Content
Voltage surges and their protection devices
2
Lightning risk
2
A few figures
2
Storm formation 2
Lightning strike phenomenon
4
Different voltage surge types
6
What is a voltage surge ?
6
The four voltage surge types
6
Different propagation modes
10
Common mode
10
Differential mode
10
Overvoltage protection devices
11
Primary 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
17 octobre 2006
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Voltage surges
Lightning risk
and their protection devices
A few figures
Between 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 fires, 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 lightning.
The cost of repairing damage caused by lightning is very high.
It is difficult 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 financial
consequences rising above the cost of the equipment destroyed by the lightning.
Storm formation
DB110866
The storm cloud is generally of the cumulo-nimbus type. It is characterised by its anvil
shape and the dark colour of its base (fig. 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 cloud
DB110867
During summer storms, the process starts by hot air rising from the ground. As it
rises, it collects water droplets until it becomes a cloud (fig. 2).
Fig. 2 - Cloud formation.
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Voltage surges
Lightning risk (cont.)
and their protection devices
Beginning of the electrification mechanism
DB110884
These 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
(fig. 3).
Fig. 3 - Beginning of the electrification mechanisms.
Beginning of the active phase
DB110869
Next, 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 (fig. 4).
Fig. 4 - Development: beginning of the active phase, lightning
inside the cloud, strong anabatic winds.
Maturity of the active phase
DB110870
This cloud forms an enormous capacitor with the ground. In the half hour following
the first lightning formed within the cloud, flashes of lightning begin to form between
the cloud and the ground. They are called strokes of lightning. The first rain appears.
This is the mature phase (fig. 5).
Fig. 5 - Maturity: intense activity within the cloud, maximum
vertical development, strong convective activity.
End of the active phase
DB110871
Next, 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 (fig. 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
Lightning strike phenomenon
and their protection devices
The electric field strength
DB110885
When the weather is fine, the natural electric field strength on the ground is
approximately 120 V/m. When an electrically charged cloud arrives, it can rise to over
15 kV/m (fig. 7).
The electric field strength is increased by protrusions in the landscape (hills, trees,
houses). These create a peak effect which amplifies the electric field strength to up to
300 times its usual value in a given place (fig. 8).
This phenomenon is called the Corona effect. It encourages a stroke of lightning to
appear in such places.
This phenomenon was observed as early as Ancient times on the end of spears or
pointed objects. Sailors call it Saint Elmo's fire; it appears at the top of boat masts.
Mountaineers know that the appearance of a glow at the end of their ice axe,
accompanied by a humming noise like a swarm of bees, heralds the risk of lightning.
Lightning stroke classification (according to K. Berger)
Lightning strokes are classified according to the way in which they develop, and the
positive or negative part of the cloud which is discharged (fig. 9).
On flat ground, lightning striking from the cloud is the most usual.
In the mountains, or in the presence of large protruding objects (high tower or factory
chimney), ascending lightning strokes may develop. These are the most dangerous,
especially the positive type.
In temperate regions, such as Europe, 90 % of lightning strokes are of the negative
descending type.
DB110886
DB108728
Fig. 7 - The electric field strength on the ground.
Fig. 9 - Classification of lightning strokes (K. Berger).
The discharge principle
Let us take the example of a negative descending stroke (fig. 10).
1. The stroke of lightning begins with a leader which develops from the cloud and
moves by successive 30 to 50 m steps towards the ground. The leader is made up of
electrical particles drawn from the cloud by the cloud-ground electrical field strength.
These particles form a luminous channel directed to ground.
2. This encourages the formation of an ionised channel which will then branch out.
Once it reaches roughly 300 m from the ground, discharges (or sparks) leave the
ground and one of them enters into contact with the tip (leader point).
3. An extremely luminous electrical arc then appears. This causes thunder and helps
the exchange of charges between the cloud-ground capacitor.
4. A succession of decreasingly intense arcs, called subsequent arcs, then follows.
Between these arcs, there is a continuous leader which makes a current of roughly
200 A, supplying the discharge of a large part of the capacitor charges.
DB110887
Fig. 8 - Electric field strength amplified by a protrusion in
the landscape.
Fig. 10 - Negative descending stroke principle.
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Voltage surges
Lightning strike phenomenon (cont.)
and their protection devices
Characteristics of discharge of lightning
Beyond peak
probability
Current peak
Gradient
Total duration
Number of
discharges
P (%)
I (kA)
S (kA/µs)
T (s)
n
95
7
9.1
0.001
1
50
33
24
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 define 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 as
considerable induction and voltage surge effects, it causes the same effects as any
other low frequency current on a conductor:
b thermal effects: fusion at the lightning impact points and joule effect, due to the
circulation of the current, causing fires
b 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 flattened wires)
b 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.
b voltage surges conducted after an impact on overhead electrical or telephone
power lines.
b voltage surges induced by the electromagnetic radiation effect of the lightning
channel which acts as an antenna over several kilometres and is crossed by a
considerable impulse current.
b 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
Different voltage surge types
and their protection devices
What is a voltage surge ?
DB108729
DB108730
A voltage surge is a voltage impulse or wave which is superposed on the rated
network voltage (fig. 1).
Fig. 1 - Voltage surge examples.
This type of voltage surge is characterised by (fig. 2):
b the rise time (tf) measured in µs
b 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 likely to destroy the equipment.
Fig. 2 - Main overvoltage characteristics.
The four voltage surge types
There are four types of voltage surge which may disturb electrical installations and
loads:
b atmospheric voltage surges
b operating voltage surges
b transient industrial frequency voltage surges
b voltage surges caused by electrostatic discharge.
Atmospheric voltage surges
DB110875
b conducted 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 (fig. 3).
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.
b induced or radiated voltage surges
An indirect stroke of lightning which falls anywhere on the ground is equivalent to a
very long antenna which radiates an electromagnetic field.
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
Different voltage surge types (cont.)
and their protection devices
Consequences
DB111652
b field to cable coupling: the electromagnetic field will couple with any cable
encountered and generate common mode and/or differential mode voltage surges.
These voltage surges are then propagated by conduction (fig. 4).
Fig. 4 - Field to cable coupling.
DB110889
b field to cable coupling:
v inductive crosstalk: in the same way, the voltage surge current circulating in a cable
generates in its turn an electromagnetic field whose electromagnetic H component
induces a voltage surge in any cable which forms a loop.
This is called inductive crosstalk.
v capacitive crosstalk.
In the same way, the electromagnetic field 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
v induction in the frame loops (fig. 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 !...
Fig. 5 - Frame loop.
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Voltage surges
Different voltage surge types (cont.)
and their protection devices
DB110890
b rise in earthing connector potential (fig. 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, causing
a 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 2 W, 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.
Fig. 6 - Rise in earth potential.
Operating voltage surges
A 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 (fig. 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:
b 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.)
b voltage surges from inductive circuits due to motors starting and stopping,
or the opening of transformers such as MV/LV substations
b voltage surges from capacitive circuits due to the connection of capacitor banks
to the network
b all devices that contain a coil, a capacitor or a transformer at the power supply inlet:
relays, contactors, television sets, printers, computers, electric ovens, filters, etc.
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Voltage surges
Different voltage surge types (cont.)
and their protection devices
Transient industrial frequency voltage surges (fig. 7)
DB108731
These voltage surges have the same frequencies as the network (50, 60 or 400 Hz):
b 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
b voltage surges due to a cable breakdown. For example, a medium voltage cable
which falls on a low voltage line
b 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.
Fig. 7 - Transient industrial frequency voltage surge.
Voltage surges caused by electrical discharge
In a dry environment, electrical charges accumulate and create a very strong
electrostatic field. 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
Different propagation modes
and their protection devices
Common mode
DB108732
Common mode voltage surges occur between the live parts and the earth:
phase/earth or neutral/earth (fig. 1).
They are especially dangerous for devices whose frame is earthed due to the risk of
dielectric breakdown.
Differential mode
Fig. 1 - Common mode.
Differential mode voltage surges circulate between phase/phase or phase/neutral
live conductors (fig. 2). They are especially dangerous for electronic equipment,
sensitive computer equipment, etc.
The table below sums up the main characteristics of voltage surges
DB108733
Type of voltage surge
Voltage surge
coefficient
Duration
Front gradient
or frequency
Industrial frequency
(insulation fault)
y 1.7
Long
30 to 1000 ms
Industrial frequency
(50-60-400 Hz)
Operating and
electrostatic discharge
2 to 4
Short
1 to 100 ms
Average
1 to 200 kHz
Atmospheric
>4
Very short
1 to 100 µs
Very high
1 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 falls
b 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
Overvoltage protection devices
and their protection devices
Two major types of protection devices are used to suppress or limit voltage surges:
they are referred to as primary protection devices and secondary protection devices.
Primary protection devices
(protection of external installations against
lightning: IEPF)
The purpose of primary protection devices is to protect installations against direct
strokes of lightning. They catch and run the lightning current into the ground. The
principle is based on a protection area determined by a structure which is higher than
the rest.
The same applies to any peak effect produced by a pole, building or very high metallic
structure.
There are three types of primary protection:
b lightning conductors, which are the oldest and best known lightning protection
device
b taut wires
b the meshed cage or Faraday cage.
DB111653
The lightning conductor
Fig. 1 - Example of IEPF protection using a lightning conductor.
The lightning conductor is a tapered rod placed on top of the building. It is earthed by
one or more conductors (often copper strips) (fig. 1).
The design and installation of a lightning conductor is the job of a specialist. Attention
must be paid to the copper strip paths, the test clamps, the crow-foot earthing to help
high frequency lightning currents run to the ground, and the distances in relation to
the wiring system (gas, water, etc.).
Furthermore, the flow of the lightning current to the ground will induce voltage
surges, by electromagnetic radiation, in the electrical circuits and buildings to be
protected. These may reach several dozen kilovolts. It is therefore necessary to
symmetrically split the down conductor currents in two, four or more, in order to
minimise electromagnetic effects.
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Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
Taut wires
DB110893
DB110892
These wires are stretched over the structure to be protected (fig. 2). They are used for
special structures: rocket launch pads, military applications and lightning protection
cables for overhead high voltage power lines (fig. 3).
Fig. 2 - Example of IEPF protection using
the taut wire lightning conductor method.
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 floors (fig. 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 fields and induction.
DB110881
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 flowing
to the earth. To reduce secondary effects,
LV surge arresters must be added on
telephone and electrical power networks.
Fig. 3 - Lightning protection ropes.
Fig. 4 - Example of IEPF protection using
the meshed cage (Faraday cage) principle.
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Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
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 classified according to the way they are connected in an
installation: serial or parallel protection.
Serial protection device
DB108734
This is connected in series to the power supply wires of the system to be protected
(fig. 5).
Fig. 5 - Serial protection principle.
Transformers
Reduce voltage surges by inductor effect and make certain harmonics disappear by
coupling. This protection is not very effective.
Filters
Based on components such as resistors, inductance coils and capacitors are suitable
for voltage surges caused by industrial and operation disturbance corresponding to a
clearly defined frequency band. This protection device is not suitable for atmospheric
disturbance.
Wave absorbers
Are 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.
Parallel protection device
The principle
The parallel protection device can adapt to the installation to be protected (fig. 6).
It is this type of overvoltage protection device that is used the most often.
DB108735
Summary
All of these serial protection devices are
specific 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.
Fig. 6 - Parallel protection principle.
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Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
DB108716
Main characteristics
DB108736
Fig. 7 - Typical U/l curve of the ideal protection device.
b the rated voltage of the protection device must correspond to the network voltage
at the installation terminals: 230/400 V
b when there is no voltage surge, a leakage current should not go through the
protection device which is on standby
b 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 (fig. 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:
b the protection device response time (tr) must be as short as possible to protect the
installation as quickly as possible
b 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
b the surge arrester protection device must be able to withstand at the rated current
In.
The products used
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 specific 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).
90273EN_Ver1.1.indd/14
Voltage 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 (fig. 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 flow 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.
17 octobre 2006
Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
The technologies used in surge
arresters
The components
Several components are used to more or less obtain the previously
described characteristics.
Zener diodes
The characteristic curve (fig. 9) is very similar to the ideal curve.
The response time is extremely fast (roughly a picosecond: 10-12 s), for
a very specific 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 tube
This is a gas-filled bulb containing two electrodes. The characteristic curve
is shown in fig. 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 (fig. 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.
Comparison
The table below sums up the main characteristics of the components used
in parallel protection devices.
Component
Symbol
Leakage
current
Energy
dissipation
Residual
voltage
Holding current
Response time
Ideal
component
0
High
Low
Zero
Low
Zener
diode
Low
Low
Low
Zero
Low
Gas
discharge
tube
0
High
High
Continuous
if not
extinguished
High
Varistor
Low
High
Low
Zero
Low
DB108719
DB108718
DB108717
DB110934
Characteristic
U/I
Fig. 9 - Comparative table.
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Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
Surge arrester layouts
Surge arrester make-up
DB108723
DB108737
There are essentially three types of components which make up surge arresters:
zener diode, gas discharge tube, varistor.
b two-way zener diode surge arresters (fig. 10) are used especially as ultra terminal
protection devices for a specific point in the installation, and never for overall
protection due to their low power stability
b surge arresters using gas discharge tubes must be associated with varistors in
order to compensate for their weak points (fig. 11).
Fig. 10 - Two-way zener diode.
Fig. 11 - Typical layout of an improved gas
discharge tube surge arrester.
DB108725
DB108726
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
b surge arresters with varistors are currently the best solution as far as the quality/
price ratio is concerned because of their simplicity and reliability (fig. 12 and 13)
Fig. 12 - Single-pole surge arrester
with varistor principle.
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Fig. 13 - Two-pole surge arrester with varistor
principle.
17 octobre 2006
Voltage surges
Overvoltage protection devices
and their protection devices (cont.)
DB110744
b 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 (fig. 14).
Fig. 14 - Typical layout of a surge arrester with its thermal disconnector.
b 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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