Overvoltage Protection

Overvoltage Protection

Created by: Glen Zhu | Updated Date: Aug 27, 2022

Every year, several hundred thousand cases of damage from lightning strikes and overvoltages, with resulting costs in the multi-million euro range. Overvoltage protection devices are part of a comprehensive protection concept for electrical installations, and reliably prevent damage from overvoltage.

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Overvoltage Protection

principle, design and installation

Damage from overvoltage

Overvoltages are brief voltage peaks of less than a thousandth of a second that exceed by many times the allowable design operating voltage of electric devices. Such overvoltage events are usually caused by lightning strikes, electrostatic discharges or power grid switching operations, and are extremely hazardous.

Protection in three levels

The protection devices shall be selected in accordance with the electrical loads at the site of installation. This concept enables implementation of overvoltage and lightning protection measures tailored to local conditions and individual requirements.

The right device for any requirement

Among other characteristics that differentiate overvoltage protection devices is their rated surge capacity and the achievable level of protection.

  • Type 1 lightning current arrester: Protects against overvoltage and high currents triggered by direct or indirect lightning strikes
  • Type 2 surge arrester: Protects against overvoltage triggered by electrical switching operations
  • Type 3 surge protective device: Protects electrical loads (consumers) against overvoltage

Overvoltage Protection

When the voltage in a system, raised beyond its rated voltage, then it is known as overvoltage. This overvoltage may be of transient or persistent nature. The main cause due to which overvoltage is produced in the power system may be conveniently grouped into two categories, namely, internal and external. Internal overvoltage has got their origin within the system itself, whereas external overvoltage is because of lightning on the lines.

Voltage Surge

Definition: Voltage surge is defined as the sudden rise in excessive voltage which damages the electrical equipment of an installation. The overvoltage in the lines occurs because of a rise in voltage between both phases and between phase and ground.

Types of Voltage Surge

The overvoltage in the power station can be caused either by internal disturbance or by atmospheric eruption. On the basis of the generation of overvoltages, the voltage surge is classified into two categories. These are

  • Internal Overvoltage
  • External Overvoltage

Internal Overvoltage

When the voltage in the system raises itself beyond the rated voltage, then such type of voltage is called internal overvoltage.

External Overvoltages

The overvoltage which is caused by the atmospheric discharge such as static discharge or lightning strokes such type of voltage is called external overvoltage.

Overvoltage protection devices in the control cabinets of industrial automation

Automated facilities commanded by industrial control cabinets rely on the components in those cabinets to work reliably even in the face of power variations. These components include local power supplies, PLCs, data loggers, networking and other communication equipment, and IO – which are all vulnerable to the detrimental effects of voltage surge.

Overvoltage Protection Devices

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)

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)

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.

Lightning stroke impacts

Lightning is a high-frequency electrical phenomenon which causes over-voltages on all conductive items, especially on electrical cabling and equipment.

Characterization of the lightning wave

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

Principle of lightning protection

General rules of lightning protection

Procedure to prevent risks of 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.

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 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).

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.

The lightning conductor with meshed cage (Faraday cage)

This protection involves placing numerous down conductors/tapes symmetrically all around the building.

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

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. 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 over-voltages in the electrical installation.

Lightning protection - Electrical installation protection system

The main objective of the electrical installation protection system is to limit over-voltages 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: metallic mesh of exposed conductive parts.

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. 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.


SPD is designed to limit transient over-voltages 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 control gear.

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.

Elements of the protection system

A 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 In = 5 kA (8/20) µs;
  • Voltage protection level UP(at In) < 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 a 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.

Protection distributed levels

Several protection levels of SPD allows the energy to be distributed among several SPDs:

  • 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 over-voltages;
  • Type 3: provides “fine” protection if necessary for the most sensitive equipment located very close to the loads.

Common characteristics of SPDs according to the installation characteristics

Selection of a Type 1 SPD

Impulse current Iimp

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

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.

Installation of Surge Protection Device

Connection of Surge Protection Device

Connections of a 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.

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.

Cabling rules of Surge Protection Device

Rule 1

The first rule to comply with is that the length of the SPD connections between the network (via the external SCPD) and the earthing terminal block should not exceed 50 cm.

Rule 2

The conductors of protected outgoing feeders:

  • should be connected to the terminals of the external SCPD or the SPD;
  • should be separated physically from the polluted incoming conductors.
Rule 3

The incoming feeder phase, neutral and protection (PE) conductors should run one beside another in order to reduce the loop surface.

Rule 4

The incoming conductors of the SPD should be remote from the protected outgoing conductors to avoid polluting them by coupling (see Fig. J44).

Rule 5

The cables should be pinned against the metallic parts of the enclosure (if any) in order to minimize the surface of the frame loop and hence benefit from a shielding effect against EM disturbances.

In all cases, it must be checked that the frames of switchboards and enclosures are earthed via very short connections.

Finally, if shielded cables are used, big lengths should be avoided, because they reduce the efficiency of shielding.

SPD for photovoltaic applications

Overvoltage may occur in electrical installations for various reasons. It may be caused by:

  • The distribution network as a result of lightning or any work carried out.
  • Lightning strikes (nearby or on buildings and PV installations, or on lightning conductors).
  • Variations in the electrical field due to lightning.

Like all outdoor structures, PV installations are exposed to the risk of lightning which varies from region to region. Preventive and arrest systems and devices should be in place.

Protection by equipotential bonding

The first safeguard to put in place is a medium (conductor) that ensures equipotential bonding between all the conductive parts of a PV installation.

The aim is to bond all grounded conductors and metal parts and so create equal potential at all points in the installed system.

Protection by surge protection devices (SPDs)

SPDs are particularly important to protect sensitive electrical equipments like AC/DC Inverter, monitoring devices and PV modules, but also other sensitive equipments powered by the 230 VAC electrical distribution network.

Installing an SPD

Surge protection technical supplements

Lightning protection standards

The IEC 62305 standard parts 1 to 4 (NF EN 62305 parts 1 to 4) reorganizes and updates the standard publications IEC 61024 (series), IEC 61312 (series) and IEC 61663 (series) on lightning protection systems.

Part 1 – General principles

This part presents general information on lightning and its characteristics and general data, and introduces the other documents.

Part 2 – Risk management

This part presents the analysis making it possible to calculate the risk for a structure and to determine the various protection scenarios in order to permit technical and economic optimization.

Part 3 – Physical damage to structures and life hazard

This part describes protection from direct lightning strokes, including the lightning protection system, down-conductor, earth lead, equipotentiality and hence SPD with equipotential bonding (Type 1 SPD).

Part 4 – Electrical and electronic systems within structures

This part describes protection from the induced effects of lightning, including the protection system by SPD (Types 2 and 3), cable shielding, rules for installation of SPD, etc.

This series of standards is supplemented by:

  • the IEC 61643 series of standards for the definition of surge protection products (see The components of a SPD);
  • the IEC 60364-4 and -5 series of standards for application of the products in LV electrical installations (see End-of-life indication of a SPD).

The components of a SPD

The SPD chiefly consists of:

  1. one or more nonlinear components: the live part (varistor, gas discharge tube [GDT], etc.);
  2. a thermal protective device (internal disconnector) which protects it from thermal runaway at end of life (SPD with varistor);
  3. an indicator which indicates end of life of the SPD; Some SPDs allow remote reporting of this indication;
  4. an external SCPD which provides protection against short circuits (this device can be integrated into the SPD).

Technology of the live part

Several technologies are available to implement the live part. They each have advantages and disadvantages:

  • Zener diodes;
  • The gas discharge tube (controlled or not controlled);
  • The varistor (zinc oxide varistor [ZOV]).

The table below shows the characteristics and the arrangements of 3 commonly used technologies.

End-of-life indication of a SPD

End-of-life indicators are associated with the internal disconnector and the external SCPD of the SPD to inform the user that the equipment is no longer protected against over-voltages of atmospheric origin.

Local indication

This function is generally required by the installation codes. The end-of-life indication is given by an indicator (luminous or mechanical) to the internal disconnector and/or the external SCPD.

When the external SCPD is implemented by a fuse device, it is necessary to provide for a fuse with a striker and a base equipped with a tripping system to ensure this function.

Integrated disconnecting circuit breaker

The mechanical indicator and the position of the control handle allow natural end-of-life indication.

Table of Contents

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Overvoltage Protection

principle, design and installation

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