What are the requirements for a protective shutdown and what functions does it perform? Safety shutdown. Electrical safety Fires and explosions
Protective shutdown is especially important when a large number of different electrical appliances are used in the house. In this article, we will consider protective shutdown devices that are recommended and used in the construction of private houses. A diagram of the residual current device will be given. Let's analyze the question of what and when to use - RCD or difavtomat (differential automaton). In addition, we will find out the main differences between circuit breakers.
Types of circuit breakers
An important step in the organization of electrical safety are protective electrical devices or, as they are more commonly called, automatic machines. Conventionally, they can be divided into three types:
- automatic switches (AB);
- differential shutdown devices (RCD);
- differential automatic switches (DAV).
Fig 1. Circuit breaker
Fig 2. Residual current device (RCD)
Fig 3. Differential circuit breaker (DAB)
The principle of operation of residual current devices
Automatic switches (AB), see Fig. 1, we install to protect electrical wiring from overcurrent, and electrical consumers from short circuits. Overcurrent leads to heating of the conductor, which leads to ignition of the wiring and its failure.
Residual current device (RCD) principle of operation(Fig. 2). We install to protect against electric shock, in the event of a breakdown of the insulation of equipment and wiring. The RCD will also protect us in case of touching open uninsulated sections of wiring or equipment that are energized with 220 V and will not allow a fire to occur if the wiring is faulty.
If there is a difference in currents, then the RCD turns off the voltage supply. It is necessary to choose an RCD according to two parameters: sensitivity and rated current. Usually for home purposes, an RCD with a sensitivity of 300 mA is chosen. The rated current is selected depending on the total power of the electrical consumers and must be equal to or be an order of magnitude lower than the rated current of the introductory circuit breaker (AB), because the RCD does not protect against short circuits and overcurrents. An RCD residual current device is usually installed in the circuit after the meter to protect all wiring in the house, see fig. 4, 5. According to modern standards, the installation of an RCD is mandatory.
Rice. 4. RCD connection diagram
Rice. 5 Wiring diagram for power supply at home using RCD
1 - w distribution flow; 2- neutral; 3 - sh ina grounding; 4 - f aza; 5 - RCD; 6 - av tomatic switch; 7 - pconsumer nutrition.
Differential circuit breakers (DAB) combine the functions of RCD and AV. The scheme of the differential machine is based on the protection of circuits from short circuits and overloads, as well as the protection of people from electric shock when touching live parts, see fig. 6.
Rice. 6. Scheme of work of DAV
These devices are widely used in household electrical networks (220/380 V), in socket networks. The differential circuit breaker consists of a fast-acting circuit breaker and a residual current device that reacts to the difference in currents in the forward and reverse directions.
The principle of operation of the differential machine. If the insulation of the electrical wiring is not damaged and there is no human contact with live parts, then there is no leakage current in the network. This means that the currents in the forward and reverse (phase-zero) load conductors are equal. These currents induce equal but oppositely directed magnetic fluxes in the magnetic core of the current transformer DAV. As a result, the current in the secondary winding is zero and does not trigger the sensitive element - the magnetoelectric latch.
If a leak occurs, for example: when a person touches a phase conductor, the balance of currents and magnetic fluxes is disturbed, an unbalance current appears in the secondary winding, which causes the magnetoelectric latch to operate, which in turn acts on the release mechanism of the machine with a contact system.
To carry out periodic monitoring of the performance of the RCD and DAV, a testing circuit is provided. By pressing the "Test" button, a differential trip current is artificially created. The operation of the protection devices means that it is generally in good order.
Choice of circuit breaker
Now, let's decide in which case and which circuit breaker we prefer:
- To protect the wiring of the lighting network, from which all our lamps are powered, we choose circuit breakers (AB) with tripping currents 16 A.
- The socket network in the house, which is used to turn on irons, table lamps, a TV, a computer, etc., must be protected by circuit breakers with differential protection (DAV).
- For the socket network, we choose DAV with a trip current of 25 A and differential current shutdown 30 mA.
- To connect an air conditioner, a dishwasher, an electric oven, a microwave oven and other powerful appliances that we need so much in everyday life, we need our own individual socket and, therefore, our own circuit breaker with differential protection. For example, to connect an electric furnace with a power of 6 kW, a differential circuit breaker with tripping currents of 32 and 30 mA is required.
Paying attention, that the sockets must all be with a grounding contact. Power equipment, such as a grinder, I advise you to connect to a circuit breaker. Since the entire network in our house is for a voltage of 220 V, we also select the listed circuit breakers for the appropriate voltage.
Let's talk about the circuit breaker, which, for security reasons, needs to be put on the input. If we protected all the outlet lines with automatic switches with differential protection, then at the input we put an automatic switch (AB) with a rated current of certain technical conditions and a single-line diagram of the project "Electrical equipment of a residential building".
But it is possible to put a residual current device (RCD) with a differential protection current of 300 mA after the introductory circuit breaker (AB). See Fig. 5 for such a switching scheme. If we choose this protection option, then it does not oblige us to install differential circuit breakers for the outlet network, but simply install a circuit breaker (AB), see the same fig. 5. Such a scheme is acceptable if we have only one outlet line with a number of outlets. But it is absolutely not rational if we have a number of independent receivers included in individual sockets.
For example: You have current leakage to the body of the washing machine and you accidentally touch it. The differential protection will immediately work and the DAV of the washing machine will turn off. It will not be difficult for you to determine and eliminate the cause. And imagine how much work is needed to find the reason for the RCD shutdown at the input.
I want to say that in the modern market of automatic switches and RCDs there is a very large selection of devices, both domestic and foreign. It should be taken into account that domestic products are distinguished by large overall dimensions, the possibility of current regulation, a lower price, and the service life in domestic conditions is almost the same.
Table 1. Cost comparison of circuit breakers
Conclusion
So, in the article we have considered the issues of electrical safety. They became especially relevant when a huge number of electrical appliances, consumer electronics and computers entered our home. The wiring carries a very high load and a protective shutdown is necessary. Modern technology is very expensive and demanding on the quality of networks. Therefore, you should not save on protection measures, because the cost of an RCD is not commensurate with the cost of equipment in your home, and even more so with the price of a human life.
Attention: Prices are valid for 2009.
Residual shutdown - high-speed protection that provides automatic shutdown of the electrical installation in the event of a danger of electric shock in it.
Such a danger may arise, in particular, when a phase is shorted to the electrical equipment case; when the insulation resistance of the phases relative to the ground drops below a certain limit; the appearance of a higher voltage in the network; touching a person to a live part that is energized. In these cases, some electrical parameters change in the network: for example, the case voltage relative to earth, phase voltage relative to earth, zero sequence voltage, etc. can change. Any of these parameters, or rather, changing it to a certain limit, at which danger arises electric shock to a person, can serve as an impulse that causes the operation of a protective shutdown device, i.e. automatic shutdown of a dangerous section of the network.
Residual current devices (RCDs) must ensure the shutdown of a faulty electrical installation in no more than 0.2 s.
The main parts of the RCD are a residual current device and a circuit breaker.
Residual current device - a set of individual elements that react to a change in any parameter of the electrical network and give a signal to turn off the circuit breaker.
A circuit breaker is a device used to turn on and off circuits under load and in case of short circuits.
RCD types.
RCDs that respond to the voltage of the case relative to the ground are designed to eliminate the danger of electric shock when an increased voltage occurs on a grounded or grounded case.
RCDs that respond to operational direct current are designed to continuously monitor the insulation of the network, as well as to protect a person who has touched the current-carrying part from electric shock.
Consider a circuit that provides protection when voltage appears on the case relative to ground.
Rice. Residual shutdown circuit at voltage on
hull relative to the ground.
The scheme works as follows. When the button P is turned on, the power circuit of the winding of the MP magnetic starter is closed, which turns on the electrical installation with its contacts and self-blocks along the circuit composed of normally closed contacts of the “stop” button C, protection relay RZ and auxiliary contacts.
When a voltage relative to the ground appears on the housing Uz, equal in magnitude to the long-term permissible contact voltage, under the action of the RZ (KRP) coil, the protection relay is activated. RZ contacts break the MP winding circuit, and the faulty electrical installation is disconnected from the network. The artificial circuit circuit, activated by the K button, serves to monitor the health of the shutdown circuit.
It is advisable to use protective shutdown in mobile electrical installations and when using hand-held power tools, since their operating conditions do not allow ensuring safety by grounding or other protective measures.
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6.4. Safety shutdown
Protective shutdown is a high-speed protection that provides automatic shutdown of an electrical installation when there is a danger of electric shock to a person in it.
Currently, protective shutdown is the most effective electrical protective tool. The experience of developed foreign countries shows that the massive use of residual current devices (RCDs) has provided a sharp reduction in electrical injuries.
Protective shutdown is increasingly used in our country. It is recommended for use as one of the means to ensure electrical safety by regulatory documents (NTD): GOST 12.1.019-79, GOST R 50571.3-94 PUE, etc. In some cases, the mandatory use of RCDs in electrical installations of buildings is required (see GOST R 5066.9 -94). The objects to be equipped with AEO include: newly built, reconstructed, overhauled residential buildings, public buildings, industrial facilities, regardless of ownership and ownership. The use of RCDs is not allowed in cases where a sudden shutdown can lead, for technological reasons, to situations that are dangerous for personnel, to turn off fire, burglar alarms, etc.
The main elements of the RCD are a residual current device and an actuator - a circuit breaker. A residual current device is a combination of individual elements that perceive the input signal, react to its change and, at a given signal value, act on the switch. The actuating device is a circuit breaker that provides shutdown of the corresponding section of the electrical installation (electrical network) upon receipt of a signal from the residual current device.The main requirements for RCD:
1) Speed - shutdown time (), summed from the time of the device (tp) and the time of the switch (tb), must meet the condition
The existing designs of devices and devices used in protective shutdown circuits provide a shutdown time tooff = 0.05 - 0.2 s.
2) High sensitivity - the ability to respond to small values of input signals. Highly sensitive RCD devices allow you to set the settings for the switches (the values of the input signals at which the switches operate), ensuring the safety of a person touching the phase.
3) Selectivity - the selectivity of the action of the RCD, i.e. the ability to disconnect from the network the area in which there is a danger of electric shock to a person.
4) Self-monitoring - the ability to respond to own faults by turning off the protected object is a desirable property for RCDs.
5) Reliability - the absence of failures in operation, as well as false positives. Reliability must be sufficiently high, since RCD failures can create situations associated with electric shock to personnel.
The scope of RCDs is practically unlimited: they can be used in networks of any voltage and with any neutral mode. RCDs are most widely used in networks up to 1000 V, where they provide safety when a phase is shorted to the case, the insulation resistance of the network relative to the ground drops below a certain limit, a person touches a live part that is energized, in mobile electrical installations, in power tools, etc. Moreover, RCDs can be used as independent protective devices, and as an additional measure for grounding or protective grounding. These properties are determined by the type of RCD used and the parameters of the protected electrical installation.
Types of residual current devices. The operation of the electrical network in both normal and emergency modes is accompanied by the presence of certain parameters that may vary depending on the conditions and mode of operation. The degree of danger of human injury in a certain way depends on these parameters. Therefore, they can be used as input signals for RCDs.
In practice, the following input signals are used to create an RCD:
Hull potential relative to earth;
earth fault current;
Zero sequence voltage;
Differential current (zero sequence current) ;
Phase voltage relative to ground;
operational current.
In addition, combined devices are also used that respond to several input signals.
Below is a diagram and operation of a residual current device that reacts to the potential of the housing relative to the ground.
The purpose of this type of RCD is to eliminate the danger of electric shock to people in the event of an increased potential on a grounded or grounded case. Usually these devices are an additional measure of protection to grounding or grounding. The device is triggered if the potential φk that has arisen on the body of the damaged equipment is higher than the potential φkdop, which is selected based on the highest continuous allowable contact voltage Upr.dop.
The sensor in this circuit is the voltage relay RN,
Fig.28. Schematic diagram of an RCD that responds to
potential of the housing connected to the earth with the help of an auxiliary earthing conductor Rvop
When a phase is shorted to a grounded (or zeroed) case, protective grounding first acts, which reduces the voltage on the case to the value Uk = Iz * Rz,
where Rz is the resistance of the protective earth.
If this voltage exceeds the relay setting voltage RN Uset, then the relay will operate due to the current Ir, opening the power supply circuit of the MP magnetic starter with its contacts. And the power contacts of the magnetic starter, in turn, will de-energize the damaged equipment, i.e. RCD will do its job.
Operative (working) switching on and off of the equipment is carried out by the START, STOP buttons. The contacts of the BC of the magnetic starter provide its power after the START button is released.
The advantage of this type of RCD is the simplicity of its circuit. The disadvantages include the need for auxiliary grounding, the lack of self-monitoring of serviceability, the non-selectivity of shutdown in the case of connecting several cases to one protective ground electrode, and the variability of the setpoint when changing Rvop.
Next, consider the second circuit that responds to differential current (or zero sequence current) - RCD (D). These devices are the most versatile, and therefore are widely used in production, public buildings, residential buildings, etc.
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Safety shutdown
Protective shutdown - a type of protection against electric shock in electrical installations, which provides automatic shutdown of all phases of the emergency section of the network. The duration of disconnection of the damaged section of the network should be no more than 0.2 s.
Fields of application of protective shutdown: addition to protective grounding or zeroing in an electrified tool; addition to zeroing to turn off electrical equipment remote from the power source; measure of protection in mobile electrical installations with voltage up to 1000 V.
The essence of the protective shutdown is that damage to the electrical installation leads to changes in the network. For example, when a phase is shorted to earth, the phase voltage changes relative to the earth - the value of the phase voltage will tend to the value of the linear voltage. This creates a voltage between the neutral source and the ground, the so-called zero-sequence voltage. The total resistance of the network relative to the ground decreases when the insulation resistance changes in the direction of its decrease, etc.
The principle of constructing protective shutdown schemes is that the listed regime changes in the network are perceived by the sensitive element (sensor) of the automatic device as signal input values. The sensor acts as a current or voltage relay. At a certain value of the input value, the protective shutdown is activated and switches off the electrical installation. The value of the input variable is called the setpoint.
The block diagram of the residual current device (RCD) is shown in fig.
Rice. Structural diagram of the residual current device: D - sensor; P - converter; KPAS - emergency signal transmission channel; IO - executive body; MOP - a source of danger of defeat
The sensor D responds to a change in the input value B, amplifies it to the value KB (K is the transfer coefficient of the sensor) and sends it to the converter P.
The converter is used to convert the amplified input value into a KVA alarm. Further, the channel for transmitting the emergency signal of the KPAS transmits the AC signal from the converter to the executive body (EO). The executive body performs a protective function to eliminate the danger of damage - it turns off the electrical network.
The diagram shows areas of possible interference that affect the operation of the RCD.
On fig. a schematic diagram of a protective shutdown using an overcurrent relay is given.
Rice. Residual current device diagram: 1 - maximum current relay; 2 - current transformer; 3 - ground wire; 4 - ground electrode; 5 - electric motor; 6 - starter contacts; 7 - block contact; 8 - starter core; 9 - working coil; 10 - testing button; 11 - auxiliary resistance; 12 and 13 - stop and turn buttons; 14 - starter
The coil of this relay with normally closed contacts is connected through a current transformer or directly into the cut of the conductor going to a separate auxiliary or common ground electrode.
The electric motor is switched on by pressing the "Start" button. In this case, voltage is applied to the coil, the starter core is retracted, the contacts close and the electric motor is connected to the network. At the same time, the auxiliary contact is closed, as a result of which the coil remains energized.
When one of the phases is shorted to the case, a current circuit is formed: the place of damage - the case - the ground wire - the current transformer - the ground - the capacitance and insulation resistance of the wires of undamaged phases - the power source - the place of damage. If the current reaches the current relay trip setting, the relay will trip (i.e., its normally closed contact will open) and break the magnetic starter coil circuit. The core of this coil will be released and the starter will turn off.
To check the serviceability and reliability of the protective shutdown, a button is provided, when pressed, the device is triggered. The auxiliary resistance limits the earth fault current to the required value. Buttons are provided to enable and disable the starter.
The system of public catering enterprises includes a large complex of mobile (inventory) buildings made of metal or with a metal frame for street trade and service (snack bars, cafes, etc.). As a technical means of protection against electrical injuries and from a possible fire in electrical installations, the mandatory use of a residual current device at these facilities is prescribed in accordance with the requirements of GOST R50669-94 and GOST R50571.3-94.
Glavgosenergonadzor recommends using for this purpose an electromechanical device of the ASTRO-UZO type, the principle of operation of which is based on the effect of possible leakage currents on a magnetoelectric latch, the winding of which is connected to the secondary winding of a leakage current transformer, with a core made of special material. The core in the normal mode of operation of the electrical network keeps the release mechanism in the on state. In the event of any malfunction in the secondary winding of the leakage current transformer, an EMF is induced, the core is retracted, and the magnetoelectric latch is activated, which is associated with the mechanism of free decoupling of contacts (the knife switch is turned off).
ASTRO-UZO has a Russian certificate of conformity. The device is included in the State Register.
A residual current device should be equipped not only with the above structures, but also with all premises with an increased or special risk of electric shock, including saunas, showers, electrically heated greenhouses, etc.
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Safety shutdown is... What is Safety shutdown?
Safety shutdownSAFETY DISCONNECTING - high-speed protection that provides automatic shutdown of an electrical installation with a voltage of up to 1000 V in the event of a danger of electric shock in it. Such a danger can arise when a phase is shorted to the case, the insulation resistance drops below a certain value, and if a person touches a live part that is energized. In such situations, a measure of protection can only be a quick shutdown of the corresponding section of the electrical network in order to break the current circuit through a person. The response time of modern residual current devices (RCD) does not exceed 0.03-0.04 s. With a decrease in the time of current flow through a person, the risk of injury is reduced. So, in household electrical installations of alternating current with a frequency of 50 Hz with a voltage of up to 1000 V, the action of a touch voltage of 100, 200 and 220 V, respectively, for 0.2, 0.1 and 0.01-0.03 s can be considered practically safe. RCDs are used in networks of any voltage and with any neutral mode, although they are most common in networks with voltages up to 1000 V. In networks with a grounded neutral, RCDs provide safety when the phase is shorted to the case and when the insulation resistance of the network drops below a certain value, and in networks with isolated neutral - also the safety of a person touching a live part of an electrical installation that is energized. However, these properties also depend on the type of RCD and the parameters of the electrical installation. There are several types of RCDs depending on the input values to which they respond: the potential of the electrical installation case, the earth fault current, the zero sequence voltage, the zero sequence current, the phase voltage relative to earth, the operational current.
Russian encyclopedia of labor protection. - M.: NTs ENAS. Ed. V. K. Varova, I. A. Vorobieva, A. F. Zubkova, N. F. Izmerova. 2007.
- Safety fence
- Protective device
See what "Safety shutdown" is in other dictionaries:
Protective shutdown - 75 Protective shutdown High-speed protection that provides automatic shutdown of the electrical installation when there is a risk of electric shock in it, as well as in emergency mode
safety shutdown - eng safety shutdown (с) eng circuit separation fra séparation (f) des circuits deu Schutztrennung (f) spa separación (f) de los circuitos … Occupational safety and health. Translation into English, French, German, Spanish
Protective shutdown - English: Earth leakage circuit High-speed protection that provides automatic shutdown of an electrical installation when there is a danger of electric shock in it (according to GOST 12.1.009 76) Source: Terms and definitions in the electric power industry. ... ... Construction dictionary
Protective shutdown in electrical installations up to 1 kV - Automatic shutdown of all phases (poles) of the network section, providing combinations of current and its passage time that are safe for humans in case of short circuits to the case or a decrease in the insulation level below a certain value Source ... Glossary-reference book of terms of regulatory and technical documentation
automatic protective shutdown - quick shutdown of power supply sources, water supply, equipment and mechanisms in an emergency. A. h. about. carried out using special automatic devices for direct or alternating current ... Russian encyclopedia of labor protection
automatic protective shutdown of electrical equipment (electrotechnical device) - A type of explosion protection of electrical equipment (electrotechnical device), which consists in removing voltage from current-carrying parts when the protective sheath is destroyed in a time that excludes the ignition of an explosive atmosphere. [GOST 12.2.020 76] Topics ... ... Technical translator's guide
Automatic protective shutdown of electrical equipment (electrical device) - 19. Automatic protective shutdown of electrical equipment (electrical device) Type of explosion protection of electrical equipment (electrical device), which consists in relieving voltage from current-carrying parts when the protective is destroyed ... ... Glossary-reference book of terms of regulatory and technical documentation
Protective shutdown - see Protective shutdown ... Russian encyclopedia of labor protection
protective shutdown - A protection system that provides automatic shutdown of all phases or poles of an emergency section of the network with a full shutdown time from the moment a one-time short circuit occurs [Terminological dictionary for construction in 12 languages (VNIIIS ... ... Technical translator's guide
protective shutdown device - A device for operational switching of power electrical circuits, which provides almost instantaneous automatic shutdown of all phases or poles of an emergency element or section of a circuit in the event of a mode dangerous for personnel maintenance ... Technical translator's guide
labor_protection.academic.ru
Why you need a residual current device for your home and how to choose one
Oleg Udaltsov
Eaton Power Distribution Components Product Specialist.
What is a residual current device
A residual current device, also known as an RCD, is a device installed in an electrical panel in an apartment or house to automatically turn off the power supply to the network in the event of a ground fault current.
Ground fault current occurs in wiring and / or electrical appliances when, for some reason, the insulation is broken in them, or when the bare parts of the wires that must be fixed in the terminals, for example, inside household electrical appliances, touch the housing of the devices - and the current begins to "leak" in the wrong direction.
This can lead to a fire due to overheating (first the wiring or device, and then everything around) or to the fact that a person or a pet will suffer from the current - the consequences can be extremely unpleasant, even death. But this will happen only if you touch the conductor or the body of the equipment, which is energized.
The main difference between an RCD and a conventional circuit breaker is that it is designed specifically to break the earth fault current that the circuit breaker cannot detect. The RCD is able to turn it off in a fraction of a second, until the moment when it becomes dangerous to a person or property.
Where and how much to install
For one- and two-room apartments - in the general electrical panel of the apartment. If the housing area is large, then in several local electrical panels distributed throughout the house.
An RCD will be required for the entire system to protect against fire, as well as for separate lines that feed groups of electrical appliances with a metal case (washing and dishwasher, electric stove, refrigerator, and so on) - to protect against electric shock. If a malfunction occurs or an accident occurs, not the entire apartment will be de-energized, but only one line, so it will be easy to determine the culprit of the RCD trip.
However, it must be borne in mind: neither RCDs nor conventional automata save from an electric arc, or an arc breakdown.
An electric arc can occur when, for example, the wire from an electric lamp is often pinched by a slamming door and the metal part of the wire inside is damaged. At the site of damage, sparks hidden from the eyes will occur, accompanied by an increase in ambient temperature and, as a result, ignition of flammable objects nearby: first the wire sheath, and then wood, fabric or plastic.
To protect against such hidden threats, it is better to choose solutions that combine the functions of an automaton, RCD and arc fault protection. In English, such a device is called an arc fault detection device (AFDD), in Russia the name “arc fault protection device” (AFDD) is used.
An electrician can include the installation of such a device in the circuit if you tell him that you need an increased degree of protection. For example, for a children's room, where a child can handle wires inaccurately, or for groups of sockets for powerful electrical appliances with flexible wires prone to breakage.
It is equally important to install protection devices where the wiring is laid in an open way and it can be damaged. And also during the planned repair, in order to avoid risks in case of accidental damage to hidden electrical wiring while drilling walls.
How to choose
A good electrician will recommend the manufacturer of the RCD and calculate the load, but you need to be sure that the recommendations are correct. And if you buy everything for repairs yourself, then all the more you need to understand what to look for when choosing a device.
Price
Do not purchase a device in the lower price range. The logic is simple: the better the components inside, the higher the price. For example, in some cheap devices there is no protection against burnout, and this can lead to ignition.
A cheap device can be made of brittle materials and break easily when you lift up the lever that has fallen when triggered. According to the RCD standard, it should be designed for 4,000 operations. This means that you will have to be puzzled by the choice only once, but only if you have purchased a quality product. By purchasing a low-quality device, you put yourself and loved ones at risk, not to mention material losses in case of fire.
Case quality
Pay attention to how tightly all parts of the device fit together. The front panel should be monolithic, and not consist of two halves. The preferred material is heat-resistant plastic.
Device weight
Give preference to heavier devices. If the RCD is light, then the manufacturer has saved on the quality of internal components.
Conclusion
To resolve issues related to electricians in the house, it is advisable to involve professionals. However, the entire responsibility should not be shifted to their shoulders. It is better to be guided by the proverb "Trust, but verify." With even basic knowledge of the subject and an understanding of the scenario for the future use of electrical appliances in the house, you can save yourself and loved ones from problems with electricity.
In networks with dead-earthed neutral voltage up to 1 kV (systems TN) protective grounding is ineffective, since even with a dead earth fault, the current depends on the grounding resistance and when it decreases, the current increases, and the touch voltage can reach dangerous values. Therefore, in systems TN protection against electric shock in case of indirect contact is provided by limiting the time of exposure to electric current on the human body. For this to be done protective automatic power off, providing protection both against overcurrents (short circuit currents) and called protective zeroing, and against leakage currents using residual current devices that respond to differential current (UZO-D).
Protective automatic power off automatic opening of the circuit of one or more phase conductors (and, if required, the zero working conductor), performed for electrical safety purposes.
Auto Power Off Assignment prevention of the appearance of contact voltage, the duration of which can be dangerous if the insulation is damaged.
For automatic power off, protective switching devices can be used that respond to overcurrents (circuit breakers) and are installed in phase conductors, or to differential current (UZO-D).
Protective nulling Intentional electrical connection of open conductive parts with a dead-earthed neutral point of the current source winding in three-phase networks. This connection is made using a null protective PE- or combined PEN-conductor.
Schematic diagram of protective earthing in a three-phase current network (system TN- S) is shown in Figure 14.8.
The principle of operation of protective zeroing transformation of a short circuit on open conductive parts (metal cases of electrical installations) into a single-phase short circuit (short circuit between phase and neutral protective conductors) in order to cause a high short circuit current I k, capable of providing protection operation and thereby automatically disconnecting the damaged electrical installation from the mains.
When shorting, for example, a phase conductor L 3 on the zeroed case (Fig. 14.8), the short-circuit current passes through the following sections of the circuit: the winding of the transformer (generator), phase L 3 and zero protective PE-the wire. The magnitude of the current is determined by the phase voltage and the impedance of the single-phase short circuit circuit:
while the transformer resistance Z t, phase wire Z f.pr and zero protective PE-wires Z n have active and inductive components.
The protection devices are fuses, automatic fuses and circuit breakers, which should provide a short circuit opening (shutdown) time.
In addition, since grounded cases (or other exposed conductive parts) are grounded through a neutral protective PE- (or combined PEN-) conductor and re-groundings R n, then in the emergency period, i.e. from the moment a short circuit to the case occurs and until the damaged electrical installation is automatically disconnected from the network, the protective property of this grounding manifests itself, as with protective grounding. Due to the flow of fault current I h through the resistance of re-grounding R n, voltage PE- conductor (or PEN-conductor), and, consequently, the cases of electrical equipment connected to it, relative to the ground decreases in the emergency period until the protection is triggered or in the event of a break PE- (or PEN-) conductor. Thus, protective grounding performs two protective actions - a quick automatic disconnection of the damaged installation from the supply network and a decrease in the voltage of the grounded metal non-current-carrying parts that are energized relative to the ground.
Re-grounding PE- or PEN- conductors on overhead lines are carried out on all branches with a length of more than 200 m and at the input to the electrical installation. In a network with a voltage of 380/220 V, the neutral grounding resistance should be no more than 4 ohms, and the total spreading resistance of the grounding conductors of all repeated groundings PE- or PEN-conductor - no more than 10 Ohm.
Protective automatic shutdown time for the system TN at rated phase voltage should not exceed the following values: 127 V - 0.8 s; 220 V - 0.4 s; 380 V - 0.2 s; more than 380 V - 0.1 s.
To achieve the specified power-off time, the single-phase short-circuit current must be at least three times the rated current of the fuse link of the nearest fuse or the operating current of the trip unit of the circuit breaker with an inverse current characteristic. When protecting the network with automatic switches with an electromagnetic release, the excess of the short-circuit current over the rated current is determined by the type of electromagnetic release: A, B, C, D.
Rice. 14.8. Schematic diagram of protective grounding.
Automatic shutdown using residual current devices (RCD ) responsive to leakage currents. At low short circuit currents, leakage currents, a decrease in the level of insulation, as well as a break in the neutral protective conductor, the protective grounding is not effective enough, therefore, in these cases, the RCD is the only means of protecting a person from electric shock. Modern residual current devices (RCDs) have a speed of 0.04 to 0.3 s.
RCDs are created on various principles of operation. The most perfect is the RCD that responds to the leakage current (differential current). Its advantage lies in the fact that it protects a person from electric shock both in the case of contact with open conductive parts of the electrical installation that are energized due to damage to the insulation, and with direct contact with live parts. It is these RCDs that can be simultaneously attributed to the means of protection both in case of indirect, as well as direct contact.
In addition, the RCD performs another important function - the protection of electrical installations from fires, the root cause of which is leakage caused by insulation deterioration. It is known that more than a third of fires arise from electrical wiring faults, therefore, RCDs are quite rightly called the "fire watchman".
The RCD consists of three functional elements: a sensor, an actuator and a switching device. The sensor detects leakage currents flowing from phase conductors to earth in the event of direct contact by a person or damage to the insulation. The signal about the presence of a leakage current enters the executive body, where it is amplified and converted into a command to turn off the switching device. The most widely used RCDs are based on the use of information about the occurrence of dangerous situations of a differential current transformer (DCT) as a sensor. The executive body of the RCD can operate on two different principles: electronic and electromechanical.
The electrical circuit of the electromechanical RCD is shown in Figure 14.9. The sensor of the device is DTT (I), the annular magnetic circuit of which covers the wires that supply the load and play the role of the primary winding. In the absence of leakage current, the operating currents (I1) in the forward (phase L) and (I2) in reverse (zero operating N) the wires are equal and induce equal but oppositely directed magnetic fluxes in the magnetic circuit; the resulting flux is zero and therefore there is no EMF in the secondary winding. RCD does not work. When a leakage current (I ) appears (for example, when a person is shorted to the case or a person touches a bare phase wire), the current in the forward wire exceeds the reverse current by the amount of leakage current I ; an unbalance magnetic flux occurs in the core, and an EMF proportional to the leakage current is induced in the secondary winding. A current flows through the winding of the magnetoelectric relay (2), causing it to operate and acting on the free trip mechanism (3), which disconnects the contacts. RCD works. This is the action of a bipolar RCD in a single-phase load circuit.
To work in a three-phase network (both three- and four-wire), the RCD is performed as a four-pole, that is, the magnetic circuit covers three phase and zero worker conductors. Some types of residual current devices (mainly foreign-made) combine the functions of an RCD and a circuit breaker, which inevitably leads to a decrease in reliability and an increase in cost due to the complexity of the circuit and an increase in the number of components.
According to the type of operating voltage (leakage current), RCDs are divided into types:
AC - only for alternating (sinusoidal) voltage;
A - for sinusoidal voltage and pulsating voltage with a constant component.
When choosing an RCD, it should be borne in mind that washing machines, personal computers, televisions, light source regulators can be a source of pulsating voltage.
RCD is a highly effective and promising method of protection. It is used in electrical installations up to 1 kV in addition to protective grounding (protective grounding), as well as the main or additional method of protection when other methods and means are inapplicable or ineffective.
Rice. 14.9. The electrical circuit of the RCD.
What is a safety shutdown used for?
The danger of electric shock is determined by the voltage of contact (£ / doya1, V) and then by the strength of the current that can pass through the human body (/ "A). As you know.
where /? A is the resistance of the human body, Ohm.
If the contact voltage at the moment a person touches the body or network phase exceeds the permissible value, then there is a real threat of electric shock and the degree of protection in this case can only be a break in the current circuit, disconnecting the corresponding section of the network. To accomplish this task, a safety shutdown is used.
A protective shutdown is a fast-acting protection that provides automatic shutdown of an electrical installation in the event of a danger of electric shock to a person.
Grounding and zeroing do not always guarantee the safety of people. Protective shutdown disconnects the damaged section of the installation much faster than zeroing, than more guaranteed protection of people from electric shock.
When is a safety switch used?
Protective shutdown is used only in electrical installations with voltages up to 1000 V as independent protection or simultaneously with grounding:
in mobile electrical installations with isolated generator neutral;
in stationary installations with isolated neutral for the protection of those working with hand power tools;
in stationary electrical installations with a dead-earthed neutral on separate high-power consumers remote from transformers, on which zeroing protection is ineffective;
where there is an increased risk of electric shock. The scope of application of residual current devices is practically unlimited. They can be used in networks of any purpose and with any neutral mode. However, they are most widely used up to 1000 V, especially where it is difficult to carry out effective grounding or grounding, when there is a high probability of accidental contact with live parts (mobile electrical installations, hand-held power tools).
What are the requirements for a protective shutdown and what functions does it perform?
Protective shutdown can be used as the main type of protection or together with grounding and zeroing.
The following requirements are placed on the residual current device: self-control, reliability, high sensitivity and short turn-off time.
Protective shutdown, alone or in combination with other means of protection, performs the following functions:
protection in case of a short circuit to the ground or the equipment case;
protection against dangerous leakage currents;
protection during the transition of higher voltage to the lower side;
automatic control of the circle of protective grounding and zeroing.
How is a safety shutdown performed?
The protective shutdown is carried out by very sensitive and fast protective emerging devices. The sensitivity and transient action of them significantly exceeds automatic switches or other measures of the elements.
In electrical circuits, protective shutdown devices use sensitive elements that respond to the appearance of current in the neutral wire, voltage on the case of damaged electrical equipment, etc.
Protective shutdown devices operate in 0.1-0.05 s, while zeroing takes 0.2 or more seconds. With such a short duration of the passage of current through the human body, a current of even 500-600 mA will be safe. Given that the resistance of the human body is 1000 ohms, then the current of the reduced value can flow through the human body only if its voltage is 500-650 V, and there cannot be such a voltage in electrical networks with a voltage of 380/220 V with a grounded neutral even in emergency mode in emergency situations.
Protective disconnection is also used in cases where the grounding device will cause significant difficulties (rocky soils) or will be impractical due to the moving front of the work.
Therefore, protective disconnecting devices are reliable protection of people from electric shock.
One of the safety measures in electrical installations is the use of low voltages of the order of 36.34.12 V or less: for local lighting lamps at machine tools; for portable lamps (12 V); power supplies for electric soldering irons, electric drills and other electric tools.
C. Safety shutdown
Purpose, principle of operation, scope. The protective shutdown is called automatic shutdown of electrical installations in case of single-phase (single-pole) contact with live parts that are unacceptable for humans, and (or) if a leakage current (short circuit) occurs in the electrical installation that exceeds the specified values.
Assignment of a protective shutdown- Ensuring electrical safety, which is achieved by limiting the time of exposure to dangerous current on a person. Protection is carried out by a special residual current device (RCD), which, operating in standby mode, constantly monitors the conditions for electric shock to a person.
Scope: electrical installations in networks with any voltage and any neutral mode.
The protective shutdown is most widely used in electrical installations used in networks with voltage up to 1 kV with a grounded or isolated neutral.
The principle of operation of the RCD is that it constantly monitors the input signal and compares it with a predetermined value (setpoint). If the input signal exceeds the setting, the device operates and disconnects the protected electrical installation from the network. As input signals of residual current devices, various parameters of electrical networks are used, which carry information about the conditions of electric shock to a person.
All RCDs are classified into several types according to the type of input signal (Fig. 4.11).
Fig.4.11. RCD classification by type of input signalIn addition, RCDs can be classified according to other criteria, for example, by design.
The main elements of any residual current device are a sensor, a converter and an actuator.
The main parameters by which this or that RCD is selected are: rated load current i.e. the operating current of the electrical installation, which flows through the normally closed contacts of the RCD in standby mode; Rated voltage; setting; device response time.
Let's consider in more detail
RCD responding to the potential of the housing relative to the ground, designed to ensure safety in the event of an increased potential on a grounded (or zeroed) electrical installation housing. The sensor in this device (Fig. 4.12) is the relay P, the winding of which is connected between the electrical installation case and the auxiliary ground electrodeR in. Auxiliary earth electrodesR c are located outside the spreading zone of the ground electrode currentsR h .
In the event of a ground fault, protective earth
R h will reduce the potential of the body relative to the ground to a value j h=I h R h. If for some reason it turns out that j c >j zdop , where j zdop - the potential of the case, at which the contact voltage does not exceed the permissible value, then the relay P is activated, which closes the power supply circuit of the coil of the switching device with its contacts and the damaged electrical installation is disconnected from the network.In fact, this type of RCD duplicates the protective properties of grounding or grounding and is used as additional protection, increasing the reliability of grounding or grounding.
This type of RCD can be used in networks with any neutral mode, when grounding or zeroing is ineffective.
RCDs that respond to differential (residual) current are widely used in all industries. Their characteristic feature is multifunctionality. Such RCDs can protect a person from electric shock with direct contact, with indirect contact, with an asymmetric decrease in the insulation of wires relative to the ground in the protection zone of the device, with ground faults and in other situations.
The principle of operation of the differential type RCD is that it constantly monitors the differential current and compares it with the setting. When the value of the differential current of the RCD setting is exceeded, it trips and disconnects the emergency consumer of electricity from the network. The input signal for three-phase RCDs is the zero sequence current. The RCD input signal is functionally related to the current flowing through the human body
I h .The scope of the RCD of the differential type is networks with a grounded neutral voltage up to 1 kV (TN-S system).
Scheme of switching on an RCD that reacts to differential current in a network with a grounded neutral type
TN-S shown in Figure 4.13.
The sensor of such a device is a zero-sequence current transformer (CTCT), on the output windings of which a signal is generated that is proportional to the current through the human body
I h . The RCD converter (P) compares the value of the input signal with the setting, the value of which is determined by the permissible current through a person, amplifies the input signal to the level necessary to control the executive body (EO). The executive body, for example, a contactor, disconnects the electrical installation from the network in the event of a danger of electric shock in the protection zone of the RCD.According to the operating conditions, differential RCDs are divided into the following types: AC, A, B,
S, G.RCD type AC is a residual current device that reacts to an alternating sinusoidal differential current that occurs suddenly or slowly increases.
RCD type A is a residual current device that responds to an alternating sinusoidal differential current and a pulsating direct differential current that occurs suddenly or slowly increases.
RCD type B is a residual current device that responds to alternating, direct and rectified differential currents.
S – residual current device, selective (with time delay). G - same as type S but with shorter delayStructurally, differential RCDs are divided into two types:
Electromechanical RCDs, functionally independent of the supply voltage. The source of energy necessary for the functioning of such RCDs - performing protective functions, including the tripping operation, is the input signal itself - the differential current to which it responds.
- Electronic RCDs functionally dependent on supply voltage. Their mechanism to carry out the shutdown operation requires energy, obtained either from a controlled network or from an external source.
