As a rule, the voltage in electrical networks should be within the limits defined by technical standards, but sometimes it can deviate from valid parameters. The maximum allowable voltage must be within ± 10% of the nominal voltage parameters, so for a single-phase network it will be from 198 to 242 V, and for a three-phase network from 342 to 418 V. And any deviations from these values will be called overvoltages.
Overvoltages are of a different nature and differ from this in duration and magnitude. Typically, long-term overvoltages occur due to some kind of breakdown of the step-down transformer at the substation or a break in the neutral wire in the network.
These overvoltages are relatively small indicators, but they act for a long time and pose a real threat to humans and to your equipment.
A long voltage rise may occur due to uneven distribution of loads across all phases in the external network . It is then that there will be a phase imbalance, in which the voltage on the loaded phase will be lower, and on the unloaded phase it will naturally be higher than the nominal.
Short-term voltage spikes can occur due to switching in the power grid or when switching on sufficiently strong reactive loads.
Strong surge voltages are caused by lightning discharges.
And the voltage can reach tens of kilovolts.
These impulses last for hundreds of microseconds, and special circuit breakers simply do not have time to respond to them, because the most modern types of machines have a response time of one milliseconds, and this can cause failure and damage to the insulation between phase and neutral.
Although this will not cause a short circuit and will not disrupt the operation of the network, it will lead to a small leakage current in the place of insulation damage. And if it passes between the phase and the neutral, it will not be detected by the circuit breakers either, and this will lead to increased heating of the insulation and an accelerated aging process.
Over time, the insulation resistance in this section of the value decreases, and the leakage current increases.
The effects of these negative factors on the electronic equipment and wiring in the house can be catastrophic, so a surge protector is needed for the home network.
The possibility of using different SPDs to perform certain protective functions is characterized by technical indicators reflected in the marking of a particular device.
It accurately determines the parameter of the residual voltage that appears at the terminals of the SPD after the passage of the discharge current.
It will be equal to the nominal voltage, taking into account the possible overvoltage at various abnormal operating modes of the entire network.
Direct current supplied to the load protected by the SPD. This parameter is important for SPDs connected to the network in series with the protected equipment.
A large number of surge protection devices are connected in parallel with the circuit, and this parameter is usually not marked with them.
For more reliable and high-quality protection of home electrical wiring from overvoltage, you need to create a multi-level protection system from SPDs of different classes. A class 1 SPD is designed for a current of 60 kA, a class 2 SPD for a current of 40 kA.
SPD of class 3 for a current of 10 kA.
When introducing a multi-stage overvoltage protection system in the network, it is necessary to ensure the appropriate power of each stage, that is, their maximum current should not exceed their nominal values.
And first of all, you need to create a high-quality grounding and overvoltage protection system.
Varistors are semiconductor resistors, and when they work, the effect of reducing the resistance is applied semiconductor material with increasing applied voltage, due to this they are more effective devices for impulse protection.
The varistor must be connected in parallel to the protected equipment and during normal operation it will be directly under the action of the operating voltage of the protected mechanism.
In operating mode, the current passing through the varistor is very small, and under these conditions it is an insulator.
When a voltage pulse appears, the varistor resistance will sharply decrease to fractions of an ohm. In this case, a current of several thousand amperes will flow through it for a short time.
After extinguishing this voltage pulse, it will again acquire a very high resistance.
The SPD is selected in accordance with the protection system. Be sure to take into account all the technical indicators of the devices, which are indicated in the catalog and printed on the front of the device case.
The UE-18/380 device is designed to protect the electrical network from short-term overvoltage caused by lightning processes.
This device provides protection and belongs to the SPD of the 3rd class and is made on varistors . For high-quality protection against long-term overvoltage associated with accidents in the electrical network, the device must be connected after the RCD and grounded.
It is with such a connection that a leakage current will be created, and the RCD will operate.
When installing and assembling an SPD, it is necessary that the distance between the protection stages is at least 10 m along the power cable.
The fulfillment of this requirement is sufficiently important for the correct switching sequence of protective devices. For class B protection, the first stage is installed outside the house in a special entrance panel.
From the protected area, all surge arresters can be divided into classes or types. Type 1 devices protect objects from external atmospheric and switching overvoltages passing through class A arresters of external electrical networks. As a rule, they are mounted on the input device of a residential building and limit the amount of overvoltage to 4.
0 kV, accompany the protection of input meters and electrical equipment of the switchboard.
A special surge protection device is necessary to prevent all kinds of damage to household appliances from strong surge voltages that are caused by various accidents in the supply network or lightning discharges. These devices are also called surge arresters (SP).
Usually they are made on the basis of arresters or varistors and have special indicator devices that signal their breakdown. SPDs based on varistors are manufactured with a special mounting on a DIN rail.
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