8. Surge protection.

Sources of impulse overvoltages.

During the summer period, a thunderstorm discharge into an air line causes the appearance of surge voltages of tens of kilovolts, which have the character of traveling waves with a large steepness and a rise time from zero to a maximum of 1.0 ÷ 8.0 μs. Once in the building’s internal distribution network, a discharge can cause breakdown, fire ignition, and electrical failure.

Lightning discharger protection devices (such as lightning conductors or Faraday cages) short the thundering discharge currents to the ground, greatly increasing the earth's potential near the buildings on which they are installed. This causes overvoltage on electrical equipment directly through the ground loop, as well as indirectly due to crosstalk on underground power supply cables.

Similar effects can cause switching overvoltages that occur when switching to substations or when starting and disconnecting high-power consumers.
Parasitic interference may, for example, be the result of work:
• Arc furnaces
• Contactors
• Welding machines
• Safety automat
• Thyristor devices
• Start motors
• And so on.

• These pickups have low energy, but their short duration, steep wave front and peak value (which can reach several kilovolts) can have a painful effect on the correct operation of sensitive equipment, causing it to breakdown or complete destruction.

Surge Protector

Surge arresters (SPD) are designed to protect the internal distribution circuits of residential and public buildings from lightning and switching overvoltages.

Structurally, the limiters contain a solid-state composite varistor of zinc carbide and a mechanism for the visual control of the degree of "wear" of the varistor with an "emergency" fuse.

Zinc carbide has the property of almost instantly reducing its resistance thousands of times when a replaceable voltage module exceeds the maximum permissible value at the terminals.

With the help of SPDs you can create a very effective and long-term protection of the object.

One of the main conditions for this is the presence of a ground loop, and for industrial premises - a system of potential equalization; because, despite the short duration, a lightning discharge carries considerable energy. The maximum peak value of the discharge current can reach 100 kA, and in the absence of potential equalization, the occurrence of a dangerous step voltage is quite possible. The three-stage protection system inside the building allows to smoothly lower the dangerous overvoltage impulse “along the way” towards the consumer to a safe value by selecting and “draining” some of the energy into the ground with high-speed arresters of each stage.

When installing arresters, it should be noted that the sequential (selective) operation of the protection stages will be ensured if the distance between the steps in the air and cable circuits is at least 7 ÷ 10 m. In this case, when a traveling discharge wave appears, the inductance of the circuit section will create the necessary time constant voltage rise delays.

The distance from the arresters installed in the subscriber switchboard of the consumer to the most remote load must not exceed 30 m.

Classification of electrical equipment for resistance to overvoltage:

I –to 1.5 kV
Special equipment, which, being connected to the existing electrical installations of buildings, requires additional surge protection devices. Surge protection devices can be integrated into category I equipment or located between this equipment and the rest of the electrical installation (for example, personal computers that are connected to the mains through extension cables with integrated surge protection devices).

II –to 2.5 kV
Equipment that is connected to existing electrical installations of buildings using receptacles and other similar connectors (for example, household electrical appliances, electronic devices, portable tools).

III –to 4.0 kV
Equipment installed inside buildings, which is part of the building’s specific electrical installation and is accessible to ordinary persons and untrained personnel. Examples of such equipment are distribution boards, wiring, switches and sockets, electric stoves.

IV –to 6.0 kV
Equipment installed close to electrical installations of buildings (inside or outside) in front of the main switchboard, which can be an input distribution device for multi-storey buildings or a flat panel for individual buildings (for example, electric meters, primary overcurrent protection devices).

Application areas of SPD in accordance with the classification voltage:

I (B) - The first stage of protection against direct or indirect lightning discharges in power lines at the input to the object. Installed at the entrance to the building in the input-distribution device (ASU) or in the main switchboard (MSB).

II (C) - The second stage of protection of the internal distribution circuits of an object from lightning discharges and switching overvoltages. Install into switchboards.

III (D) - The third stage of protection of electrical equipment of an object from residual voltage surges, protection against differential (asymmetrical) overvoltages. Install in close proximity to electrical consumers (electrical appliances).

Installing an arrester in the TN-CS 220/380 V network

In order to reliably protect the object from the effects of any kind of overvoltage, it is first necessary to create an effective grounding and potential equalization system with the TN-S or TN-CS power supply system.

This is important not only from the point of view of protection against impulse overvoltages, but also to protect people from electric shock (RCD is possible).


The next step is to install security devices.

The basic principles of the use of SPDs in the domestic regulatory framework are discussed in GOST R 50571.26-2002. When installing protective devices, it is necessary that the distance between adjacent protection steps be at least 10 m along the power supply cable. Compliance with this requirement is very important for the proper operation (coordination of the operation) of protective devices. At the moment when a pulsed lightning surge arises in the power cable due to an increase in the inductive resistance of the metal cores of the cable when a current pulse flows through it, a voltage drop occurs that is applied to the first protection stage. Thus, its primary operation is achieved (the necessary time delay in the rise of the overvoltage pulse at the next stage of protection is ensured).


Overvoltages caused by lightning discharges create very large currents (several tens of kiloampers), and these currents run for a very short time (several microseconds). As a result, the connecting cables represent a very high impedance.

(impedance). The voltage that occurs at 1 meter of cable is:
U = L di / dt = 1200 V (assuming that the inductance of 1 meter of the wire is 1 μH, and the current waveform corresponds to 8/20).

The length of the connecting conductors between the SPD and the live conductors, as well as between the SPD and the ground terminal, should be as short as possible, since the impedance of these conductors will significantly reduce the effectiveness of the protection.


The cross section of the cable cores depends on the expected short-circuit current, which can go from the power supply network to the electrical installation. The cross section of cables must be not less than the cross section of conductors in the rest of the electrical installation.


The grounding conductors of all interconnected surge protection devices and equipment must have equal grounding potentials to prevent any potential difference between local grounding points, which can negate the level of protection provided by overvoltage protection devices.


A characteristic feature of the current-voltage characteristic of a varistor is the presence of a section of small currents (from zero to several milliamps), which contains the working point of the varistor and a section of large currents (up to thousands of amperes), which in some cases is called tunneling.

The tunnel section largely determines the functional properties and, in particular, the limiting voltage, i.e. maximum impulse voltage acting on the protected electrical equipment when it is shunted by a varistor.

One of the characteristics of the varistor is the classification voltage (Ukl). Voltage at current 1.5 mA is indicated as a classification one.

Application of SPD in regulatory documents:

GOST R 50571.19-2000 Issues of protection of electrical installations of buildings from lightning and switching overvoltages.
GOST R 50571.20-2000 Protection against overvoltage caused by electromagnetic effects.
GOST R 50571.26-2002 Selection and installation of devices for protection against impulse overvoltages in electrical installations of buildings.
GOST R 51992-2002 Health requirements and test methods for devices for protection against impulse overvoltages - SPD.
GOST R 50571.21-2000 GOST R 50571.22-2000 The use of SPD in electrical installations containing information processing equipment, in addition to the requirements for the protection of information technology equipment from failures.
EMP (chapter 7.1, ed. 7th) During the air entry into residential, public and other buildings, surge suppressors should be installed.