Thunderstorm and surge protection. Part 2

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In the first part, we looked at how to protect a house from a lightning strike using a lightning rod, but protective measures are not limited to this. What else threatens us and how to protect ourselves from these threats, we will tell in this article..

Lightning and surge protection

As it was said at the very beginning, the organization of external protection is not enough. We remove only the risks of a direct lightning strike into the house and other objects located on the personal plot. Unfortunately, thunderstorms can affect objects that may even be outside the site. But the result of such an impact poses a serious danger to the home. In real conditions, this effect is more common than lightning striking directly into the house..

Internal surge protection

The channel through which a dangerous effect can be exerted is external electrical and communication networks. So, if lightning hit, for example, electrical networks even a few kilometers from a country house, the damage can be significant. From the failure of electronic devices and electrical equipment to a real fire. This effect is usually called impulse overvoltage. It should be noted that in addition to a thunderstorm, such overvoltage can be caused by other reasons, for example, an accident at a substation.

There are usually two reasons for overvoltage caused by a thunderstorm. The first is a direct lightning strike into a network, most often an electric one. The second is a lightning strike near the network. The fact is that with such a shock, an electric field arises, and we will receive an induced electric current, which causes an overvoltage. Lightning can strike near the house, or outside the infield. Hence the conclusion that it may be impossible to protect external networks from such an impact, therefore it is necessary to protect networks directly in the house.

There are two important points to note. First, for such a protection system to work, first of all, the electrical networks themselves must be performed at the proper level, in particular, a full-fledged potential equalization system must be implemented. The second important point is that there is no universal surge protector. Therefore, the zone principle is applied, and all protection devices are divided into classes and categories. Class “A” is not of interest to ordinary users, such equipment is intended for installation at substations. To protect a country house, equipment of class “B” to “D” is used.

Home protection

At the entrance to the building, the first level of protection is usually organized. For these purposes, class B equipment is used, its task is to limit the overvoltage to 2.5 kV. Typically, arresters of different types are used for such purposes. They are arranged simply, schematically, these are two contacts, between which the required gap is established. Under normal conditions, this gap acts as a dielectric. When a critical value is reached, a breakdown occurs, an arc discharge is formed between the contacts and the overvoltage is extinguished to ground.

Arrester Arrester for installation at the input

Such arresters are installed at the very entrance to the house. This is done in order to avoid affecting the protective conductor and the potential equalization junction. Dischargers are open and gas-filled. The parameters of open arresters depend on external influences, such as air humidity. In winter, the humidity is lower, but in winter, thunderstorms are very rare. Therefore, such an arrester must protect against accidents at the transformer substation. The overvoltage parameters in this case are known, which makes it possible to select the required device. In the summer, when a thunderstorm is to be expected, the air humidity rises, which means that the level of the arrester actuation decreases. At the same time, the arrester selected based on winter conditions will provide reliable protection in summer.

Gas-filled arrester Gas-filled arrester

In a gas-filled spark gap, the contacts are isolated from the external environment, and the container is filled with an inert gas at low pressure. Such devices have stable parameters, although they are more expensive..

Line protection

If for the whole house the voltage limitation of 2.5 kV may be justified, then for individual house lines it is excessive. Therefore, the next line is needed, which will protect the individual lines. Unfortunately, there is an opinion that simple machines are enough to protect. This is a dangerous delusion. The thing is that the machines have a slightly different purpose – they protect against abnormal situations on the line, for example, a short circuit. But, they cannot protect from external influences.

Varistors Varistors

Varistors are used to protect the lines; these are devices of class “C”, which protect against surges up to 1.5 kV. A varistor, or semiconductor resistor, is most often produced in a ceramic design. In normal mode, they have a resistance of a few GΩ, that is, there is practically no current flowing through them. When the critical value of the voltage is reached, the resistance drops sharply to tens of ohms; with a further increase in the voltage, the resistance only decreases, so the discharge is extinguished to the ground. For home networks (voltage 220/380 V 50 Hz), the critical voltage value is 470-560 V. Varistors are installed in distribution boards for each line that must be protected.

Protection of a specific device

The last line of defense is the protection of a particular household appliance. Class D devices are used for these purposes. This is especially true for electronic equipment that is sensitive to power surges. Uninterruptible power supply devices for computers that are familiar to us, and even surge protectors can have built-in protection of the required level.

Surge protection surge protector Surge protection surge protector

Usually, each device is not protected from such surges – for some household appliances such surges do not harm, the cost of others is simply much lower than the organization of such protection. For example, it is easier to replace an incandescent light bulb than to protect it from occasional power surges. In the same case, when protection is required, there are devices that allow you to protect even a single outlet. Most often, this is an arrester already familiar to us, however, designed for a lower critical level of impulse overvoltage. Varistors can also be used, also specialized.

It is important to remember that without organizing the protection of the upper levels, and this is the protection of the house and lines, it is still not worth hoping for reliable protection of a particular device.

Street electrical networks

We have practically figured out the electrical networks. Only the last case remains. The methods described above are designed to protect internal networks from overvoltage that is generated in the external network. But overvoltage can also occur in the internal network itself. This happens, for example, when it is necessary to connect devices located on the street to the electrical network. For example, this could be street lighting or an anti-icing system..

In such cases, the output of electrical networks outside the house must be organized as a separate line. And as an additional protective device, an arrester is installed, similar to that which is installed at the entrance to the house.

Protection of low-current networks

In a modern house, in addition to electrical, there are also low-current networks. They do not require protection from thunderstorms inside the house. But in the event that such networks are brought out of the house, then protection is necessary. An obvious example is a television antenna. A direct lightning strike is likely. Other low-current networks can also be brought outside the house. For example, there are two separate buildings to connect to a home computer network. And it is possible that such a network will be laid to control automatic irrigation or to organize video surveillance. If the cable is laid underground, there will be no direct lightning strike. Nevertheless, if we recall the inductive shock, it becomes clear that this will not protect against impulse overvoltage..

Lightning and surge protection Device for protection of low-current networks on a din rail

To protect low-current networks, both arresters and varistors can be used, of course, with appropriate parameters. But the equipment using such networks is very sensitive to overvoltage, therefore, more often combined devices are used that contain both a gas surge arrester and a varistor..

Lightning and surge protection Protection device for low-current networks of free installation

Protective devices are placed in low-current shields, on din rails. If, of course, SCS (structured cabling system) is organized in the house. If not, then they use free-standing devices, such small boxes designed to be fixed on the wall. It is convenient that the devices can be designed for several channels at once, usually no more than four.

Now the reader knows everything about protecting his country house from thunderstorms. It remains only to realize this knowledge in life.

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