductive structure, he will give off an electrical discharge of several

amperes in a very short rise time of a few nanoseconds. If the structure contains

sensitive electronics, a computer for example, its components or circuit boards may

be destroyed.

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Voltage surges

and their protection devicesDifferent propagation modes

Common modeCommon mode voltage surges occur between the live parts and the earth:

phase/earth or neutral/earth (g. 1).

They are especially dangerous for devices whose frame is earthed due to the risk of

dielectric breakdown.

Differential modeDifferential mode voltage surges circulate between phase/phase or phase/neutral

live conductors (g. 2). They are especially dangerous for electronic equipment,

sensitive computer equipment, etc.

The table below sums up the main characteristics of voltage surgesFig. 1 - Common mode.

Type of voltage surge Voltage surgecoefcient

Duration Front gradientor frequency

Industrial frequency(insulation fault)

y 1.7 Long

30 to 1000 ms

Industrial frequency

(50-60-400 Hz)

Operating andelectrostatic discharge

2 to Short

1 to 100 ms

Average

1 to 200 kHz

Atmospheric > Very short 1 to 100 µs

Very high1 to 1000 kV/µs

Fig. 2 - Dfferential mode.

Summary

Three points must be kept in mind:

b a direct or indirect lightning stroke may have

destructive consequences on electrical installations

several kilometres away from where it fallsb industrial or operating voltage surges also cause

considerable damage

b the fact that a site installation is underground in no way

protects it although it does limit the risk of a direct strike.

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Voltage surges

and their protection devicesOvervoltage protection devices

Taut wiresThese wires are stretched over the structure to be protected (g. 2). They are used

for special structures: rocket launch pads, military applications and lightning

protection cables for overhead high voltage power lines (g. 3).

Fig. 2 - Example of IEPF protection using the taut wire lightning conductor method.

Fig. 3 - Lightning protection ropes.

Summary

Primary lightning conductor protection devices (IEPF)

such as a meshed cage or taut wires are used to

protect against direct strokes of lighting.These

protection devices do not prevent destructive

secondary effects on equipment from occurring.

For example, rises in earth potential and electromagnetic

induction which are due to currents owing to the earth.To reduce secondary effects, LV surge arresters must be

added on telephone and electrical power networks.

The meshed cage (Faraday cage)This principle is used for very sensitive buildings housing computer or integrated

circuit production equipment. It consists in symmetrically multiplying the number of

down strips outside the building. Horizontal links are added if the building is high; for

example every two oors (g. 4). The down conductors are earthed by frog's foot

earthing connections. The result is a series of interconnected 15 x 15 m or 10 x 10 m

meshes. This produces better equipotential bonding of the building and splits

lightning currents, thus greatly reducing electromagnetic elds and induction.

Fig. 4 - Example of IEPF protection using the meshed cage (Faraday cage) principle.

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Voltage surges


Secondary protection devices

(protection of internal installations against

lightning: IIPF)These handle the effects of atmospheric, operating or industrial frequency voltage

surges. They can be classied according to the way they are connected in an

installation: serial or parallel protection.

Serial protection deviceThis is connected in series to the power supply wires of the system to be protected

(g. 5).

Fig. 5 - Serial protection principle.

TransformersReduce voltage surges by inductor effect and make certain harmonics disappear by

coupling. This protection is not very effective.

FiltersBased on components such as resistors, inductance coils and capacitors are

suitable for voltage surges caused by industrial and operation disturbance

corresponding to a clearly dened frequency band. This protection device is not

suitable for atmospheric disturbance.

Wave absorbersAre essentially made up of air inductance coils which limit the voltage surges, and

surge arresters which absorb the currents. They are extremely suitable for protecting

sensitive electronic and computing equipment. They only act against voltage surges.

They are nonetheless extremely cumbersome and expensive. They cannot

completely replace inverters which protect loads against power cuts.

Network conditioners and static uninterrupted power supplies

(UPS)These devices are essentially used to protect highly sensitive equipment, such as

computer equipment, which requires a high quality electrical power supply. They can

be used to regulate the voltage and frequency, stop interference and ensure a

continuous electrical power supply even in the event of a mains power cut (for the

UPS). On the other hand, they are not protected against large, atmospheric type

voltage surges against which it is still necessary to use surge arresters.

Summary

All of these serial protection devices are specic

to a device or application. They must be sized in

accordance with the power rating of the installation to

be protected. Most of them require the additional

protection of a surge arrester.

Parallel protection device

The principleThe parallel protection device can adapt to the installation to be protected (g. 6).

It is this type of overvoltage protection device that is used the most often.

Fig. 6 - Parallel protection principle.

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Voltage surges


Main characteristicsThe rated voltage of the protection device must correspond to the network voltage

at the installation terminals: 230/400 V.

When there is no voltage surge, a leakage current should not go through the

protection device which is on standby.

When a voltage surge above the allowable voltage threshold of the installation to

be protected occurs, the protection device violently conducts the voltage surge

current to the earth by limiting the voltage to the desired protection level Up (g. 7).

When the voltage surge disappears, the protection device stops conducting and

returns to standby without a holding current. This is the ideal U/I characteristic curve:

The protection device response time (tr) must be as short as possible to protect

the installation as quickly as possible.

The protection device must have the capacity to be able to conduct the energy

caused by the foreseeable voltage surge on the site to be protected.

The surge arrester protection device must be able to withstand at the rated current In.

The products usedVoltage limiters

Are used in MV/LV substations at the transformer outlet.Because they are only used for insulated or impedant neutral layouts, they can run

voltage surges to the earth, especially industrial frequency surges (g. 8)

LV surge arresters

This term designates very different devices as far as technology and use are

concerned.

Low voltage surge arresters come in the form of modules to be installed inside a LV

switchboard. There are also plug-in types and those that protect power points.

They ensure secondary protection of nearby elements but have a small ow

capacity. Some are even built into loads although they cannot protect against strong

voltage surges

Low current surge arresters or overvoltage protectors

These protect telephone or switching networks against voltage surges from

the outside (lightning), as well as from the inside (polluting equipment, switchgear

switching, etc.).

Low current voltage surge arresters are also installed in distribution boxes or built

into loads.

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Fig. 7 - Typical U/l curve of the ideal protection device.

Fig. 8 - Voltage limiter.

Summary

There are numerous types of secondary protection

devices to be used against voltage surges. They are

classed in two categories: serial protection and parallel

protection. Serial protection devices are designed for a

very specic need. Whatever this need, most of the

time they are additional to parallel protection devices.Parallel protection devices are used the most often,

whatever the installation to be protected: power supply

network, telephone network, switching network (bus).

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Voltage surges


The technologies used in surge arresters

The componentsSeveral components are used to more or less obtain the previously

described characteristics.

Zener diodesThe characteristic curve (g. 9) is very similar to the ideal curve.

The response time is extremely fast (roughly a picosecond: 10-12 s), for a

very specic threshold voltage (Us). The leakage current is negligible

although the zener diode has the disadvantage of dissipating very low

energy. This component is never placed at the head of the installation but

as an ultra terminal protection device in association with another surge

arrester.

The gas discharge tubeThis is a gas-lled bulb containing two electrodes. The characteristic

curve is shown in g. 9. This component was widely used until just

recently.

It has the advantage of having a high energy dissipation capacity and a

leakage current which is negligible in time thus reducing ageing by

overheating. Its drawbacks are a long response time, linked to the voltage

surge wave front and the maximum voltage to be reached, which is higher

than the threshold voltage, in order to be able to ionise the gas and start

the spark-gap conducting. Finally, when the voltage disappears at its

terminals, the spark-gap remains ionised and a holding current continues

to circulate.

The varistor (in zinc oxide)Its characteristic curve (g. 9) is similar to the ideal curve. The response

time is low, roughly a nanosecond (10-9 s). The energy dissipated is high.

The holding current is zero. The drawback is the leakage current which,

although low at the beginning, increases with each voltage surge impulse

and ends up overheating the component which must be disconnected

from the installation. An end-of-life lamp indicates disconnection.

ComparisonThe table below sums up the main characteristics of the components used

in parallel protection devices.

CharacteristicU/I

Component Symbol Leakagecurrent

Energydissipation

Residualvoltage

Holding current Response time

Ideal

component0 High Low Zero Low

Zener diode

Low Low Low Zero Low

Gas dischargetube

0 High High Continuousif not

extinguished

High

Varistor Low High Low Zero Low

Fig. 9 - Comparative table.

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Voltage surges


Surge arrester layouts

Surge arrester make-upThere are essentially three types of components which make up surge arresters:

zener diode, gas discharge tube, varistor.

Two-way zener diode surge arresters (g. 10) are used especially as ultra terminal

protection devices for a specic point in the installation, and never for overall

protection due to their low power stability

Surge arresters using gas discharge tubes must be associated with varistors in

order to compensate for their weak points (g. 11).

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Fig. 10 - Two-way zener diode. Fig. 11 - Typical layout of an improved gasdischarge tube surge arrester.

Varistor V1, which is in series with the gas discharge tube, extinguishes the spark at

the end of the voltage surge thus avoiding the holding current.

Varistor V2 conducts the voltage surge when it appears. It allows the voltage surge

to be absorbed as soon as it appears and helps the gas discharge tube to later arc

without causing damage to the installation.This is a relatively complex layout and therefore expensive.

Used alone, the gas discharge tube would cause the circuit protection or residual

current devices to operate because of the holding current.

Surge arresters with varistors are currently the best solution as far as the quality/

price ratio is concerned because of their simplicity and reliability (g. 12 and 13)

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Fig. 12 - Single-pole surge arrester with varistor principle.

Fig. 13 - Two-pole surge arrester with varistor principle.

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Voltage surges


Disconnection

The French standard NF C 15-100/1995 stipulates that an end-of-life disconnector,

either built into or placed outside the surge arrester, must be used. An end-of-life

indicator, can be added to facilitate maintenance (g. 14).

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Fig. 14 - Typical layout of a surge arrester with its thermal disconnector.

Connections

A single-pole surge arrester limits voltage surges between phase and earth or

between neutral and earth in common mode.

It also limits voltage surges between phase and neutral in differential mode.

As many single-pole surge arresters must be added for protection in common mode

as in differential mode (dotted line in diagrams 10, 11 and 13).

The modular surge arrester includes both of these types of protection.

The standard does not stipulate differential mode protection. It is, however, strongly

recommended for surge arresters installed in TT or TN-S layouts.

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