Lightning current surge and overvoltage protection

Lightning current surge and overvoltage protection

Overvoltage of atmospheric origin
Overvoltage definitions

Overvoltage (in a system) any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment definition from the International Electrotechnical Vocabulary (IEV 604-03-09)

Various types of overvoltage

An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network (see Fig. J1)

This type of overvoltage is characterized by (see Fig. J2):

• the rise time tf (in μs);
• the gradient S (in kV/μs).

An overvoltage disturbs equipment and produces electromagnetic radiation. Moreover, the duration of the overvoltage (T) causes an energy peak in the electric circuits which could destroy equipment.
Fig. J2 – Main characteristics of an overvoltage

Four types of overvoltage can disturb electrical installations and loads:

• Switching surges: high-frequency overvoltages or burst disturbance (see Fig. J1) caused by a change in the steady-state in an electrical network (during operation of switchgear).
• Power-frequency overvoltages: overvoltages of the same frequency as the network (50, 60, or 400 Hz) caused by a permanent change of state in the network (following a fault: insulation fault, breakdown of the neutral conductor, etc.).
• Overvoltages caused by electrostatic discharge: very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges (for example, a person walking on a carpet with insulating soles is electrically charged with a voltage of several kilovolts).
• Overvoltages of atmospheric origin.

Overvoltage characteristics of atmospheric origin

Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Figure J4)

• of several thousand amperes (and several thousand volts)
• of high frequency (approximately 1 megahertz)
• of short duration (from a microsecond to a millisecond)

Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year.

The table in Figure J3 shows some lightning strike values with their related probability. As can be seen, 50% of lightning strokes have a current exceeding 35 kA and 5% a current exceeding 100 kA. The energy conveyed by the lightning stroke is therefore very high.

Fig. J3 – Examples of lightning discharge values given by the IEC 62305-1 standard (2010 – Table A.3)

Fig. J4 – Example of lightning current

Lightning also causes a large number of fires, mostly in agricultural areas (destroying houses or making them unfit for use). High-rise buildings are especially prone to lightning strokes.

Effects on electrical installations

Lightning damages electrical and electronic systems in particular: transformers, electricity meters and electrical appliances on both residential and industrial premises.

The cost of repairing the damage caused by lightning is very high. But it is very hard to assess the consequences of:

• disturbances caused to computers and telecommunication networks;
• faults generated in the running of programmable logic controller programs and control systems.

Moreover, the cost of operating losses may be far higher than the value of the equipment destroyed.

Lightning stroke impacts

Lightning is a high-frequency electrical phenomenon that causes overvoltages on all conductive items, especially on electrical cabling and equipment.

Lightning strikes can affect the electrical (and/or electronic) systems of a building in two ways:

• by the direct impact of the lightning strike on the building (see Fig. J5 a);
• by indirect impact of the lightning strike on the building:
• A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can spread several kilometers from the point of impact.
• A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network. In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network.

A lightning stroke can fall near a building (see Fig. J5 d). The earth’s potential around the point of impact rises dangerously.

Fig. J5 – Various types of lightning impact

In all cases, the consequences for electrical installations and loads can be dramatic.

Fig. J6 – Consequence of a lightning stroke impact

Lightning falls on an unprotected building.	Lightning falls near an overhead line.	Lightning falls near a building.
The lightning current flows to earth via the more or less conductive structures of the building with very destructive effects: thermal effects: Very violent overheating of materials, causing fire mechanical effects: Structural deformation thermal flashover: The extremely dangerous phenomenon in the presence of flammable or explosive materials (hydrocarbons, dust, etc.)	The lightning current generates overvoltages through electromagnetic induction in the distribution system. These overvoltages are propagated along the line to the electrical equipment inside the buildings.	The lightning stroke generates the same types of overvoltage as those described opposites. In addition, the lightning current rises back from the earth to the electrical installation, thus causing equipment breakdown.
The building and the installations inside the building are generally destroyed	The electrical installations inside the building are generally destroyed.

The various modes of propagation

Common mode

Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7 ). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.

Fig. J7 – Common mode

Differential mode

Differential-mode overvoltages appear between live conductors:

phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.

Fig. J8 – Differential mode

Characterization of the lightning wave

Analysis of the phenomena allows the definition of the types of lightning current and voltage waves.

• 2 types of current wave are considered by the IEC standards:
• 10/350 µs wave: to characterize the current waves from a direct lightning stroke (see Fig. J9);

Fig. J9 – 10/350 µs current wave

• 8/20 µs wave: to characterize the current waves from an indirect lightning stroke (see Fig. J10).

Fig. J10 – 8/20 µs current wave

These two types of lightning current waves are used to define tests on SPDs (IEC standard 61643-11) and equipment immunity to lightning currents.

The peak value of the current wave characterizes the intensity of the lightning stroke.

The overvoltages created by lightning strokes are characterized by a 1.2/50 µs voltage wave (see Fig. J11).

This type of voltage wave is used to verify equipment withstand to overvoltages of atmospheric origin (impulse voltage as per IEC 61000-4-5).

Fig. J11 – 1.2/50 µs voltage wave

Principle of lightning protection
General rules of lightning protection

Procedure to prevent risks of the lightning strike
The system for protecting a building against the effects of lightning must include:

• protection of structures against direct lightning strokes;
• protection of electrical installations against direct and indirect lightning strokes.

The basic principle for the protection of installation against the risk of lightning strikes is to prevent the disturbing energy from reaching sensitive equipment. To achieve this, it is necessary to:

• capture the lightning current and channel it to earth via the most direct path (avoiding the vicinity of sensitive equipment);
• perform equipotential bonding of the installation; This equipotential bonding is implemented by bonding conductors, supplemented by Surge Protection Devices (SPDs) or spark gaps (e.g., antenna mast spark gap).
• minimize induced and indirect effects by installing SPDs and/or filters. Two protection systems are used to eliminate or limit overvoltages: they are known as the building protection system (for the outside of buildings) and the electrical installation protection system (for the inside of buildings).

Building protection system

The role of the building protection system is to protect it against direct lightning strokes.
The system consists of:

• the capture device: the lightning protection system;
• down-conductors designed to convey the lightning current to earth;
• “crow’s foot” earth leads connected together;
• links between all metallic frames (equipotential bonding) and the earth leads.

When the lightning current flows in a conductor, if potential differences appear between it and the frames connected to earth that are located in the vicinity, the latter can cause destructive flashovers.

The 3 types of the lightning protection system
Three types of building protection are used:

The lightning rod (simple rod or with triggering system)

The lightning rod is a metallic capture tip placed at the top of the building. It is earthed by one or more conductors (often copper strips) (see Fig. J12).

Fig. J12 – Lightning rod (simple rod or with triggering system)

The lightning rod with taut wires

These wires are stretched above the structure to be protected. They are used to protect special structures: rocket launching areas, military applications and protection of high-voltage overhead lines (see Fig. J13).

Fig. J13 – Taut wires

The lightning conductor with meshed cage (Faraday cage)

This protection involves placing numerous down conductors/tapes symmetrically all around the building. (see Fig. J14).

This type of lightning protection system is used for highly exposed buildings housing very sensitive installations such as computer rooms.

Fig. J14 – Meshed cage (Faraday cage)

Consequences of building protection for the electrical installation’s equipment

50% of the lightning current discharged by the building protection system rises back into the earthing networks of the electrical installation (see Fig. J15): the potential rise of the frames very frequently exceeds the insulation withstand capability of the conductors in the various networks (LV, telecommunications, video cable, etc.).

Moreover, the flow of current through the down-conductors generates induced overvoltages in the electrical installation.

As a consequence, the building protection system does not protect the electrical installation: it is, therefore, compulsory to provide for an electrical installation protection system.

Fig. J15 – Direct lightning back current

Lightning protection – Electrical installation protection system

The main objective of the electrical installation protection system is to limit overvoltages to values that are acceptable for the equipment.

The electrical installation protection system consists of:

• one or more SPDs depending on the building configuration;
• the equipotential bonding: a metallic mesh of exposed conductive parts.

Implementation

The procedure to protect the electrical and electronic systems of a building is as follows.

Search for information

• Identify all sensitive loads and their location in the building.
• Identify the electrical and electronic systems and their respective points of entry into the building.
• Check whether a lightning protection system is present on the building or in the vicinity.
• Become acquainted with the regulations applicable to the building’s location.
• Assess the risk of lightning strikes according to the geographic location, type of power supply, lightning strike density, etc.

Solution implementation

• Install bonding conductors on frames by a mesh.
• Install an SPD in the LV incoming switchboard.
• Install an additional SPD in each subdistribution board located in the vicinity of sensitive equipment (see Fig. J16).

Fig. J16 – Example of protection of a large-scale electrical installation

The Surge Protection Device (SPD)

Surge Protection Devices (SPD) are used for electric power supply networks, telephone networks, and communication and automatic control buses.

The Surge Protection Device (SPD) is a component of the electrical installation protection system.

This device is connected in parallel on the power supply circuit of the loads that it has to protect (see Fig. J17). It can also be used at all levels of the power supply network.

This is the most commonly used and most efficient type of overvoltage protection.

Fig. J17 – Principle of protection system in parallel

SPD connected in parallel has a high impedance. Once the transient overvoltage appears in the system, the impedance of the device decreases so surge current is driven through the SPD, bypassing the sensitive equipment.

Principle

SPD is designed to limit transient overvoltages of atmospheric origin and divert current waves to earth, so as to limit the amplitude of this overvoltage to a value that is not hazardous for the electrical installation and electric switchgear and controlgear.

SPD eliminates overvoltages

• in common mode, between phase and neutral or earth;
• in differential mode, between phase and neutral.

In the event of an overvoltage exceeding the operating threshold, the SPD

• conducts the energy to earth, in common mode;
• distributes the energy to the other live conductors, in differential mode.

The three types of SPD

Type 1 SPD
The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage.
It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.
Type 1 SPD is characterized by a 10/350 µs current wave.

Type 2 SPD
The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of overvoltages in the electrical installations and protects the loads.
Type 2 SPD is characterized by an 8/20 µs current wave.

Type 3 SPD
These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.
Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).

SPD normative definition

Fig. J18 – SPD standard definition

Note 1: There exist T1+ T2 SPD (or Type 1 + 2 SPD) combining the protection of loads against direct and indirect lightning strokes.

Note 2: some T2 SPD can also be declared as T3

Characteristics of SPD

International standard IEC 61643-11 Edition 1.0 (03/2011) defines the characteristics and tests for SPD connected to low voltage distribution systems (see Fig. J19).

In green, the guaranteed operating range of the SPD.
Fig. J19 – Time/current characteristic of a SPD with varistor

Common characteristics

• U_C: Maximum continuous operating voltage. This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.
• U_P: Voltage protection level (at I_n). This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the overvoltage withstand capability of the loads. In the event of lightning strikes, the voltage across the terminals of the SPD generally remains less than U_P.
• In: Nominal discharge current. This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging a minimum 19 times.

Why is In important?
In corresponds to a nominal discharge current that an SPD can withstand at least 19 times: a higher value of In means a longer life for the SPD, so it is strongly recommended to choose higher values than the minimum imposed value of 5 kA.

Type 1 SPD

• I_imp: Impulse current. This is the peak value of a current of 10/350 µs waveform that the SPD is capable of discharging of discharging at least one time.

Why is I_imp important?
IEC 62305 standard requires a maximum impulse current value of 25 kA per pole for the three-phase system. This means that for a 3P+N network the SPD should be able to withstand a total maximum impulse current of 100kA coming from the earth bonding.

• I_fi: Autoextinguish follow current. Applicable only to the spark gap technology. This is the current (50 Hz) that the SPD is capable of interrupting by itself after flashover. This current must always be greater than the prospective short-circuit current at the point of installation.

Type 2 SPD

• Imax: Maximum discharge current. This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging once.

Why is Imax important?
If you compare 2 SPDs with the same In, but with different Imax: the SPD with higher Imax value has a higher “safety margin” and can withstand higher surge current without being damaged.

Type 3 SPD

• U_OC: Open-circuit voltage applied during class III (Type 3) tests.

Main applications

• Low Voltage SPD. Very different devices, from both a technological and usage viewpoint, are designated by this term. Low voltage SPDs are modular to be easily installed inside LV switchboards. There are also SPDs adaptable to power sockets, but these devices have a low discharge capacity.

• SPD for communication networks. These devices protect telephone networks, switched networks and automatic control networks (bus) against overvoltages coming from outside (lightning) and those internal to the power supply network (polluting equipment, switchgear operation, etc.). Such SPDs are also installed in RJ11, RJ45, … connectors or integrated into loads.

Notes

1. Test sequence according to standard IEC 61643-11 for SPD based on MOV (varistor). A total of 19 impulses at I_n:

• One positive impulse
• One negative impulse
• 15 impulses synchronised at every 30°on the 50 Hz voltage
• One positive impulse
• One negative impulse

2. for type 1 SPD, after the 15 impulses at I_n (see previous note):

• One impulse at 0.1 x I_imp
• One impulse at 0.25 x I_imp
• One impulse at 0.5 x I_imp
• One impulse at 0.75 x I_imp
• One impulse at I_imp

Design of the electrical installation protection system
Design rules of the electrical installation protection system

To protect an electrical installation in a building, simple rules apply for the choice of

• SPD(s);
• its protection system.

For a power distribution system, the main characteristics used to define the lightning protection system and select a SPD to protect an electrical installation in a building are:

• SPD
• quantity of SPD
• type
• level of exposure to define the SPD’s maximum discharge current Imax.
• The short circuit protection device
• maximum discharge current Imax;
• short-circuit current Isc at the point of installation.

The logic diagram in Figure J20 below illustrates this design rule.

Fig. J20 – Logic diagram for selection of a protection system

The other characteristics for the selection of an SPD are predefined for electrical installation.

• number of poles in SPD;
• voltage protection level U_P;
• U_C: Maximum continuous operating voltage.

This sub-section Design of the electrical installation protection system describes in greater detail the criteria for selection of the protection system according to the characteristics of the installation, the equipment to be protected and the environment.

Elements of the protection system

SPD must always be installed at the origin of the electrical installation.

Location and type of SPD

The type of SPD to be installed at the origin of the installation depends on whether or not a lightning protection system is present. If the building is fitted with a lightning protection system (as per IEC 62305), a Type 1 SPD should be installed.

For SPD installed at the incoming end of the installation, the IEC 60364 installation standards lay down minimum values for the following 2 characteristics:

• Nominal discharge current I_n = 5 kA (8/20) µs;
• Voltage protection level U_P(at I_n) < 2.5 kV.

The number of additional SPDs to be installed is determined by:

• the size of the site and the difficulty of installing bonding conductors. On large sites, it is essential to install an SPD at the incoming end of each subdistribution enclosure.
• the distance separating sensitive loads to be protected from the incoming end protection device. When the loads are located more than 10 meters away from the incoming-end protection device, it is necessary to provide for additional fine protection as close as possible to sensitive loads. The phenomena of wave reflection is increasing from 10 meters see Propagation of a lightning wave
• the risk of exposure. In the case of a very exposed site, the incoming-end SPD cannot ensure both a high flow of lightning current and a sufficiently low voltage protection level. In particular, a Type 1 SPD is generally accompanied by a Type 2 SPD.

The table in Figure J21 below shows the quantity and type of SPD to be set up on the basis of the two factors defined above.

Fig. J21 – The 4 cases of SPD implementation

Protection distributed levels

Several protection levels of SPD allows the energy to be distributed among several SPDs, as shown in Figure J22 in which the three types of SPD are provided for:

• Type 1: when the building is fitted with a lightning protection system and located at the incoming end of the installation, it absorbs a very large quantity of energy;
• Type 2: absorbs residual overvoltages;
• Type 3: provides “fine” protection if necessary for the most sensitive equipment located very close to the loads.

Note: The Type 1 and 2 SPD can be combined in a single SPD
Fig. J22 – Fine protection architecture

Common characteristics of SPDs according to the installation characteristics
Maximum continuous operating voltage Uc

Depending on the system earthing arrangement, the maximum continuous operating voltage U_C of SPD must be equal to or greater than the values shown in the table in Figure J23.

Fig. J23 – Stipulated minimum value of U_C for SPDs depending on the system earthing arrangement (based on Table 534.2 of the IEC 60364-5-53 standard)

N/A: not applicable
U: line-to-line voltage of the low-voltage system
a. these values are related to worst-case fault conditions, therefore the tolerance of 10 % is not taken into account.

The most common values of UC chosen according to the system earthing arrangement.
TT, TN: 260, 320, 340, 350 V
IT: 440, 460 V

Voltage protection level U_P (at I_n)

The IEC 60364-4-44 standard helps with the choice of the protection level Up for the SPD in function of the loads to be protected. The table of Figure J24 indicates the impulse withstand capability of each kind of equipment.

Fig. J24 – Required rated impulse voltage of equipment Uw (table 443.2 of IEC 60364-4-44)

a. According to IEC 60038:2009.
b. This rated impulse voltage is applied between live conductors and PE.
c. In Canada and the USA, for voltages to earth higher than 300 V, the rated impulse voltage corresponding to the next highest voltage in this column applies.
d. For IT systems operations at 220-240 V, the 230/400 row shall be used, due to the voltage to earth at the earth fault on one line.

Fig. J25 – Overvoltage category of equipment

	Equipment of the overvoltage category I am only suitable for use in the fixed installation of buildings where protective means are applied outside the equipment – to limit transient overvoltages to the specified level. Examples of such equipment are those containing electronic circuits like computers, appliances with electronic programs, etc.
	Equipment of overvoltage category II is suitable for connection to the fixed electrical installation, providing a normal degree of availability normally required for current-using equipment. Examples of such equipment are household appliances and similar loads.
	Equipment of overvoltage category III is for use in the fixed installation downstream of, and including the main distribution board, providing a high degree of availability. Examples of such equipment are distribution boards, circuit-breakers, wiring systems including cables, bus-bars, junction boxes, switches, socket-outlets) in the fixed installation, and equipment for industrial use and some other equipment, e.g. stationary motors with a permanent connection to the fixed installation.
	Equipment of overvoltage category IV is suitable for use at, or in the proximity of, the origin of the installation, for example upstream of the main distribution board. Examples of such equipment are electricity meters, primary overcurrent protection devices, and ripple control units.

The “installed” U_P performance should be compared with the impulse withstand capability of the loads.

SPD has a voltage protection level U_P that is intrinsic, i.e. defined and tested independently of its installation. In practice, for the choice of U_P performance of an SPD, a safety margin must be taken to allow for the overvoltages inherent in the installation of the SPD (see Figure J26 and Connection of Surge Protection Device).

Fig. J26 – Installed U_P

The “installed” voltage protection level U_P generally adopted to protect sensitive equipment in 230/400 V electrical installations is 2.5 kV (overvoltage category II, see Fig. J27).

Note:
If the stipulated voltage protection level cannot be achieved by the incoming-end SPD or if sensitive equipment items are remote (see Elements of the protection system#Location and type of SPD Location and type of SPD, additional coordinated SPD must be installed to achieve the required protection level.

Number of poles

• Depending on the system earthing arrangement, it is necessary to provide for an SPD architecture ensuring protection in common-mode (CM) and differential-mode (DM).

Fig. J27 – Protection needs according to the system earthing arrangement

a. The protection between phase and neutral can either be incorporated in the SPD placed at the origin of the installation or be remoted close to the equipment to be protected
b. If neutral distributed

Note:

Common-mode overvoltage
A basic form of protection is to install a SPD in common mode between phases and the PE (or PEN) conductor, whatever the type of system earthing arrangement used.

Differential-mode overvoltage
In the TT and TN-S systems, earthing of the neutral results in an asymmetry due to earth impedances which leads to the appearance of differential-mode voltages, even though the overvoltage induced by a lightning stroke is common-mode.

2P, 3P and 4P SPDs
(see Fig. J28)
These are adapted to the IT, TN-C, TN-C-S systems.
They provide protection merely against common-mode overvoltages

Fig. J28 – 1P, 2P, 3P, 4P SPDs

1P + N, 3P + N SPDs
(see Fig. J29)
These are adapted to the TT and TN-S systems.
They provide protection against common-mode and differential-mode overvoltages

Fig. J29 – 1P + N, 3P + N SPDs

Selection of a Type 1 SPD
Impulse current Iimp

• Where there are no national regulations or specific regulations for the type of building to be protected: the impulse current Iimp shall be at least 12.5 kA (10/350 µs wave) per branch in accordance with IEC 60364-5-534.
• Where regulations exist: standard IEC 62305-2 defines 4 levels: I, II, III and IV

The table in Figure J31 shows the different levels of I_imp in the regulatory case.

Fig. J30 – Basic example of balanced I_imp current distribution in 3 phase system

Fig. J31 – Table of I_imp values according to the building’s voltage protection level (based on IEC/EN 62305-2)

Autoextinguish follow current I_fi

This characteristic is applicable only for SPDs with spark gap technology. The autoextinguish follow current I_fi must always be greater than the prospective short-circuit current I_sc at the point of installation.

Selection of a Type 2 SPD
Maximum discharge current Imax

The maximum discharge current Imax is defined according to the estimated exposure level relative to the building’s location.
The value of the maximum discharge current (Imax) is determined by risk analysis (see the table in Figure J32).

Fig. J32 – Recommended maximum discharge current Imax according to the exposure level

Selection of external Short Circuit Protection Device (SCPD)

The protection devices (thermal and short circuit) must be coordinated with the SPD to ensure reliable operation, i.e.
ensure continuity of service:

• withstand lightning current waves
• not generate excessive residual voltage.

ensure effective protection against all types of overcurrent:

• overload following thermal runaway of the varistor;
• short circuit of low intensity (impedant);
• short circuit of high intensity.

Risks to be avoided at end of life of the SPDs
Due to ageing

In the case of natural end of life due to ageing, protection is of the thermal type. SPD with varistors must have an internal disconnector which disables the SPD.
Note: End of life through thermal runaway does not concern SPD with gas discharge tube or encapsulated spark gap.

Due to a fault

The causes of end of life due to a short-circuit fault are:

• Maximum discharge capacity exceeded. This fault results in a strong short circuit.
• A fault due to the distribution system (neutral/phase switchover, neutral disconnection).
• Gradual deterioration of the varistor.
  The latter two faults result in an impedant short circuit.
  The installation must be protected from damage resulting from these types of fault: the internal (thermal) disconnector defined above does not have time to warm up, hence to operate.
  A special device called “external Short Circuit Protection Device (external SCPD)”, capable of eliminating the short circuit, should be installed. It can be implemented by a circuit breaker or fuse device.

Characteristics of the external SCPD

The external SCPD should be coordinated with the SPD. It is designed to meet the following two constraints:

Lightning current withstand

The lightning current withstand is an essential characteristic of the SPD’s external Short Circuit Protection Device.
The external SCPD must not trip upon 15 successive impulse currents at In.

Short-circuit current withstand

• The breaking capacity is determined by the installation rules (IEC 60364 standard):
  The external SCPD should have a breaking capacity equal to or greater than the prospective short-circuit current Isc at the installation point (in accordance with the IEC 60364 standard).
• Protection of the installation against short circuits
  In particular, the impedant short circuit dissipates a lot of energy and should be eliminated very quickly to prevent damage to the installation and to the SPD.
  The right association between a SPD and its external SCPD must be given by the manufacturer.

Installation mode for the external SCPD
Device “in series”

The SCPD is described as “in series” (see Fig. J33) when the protection is performed by the general protection device of the network to be protected (for example, connection circuit breaker upstream of an installation).

Fig. J33 – SCPD “in series”

Device “in parallel”

The SCPD is described as “in parallel” (see Fig. J34) when the protection is performed specifically by a protection device associated with the SPD.

• The external SCPD is called a “disconnecting circuit breaker” if the function is performed by a circuit breaker.
• The disconnecting circuit breaker may or may not be integrated into the SPD.

Fig. J34 – SCPD “in parallel”

Note:
In the case of an SPD with a gas discharge tube or encapsulated spark gap, the SCPD allows the current to be cut immediately after use.

Guarantee of protection

The external SCPD should be coordinated with the SPD and tested and guaranteed by the SPD manufacturer in accordance with the recommendations of the IEC 61643-11 standard. It should also be installed in accordance with the manufacturer’s recommendations. As an example, see the Electric SCPD+SPD coordination tables.

When this device is integrated, conformity with product standard IEC 61643-11 naturally ensures protection.

Fig. J35 – SPDs with external SCPD, non-integrated (iC60N + iPRD 40r) and integrated (iQuick PRD 40r)

Summary of external SCPDs characteristics

A detailed analysis of the characteristics is given in section Detailed characteristics of the external SCPD .
The table in Figure J36 shows, on an example, a summary of the characteristics according to the various types of external SCPD.

Fig. J36 – Characteristics of end-of-life protection of a Type 2 SPD according to the external SCPDs

Installation mode for the external SCPD	In series	In parallel
		Fuse protection-associated	Circuit breaker protection-associated	Circuit breaker protection integrated
Surge protection of equipment	=	=	=	=
	SPDs protect the equipment satisfactorily whatever the kind of associated external SCPD
Protection of installation at the end of life	–	=	+	+ +
	No guarantee of protection possible	Manufacturer’s guarantee		Full guarantee
		Protection from impedance short circuits not well ensured	Protection from short circuits perfectly ensured
Continuity of service at the end of life	– –	+	+	+
	The complete installation is shut down	Only the SPD circuit is shut down
Maintenance at end of life	– –	=	+	+
	The shutdown of the installation required	Change of fuses	Immediate resetting

SPD and protection device coordination table

The table in Figure J37 below shows the coordination of disconnecting circuit breakers (external SCPD) for Type 1 and 2 SPDs of the XXX Electric brand for all levels of short-circuit currents.

Coordination between SPD and its disconnecting circuit breakers, indicated and guaranteed by Electric, ensures reliable protection (lightning wave withstand, reinforced protection of impedance short-circuit currents, etc.)

Fig. J37 – Example of a coordination table between SPDs and their disconnecting circuit breakers. Always refer to the latest tables provided by manufacturers.

Coordination with upstream protection devices

Coordination with overcurrent protection devices
In an electrical installation, the external SCPD is an apparatus identical to the protection apparatus: this makes it possible to apply selectivity and cascading techniques for technical and economic optimization of the protection plan.

Coordination with residual current devices
If the SPD is installed downstream of an earth leakage protection device, the latter should be of the “si” or selective type with an immunity to pulse currents of at least 3 kA (8/20 μs current wave).

Installation of Surge Protection Device
Connection of Surge Protection Device

Connections of an SPD to the loads should be as short as possible in order to reduce the value of the voltage protection level (installed Up) on the terminals of the protected equipment.

The total length of SPD connections to the network and the earth terminal block should not exceed 50 cm.

One of the essential characteristics for the protection of equipment is the maximum voltage protection level (installed Up) that the equipment can withstand at its terminals. Accordingly, a SPD should be chosen with a voltage protection level Up adapted to the protection of the equipment (see Fig. J38). The total length of the connection conductors is

L = L1+L2+L3.

For high-frequency currents, the impedance per unit length of this connection is approximately 1 µH/m.

Hence, applying Lenz’s law to this connection: ΔU = L di/dt

The normalized 8/20 µs current wave, with a current amplitude of 8 kA, accordingly creates a voltage rise of 1000 V per metre of cable.

ΔU =1 x 10-6 x 8 x 103 /8 x 10-6 = 1000 V

Fig. J38 – Connections of a SPD L < 50 cm

As a result the voltage across the equipment terminals, U equipment, is:
U equipment = Up + U1 + U2
If L1+L2+L3 = 50 cm, and the wave is 8/20 µs with an amplitude of 8 kA, the voltage across the equipment terminals will be Up + 500 V.

Connection in plastic enclosure

Figure J39 below shows how to connect a SPD in plastic enclosure.

Fig. J39 – Example of connection in plastic enclosure

Connection in metallic enclosure

In the case of a switchgear assembly in a metallic enclosure, it may be wise to connect the SPD directly to the metallic enclosure, with the enclosure being used as a protective conductor (see Fig. J40).
This arrangement complies with standard IEC 61439-2 and the Assembly manufacturer must make sure that the characteristics of the enclosure make this use possible.

Fig. J40 – Example of connection in metallic enclosure

Conductor cross section

The recommended minimum conductor cross section takes into account:

• The normal service to be provided: Flow of the lightning current wave under a maximum voltage drop (50 cm rule).
  Note: Unlike applications at 50 Hz, the phenomenon of lightning being high-frequency, the increase in the conductor cross section does not greatly reduce its high-frequency impedance.
• The conductors’ withstand to short-circuit currents: The conductor must resist a short-circuit current during the maximum protection system cutoff time.
  IEC 60364 recommends at the installation incoming end a minimum cross section of:
• 4 mm2(Cu) for connection of Type 2 SPD;
• 16 mm2(Cu) for connection of Type 1 SPD (presence of lightning protection system).

Examples of good and bad SPD installations

Fig. J41 – Examples of good and bad SPD installations