Impulse voltage when it enters the network. Uzip (devices for protection against surges and interference). How the Uzip is arranged and how it works

Modern household appliances often have built-in surge protection in their power supplies, however, the resource of typical solutions on varistors is limited to a maximum of 30 trips, and even then if the current in an emergency does not exceed 10 kA. Sooner or later, the protection built into the device can fail, and devices that are not protected from overvoltage will simply fail and bring a lot of trouble to their owners. Meanwhile, the causes of dangerous surge surges can be: a thunderstorm, repair work, surges when switching powerful reactive loads, and you never know what else.

To prevent such unpleasant situations, surge protection devices (abbreviated as SPDs) are designed, which take on an emergency overvoltage impulse, preventing it from damaging electrical appliances connected to the network.

The principle of operation of the SPD is quite simple: in normal mode, the current inside the device flows through a conductive shunt, and then through the load connected to the network at that moment; but a protective element is installed between the shunt and grounding - a varistor or a spark gap, the resistance of which in normal mode is megaohms, and if an overvoltage suddenly occurs, the protective element will instantly go into a conductive state, and the current will rush through it to ground.

At the moment the SPD is triggered, the resistance in the phase-zero loop will drop to critical, and the household appliances will be saved, because the line will be practically short-circuited through the protective element of the SPD. When the voltage in the line stabilizes, the protective element of the SPD will again go into a non-conductive state, and the current will again flow to the load through the shunt.

Three classes of surge protection devices exist and are widely used:

Class I protection devices are designed to protect against overvoltage impulses with a wave characteristic of 10/350 µs, which means that the maximum allowable rise time of an overvoltage impulse to a maximum and a fall to a nominal value should not exceed 10 and 350 microseconds, respectively; at the same time, a short-term current from 25 to 100 kA is acceptable, such pulsed currents occur during a lightning discharge when it enters a power line at a distance closer than 1.5 km to the consumer.

Devices of this class are carried out on arresters, and their installation is carried out in the main switchboard or input-distribution device at the entrance to the building.

SPDs of class II are designed to protect against short-term impulse noise, and are installed in switchboards. They are able to provide protection against overvoltage impulses with parameters of 8/20 µs, with a current strength of 10 to 40 kA. SPDs of this class use varistors.

Since the resource of varistors is limited, a mechanical fuse is added to the design of SPDs based on them, which simply unsolders the shunt from the varistor when its resistance ceases to be adequate to the safe protective mode. This is, in fact, a thermal protection that protects the device from overheating and fire. On the front of the module there is a color indicator associated with the fuse, and if the varistor needs to be replaced, it will be easy to understand.

SPDs of class III are arranged in a similar way, with the only difference that the maximum current of the internal varistor should not exceed 10 kA.

The traditional impulse protection circuits built into household appliances have the same parameters, however, when duplicating them with an external Class III SPD, the probability of premature equipment failure is minimized.

In fairness, it should be noted that for reliable protection of equipment, it is important to install SPDs of both I, II and III protection classes. This must be observed, since a powerful class I SPD will not operate with short impulses of a low overvoltage simply because of its low sensitivity, and a less powerful one will not cope with a large current that a class I SPD can handle.

Many of the processes that take place in our home, we do not even assume that this happened due to overvoltage. Our Philips TV burned out, and we sin against the manufacturer's company that we had to buy Samsung. And why it burned down - we don’t even think about it.

What is surge voltage?

An overvoltage is a short-term increase in voltage at a power point in excess of the permissible value. After this jump, the voltage in the network is restored to its original value. The degree of voltage distortion in this case is characterized by an indicator of impulse voltage.

For example, a sinusoidal voltage of 220 V is supplied to our apartment. Surge overvoltages can occur in the electrical network (we will consider the reason for their occurrence a little later), this is when an overvoltage surge occurs, lasting several milliseconds, but the amplitude (maximum value) can reach up to 10 thousand. AT.

Why is surge voltage dangerous for household electrical appliances?

The insulation of any electrical appliance is designed for a certain voltage level. As a rule, electrical appliances with a voltage of 220 - 380 V are designed for an overvoltage impulse of about 1000 V. And if overvoltages occur in the network with an impulse of 3000 V? In this case, insulation breakdown occurs. A spark appears - an ionized gap of air through which an electric current flows. As a result, an electric arc, short circuit and fire.

Note that insulation surge may occur even if you have all appliances unplugged from outlets. Under voltage in the house, electrical wiring, junction boxes, and the same sockets will still remain. These network elements are also not protected against surge voltage.

Causes of impulse overvoltage

One of the causes of impulse overvoltage is lightning discharges (lightning strike). Switching surges that occur as a result of switching on / off consumers with a large load. In case of phase imbalance as a result of a short circuit in the network.

Protecting your home from surges

It is impossible to get rid of impulse overvoltages, but in order to prevent insulation breakdown, there are devices that reduce the magnitude of impulse overvoltage to a safe value.

These protective devices are SPD - surge protection device.

Exists partial and full protection by SPDs.

Modern man, trying to keep up with the times, saturates his house with electrical appliances for various purposes. But not every homeowner thinks that in the event of even a very short-term impulse voltage in the network that is several times higher than the nominal one, his entire expensive fleet of electrical engineering and electronics may fail. Remarkably, the impact of overvoltage on electrical consumers is detrimental in that the affected equipment, as a rule, becomes unsuitable for repair. This force majeure, though not often, but guaranteed, can be the result of overvoltage in networks caused by thunderstorms, emergency phase overlap or switching processes. The so-called surge protection devices are designed to protect electrical equipment. The principle of SPD operation, classes and the difference between them are discussed below.

SPD classification

Surge protection devices are a broad and generalized concept. This category of devices includes devices that can be divided into classes:

• I class. Are intended for protection against direct influence of a lightning category. These devices must be equipped with input-distribution devices (ASU) of administrative and industrial buildings and residential apartment buildings.
• II class. They provide protection of electrical distribution networks from overvoltages caused by switching processes, as well as performing the functions of the second stage of protection against the effects of a lightning strike. Mounted and connected to the network in switchboards.
• III class. They are used to protect equipment from surge voltages caused by residual voltage surges and asymmetric voltage distribution between the phase and the neutral wire. Devices of this class also work in the mode of high-frequency noise filters. The most relevant for the conditions of a private house or apartment, they are connected and installed directly at consumers. Particularly popular are devices that are manufactured as modules equipped with a quick-release mount for installation on, or have the configuration of electrical sockets or power plugs.

Device types

All devices providing surge protection are divided into two types, which differ in design and principle of operation. Consider how SPDs of different types work.

Valve and spark gaps. The principle of operation of the arresters is based on the use of the effect of spark gaps. The design of the arresters provides for an air gap in the jumper connecting the phases of the power line with the ground loop. At the nominal voltage value, the circuit in the jumper is broken. In the event of a lightning discharge, as a result, a breakdown of the air gap occurs in the power transmission line, the circuit between the phase and the ground is closed, the high voltage pulse goes directly to the ground. The design of the valve arrester in a circuit with a spark gap provides for a resistor on which the high-voltage pulse is quenched. Arresters in most cases are used in high voltage networks.

Surge arresters (SPDs). These devices have replaced the outdated and bulky arresters. In order to understand how the limiter works, you need to remember the properties of non-linear resistors, built on the use of their current-voltage characteristics. Varistors are used as non-linear resistors in SPDs. For people who are not experienced in the intricacies of electrical engineering, a little information about what it consists of and how it works. The main material for the manufacture of varistors is zinc oxide. In a mixture with oxides of other metals, an assembly is created, consisting of p-n junctions, which has current-voltage characteristics. When the voltage value in the network corresponds to the nominal parameters, the current in the varistor circuit is close to zero. At the moment of overvoltage on p-n junctions, a sharp increase in current occurs, which leads to a decrease in voltage to the nominal value. After the normalization of the network parameters, the varistor returns to the non-conducting mode and does not affect the operation of the device.

The compact dimensions of the surge arresters and a wide range of varieties of these devices made it possible to significantly expand the scope of these devices, it became possible to use SPDs as a means of surge protection for a private house or apartment. However, surge voltage limiters assembled on varistors, despite all their advantages compared to arresters, have one significant drawback - the limitation of the service life. Due to the thermal protection built into them, the device after operation remains inoperative for some time, for this reason, a quick-detachable device is provided on the SPD housing, which allows for a quick replacement of the module.

You can learn more about what an SPD is and what its purpose is from the video:

How to arrange protection?

Before proceeding with the installation and connection of surge protection equipment, it is necessary, otherwise all work on the arrangement of SPDs will lose all meaning. The classic scheme provides 3 levels of protection. Arresters (SPD class I) are installed at the input, providing lightning protection. The next protective device is class II, usually the surge arrester is connected in the switchboard of the house. The degree of its protection should ensure the reduction of the magnitude of the overvoltage to the parameters safe for household appliances and the lighting network. In the immediate vicinity of electronic products sensitive to fluctuations in current and voltage, class III is desirable.

When connecting an SPD, it is necessary to provide for their current protection and protection against short circuits by an introductory circuit breaker or fuses. We will tell you more about the installation of these protective devices in a separate article.

So we have considered the principle of operation of SPDs, classes and the difference between them. We hope the information provided was useful to you!