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.
principle, design and installation
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.
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.
Among other characteristics that differentiate overvoltage protection devices is their rated surge capacity and the achievable level of 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.
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.
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
When the voltage in the system raises itself beyond the rated voltage, then such type of voltage is called internal overvoltage.
The overvoltage which is caused by the atmospheric discharge such as static discharge or lightning strokes such type of voltage is called external overvoltage.
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 (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)
Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Figure J4)
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 is a high-frequency electrical phenomenon which causes over-voltages on all conductive items, especially on electrical cabling and equipment.
Analysis of the phenomena allows definition of the types of lightning current and voltage waves.
Procedure to prevent risks of lightning strike
The system for protecting a building against the effects of lightning must include:
The role of the building protection system is to protect it against direct lightning strokes.
The system consists of:
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 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.
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.
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.
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:
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.
To protect an electrical installation in a building, simple rules apply for the choice of
A SPD must always be installed at the origin of the electrical installation.
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:
The number of additional SPDs to be installed is determined by:
Several protection levels of SPD allows the energy to be distributed among several SPDs:
Impulse current Iimp
standard IEC 62305-2 defines 4 levels: I, II, III and IV
The maximum discharge current Imax is defined according to the estimated exposure level relative to the building’s location.
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.
The recommended minimum conductor cross section takes into account:
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.
The conductors of protected outgoing feeders:
The incoming feeder phase, neutral and protection (PE) conductors should run one beside another in order to reduce the loop surface.
The incoming conductors of the SPD should be remote from the protected outgoing conductors to avoid polluting them by coupling (see Fig. J44).
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.
Overvoltage may occur in electrical installations for various reasons. It may be caused by:
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.
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.
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.
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 SPD chiefly consists of:
Several technologies are available to implement the live part. They each have advantages and disadvantages:
The table below shows the characteristics and the arrangements of 3 commonly used technologies.
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.
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.
The mechanical indicator and the position of the control handle allow natural end-of-life indication.
principle, design and installation