Devices that respond to housing voltage relative to ground. Safety shutdown

RCD(Residual Disconnection Device) is a switching device designed to protect an electrical circuit from leakage currents, that is, currents flowing along undesirable, under normal operating conditions, conductive paths, which in turn provides protection from fires (electrical wiring fires) and from injury to humans electric shock.

The definition of “switching” means that this device can turn on and off electrical circuits, in other words, switch them.

RCD also has other names, for example: differential switch, differential current switch, (abbreviated as differential current switch), etc.

Design and operating principle of RCD

And so, for clarity, let’s imagine the simplest scheme connections via RCD light bulbs:

The diagram shows that during normal operation of the RCD, when its moving contacts are closed, a current I 1 of value, for example, 5 Amperes from the phase wire passes through the magnetic circuit of the RCD, then through the light bulb, and returns to the network via the neutral conductor, also through magnetic circuit of the RCD, and the value of the current I 2 is equal to the value of the current I 1 and is 5 Amperes.

In such a situation, part of the electrical circuit current coming from the phase wire will not return to the network, but passing through the human body will go into the ground, therefore the current I 2 will return to the network through the magnetic circuit of the RCD along neutral wire will be less than the current I 1 entering the network, respectively, the value of the magnetic flux F 1 will become greater than the value of the magnetic flux F 2, as a result of which the total magnetic flux in the RCD magnetic circuit will no longer be equal to zero.

For example, current I 1 = 6A, current I 2 = 5.5A, i.e. 0.5 Ampere flows through the human body into the ground (i.e. 0.5 Ampere is the leakage current), then the magnetic flux Ф 1 will be equal to 6 conventional units, and the magnetic flux Ф 2 will be 5.5 conventional units, then the total magnetic flux will be equal to:

F sums = F 1 + F 2 =6+(-5.5)=0.5 arb. units

The resulting total magnetic flux induces an electric current in the secondary winding, which, passing through the magnetoelectric relay, causes it to work, and it, in turn, opens the moving contacts, turning off the electrical circuit.

The functionality of the RCD is checked by pressing the “TEST” button. Pressing this button artificially creates a current leak in the RCD, which should lead to the RCD turning off.

RCD connection diagram.

IMPORTANT! Since the RCD does not have overcurrent protection, any circuit for its connection must also include an installation to protect the RCD from overload and short circuit currents.

RCD connection carried out according to one of the following schemes, depending on the type of network:

Connecting an RCD without grounding:

This scheme is used, as a rule, in buildings with old electrical wiring (two-wire), in which there is no ground wire.

Connecting an RCD with grounding:

N-C-S(when the neutral conductor is divided into zero working and zero protective):

Connection diagram of the RCD in the electrical network(when the neutral working and neutral protective conductors are separated):

IMPORTANT! In the coverage area of the RCD, you cannot combine the neutral protective (grounding wire) and the neutral working conductors! In other words, it is impossible in the circuit, after the installed RCD, to connect the working zero (blue wire in the diagram) and the ground wire (green wire in the diagram).

Errors in connection diagrams due to which the RCD trips.

As mentioned above, the RCD is triggered by leakage currents, i.e. if the RCD has tripped, this means that a person has come under voltage or for some reason the insulation of the electrical wiring or electrical equipment has been damaged.

But what if the RCD trips spontaneously and there is no damage anywhere, and the connected electrical equipment is working properly? Perhaps the whole point is one of the following errors in the network diagram of the protected RCD.

One of the most common mistakes is combining the neutral protective and neutral working conductors in the coverage area of the RCD:

In this case, the amount of current leaving the network through the RCD along the phase wire will be greater than the amount of current returning to the network through the neutral conductor because part of the current will flow past the RCD along the grounding conductor, which will cause the RCD to trip.

Also, there are often cases of using a grounding conductor or a third-party conductive grounded part as a neutral working conductor (for example, building fittings, a heating system, water pipe). This connection usually occurs when the neutral working conductor is damaged:

Both of these cases lead to the RCD tripping, because The current leaving the network through the phase wire does not return back to the network through the RCD.

How to choose an RCD? Types and characteristics of RCD.

To choose the right RCD and eliminate the possibility of error, use ours.

The RCD is selected according to its main characteristics. These include:

Rated current— the maximum current at which the RCD can operate for a long time without losing its functionality;
Differential current— the minimum leakage current at which the RCD will disconnect the electrical circuit;
Rated voltage- voltage at which the RCD is able to operate for a long time without losing its functionality
Current type— constant (denoted by “-“) or variable (denoted by “~”);
Conditional short circuit current- current that the RCD can withstand for a short time until the protective equipment (fuse or circuit breaker) trips.

RCD selection based on following criteria:

— By rated voltage and network type: The rated voltage of the RCD must be greater than or equal to the rated voltage of the circuit it protects:

Unom. RCD⩾ Unom. networks

At single-phase network required two-pole RCD, at three-phase network — four-pole.

— By rated current: according to clause 7.1.76. PUE, the use of RCDs in group lines that do not have protection from, without an additional device that provides this protection is not allowed, and a calculated check of the RCD in overcurrent modes is necessary, taking into account the protective characteristics of the higher-level device that provides overcurrent protection.

From the above it follows that in front of the RCD there must be a protection device (or) it is according to the current of this higher-level protection device that it is necessary to select the rated current of the RCD based on the condition that the rated current of the RCD must be greater than or equal to the rated current of the protection device installed before it:

I no. RCD ⩾ I nom. protection device

In this case, it is recommended that the rated current of the RCD be one step higher than the rated current of the higher-level protection device (for example, if a 25 Ampere circuit breaker is installed in front of the RCD, it is recommended to install the RCD with a rated current of 32 Amps)

For reference, standard values of RCD rated currents: 4A, 5A, 6A, 8A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, etc.,

— By differential current:

Differential current is one of the main characteristics of the RCD, which shows at what value of leakage current the RCD will turn off the circuit.

In accordance with paragraph 7.1.83. PUE: The total leakage current of the network, taking into account the connected stationary and portable electrical receivers in normal operation, should not exceed 1/3 of the rated current of the RCD. In the absence of data, the leakage current of electrical receivers should be taken at the rate of 0.4 mA per 1 A of load current, and the network leakage current at the rate of 10 μA per 1 m of phase conductor length. Those. The differential network current can be calculated using the following formula:

Δ I network =((0.4*I network)+(0.01*L wire))*3, milliampere

Where: Inetworks— network current (calculated using the formula above), in Amperes; Lwires— total length of the protected electrical network wiring in meters.

Having calculated Δ I network we accept the nearest higher standard value of the residual current of the RCD Δ I RCD:

Δ I RCD ⩾ Δ I network

The standard values of the residual current of the RCD are: 6, 10, 30, 100, 300, 500mA

Differential currents: 100, 300 and 500 mA are used for protection against fires, and currents: 6, 10, 30 mA are used to protect against electric shock. In this case, currents of 6 and 10 mA are used, as a rule, to protect individual consumers and, and a differential current of 30 mA is suitable for general protection electrical networks.

If an RCD is needed to protect against electric shock, and according to the calculation, the leakage current is more than 30 mA, it is necessary to provide for the installation of several RCDs on different groups of lines, for example, one RCD to protect sockets in rooms, and a second to protect sockets in the kitchen, thereby reducing the most power passing through each RCD and, as a result, reducing the leakage current of the network, i.e. in this case, the calculation will need to be made for two or more RCDs that will be installed on different lines.

— By type of RCD:

There are two types of RCDs: electromechanical And electronic. We discussed the principle of operation of an electromechanical RCD above; its main working element is a differential transformer (magnetic core with a winding) that compares the magnitude of the current going into the network and the current returning from the network, and in an electronic device this function is performed by an electronic board that requires voltage to operate.

Safety shutdown- a type of protection against electric shock in electrical installations, providing 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.

Application areas of residual current: addition to protective grounding or zeroing in an electrified tool; addition to grounding to disconnect electrical equipment remote from the power source; a measure of protection in mobile electrical installations with voltages 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 ground, the phase voltage relative to ground changes - the value of the phase voltage will tend to the value of the line voltage. In this case, a voltage arises between the neutral of the source and the ground, the so-called zero sequence voltage. Decreasing total resistance network relative to the ground when the insulation resistance changes towards its decrease, etc.

The principle of constructing protective shutdown circuits is that the listed operating changes in the network are perceived by the sensitive element (sensor) automatic device as signal input quantities. The sensor acts as a current relay or voltage relay. At a certain value of the input value, the protective shutdown is triggered and turns off the electrical installation. The value of the input quantity is called the setpoint.

The block diagram of a residual current device (RCD) is shown in Fig.

Rice. Block diagram of the residual current device: D - sensor; P - converter; KPAS - alarm signal transmission channel; EO - executive body; MOP is a source of danger of injury

Sensor D reacts to a change in the input value B, amplifies it to the value KB (K is the sensor transmission coefficient) and sends it to the converter P.

The converter is used to convert the amplified input value into a KVA alarm signal. Next, the emergency signal transmission channel CPAS transmits the AC signal from the converter to the executive body (EO). The executive body carries out protective function to eliminate the danger of damage - turns off the electrical network.

The diagram shows areas of possible interference that affect the operation of the RCD.

In Fig. given circuit diagram protective shutdown using an overcurrent relay.

Rice. Residual current circuit diagram: 1 - maximum current relay; 2 - current transformer; 3 - ground wire; 4 - grounding conductor; 5 - electric motor; 6 - starter contacts; 7 - block contact; 8 - starter core; 9 - working coil; 10 - test button; 11 - auxiliary resistance; 12 and 13 - stop and start buttons; 14 - starter

The coil of this relay with normally closed contacts is connected through a current transformer or directly into a conductor cut leading to a separate auxiliary or common grounding conductor.

The electric motor is put into operation by pressing the “Start” button. In this case, voltage is applied to the coil, the starter core is retracted, the contacts are closed and the electric motor is switched on. At the same time, the block contact closes, as a result of which the coil remains energized.

When one of the phases is short-circuited to the housing, a current circuit is formed: the location of the damage - the housing - the grounding wire - the current transformer - the ground - the capacitance and insulation resistance of the wires of the undamaged phases - the power source - the location of the damage. If the current reaches the current relay operating setting, the relay will operate (that is, its normally closed contact will open) and break the circuit of the magnetic starter coil. 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 activated. The auxiliary resistance limits the fault current to the frame to the required value. There are buttons to turn the starter on and off.

To the enterprise system catering included large complex mobile (inventory) buildings made of metal or with metal frame for street trade and service (snack bars, cafes, etc.). As technical means protection from electrical injuries and from possible fire in electrical installations, the mandatory use of residual current devices 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 operating principle 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 a special material. Core in normal operation electrical network keeps the release mechanism engaged. If any malfunction occurs in the secondary winding of the leakage current transformer, an EMF is induced, the core is retracted, and the magnetoelectric latch associated with the mechanism for freely releasing the contacts is activated (the switch is turned off).

ASTRO-UZO has a Russian certificate of conformity. The device is included in the State Register.

Not only the above structures must be equipped with a residual current device, but also all premises with an increased or special risk of electric shock, including saunas, showers, electrically heated greenhouses, etc.

Protective shutdown is designed to quickly and automatically shut down a damaged electrical installation in cases of a phase short circuit to the housing, a decrease in the insulation resistance of conductors, or when a person is short circuited to conductive elements.

The scope of application of residual current devices (RCDs) is practically unlimited: they can be used in networks of any voltage and with any neutral mode. RCDs are most widespread in networks with voltages up to 1000 V in installations with a high degree of danger, where the use of protective grounding or grounding is difficult for technical or other reasons, for example, on test or laboratory benches.

The advantages of RCDs include: simplicity of the circuit, high reliability, high speed (response time t = 0.02¸0.05 s), high sensitivity and selectivity.

According to the principle of operation, RCDs differ as follows:

Direct action:

1. RCD that responds to housing voltage U To;

2. RCD responding to body current I To.

Indirect action:

3. RCD that responds to phase voltage asymmetry - zero sequence voltage U O;

4. RCD that responds to asymmetry of phase currents - zero-sequence current I O;

5. RCD responding to operational current I op.

Let's consider the listed types of residual current devices.

1. RCD that responds to housing voltage.

Operation of the RCD circuit shown in Fig. 7.29 is carried out as follows.

The power plant is put into operation by pressing the “START” button with normally open contacts. In this case, the trip coil is OK, having received power from the phase conductors 2 And 3 , compressing the spring P and retracting the rod, closes all four contacts of the MP magnetic starter. The “START” button is released, and further power supply to the OK when the EC is running is carried out through the LS self-feeding line through the MK contact. When a phase conductor, such as a conductor, is short-circuited 2 , to the power plant housing through a voltage relay RN installed on the additional grounding line ( r g), current will flow. In this case, the normally closed contacts of the RN voltage relay will open, the OK coils will be de-energized, and with the help of a mechanical spring P, the contacts of the magnetic starter will open and the damaged installation will be disconnected from the network. Eliminates the risk of injury service personnel electric shock. To check the functionality of the RCD circuit, a self-testing operation is performed on idling electrical installation work. When you press the KS button connected to the phase conductor 1 and a protective grounding line through a resistance R with, the power supply housing will be energized. If the RCD circuit is in good condition and there are no defects, the entire installation will be switched off, as described above. Using a self-feeding line LS with an additional mechanical contact MK, the RCD circuit shown in Fig. 7.29, allows for zero protection - protection against self-starting of the electrical installation

with a sudden disappearance and sudden reappearance of voltage.

Rice. 7.28. Schematic diagram of the residual current device,
reacting to the potential of the body:

MP - magnetic starter; OK - trip coil with spring P; RN - voltage relay with normal closed contacts RN; r 3 - resistance of the main protective grounding; r g- resistance of additional grounding; LS - self-feeding line; MK - additional mechanical contact; P - “START” button; C - “STOP” button; KS - “SELF-CONTROL” button; R c- resistance to self-control; a 1 , a 2 - contact coefficients of the main and additional groundings

The selection of the response voltage of the RCD that responds to the housing voltage is made according to the formula:

(7.25)

Where U pr add – permissible touch voltage, taken equal to 36 V with a duration of current exposure to a person of 3¸10 s. (Table 7.2); R p, X L– active and inductive resistance of the LV; a 1 , a 2 – contact coefficients of the corresponding grounding conductors; r g– resistance of additional grounding.

Calculation using formula (7.25) reduces to determining the quantity r g in this case, the response voltage of the RCD circuit should be less than the touch voltage, i.e. U Wed< U Ave.

2. RCD that responds to body current.

The principle of operation of the circuit breaker circuit, which responds to the body current, is similar to the operation of the RCD circuit, triggered by the body voltage, described above. This scheme does not require the installation of additional grounding. Instead of a voltage relay RN, a current relay RT is installed on the main protective grounding line. Other devices and circuit elements remain unchanged, as in Fig. 7.20. Trigger current selection I The average of the RCD reacting to the current of the EC housing is made according to the formula:

I av = (7.26)

Where Z RT – total resistance of the current relay, r 3 – protective grounding resistance; U– permissible touch voltage (7.25).

3. RCD that responds to phase voltage asymmetry.

Rice. 7.30. Schematic diagram of the residual current device,
responding to phase voltage asymmetry:

A- zero sequence filter with common point 1 ; RN - voltage relay;
Z 1 , Z 2 , Z 3 - impedances of phase conductors 1, 2 and 3; r zm1, r zm2 - resistance
short circuit of phase conductors 1 and 2 to ground; Uо =φ 1 - φ 2  – zero sequence voltage (φ 1 – potential at point 1 , φ 2  - potential at a point 2 )

The sensor in this RCD circuit is a zero-sequence filter consisting of capacitors connected in a star.

Let's consider the operation of the RCD circuit shown in Fig. 7.30.

If the resistances of the phase conductors relative to the ground are equal to each other, i.e. Z 1 = Z 2 = Z 3 = Z, then the zero sequence voltage is zero, U o = φ 1 - φ 2  = 0. In this case, this RCD circuit does not work.

If there is a symmetrical decrease in the resistance of phase conductors by the amount n> 1, i.e. , then the voltage U o will also be equal to zero and the RCD will not work.

If asymmetrical deterioration of the insulation of phase conductors occurs Z 1¹ Z 2¹ Z 3, then in this case the zero-sequence voltage will exceed the circuit’s response voltage and the residual current device will turn off the network, U o > U Wed

If one phase conductor is shorted to ground, then with a low resistance value the short circuit r zm1 zero sequence voltage will be close to the phase voltage, U f > U Wed, which will trigger a protective shutdown.

If two conductors are shorted to ground at the same time, then at low values r zm1 and r zm2 the zero-sequence voltage will be close to the value, which will also lead to a network shutdown. Thus, the advantages of an RCD circuit that responds to voltage U o include:

Reliability of operation of the circuit in case of asymmetrical deterioration of the insulation of phase conductors;

Reliability of operation during single- or two-phase conductor-to-ground faults.

The disadvantages of this RCD circuit are absolute insensitivity with a symmetrical deterioration of the insulation resistance of phase conductors and the lack of self-control in the circuit, which reduces service safety electrical systems and installations.

4. RCD that responds to phase current asymmetry

A) b)

Rice. 7.31. Schematic diagram of the residual current device,
responding to phase current asymmetry:

A- circuit of the zero-sequence current transformer TTNP; b - I 1 , I 2 , I 3 - currents of phase conductors 1 , 2 , 3 ; RT - current relay; OK - trip coil; 4 - TTNP magnetic circuit;
5 - secondary winding TTNP

The sensor in the RCD circuit of this type is the zero-sequence current transformer TTNP, schematically shown in Fig. 7.31, b. The secondary winding of the TTNP gives a signal to the RT current relay even at zero sequence current I 0, equal to or greater than the installation current, the electrical installation will shut down.

Let us consider the effect of the RCD shown in Fig. 7.31.

If the insulation resistances of the phase conductors are equal Z 1 = Z 2 = Z 3 = Z and symmetrical load on phases I 1 = I 2 = I 3 = I zero sequence current I 0 will be equal to zero, and therefore the magnetic flux in the magnetic core 4 (Fig. 7.31, A) and EMF in the secondary winding 5 TTNP will also be equal to zero. The protection circuit is not working.

With symmetrical deterioration of the insulation of phase conductors and a symmetrical change in phase currents, this RCD circuit also does not respond, since the current I 0 = 0 and there is no EMF in the secondary winding.

If the insulation of phase conductors is asymmetrically deteriorated or if they are shorted to the ground or to the power plant housing, a zero-sequence current will occur I 0 > 0 and a current is generated in the secondary winding of the TTNP that is equal to or greater than the operation current. As a result, the damaged area or installation will be disconnected from the network, which is the main advantage of this RCD circuit. Disadvantages of the circuit include design complexity, insensitivity to symmetrical insulation degradation, and lack of self-monitoring in the circuit.

5. RCD that responds to operational current.

The sensor in this RCD circuit is a current relay with low operating currents (several milliamps).

Rice. 7.32. Schematic diagram of the residual current device,
responsive to operating current:

D 1, D 2, D 3 - three-phase choke with a common point 1 ; D r - single-phase choke; I op - operational current from an external source; RT - current relay; Z 1 , Z 2 , Z 3 - impedance of phase conductors 1 , 2 And 3 ; r zm - phase conductor circuit resistance;
- operational current path

A constant operating current is supplied to the protection circuit I op from an external source that passes through a closed circuit: source - ground - insulation resistance of conductors Z 1 , Z 2 and Z 3 – the conductors themselves – three-phase and single-phase chokes – winding of the RT current relay.

During normal operation, the insulation resistance of the conductors is high, and therefore the operating current is insignificant and less than the operating current, I op< I Wed

In the event of any decrease in the resistance (symmetrical or asymmetrical) of the insulation of phase conductors or as a result of human contact with them, the total resistance of the circuit Z will decrease, and the operating current I op will increase and if it exceeds the operating current I Wed, the network will be disconnected from the power source.

The advantage of an RCD that responds to operational current is the provision of a high degree of safety for people in all modes of network operation due to current limitation and the ability to self-monitor the health of the circuit.

The disadvantage of these devices is the complexity of the design, since a constant current source is required.

Protective shutdown is a protection system that automatically turns off an electrical installation when there is a danger of electric shock to a person (in the event of a ground fault, a decrease in insulation resistance, a grounding fault or grounding). Protective shutdown is used when it is difficult to ground or neutralize, and also in addition to it in some cases.

Depending on what is the input quantity to which the protective shutdown reacts, protective shutdown circuits are distinguished: for the housing voltage relative to the ground; for ground fault current; for zero sequence voltage or current; on phase voltage relative to ground; for direct and alternating operating currents; combined.

One of the protective shutdown circuits for body voltage relative to ground is shown in Fig. 13.2.

Rice. 13.2. Protective shutdown circuit for case voltage relative to ground

The main element of the circuit is the protective relay RZ. When one phase is short-circuited to the housing, the housing will be under a voltage higher than permissible, the relay core of the relay is drawn in and closes the power circuit of the coil circuit breaker AB, as a result of which the electrical installation is switched off.

The advantage of the scheme is its simplicity. Disadvantages: the need to have auxiliary grounding RB; non-selective shutdown in case of connecting several buildings to one ground; inconstancy of the setting when resistance RB changes. Residual current devices that respond to zero-sequence current are used for any voltage, both with a grounded and an insulated neutral.

Fires and explosions

Fires and explosions are the most common emergency events in modern industrial society.

Most often and, as a rule, with severe social and economic consequences, fires occur at fire-hazardous and fire-explosion sites.

Objects where explosions and fires are most likely include:

Enterprises of the chemical, oil refining and pulp and paper industries;

Enterprises using gas and oil products as raw materials for energy resources;

Gas and oil pipelines;

All types of transport transporting explosive and fire hazardous substances;

Fuel stations;

Enterprises food industry;

Enterprises using paint and varnish materials etc.

EXPLOSION AND FIRE HAZARDOUS substances and mixtures are;

Explosives and gunpowders used for military and industrial purposes, manufactured at industrial enterprises, stored in warehouses separately and in products and transported various types transport;

Mixtures of gaseous and liquefied hydrocarbon products (methane, propane, butane, ethylene, propylene, etc.), as well as sugar, wood, flour, etc. dust with air;

Vapors of gasoline, kerosene, natural gas on various vehicles, fuel stations, etc.

Fires in enterprises can also occur due to damage to electrical wiring and live machines, furnaces and heating systems, containers with flammable liquids, etc.

There are also known cases of explosions and fires in residential premises due to malfunction and violation of the operating rules of gas stoves.

Characteristics of flammable substances

Substances that can burn independently after removing the source of ignition are called combustible, in contrast to substances that do not burn in air and are called non-flammable. An intermediate position is occupied by difficultly combustible substances that ignite when exposed to an ignition source, but stop burning after the latter is removed.

All flammable substances are divided into the following main groups.

1. COMBUSTIBLE GASES (GG) - substances capable of forming flammable and explosive mixtures with air at temperatures not exceeding 50° C. Combustible gases include individual substances: ammonia, acetylene, butadiene, butane, butyl acetate, hydrogen, vinyl chloride, isobutane, isobutylene , methane, carbon monoxide, propane, propylene, hydrogen sulfide, formaldehyde, as well as vapors of flammable and combustible liquids.

2. FLAMMABLE LIQUIDS (FLFL) - substances capable of burning independently after removal of the ignition source and having a flash point not higher than 61 ° C (in a closed crucible) or 66 ° (in an open crucible). These liquids include individual substances: acetone, benzene, hexane, heptane, dimethylforamide, difluorodichloromethane, isopentane, isopropylbenzene, xylene, methyl alcohol, carbon disulfide, styrene, acetic acid, chlorobenzene, cyclohexane, ethyl acetate, ethylbenzene, ethyl alcohol, as well as mixtures and technical products gasoline, diesel fuel, kerosene, white alcohol, solvents.

3. FLAMMABLE LIQUIDS (FL) - substances capable of burning independently after removal of the ignition source and having a flash point above 61° (in a closed crucible) or 66° C (in an open crucible). Flammable liquids include the following individual substances: aniline, hexadecane, hexyl alcohol, glycerin, ethylene glycol, as well as mixtures and technical products, for example, oils: transformer oil, vaseline, castor oil.

4. COMBUSTIBLE DUSTS (GP) - solid substances in a finely dispersed state. Combustible dust in the air (aerosol) can form explosive mixtures with it. Dust (aerogel) settled on walls, ceilings, and equipment surfaces is a fire hazard.

Combustible dusts are divided into four classes according to the degree of explosion and fire hazard.

Class 1 - the most explosive - aerosols with a lower concentration limit of flammability (explosiveness) (LCEL) of up to 15 g/m3 (sulphur, naphthalene, rosin, mill dust, peat, ebonite).

Class 2 - explosive - aerosols with an LEL value from 15 to 65 g/m3 (aluminum powder, lignin, flour dust, hay dust, shale dust).

3rd class - the most fire hazardous - aerogels with an LFL value greater than 65 g/m3 and a self-ignition temperature of up to 250 ° C (tobacco, elevator dust).

4th class - fire hazardous - aerogels with an LFL value greater than 65 g/m3 and a self-ignition temperature greater than 250 ° C ( sawdust, zinc dust).

In accordance with NPB 105-03, buildings and structures in which production is located are divided into five categories.

Room category	Characteristics of substances and materials located (circulating) in the room
And explosive and fire hazardous	Combustible gases, flammable liquids with a flash point of not more than 28 ° C in such quantities that they can form explosive vapor-gas mixtures, upon ignition of which the calculated overpressure explosion in a room exceeding 5 kPa. Substances and materials capable of exploding and burning when interacting with water, air oxygen, or one with the other in such quantities that the calculated excess explosion pressure in the room exceeds 5 kPa.
B explosive and fire hazardous	Combustible dusts or fibers, flammable liquids with a flash point of more than 28 ° C, flammable liquids in such quantities that they can form explosive dust or steam-air mixtures, the ignition of which develops a calculated excess explosion pressure in the room exceeding 5 kPa.
B1 - B4 fire hazardous	Flammable and low-flammable liquids, solid flammable and low-flammable substances and materials that can only burn when interacting with water, air oxygen or one another, provided that the premises in which they are available or handled do not belong to categories A or B
G	Non-flammable substances and materials in a hot, incandescent or molten state, the processing of which is accompanied by the release of radiant heat, sparks and flames, flammable gases, liquids and solids that are burned or disposed of as fuel
D	Non-combustible substances and materials in a cold state

EXAMPLES of production facilities located in premises of categories A, B, C, D and D.

Category A: shops for processing and using metallic sodium and potassium, oil refining and chemical production, warehouses for gasoline and cylinders for flammable gases, premises for stationary acid and alkaline battery installations, hydrogen stations, etc.

The nature of the development of a fire and subsequent explosion largely depends on the fire resistance of structures - the properties of structures to maintain load-bearing and enclosing capacity in fire conditions. In accordance with SNiP 2.01.02.85, there are five degrees of fire resistance of buildings and structures: I, II, III, IV, V.

The fire resistance of building structures is characterized by following parameters:

1) the minimum fire resistance limit of a building structure - the time in hours from the beginning of the impact of fire on the structure until through cracks form in it or a temperature of 200 ° C is reached on the surface opposite to the impact of fire.

2) maximum limit of fire spread building structures visually determined size of damage in centimeters, which is considered to be charring or burning of materials, as well as melting of thermoplastic materials outside the heating zone.

All building materials According to flammability, they are divided into three groups: NON-COMBUTTABLE, DIFFICULTLY COMBUSTIBLE and COMBUSTIBLE.

COMBUSTIBLE materials and structures include metals and inorganic materials used in construction mineral materials and products made from them: sand, clay, gravel, asbestos, brick, concrete, etc.

NON-COMBUTTABLE include materials and products made from them, consisting of combustible and non-combustible components: adobe brick, gypsum dry plaster, fiberboard, lenolium, ebonite, etc.

COMBUSTIBLE include all materials of organic origin: cardboard, felt, asphalt, roofing felt, roofing felt, etc.

Basic concepts about fires and explosions.

FIRE is an uncontrolled combustion outside a special fireplace, causing material damage.

BURNING - chemical reaction oxidation, accompanied by the release of a large amount of heat and usually glow. For combustion to occur, the presence of a flammable substance, an oxidizer (usually atmospheric oxygen, as well as chlorine, fluorine, iodine, bromine, nitrogen oxides) and an ignition source are necessary. In addition, it is necessary that the combustible substance be heated to a certain temperature and be in a certain quantitative ratio with the oxidizer, and that the ignition source has sufficient energy.

EXPLOSION - an extremely rapid release of energy in a limited volume, associated with a sudden change in the state of a substance and accompanied by the formation of a large amount of compressed gases capable of producing mechanical work.

An explosion is a special case of combustion. But the only thing it has in common with combustion in the usual sense is that it oxidation reaction. The explosion is characterized by the following features:

High speed chemical transformation;

Large quantity gaseous products;

Powerful crushing (blasting) action;

Strong sound effect.

The duration of the explosion is about 10-5...10-6 s. Therefore, its power is very high, although the reserves of internal energy of explosives and mixtures are no higher than those of flammable substances that burn under normal conditions.

When analyzing explosive phenomena, two types of explosion are considered: explosive combustion and detonation.

The first includes explosions of fuel-air mixtures (a mixture of hydrocarbons, petroleum product vapors, as well as sugar, wood, flour and other dust with air). Characteristic feature Such an explosion has a burning speed of the order of several hundred m/s.

DETONATION - very rapid decomposition of an explosive (gas-air mixture). propagating along it at a speed of several km/s and characterized by features inherent in any explosion mentioned above. Detonation is typical for military and industrial explosives, as well as for fuel-air mixtures in a closed volume.

The difference between explosive combustion and detonation is the rate of decomposition; in the latter it is an order of magnitude higher.

In conclusion, three types of decomposition should be compared: conventional combustion, explosive and detonation.

NORMAL COMBUSTION processes proceed relatively slowly and at variable speeds - usually from fractions of a centimeter to several meters per second. The burning rate depends significantly on many factors, but mainly on external pressure, increasing noticeably with increasing pressure. On outdoors this process proceeds relatively sluggishly and is not accompanied by any significant sound effect. In a limited volume, the process proceeds much more energetically, is characterized by a more or less rapid increase in pressure and the ability of gaseous combustion products to produce work.

EXPLOSIVE COMBUSTION, compared to conventional combustion, is a qualitatively different form of process propagation. Distinctive features explosive combustion are: a sharp jump in pressure at the site of the explosion, a variable speed of propagation of the process, measured in hundreds of meters per second and relatively little dependent on external conditions. The nature of the explosion is a sharp impact of gases on environment, causing crushing and severe deformation of objects at a relatively short distances from the explosion site.

DETONATION is an explosion propagating at the maximum possible speed for a given substance (mixture) and given conditions (for example, concentration of the mixture), exceeding the speed of sound in a given substance and measured in thousands of meters per second. Detonation does not differ in the nature and essence of the phenomenon from explosive combustion, but represents its stationary form. The detonation speed is a constant value for a given substance (mixture of a certain concentration). Under detonation conditions, the maximum destructive effect explosion.

Protective shutdown is a type of protection against electric shock in electrical installations, providing 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.

Areas of application of protective shutdown: addition to protective grounding or grounding in an electrified tool; addition to grounding to disconnect electrical equipment remote from the power source; a measure of protection in mobile electrical installations with voltages 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 ground, the phase voltage relative to ground changes - the value of the phase voltage will tend to the value of the line voltage. In this case, a voltage arises between the neutral of the source and the ground, the so-called zero sequence voltage. The total resistance of the network relative to ground decreases when the insulation resistance changes towards its decrease, etc.

The principle of constructing protective shutdown circuits is that the listed operating changes in the network are perceived by the sensitive element (sensor) of the automatic device as signal input quantities. The sensor acts as a current relay or voltage relay. At a certain value of the input value, the protective shutdown is triggered and turns off the electrical installation. The value of the input quantity is called the setpoint.

The block diagram of a residual current device (RCD) is shown in Fig.

Rice. Block diagram of the residual current device: D - sensor; P - converter; KPAS - alarm signal transmission channel; EO - executive body; MOP is a source of danger of injury

Sensor D reacts to a change in the input value B, amplifies it to the value KB (K is the sensor transmission coefficient) and sends it to the converter P.

The converter is used to convert the amplified input value into a KVA alarm signal. Next, the emergency signal transmission channel CPAS transmits the AC signal from the converter to the executive body (EO). The executive body carries out 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.

In Fig. A schematic diagram of protective shutdown using an overcurrent relay is shown.

The coil of this relay with normally closed contacts is connected through a current transformer or directly into a conductor cut leading to a separate auxiliary or common grounding conductor.

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 services (snack bars, cafes, etc.). As a technical means of protection against electrical injuries and against possible fire in electrical installations, the mandatory use of residual current devices 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 operating principle 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 a special material. During normal operation of the electrical network, the core keeps the release mechanism in the on state. If any malfunction occurs in the secondary winding of the leakage current transformer, an EMF is induced, the core is retracted, and the magnetoelectric latch associated with the mechanism for freely releasing the contacts is activated (the switch is turned off).

ASTRO-UZO has a Russian certificate of conformity. The device is included in the State Register.

Design and operating principle of RCD

RCD connection diagram.

Errors in connection diagrams due to which the RCD trips.

How to choose an RCD? Types and characteristics of RCD.