According to requirements regulatory documentsThe safety of electrical installations is ensured by the following main measures:
- 1) inaccessibility of current parts;
- 2) proper, and in some cases of increased (double) isolation;
- 3) grounding or reducing electrical equipment and elements of electrical installations that can be under voltage;
- 4) reliable and high-speed automatic protective shutdown;
- 5) the use of reduced stresses (42 V and below) to power portable current collectors;
- 6) protective separation of chains;
- 7) locking, warning alarms, inscriptions and posters;
- 8) the use of protective equipment and devices;
- 9) carrying out planned-warning repairs and preventive tests of electrical equipment, apparatus and networks in operation;
- 10) carrying out a number of organizational events ( special learning, certification and rertesting of persons electrical personnel, instructions, etc.).
The following technical methods and means of protection are used to provide electrical safety at meat and dairy industries: protective grounding, reinforcement, low voltage, use of insulation of windings, means individual protection and safety devices, protective disconnecting devices.
Protective grounding - This is a deliberate electrical connection with the Earth or its equivalent of metal inadvertent parts, which may be energized. It protects against electric shock when touched by metal hardware, metal structures of electrical installations, which, due to disruption of electrical insulation, are substituted.
The essence of protection lies in the fact that when closing the current passes on both parallel branches and is distributed in between them inversely proportionate to their resistance. Since the resistance of the "Land" chain is many times the resistance of the chain "Corps-Earth", the strength of the current passing through a person is reduced.
Depending on the placement of the grounding agent relative to grounded equipment, remote and contour grounding devices are distinguished.
Remote earthingers are located at some distance from the equipment, while the grounded electrical installation housings are located on Earth with zero potential, and a person touching the housing is under full earthing voltage.
Contour earthingers are located along the contour around the equipment in close proximity, so the equipment is located in the zone of current spreading. In this case, when you closure to the body, the soil potential on the territory of the electrical installation (for example, substation) acquires values \u200b\u200bclose to the grounding potential and grounded electrical equipment, and the touch voltage is reduced.
Stage - This is a deliberate electrical connection with a zero protective conductor of metal inadequate parts, which may be energized. With such an electrical connection, if it is reliably completed, any closure on the body turns into a single-phase short circuit (i.e., the closure between the phases and zero wire). At the same time, there is a current of such a force at which the protection of protection (fuse or machine) is ensured and the automatic shutdown of the damaged installation from the supply network.
Small tension - Voltage not more than 42 V, used in order to reduce the risk of electric shock. Small alternating voltages are obtained using lower transformers. It is used when working with portable power tools, when using portable luminaires during installation, dismantling and repairing equipment, as well as in remote control schemes.
Isolation of workplace - This is a set of measures to prevent the occurrence of the current chain of the Land and an increase in the value of transitional resistance in this chain. This protection measure is applied in cases of increased danger of electric shock and usually in combination with a separation transformer.
Allocate the following types of insulation:
- · Working - electrical insulation of current-carrying electrical installation parts, providing its normal operation and protection against electric shock;
- · Additional - electrical insulation provided in addition to working isolation to protect against electric shock in case of damage to working isolation;
- · Double - electrical insulation consisting of working and additional isolation. Double insulation consists in one electrical appliance of two independent ones from another insulation steps (for example, a coating of electrical equipment with a layer of insulating material - paint, film, varnish, enamel, etc.). The use of double insulation is the most rational when, in addition to the working electrical insulation of the current-carrying parts, the electrical receipt housing is made of insulating material (plastics, fiberglass).
Protective shutdown - This is a high-speed protection that provides automatic shutdown of the electrical installation in it occurs in it the danger of electric shock.
It should ensure an automatic shutdown of electrical installations with single-phase (single-pole) touch to parts under voltage, not allowed for humans, and (or) when the leakage current occurs in the electrical installation of the leakage current (closure) exceeding the specified values.
Protective shutdown is recommended as the main or additional measure Protection, if security cannot be provided when grounding or reworked, or if the grounding or zero is difficult, or inexpedient for economic considerations. Devices (devices) for a protective shutdown with respect to the reliability of action must meet special specifications.
Individual protection products are divided into insulating, auxiliary and enclosing.
Insulating protective funds Provide electrical isolation of a person from current-carrying parts and land. They are divided into main ( dielectric gloves, tool with insulated handles) and additional (dielectric galoshes, mats, stands)
Auxiliary can be attributed to glasses, gas masks, masks designed to protect against light, thermal and mechanical impacts.
The enclosure includes portable shields, cells, insulating lining, portable grounding and posters. They are intended mainly for the temporary fence of the current-carrying parts to which the touch of working.
Safety when working with electrical installations is provided by the use of various technical and organizational measures. They are regulated by the operating rules of the electrical installation device (PUE). The technical means of protection against electric shock are divided into collective and individual, funds warning the touch of people to the elements of a network that are under voltage, and the means that ensure safety if the touch still happened.
Main ways and means electroplature:
Insulation of conductive parts and its continuous control;
Installation of protective devices;
Warning alarm and blocking;
Use of safety signs and warning posters;
Using low voltages;
Electrical separation of networks;
Protective grounding;
Equalization of potentials;
Zero;
Protective shutdown;
Individual electric power facilities.
Isolation of conductive parts is one of the main electrical safety measures. According to PUE, the insulation resistance of the conductive parts of the electrical settings relative to the Earth should be at least 0.5-10 MΩ 1. Distinguish working, double and enhanced working isolation.
Workers It is called insulation that ensures the normal operation of the electrical installation and protection of personnel from electric shock. Double isolation, consisting of working and optional, is used in cases where an increased electrical safety of equipment is required (for example, manual power tools of household electrical appliances, etc.). The resistance of double insulation should be at least 5 MΩ, which is 10 times the resistance of the usual working. In some cases, working insulation is carried out so reliably that its electrical resistance is at least 5 MΩ and therefore it provides the same protection against shock, as well as double. Such insulation is called enhanced working isolation.
There are basic and additional insulating agents. The main one is called such electric cavalries, the isolation of which reliably can withstand the working voltage. Additional electrical equipment reinforce the isolation of a person from conductive parts and ground. In tab. 20.2 provides basic information about insulating electrical equipment.
Uninsulated conductive parts of electrical installations operating under any voltage must be reliably fenced or located in an inaccessible height to eliminate the accidental touch to them. Structurally, the fences are made of solid metal sheets or metal grids.
To prevent the danger of electric shock, various sound, light and color alarms, installed in the visibility zones and audibility of personnel, use. In addition, blockages are provided in the structures of electrical installations - automatic devices that block the path to a hazardous zone or is preventing incorrect, dangerous to humans. Blocking can be mechanical (stoppers, latches, curly cuts), electrical or electromagnetic. For hazard personnel, warning posters are used, which in accordance with the appointment are divided into warns prohibiting, allowing and resembling. Parts of equipment that are dangerous for people are painted in signaling colors and security sign (in accordance with GOST 12.4.026-76 "Colors Signal and Safety Signs"). Red color buttons and emergency shutdown levers of electrical installations.
Table 2. Classification of insulating electric power facilities
To reduce the risk of lesion of people working with portable power tools and lighting lamps, use small voltage not exceeding 42 V. In some cases, for example, when working in a metal tank, 12 V. voltage is used to power the hand portable lamps.
To enhance the safety, the electrical separation of networks is carried out into separate short electrically unambiguous areas with the help of separating transformers. Such separated networks have low capacity and high insulation resistance. Separate meals are used when working with portable electrical devices, on construction sites, during repairs on power plants, etc.
When the current closures on the structural parts of the electrical equipment (closure on the body), voltages appear on them sufficient to defeat people or the occurrence of fire. To protect against electric shock and fire in this case, in this case, you can three ways: protective grounding, rewarding and protective disconnection.
Protective grounding -this is a deliberate connection with the Earth or its equivalent of metal inadvertent parts of electrical equipment, which are not under voltage in normal state, but may be under it with a random connection to the current-friendly parts.
If the closure occurred and the electrical installation body was under voltage, then a person touched him falls under the tension voltage (U. etc ), which is determined by the expression:
(9)
where V. 3 - full voltage on the housing of the electrical installation, in;
Vx - potential of the surface of the Earth or Paul, V.
Thus, the tension of the touch is called the voltage between the two points of the circuit of the current, which a person can touch at the same time.
Consider a scheme of action protective grounding On the example of a three-phase network with an isolated neutral (Fig. 3).
If a person comes to a grounded electrical installation, which is energized, it will fall under the tension voltage determined by the formula:
where and pr - tension coefficient of touch or just a coefficient of touch (and< 1 и зависит от вида заземлителя);
З - Current current, and;
Rz is the resistance of the protective ground, Ohm.
The current passing through the human body that has incurred under the tension voltage (I A person, a) will be:
where R. from- resistance to spreading current in the ground, depending on the resistivity of the Earth and the resistance of the sole of man's shoes, Ohm.
If a person is in high humidity (R from -> 0), the previous formula can be simplified:
Calculate I and people for the case if iz \u003d 4 A, Rz \u003d 4 Ohm and Apr \u003d 0.4 (contoured earthing):
This current is safe for humans, since it does not exceed the inclusive current (10 mA).
Thus, the principle of the protection of the protective ground is to reduce the safe values \u200b\u200bof the touch voltage (and step voltage) caused by a closure on the body.
Protective grounding (bottom) is subjected to metal parts of electrical installations and equipment available for human touch and have no other types of protection, such as electrical machinery, transformers, lamps, switchboards, metal pipes, and electrical wiring casing, as well as metal portable electrical receivers.
Be sure to ground electrical installations operating under the voltage of 380 V and above AC and feed on the DC source with a voltage of 440 V and above. In addition, in rooms of increased and special hazards, installations with voltage from 42 to 380 V AC and from 110 to 440 V DC are ground and from 110 to 440 V DC.
Grounding device -this is a combination of earthing - metal conductors in contact with the ground, and grounding conductors connecting grounded parts of the electrical installation with the earthing. Depending on the mutual arrangement of earthing and grounded equipment, remote and contour grounding devices distinguish. The first of them are characterized by the fact that the grounds are carried out beyond the site, on which grounded equipment is placed, or focused on some part of this site (Fig. 4).
The contour grounding device (Fig. 5), the entrancers of which are located along the contour (perimeter) around the grounded equipment at a short distance from each other (several meters), provides a better degree of protection than the previous one.
Grounders are artificial, which are used only for grounding purposes, and natural, which use pipelines in Earth (with the exception of pipelines of combustible liquids or gases), metal structures, reinforced concrete structures, lead shells of cables, etc. Artificial earthingers are made of steel pipes, corners, rods or strip tissue.
Requirements for protective grounding resistance are regulated by PUE. At any time of the year, this resistance should not exceed:
4 ohms - in installations operating under voltage up to 1000 V; If the power of the current source is 100 kV * A and less, then the resistance of the grounding device can reach 10 ohms;
0.5 Ohms - in installations operating under voltage above 1000 V with effectively grounded neutral. The greatest resistance of the grounding device (R., OM) There should be no more than 250 / з (but not more than 10 ohms) in the installations with a voltage above 1000 V with an isolated neutral. When using a grounding device at the same time for voltage settings up to 1000 V, R. there should be no more than 125 / з (but not more than 4 or 10 ohms, respectively). In these formulas, the zaming current to the ground, A.
Protective zeroit is intended to protect in three-phase four-wire networks with a deaf-andled neutral, operating under voltage up to 1000 V, since in these networks, the use of protective grounding is inefficient. Usually it is 220/127, 380/220 and 660/380 V.
Consider the action protective reassembly Read more. Let there be a three-phase three-wire network operating under a voltage of up to 1000 V with a grounded neutral (Fig. 6).
If in such a scheme, one of the phases will be closed on the electrical wiring housing (shown in the lightning arrow scheme), the current value (z, a) flowing on the network is determined from the following dependency:
(14)
where VF -phase voltage, in;
Ro. - neutral grounding resistance, Ohm;
З. - resistance of the electrical installation case, Ohm.
In this case, on the housing of the electrical installation there is a voltage relative to the Earth (VK), determined by the following formula:
(15)
Calculate the magnitude of the short circuit current (1 to,A) for values \u200b\u200bV f \u003d 220 V and R 0 = RZ = 4 ohms:
The short circuit current / 3 may be insufficient to trigger protection, and the electrical installation may not be disconnected. The housing of the electrical installation is under dangerous voltage. If a person accidentally communicates to the electrical installation case under this voltage, the current flowing through the human body will be:
(17)
where a pr is the tension coefficient of touch.
If and pr \u003d 1 and V. K. = 110 V, then I person \u003d 110/1000 \u003d 0.11 A \u003d 110 mA. This current exceeds the value of fibrillation, therefore it is deadly. Thus, the protective grounding in this case does not provide reliable human protection, therefore it is not grounded, but a zealing.
Rejectionthey call a way to protect against damage to the current automatic disconnection of the damaged area of \u200b\u200bthe network and at the same time a decrease in the voltage on the equipment enclosures at the time until the disconnecting device stops (fuses, automata, etc.). The reinforcement is a deliberate connection with a zero protective conductor of metal networked parts, which may be under voltage (Fig. 7).
Explorer (1), which connects the reinforced parts of the elestation with a deaf-free neutral point of the transformer winding, is called zero protective. The purpose of this conductor consists in creating a short circuit current with a small electrical resistivity (the chain is indicated in the figure of I - II - III - II - V)) so that this current is sufficient to quickly disable damage from the network. This is achieved by the operation of the network protection element from the short circuit current (in the figure, this element is indicated by a number 2).
The circuit of the I - II - III - II - V - V is a very small electrical resistance (the share of OM). A short circuit current occurs when the housing occurs and passing along the circuit of the reinforcement, reaches a large value (several hundred amps), which provides fast and reliable triggering elements.
To eliminate the risk of cliff of the zero wire, it is arranged by its repeated multiple work grounding every 250 m.
The basic security requirement for the downward: it should ensure reliable and fast protection. This requires the following condition:
I. K. z. \u003e K I. HOM , (18)
where IO is the rated value of the current at which the protection element is triggered;
k. - the coefficient characterizing the multiplicity of the short circuit current relative to the nominal value of the current at which the protection element is triggered.
The response time of protection elements depends on the current force. So, for fuses and heat machines with k. \u003d 10 The response time of the fuse is 0.1 s, and when k. = 3-0.2 s. Electromagnetic circuit breaker dealers a network for 0.01 s. According to PUE requirements in premises with normal conditions k. must be within 1.2-3, and in explosive rooms - k. = 1,4-6.
Another protection system - protective shutdown -this is a protection against electric shock in electrical installations, working under voltage up to 1000 V, automatic shutdown of all phases of the emergency section of the network during the time allowed by safety conditions for a person.
The main characteristic of this system is a speed, it should not exceed 0.2 s. The principle of protection is based on limiting the time of the flow of hazardous current through the human body. There are various protective shutdown schemes, one of them, based on the use of the voltage relay, is presented in Fig. 20.8.
When the phase wire is closed on a grounded or unfolded electrical installation case on it there is a case voltage V. K. . If it exceeds a predetermined maximum allowable voltage V. K. extra (i.e. if V. K. \u003e U. to Ext), triggered a protective disconnecting device. The scheme works as follows.
Due to the potential difference between the electrical installation case 1 and the land occurs current I P, which, passing through the relay 5, closes its contacts, feeding the power to the non-coil 3. Under the influence of an electromagnetic field that arose inside it is drawn by a core 4, causes off the circuit breaker 2, and the installation is de-energized.
In mobile settings voltage up to 1000 V;
To disable electrical equipment removed from the power supply, as an addition to the downward;
In an electrified tool as an addition to | protective grounding or rewarding;
In rock and frozen soils, if it is impossible to perform the necessary grounding.
1 - electrical installation case; 2 - circuit breaker; 3 - turning coil; 4 - core coil; 5 - maximum relay
voltage; R h is the resistance of the protective ground; I 3 - closure current; I P - current flowing through the relay; R 1 - Auxiliary Ground Resistance
Fig. 8. Protective shutdown scheme
Consider briefly organizational measures ensuring the safe operation of electrical installations.These include the design of the appropriate work or orders, admission to work, supervision of work, strict observance of the regime and recreation, transitions to other works and the end of work.
An outfit for carrying out work in electrical installations is called composed on a special form of a task for its safe production, which determines the content, place, start time and end of work, the necessary security measures, the composition of the brigades and persons responsible for the safety of work. By order they call the same task for safe work, but indicating the content, place, time and persons who are entrusted with its implementation.
All works on conductive parts of electrical installations under voltage and with removal of voltage are performed by alongside, except for short-term work (no more than 1 hour) requiring the participation of no more than three people. These works are performed by order.
Organizational activities also include personnel training with the right techniques for working with the assignment of employees serving electrical installations corresponding to qualification groups. Information on personnel qualifying groups is presented in Table. 3.
In some cases, a significant danger to a person represents static electricity,under which the combination of phenomena associated with the occurrence, preservation and relaxation (weakening) of the free electric charge on the surface and in the volume of dielectric substances, materials, products, or on isolated conductors. The flow of various technological processes, such as grinding, spraying, filtration and others, is accompanied by electrification of materials and equipment, and the electrical potential arising on them reaches the values \u200b\u200bof thousands and ten thousand volts. The effect of static electricity on the human body is manifested in the form of a weak long flowing current or in the form of a short-term discharge through the human body, as a result of which an accident may occur.
The harmful effect on the human body has electrical field of increased tension.It causes functional changes in central nervous, cardiovascular and some other organism systems.
Static electricity protectionthey are carried out in two main areas: a decrease in the generation of electrical charges and the elimination of static electricity charges. To implement the first direction, it is necessary to properly select structural materials from which machines, aggregates and other technological equipment are manufactured. These materials should be weakly electrochatized or non-election. For example, a synthetic material consisting by 40% of the nylon and 60% Dacron is not electrified by friction about the chrome-plated surface.
Table 3. Qualification groups of personnel serving electrical installation
To remove static electricity charges from the surface of the technological equipment, it is necessarily ground.
In addition to the listed methods of protection against static electricity, the decrease in the specific surface electrical resistance of the processed materials is of great importance. This is achieved by increasing the relative humidity in the room, where the processing of the water absorbing materials (wood, paper, cotton fabric, etc.) is processed, up to 65-70%, applying special antistatic compositions to their surface, introducing into solid dielectrics of electrically conductive materials (graphite, carbon fibers, aluminum powder, etc.). There are other methods of protection against static electricity.
Basic technical means The protection of a person from electric shock used separately or in combination with each other is: protective grounding, reinforcement, protective shutdown, electrical network separation, low voltage, electric power supply, potential equalization, double insulation, warning alarm, blocking, safety signs.
Protective grounding - this is a deliberate electrical connection with the ground of the Earth of the metal inactive elements of electrical installations, which in emergency situations may be energized.
Scope of Protective Grounding - electrical installations with voltages up to 1000 V feeding on the joints. At the same time, in the premises without increased danger, the protective grounding is mandatory at the rated voltage of electrical installations 380 V and above AC and 440 V and above DC, and in rooms with increased danger and especially dangerous, as well as in the outer installations - at a voltage above 42 V alternating and above 110 V DC.
Protective grounding is specifically designed to provide electrical safety and allows you to reduce the voltage applied to the human body, up to a long permissible value.. Protective grounding is subject to mobile non-flowing elements of electrical installations, which can be energized, for example, due to damage to the isolation of the network phase conductor. The protective grounding scheme is presented in Fig. 4.10.
Figure by dashed lines shows equivalent resistance Z out / 3which replaces the complex resistance of phase insulation in case of their equality, but is connected to neutral N electrical network.
In the case of a phase breakdown on the housing of the closure current is determined by the formula
in which the effect of parallel compound Rz and R H. can be neglected ( R h || r h<< Z из /3 ), t. Rz<< Z из . As a result, the closure current into the land in the sync voltage to 1000 in practically does not exceed 5 A, and in most cases it is many times less.
To ensure an acceptable safety of touching the damaged electrical installation in Xin (closing the phase on the body), it is necessary to ensure a sufficiently small value of the ground resistance at any time of the year.
Protective grounding is carried out using grounding devicewhich is a totality of earthing (natural or artificial) and grounding conductors.
Natural entrancers - It is directly in contact with the soil electrically conductive elements of communications, buildings and structures, specially not intended for grounding purposes, but used as earthing. These include reinforced concrete foundation fittings, metallic water pipes, laid in the ground, dummy pipes. It is forbidden to use pipelines of combustible liquids, explosive or combustible gases as natural earthing.
Artificial entrancers - It is a steel electrodes (pipes, rods, corners) to several meters long, which have direct contact with the soil. They are used if natural earthingers are absent or their resistance to spreading currents do not meet the requirements.
Ground conductors - These are electrical conductors connecting earthing machines with ground elements of electrical installations.
Pue and GOST 12.1.030-81 * establish, in particular, that in networks S. U F.\u003d 220 in the resistance of the grounding device should not exceed 4 ohms (Rz ≤ 4 Ohm.). If the power of the power source (transformer, generator) does not exceed 100 kVA, then Rz ≤ 10 Ohm.. Thus provide voltage on the emergency electrical installation case, not exceeding 20 V, which is considered permissible.
Stage - This is a deliberate electrical connection of the inadvertent parts of electrical installations, which in emergency situations may be energized, with a deaf-and loose neutral of the electrical network with a zero protective conductor (NWP).
Scope of application - electrical installation with voltages up to 1000 V, feed from SZN. At the same time, indoors without increased danger is mandatory under the rated voltage of electrical installations 380 V and above AC and 440 V and above DC, and indoors with increased danger and especially dangerous, as well as in external installations - at a voltage above 42 in alternating and above 110 V DC. The scheme of the option of the rise in the SZN is shown in Fig. 4.11, where PR1 and PR2 are fuses of the power supply and electrical installation.
Zero Protective Conductor NZP must be distinguished from zero working conductor N. Zero working conductor, if necessary, can be used to power electrical installations. In a real network, it can be combined with NZP, with the exception of the power of portable electrical receivers, if it complies with additional requirements for NWP. The guaranteed continuity of the NCP should be ensured throughout the element of the power supply unit to neutral. This is ensured by the lack of elements of protection (fuses and circuit breakers) as well as a different kind of disconnectors. All NWP connections must be made on the basis of welding or be threaded. The total conductivity of the NWP should be at least 50% of the conductivity of the phase conductor.
When one of the phases is closed on a reinforced electrical installation case, a short circuit contour arises, formed by the phase voltage source and complex phase (ż φ) and zero protective (ż NZP) of the conductors, the current value in which ensures the rapid triggering of the nearest protection element (PR2) . In order to further increase the level of electrical safety, for example, when the NCP breaks, it is re-ground (in Fig. 4.11 R P. - resistance of re-earthing). With absence R P. The voltage on the damaged installation case may exceed 0.5U f, and in the case of applying a repeated earthing, it can be somewhat reduced.
Thus, when reducing the safety of a person relating to the damaged installation housing is provided by reducing the time of the danger voltage, acting until the protection element is triggered.
In order to ensure quick disabling the emergency installation, a short circuit current according to PUE requirements should not less than 3 times higher than the rated current of the intake of the nearest fuse or 1.4 times exceed the circuit breaker setting current.
In SZN, it is impossible to ground the installation case, without connecting it before the NWP.
Protective shutdown - This is an automatic shutdown of all phases of the controlled network section, providing secure combinations of the current and time of its effects in the event of danger of human damage (Pue-99). Such situations arise, for example, in cases of ground shortening, reduce insulation resistance, grounding or reassembly devices, as well as with a single-phase touch of a person to the current elements of the installations. Protective shutdown can be used as an independent measure of protection, as well as in combination with a change in or protective grounding to ensure greater safety.
Protective Disconnection Device (UZO) Can be used in networks with any neutral mode. It is connected to the controlled electrical installation and with an invincible deterioration of the electrical safety parameters, turns off it from the power supply. The principle of protection using the UZO is to reduce the time of the dangerous current through a person. The efficiency of the RCD is determined by its speed. In any case, the combination of tension voltage acting until the time of shutdown, and the response time must comply with the requirements of GOST 12.1.038-82 *. All RCOs are built on a single functional principle (Fig. 4.12). Sensor D. reacts to changes in one or more parameters Ueh. characterizing electrical safety. Its output U D. It is proportional to the UDO input signal to which it reacts. In alarm formator UAA. (FAS) Sensor Signal U D. Compared with the established level of response UP. It is proportional to the setpoint of the UDO, i.e. The input signal value in which it works. If a U d\u003e up, then signal U AC Through the reconciliation element (by power, voltage), the EC leads to the opening of the contacts of the disconnecting device OU. The practical diversity of the Uzo is determined by the input signals used and the selected structural elements. The input signals may be a zero sequence current (with the asymmetry of the leakage currents), the zero sequence voltage (with phase voltage asymmetry relative to the Earth), the voltage of the installation case relative to the Earth, the closure current (leakage) to the ground.
Electrical separation of the network. Real electrical networks may have a deaf-loose neutral, to be extended and branched, which dramatically increases the danger of lesion with a single-phase touch of a person. In fig. 4.13 shows an example of an extensive single-phase network containing N branches with appropriate insulation resistance. Resulting insulation resistance Z Networks are defined as the result of a parallel connection of insulation resistance N of individual sections and insulation resistance Z Eu electrical installations. It may be insufficient to ensure an acceptable safety of a single-phase touch and can be tens of com.
In order to improve security, in such cases, an electrical separation of an extensive network is used to a number of sections with the help of special separation transformers RT (Fig. 4.14). From each of each of them, only one electrical appliance with a nominal current of the protection element is not more than 15 A. The network plot connected to the secondary winding of the RT has low length and extension. Therefore, a large resistance of the power conductors relative to the Earth is easily ensured. Separating transformers can be part of the power supply (voltage converters) of radio-electronic devices. It should be borne in mind that the conclusions of the RT secondary winding must be isolated from the ground.
Using small stresses. A significant increase in the level of electrical safety can be achieved by reducing the operating stresses of electrical installations. If the nominal electrical installation voltage does not exceed a long-permissible amount of touch voltage, even the simultaneous contact of a person with current-friendly parts of different phases can be relatively secure.
Small called tension Not more than 42 in variable and no more than 110 V DC, used in order to reduce the risk of electric shock. The greatest degree of security is achieved at stresses up to 12 V., t. With such stresses, the human body resistance is usually at least 6 com and, therefore, the current passing through the human body will not exceed 2 mA. Such a current can be considered conditionally safe. In terms of production conditions, small voltages 36 V (in rooms with increased hazard) and 12 V (in particularly dangerous rooms) are used to improve the safety of operation of portable electrical installations. However, in any case, small stresses are only relatively safe, because In the worst case, the current through the human body may exceed the value of the threshold irregular one.
Sources of low voltage are galvanic elements, batteries, voltage converters or transformers. Getting low voltages using autotransformers is not allowed, T. K. Current elements of a small voltage network in this case are galvanically connected to the main electrical network.
The scope of small stresses is limited mainly by manual electrified tool, portable lamps, lamps of local illumination in rooms, both with increased danger and especially dangerous.
Electroplature -these are the means of individual protection that serve to protect people from electric shock, from exposure to an electrical arc and an electromagnetic field. In its purpose, the means of protection is conventionally divided into insulating, enclosing and safety.
Insulating remedies Designed for the isolation of a person from parts of electrical installations under tension, and from the ground. Distinguish basic and additional insulating agents. Basic insulating tools They have insulation, which is capable of to withstand the operating voltage of the electrical installation for a long time, and, therefore, with their help, it is possible to concern the current-carrying parts that are under voltage. The main insulating means for electrical installations by voltage up to 1000 V are insulating rods, insulating and electrical metering mites, dielectric gloves, a fitting and assembly tool with insulating handles, voltage pointers. Additional insulating tools Apply only complete with basic means to ensure greater safety. These include dielectric bots and galoishes, insulating stands and rugs. All insulating tools should be subject to testing after the manufacture and periodically during operation, which the corresponding mark is made on them.
Fencing protective equipment Designed for the temporary fencing of current-carrying parts under voltage (insulating lining, shields, barriers, fencing cells), as well as to prevent the hazardous voltage on the disconnected circuit parts (portable grounding devices).
Safety protective equipment Serve to protect staff from factors associated with its work with electrical installations. These include the means of protection against falling from height (safety belts), when lifting to height (Montarian claws, stairs), from light, heat, mechanical, chemical influences (safety glasses, shields, mittens) and electromagnetic fields (shielding helmets, costumes ).
Adjusting potentialsapply in rooms that have grounded or imposed electrical installations to improve the level of security. At the same time, elements of production structures, pipelines are connected to the grounding or reinforcement network. With the same purpose in the bathrooms of residential buildings, the metal baths of the baths should be galvanically connected to the metal pipes of the water supply.
Double isolation It is a combination of working and protective (additional) isolation, in which the metallic parts of the electrical installation are available to the touch do not acquire a hazardous voltage during damage only to the working or only protective insulation. According to the requirements of GOST 12.2.006-87, dual isolation must have devices of household or similar general applications. Double insulation installations should not be grounded or reset, so they do not have the corresponding connecting elements. As an additional isolation, plastic cases, knobs, sleeves are used. If a double insulation device has a metal housing, it should be isolated from the structural parts of the installation, which may be energized (chassis, the axis of the regulators, the states of the electric motors) insulating elements.
Warning alarmit serves to issue a hazard signal when approaching high voltage parts.
Lockprevent access to unconnected current-generating parts of the electrical installation, for example, during repair. Electric blocking Perform a rupture of the chain by contacts that open when the hardware door is opened or does not allow it to open if the high voltage is not removed from the current-carrying parts. Mechanical blocking They have structural elements that do not allow will turn on the device with a lid open or open the machine when it is turned on.
Safety signs and posters Designed to attract attention to the danger of damage to current, prescriptions, permits of certain actions and guidelines in order to ensure security. They are prohibiting warning, prescribing and indicable.
6.4. Electric shock protection measures
electrical safety Provided by the design of electrical installations, technical methods and means of protection, organizational and technical measures.
Construction of electrical installations must meet the conditions of their operation and provide personnel protection against contact with current and moving parts, but equipment - From entering inside out of foreign solids and water.
Methods and electrical safety equipment: protective grounding, reinforcement, protective shutdown, potential leveling, small voltage, insulation of current-handing parts, electrical separation of networks, fenception devices, locks, warning alarm, safety signs, warning posters, electric power facilities.
Protective grounding - this is intentional electrical connection with earth or its equivalent metal netokyadpartswhich may be energized as a result of damage to the insulation of the electrical installation.
Principle of protective grounding: reducing the safe values \u200b\u200bof the tension voltage and current force passing through a person caused by closure on the body.When the housing is grounded, the ground is closed to the ground and the touch to the grounded body causes the appearance of a parallel branch, along which part of the closure current passes into the ground through the human body (Fig.6.5). The current strength in parallel chains is inversely proportional to the resistance of the chains, so the current through person (I h) is not dangerous.
Scope of Protective Grounding- three-phase voltage networksup to 1 kV with an isolated neutral and network voltage above 1 kVany neutral regime.
The resistance of the grounding device used for grounding electrical equipment in electrical installations by voltage to 1 kVC isolated neutral should be no more than 4 ohms .
With the power of generators and transformers 100 kvi less, grounding devices may have no more than 10 ohm resistance .
A grounding device in electrical installations with a voltage above 1 kVC deaf-market neutral must have resistance of not more than 0.5 ohms , and in electrical installations with an isolated neutral - no more than 10 ohm .
Calculation of protective grounding It is to determine the parameters of the vertical and horizontal elements of grounding, subject to the obverse of the allowable resistance value of the grounding device. Grounding device consists of a ground (one or more metal elements immersed on a certain depth of the ground) and conductors connecting grounded equipment with a grounding.
Stage - this is intentional electrical connectionwith a zero protective conductor of metal inadvertent parts, which may be energized.
Problem of the Range: eliminating the danger of damage to the current in the case of contact with the case and other inactive metal parts of the electrical installation, caused by stress due to the closure. The task is solved fast disconnection of damaged electrical installation from the network (Fig.6.6).
Principle of action of the rise lies in turning the closure bycase into single-phase short circuit(between phase and zero wires) in order to cause high current, ensuring the operation of protection, and thereby automatically disable damaged mustachenetwork from the supply network.
Calculation of reassembly lies in definition of the cross section of the zero wire that satisfies the condition for the operation of maximum current protection. Such protection can be fuses, magnetic starters with built-in thermal protection, contactors in combination with thermal relays, automatic protection machines simultaneously from short circuit currents and overload.
Zauluyiaapply in three-phase four-wire networks to 1 kV voltagewith a deaf-free neutral.
Protective grounding or rewardingelectrical installations is obligatorycurrent in premises without increased danger of lesion current at nominal voltage 380 INand above alternating current as well as 440 INand above DC.
In rooms with increased danger and especially dangerous, it is necessary to ground or reinstall installations with a memorial voltage of 42 V above the alternating current, as well as 110 V and above DC. In explosive rooms, grounding or reassembling settings is definitely independent of the network voltage.
Protective shutdown - this is high-speed protection, providing automatic shutdown of electrical installation whenthe occurrence of the danger of damage to the current. When applying this type of protection, safety is provided by high-speed (not more than 0.2 seconds) by disconnecting the emergency area or the entire network with single-phase closure to the ground or on electrical equipment elements, normally isolated from the Earth, as well as when touched a person to parts under voltage.
Schemes and design devices of protective shutdown.
Scheme Protectiveth shutdown, triggered when the voltage appears on the caseregarding land (Fig. 6.7). In the diagrams of this type, the sensor serves a voltage relay included between the housing and the auxiliary earthing.
Alignment of potential - the method of reducing the tension voltage and step between the points of the electrical circuit to whichperhaps simultaneous touch or on which a person can stand at the same time.
For equalizing the potential to the ground, steel strips in the form of a mesh across the area occupied by the equipment are placed. In the industrial premises of the body of electrical equipment and production equipment to one degree or another are interconnected. When closed on the body in any of the electrical acceptors, all metal parts receive a close voltage relative to the Earth. As a result, the voltage between the casing of the electrical receipt and the floor decreases, the potential is equalized throughout the area of \u200b\u200bthe room and the person in this circuit of the circuit occurs under relatively low voltage.
Small tension - nominal voltage no more42 B.This is used to power the power tools, fixtures of stationary lighting, portable lamps in rooms with increased danger, especially dangerous and exterior installations. Sources of low voltage can be special reduction transformers with secondary voltage 12-42 V .
Reactinsulation - this is the underlying condition ensuring the safety of operationand reliability of power supply electrical installation. For insulation of current-carrying partselectrical installations apply working and additional insulation.
Working isolationis enamel and braid winding wires, impregnating varnishes and compounds, etc. Additional insulation There may be a plastic housing of the machine, insulating sleeve, etc.
Electrical insulation consisting of working and optional,called double. It is considered sufficient to provide electrical safety, therefore double-insulated devices are allowed to use without the use of other protective agents.
Controlinsulation resistance can be periodic and continuous.The insulation resistance of power and lighting electrical conductors should be at least 0.5 MΩ.
Electrical separation of networks - network separationon separate electrically unnecessied areas with petssite separating transformerwhich isolates the electrical acceptor from the primary network and the grounding network (Fig. 6.8).
From the separating transformer can be powered only by onea tristener with protective weling insert (The current of the insertion of the machine on the primary side should not exceed 15a), secondary voltagetransformer must be no higher than 380 V. The secondary winding of the transformer and the electrical receiving body should not have grounding or communication with the reinforcement network. In this case, when touched, a danger is not created when touched into parts that are under voltage or to the body with damaged insulation, since the secondary chain of the short and the strength of the leakage currents in it and capacitive currents are small.
The protective separation of networks is used in electrical installations by voltage up to 1000 V,operation of which is associated with special and increased danger (mobile electrical installations, manual electrified tool, etc.).
To eliminate random touch to current parts of electrical installations apply protective solid and netstop devices.
Solid fences required for electrical installations, sizespared in industrial (non-electric) premises. Mesh fences Apply B. electrical installations available to qualified electrical personnel.
In cases where the isolation and the fencing of the current-carrying parts are inappropriate (for example, high voltage air lines), they are placed on an inaccessible height. Inside the production premises, non-frame uninsulated current-carrying parts are paired by a height of at least 3.5 m from the floor.
Lock - protection against penetrationzone where installation is. It allows you to automatically remove the voltage from all elements of the installation, the approach to which threatens human life. Blocking apply to Elekt.in the maintenance of which increased security measures must be respected, in electrical equipmentlocated in available for non-electric staffpremises.
This article will contain examples of those methods and methods of protection, thanks to which it is possible to significantly protect themselves and others when performing electrical works, thereby reducing the chance of an accident to a minimum.
Application of protective fences
The touch of a person to the uninsulated current-carrying part is under voltage, is dangerous - this is a fact. Even knowing the presence of tension in various places, there is a chance of a random touch.
In order to avoid such cases, protective fences around dangerous zones (systems, equipment, parts, etc.) is taken to ensure the electrical safety of working personnel.
Use protective blocks
Locks, perhaps, more refer to electrical protection against accidental electric shock by electric shock or from a sudden equipment inclusion, which can also entail an accident.
When they are installed, those cases that may occur in the event of an erroneous and improper behavior of people working or servicing electrical systems and devices are taken into account.
When blocking is triggered, a compulsory disconnection and de-energization of electrical equipment in order to prevent emergency situations.
Portable grounds
Portable earthingers are temporary means of protection. They are used to provide additional security (protection of working personnel from electric shock) when working on disabled sections of electrical systems, equipment, devices, etc. In the event that suddenly there will be a voltage in these areas, where people still work, these portable grounding (conductors relating to the Earth) will send electricity to the Earth.
Using protective insulation
Another important method of technical protection against electric shock is the use of protective insulation on its workplace.
Insulation of the workplace suggests a certain organization of activities aimed at preventing the emergence of the Earth's electric chain.
The main task of this method is to increase the resistance (transition) for this electrocipy.
This option involves the use of rubber carpets, isolation of current-carrying parts of electrical equipment in the most electrically dangerous places, etc.
Technical measures to protect against electric shock
Technical protection measures can be divided into 2 main groups.
The first can be attributed to the separation of power grids, the use of low voltages, timely insulation control, protective grounding, reinforced isolation (using double insulation) and so on. The use of such protection measures gives a person maximum protection against electric shock.
To the second group, we will draw a protective disconnection and reassessment:
- Separation of power grids. Transformers use transformers for separating the power grid. They allow you to split the total chain into separate chains and areas (electrically unable to each other). In power grids, which uses isolated neutral, it increases the insulating resistance and lowers the capacity relative to the Earth, comparing with the power grid in general. When separated by the power grid, the use of autotransformers is unacceptable.
- Use low power supply voltages. In accordance with GOST, low voltage can be considered voltage up to 42 V. It is used in order to increase safety from electricity damage. Low voltages are usually obtained using transformers (lowering).
- Isolation, its control, damage detection, prevention.
- Control over the state of the insulating coating is carried out by a periodic measurement of its resistance. The purpose of this procedure is the detection of defective places and the timely warning of short circuits to the Earth.
- Protective grounding. The protective ground is called a deliberate electrical connection from the earth (or its equivalent). The grounding task is to reduce voltage values \u200b\u200brelative to the earth itself. It is used in power grids with voltages up to 1000 V (with an isolated neutral). The protective grounding implies the redistribution of voltage drops on the electrical circuit sites: "Case - Earth" and "Phase - Earth".
- Using double isolation. Under double insulation it is understood to combine working and additional insulation together. This significantly increases the overall reliability of protection against damage. Electrical equipment made with such insulation is usually marked by special signs. Effectively shows double insulation in various electrical instruments.
- Apply a protective shutdown. Protective shutdown is a fairly effective measure of protection against electric shock. It is a high-speed protection, which provides premature automatic triggering and turns off electrical equipment.
- Range. Protective reinforcement is a deliberate (special) electrical connection with a zero conductor of inadequate metal parts, which can potentially be under voltage (with malfunctions, insulation breakdowns, etc.). It is used in power grids up to 1000 V (with a deaf grounded neutral). The main task of such a reinforcement is to reduce the likelihood of damage to the electrical current of a person with an emergency test of electrical equipment on the housing along one of the phases of the power grid.