An overvoltage is an excess of the maximum voltage rating for a particular network. Surge voltage refers to a sudden surge in voltage between phase and earth, which takes a fraction of a second. Such a voltage drop is dangerous not only for the line, but also for electrical appliances connected to it. To prevent this situation, a surge protection device is used.
Content
What is an SPD and why is it needed?
SPD is a surge protection device that provides protection for electrical installations up to 1 kV.The device protects against overvoltages in the mains, as well as against lightning effects by diverting current pulses to the ground.
SPDs are used only in low-voltage power distribution systems. This device is suitable for both industrial enterprises and residential buildings.
There are two types of SPDs:
- OPS - network surge arrester;
- SPE - surge voltage limiter.
Principle of operation and device
The principle of operation of the SPD is the use of varistors - a non-linear element in the form of a semiconductor resistance resistor against the applied voltage.
SPD has two types of protection:
- Unbalanced (common mode) - in case of overvoltage, the device sends impulses to the ground (phase - ground and neutral - ground);
- Symmetrical (differential) - in case of overvoltage, energy is directed to another active conductor (phase - phase or phase - neutral).
To better understand the principle of operation of SPDs, we present a small example.
The normal voltage of the circuit is 220 V, and when an impulse occurs in this very circuit, the voltage rises sharply, for example, during a lightning strike. With a sharp power surge, the resistance in the SPD decreases, which leads to a short circuit, which in turn leads to the operation of the circuit breaker and subsequently to the disconnection of the circuit itself. Thus, protection of electrical equipment from sudden voltage drops is ensured, preventing a high voltage pulse from flowing through it.
Varieties of SPD
Surge protection devices come with one and two inputs, and subdivided into:
- Commuting;
- limiting;
- Combined.
Switching protective devices
A characteristic feature of switching devices is a high resistance, which, when a strong impulse occurs in the voltage, instantly drops to zero. The principle of operation of switching devices is based on arresters.
Mains overvoltage limiters (SPD)
The mains voltage limiter is also characterized by high resistance. Its difference from the switching device is only that the decrease in resistance occurs gradually. The surge arrester is based on the operation of the varistor (resistor), which is used in its design. The resistance of the varistor is in a non-linear dependence on the voltage acting on it. With a sharp increase in voltage, there is also a sharp increase in the current strength, which passes directly through varistor and so electrical impulses are smoothed out in this way, after which the mains voltage limiter returns to its original state.
Combined SPDs
SPDs of combined type combine arresters and varistors, and can perform both the function of an arrester and a limiter.
SPD classes
There are only three classes of devices according to the degree of protection:
- Class I device (overvoltage category IV) - protects the system from direct lightning strikes, and is installed in the main switchboard or in the input distribution device (ASU). Be sure to use this device if the building is located in an open area and is surrounded by many tall trees, which increases the risk of lightning exposure.
- Class II device (overvoltage category III) - used as an addition to a class I device to protect the network from switching effects, i.e. from internal network overvoltage. Installed in the switchboard.
- Class III device (overvoltage category II) - used to protect against residual atmospheric and switching surges, as well as to eliminate high-frequency interference that has passed through a class II device. Installation is carried out both in ordinary sockets or junction boxes, and in the electrical appliances themselves, which must be secured.
Classification according to the degree of current discharge:
- Class B - air or gas discharges with a discharge current from 45 to 60 kA. They are installed at the entrance to the building in the main shield or in the input switchgear.
- Class C - varistor modules with discharge currents of the order of 40 kA. Are established in additional boards.
- Classes C and D are used in tandem when underground cable entry is required.
IMPORTANT! The distance between SPDs must be at least 10 meters along the length of the wiring.
How to choose an SPD?
The first thing to do when choosing an SPD is to determine the earthing system that is used in the building.
There are three types of grounding system:
- TN-S single phase;
- TN-S with three phases;
- TN-C or TN-C-S with three phases.
It is equally important to pay attention to the withstand temperature when purchasing the device. Most SPDs are designed to operate at temperatures down to -25. If your area has a very cold climate and winters are harsh, then the electrical panel should not be located outside, otherwise the device will fail.
When choosing an SPD, the following factors should also be taken into account:
- Importance of protected equipment;
- Risk of impact on the object: terrain (city or suburb, flat open area), zone with special risk (trees, mountains, reservoir), zone of special impacts (lightning rod at a distance of less than 50 meters from the building, which is dangerous).
In connection with the situation in which it became necessary to install an SPD, a suitable class (I, II, III) is selected.
It is also important to consider the voltage withstand of the device. For class I devices, this figure does not exceed 4 kV. A class II device withstands voltage levels up to 2.5 kV, and a class III device up to 1.5 kV.
Another important parameter when choosing an SPD is the maximum continuous operating voltage - the effective value of alternating or direct current, which is continuously applied to the SPD. This parameter must be equal to the rated voltage in the network. Details can be found in the information in IEC 61643 - 1, Appendix 1.
When connecting an SPD to protect equipment, it is important to take into account its rated direct or alternating current, which can be loaded.
How to connect an SPD in a private house?
The SPD is installed depending on the voltage indicator: 220V (one phase) and 380V (three phases).
The wiring diagram can be aimed at continuity or security, you need to prioritize. In the first case, lightning protection may be temporarily disabled in order to prevent interruption in the supply of consumers. In the second case, it is unacceptable to turn off lightning protection, even for a few seconds, but a complete shutdown of the supply is possible.
Connection diagram in a single-phase network of the TN-S earthing system
When using a single-phase TN-S network, a phase, zero working and zero protective conductor must be connected to the SPD. Phase and zero are first connected to the corresponding terminals, and then by a loop to the equipment line. A grounding conductor is connected to the protective conductor. SPD is installed immediately after the introductory machine. To facilitate the connection process, all contacts on the device are marked, so there should be no difficulties.
Explanation for the scheme: A, B, C - phases of the electrical network, N - working neutral conductor, PE - protective neutral conductor.
REFERENCE. It is recommended to use fuses for additional protection of the SPD, which are installed directly on the device itself.
Wiring diagram in a three-phase network of the TN-S earthing system
A distinctive feature of a three-phase TN-S network from a single-phase one is that five conductors come from the power source, three phases, a working neutral and a protective neutral conductor. Three phases and a neutral wire are connected to the terminals. The fifth protective conductor is connected to the body of the electrical appliance and the ground, that is, it serves as a kind of jumper.
Connection diagram in a three-phase network of the TN-C earthing system
In the TN-C earthing connection system, the working and protective conductors are combined into one wire (PEN), this is the main difference from the TN-S earthing.
The TN-C system is simpler and already quite outdated, and is common in an outdated housing stock. According to modern standards, the TN-C-S grounding system is used, in which there are separately zero working and zero protective conductors.
The transition to a newer system is necessary in order to avoid electric shock to service personnel and fire situations. And of course, in the TN-C-S system, protection against sudden surge surges is better.
In all three connection options, in case of overvoltage, the current is directed to earth through the earth cable or through a common protective conductor, which prevents the impulse from harming the entire line and equipment.
Connection errors
1. Installation of an SPD in a switchboard with a poor ground loop.
If you make such a mistake, you can lose not only all electrical appliances, but also the switchboard itself at the first lightning strike, since there will be no sense from protection with a bad ground loop, and, accordingly, no protection.
2. Incorrectly selected SPD that does not fit the earthing system used.
Before buying a device, be sure to find out which grounding system is used in your home, and when buying, carefully read its technical documentation to avoid mistakes.
3. Use of an SPD of the wrong class.
As already discussed above, there are 3 classes of surge protection devices. Each class corresponds to a specific switchboard, and must be installed in accordance with the rules and regulations.
4. Installation of SPDs of only one class.
It is often not enough to install an SPD of one class for reliable protection.
5. The class of the device and its destination are confused.
It also happens that class B devices are placed in the switchboard of the apartment, class C devices in the ASU of the building, and class D devices in front of electronic equipment.
SPD is certainly a good and necessary thing, but its use in the power supply at home is not mandatory.In the case of connecting this device, it is worth remembering that it is selected individually for each grounding system. It is for this reason that immediately before buying it is recommended to use the services of an experienced electrician in order to avoid trouble.
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