Single phase inclusion of a person in the electrical network. Schemes for inclusion of a person in the electric circuit of the current Analysis of the lesion of the current in single-phase circuits

Flow permanent the current of the human body causes a painful sensation in the place of touch and in the joints of the limbs. As a rule, the impact of DC to the human body causes burning or painful shock, which in severe cases can lead to stopping breathing or heart.

In the event of a person touching a single-phase or two-phase AC networks at any network mode relative to the Earth (isolated from the ground, with a grounded pole, with a grounded midpoint), because In this case, the current flowing through a person is determined only by the electrical resistance of its body.

The degree of danger and the outcome of electric shock dependes: from the "Connection" scheme of a person in an electrical circuit; On the electrical network:

three-phase four-wire with a grounded neutral;

three-phase with neutral isolated.

Man damage to electric current can be caused by single-pole (single-phase) or two-pole (two-phase) touch to the current-carrying part of the installation.

Single-phase connection is less dangerous than two-phase, but it occurs significantly more often and is the main cause of electric shock. The outcome of the lesion in this case has a decisive effect of the neutral regime of the power grid.

When touched to one of the network phases with an isolated neutral, the insulation resistance and capacity relative to the Earth of the two other intact phases are converted to one of the phases.

Scheme of human touch to one network phase with grounded neutral

With an increase in insulation resistance, the danger of electric shock decreases.

When an emergency mode of operation of the same network, when the deaf closing phase on the ground occurs, the voltage in the neutral point can reach the phase voltage, the voltage of the intact phases relative to the Earth becomes equal to linear voltage. In this case, if a person comes to the same phase, it will be under linear voltage, the current will go along the way "Hand - Leg". In this situation on the outcome of the damage, the insulation resistance of the wires does not play any role. Such defeat current most often leads to a fatal outcome.

Examples indicate that, with other things being equal, a single-phase connection of a person into a network with an isolated neutral is less dangerous than the network with a grounded neutral.

The most dangerous is the two-phase connection of a person into the electrical network, as it falls under the linear network voltage, regardless of the neutral mode and operating conditions of the network.

7.9. Duration of current exposure.

The duration of the current is often a factor on which the final outcome of the lesion depends. The longer the effect of electric current on the human body, the harder the effects of the lesion. After 30s, the human body resistance to the flow of current drops by about 25%, and after 90s - by 70%.

It has been established that electric shock is possible only in standing in full peace of the human heart, when there is no compression (systole) or relaxation (diastole) of the ventricles of the heart and atrial. Therefore, at a low time, the exposure of the current may not coincide with the phase of complete relaxation, but everything that increases the tempo of the heart of the heart, contributes to increasing the probability of stopping the heart when the current strikes any duration. Such reasons include: fatigue, arousal, hunger, thirst, fright, adopting alcohol, drugs, some drugs, smoking, disease, etc.

The passage of current through a person is a consequence of its touch no less than two points of the electrical circuit, between which there is some potential difference (voltage).

The danger of such a touch is ambiguous and depends on a number of factors:

personal circuits in electrical circuit;

network voltage;

network schemes;

network neutral modes;

degrees of insulation of current parts from the ground;

capacity of current-carrying parts relative to the Earth.

Classification of networks voltage up to 1000 V

Single-phase networks

Single-phase networks are divided into two-wire and single-wire.

Two-wire

Two-wire networks are divided into isolated from the ground and with ground wire.

Isolated from Earth

With grounded wire

These networks are widely used in the folk economy, starting with a low-voltage of the portable tool and ending with powerful single-phase consumers.

Single-wire

In the case of a single-wire network, the role of the second wire is performed by land, rail, etc.

Single-phase network. Single-wire

The main application of these networks received in electrified transport (electric locomotives, trams, metro, etc.).

Three-phase networks

Depending on the neutral mode of the current source and the presence of a neutral or zero conductor can be performed in four schemes.

Neutral current source point - Point, voltage on which relative to all phases are the same in absolute value.

Zero point of the current source - grounded neutral point.

The conductor attached to the neutral point is called a neutral conductor (neutral), and to zero point - zero conductor.

1. Three-wire network with neutral isolated

2. Three-wire sitting with ground neutral

3. Four wired network with isolated neutral

4. Four Wired Network with Ground Neutral

At a voltage of up to 1000V, the schemes "1" and "4" are used in our country.

Human inclusion schemes in electrical circuit

Two-phase touch - Between the two phases of the electrical network. As a rule, the most dangerous because there is a linear voltage. However, these cases are quite rare.

Single-phase touch - between phase and earth. This assumes the existence of an electrical connection between the network and the earth.

Read more about the schemes of inclusion of a person in a chain, see Vallea P.A. Fundamentals Safety in electrical installations.

Single-phase networks

Isolated from Earth

Normal mode

The better the insulation of the wires relative to the ground, the less danger of one-phase touch to the wire.
The touch of a person to a wire with high electrical insulation resistance is more dangerous.

Emergency mode

When the wire is closed to the Earth, the person touched a working wire turns out to be energized equal to the almost total line voltage, regardless of the insulation resistance of the wires.

With grounded wire

Touching uncontrolled wire

In this case, the person turns out to be practically under the total voltage of the network.

Touching grounded wire

Under normal conditions, the touch towards the grounded wire is practically not dangerous.

Touching grounded wire. Emergency mode of operation

With a short closure, the voltage on the ground wire can reach dangerous values.

Three-phase networks

With isolated neutral

Normal mode

The danger of touch is determined by the total electrical resistance of the wires relative to the Earth, with increasing resistance, the risk of touch decreases.

Emergency mode

Touch voltage is almost equal to linear network voltage. The most dangerous case.

With ground neutral

Normal mode

The person in this case turns out to be almost under the phase voltage of the network.

Emergency mode

The tension value of the touch lies between the linear and phase voltage depends on the relationship between the ground-circuit resistance to the ground and the ground resistance.

Electrical safety measures

Elimination of a person's contact with current-time parts.
It is relazsed by the location of the current-handing parts in the inaccessible places (at height, in cable channels, boxes, pipes, etc.)

Using low voltages (12, 24, 36 V).
For example, to power the hand tools in rooms with an increased danger of electric shock.

Using double isolation.
For example, the execution of the electrical installation case from the dielectric.

Application of personal protective equipment.
Before use, it is necessary to make sure that they are configured, integrity, as well as check the deadlines of the previous and subsequent tool calibration.

Basic protective equipment Provide direct protection against electric shock.
Additional protective equipment Can not independently provide security, but can help when using fixed assets.

Control of equipment isolation and networks.
- Output control.
- planned.
- extraordinary, etc.

Safety separation of networks.
Allows you to reduce the capacity of lines near the consumers of electrical energy.

Protective grounding is a deliberate electrical connection of metal inad -ide parts that can be energized, with land or its equivalent (popular about grounding on geektimes.ru).

In networks up to 1000 in a protective grounding applied in networks with isolated Neutral.
The principle of operation consists in reducing to the safe value of tension.

When the grounding is impossible, in order to protect the potential of the base on which there is a person and equipment, by increasing. For example, connecting the repair basket with a phase conductor of the LPP.

Grounders are divided into:
a. Artificial, intended for grounding purposes directly.
b. Natural, in the ground metal items of other purposes, which can be used as earthing. Exceptions on the criterion of recovery hazardos (gas pipelines, etc.).

Ground resistance should be no more than a few OM. At the same time, with time, as a result of corrosion, the earthing resistance increases. Therefore, its value must be periodically monitored (winter / summer).

Protective reinforcement is a deliberate connection of metal inadvertent parts that can be under voltage, with a repeated ground-grounded zero protective conductor.

Scope - electrical installation with ground neutral with voltage up to 1000V.

Principle of operation - converting the closure on the equipment body into a single-phase short circuit, followed by disabling equipment to exceed the maximum allowable current.

Clean protection is implemented either using circuit breakers or fuses. Special attention should be paid to the choice of the thickness of the zero protective wire sufficient to carry out a short circuit current.

Application of Uzo (protective shutdown devices).

This type of protection is triggered when the currents incoming and outgoing in the tracking circuit do not coincide in size, that when there is a current leakage. For example, when touched a person to a phase wire, part of the current goes past the main circuit into the ground, which causes the power supply to the controlled circuit. Read more ,.

The degree of danger and the outcome of electric shock dependes: from the "Connection" scheme of a person in an electrical circuit; On the electrical network:

three-phase four-wire with a grounded neutral;

three-phase with neutral isolated.

The neutral point of the transformer (generator) is called the point of connecting the winding of the supply transformer. During the normal mode of operation of the electrical network at this point, the voltage is 0. The neutral of the power supply can be grounded and isolated from the ground, it determines its mode of operation. Grounding neutral is called a working ground R 0.

The selection of the network scheme and the current source neutral mode is carried out depending on the technological requirements and safety conditions.

By technological requirements Preference is given to the four-wire network, since this network is characterized by two voltages - linear and phase (380/220 V). The linear voltage of 380 V feeds the power load - include electric motors of production equipment between phase wires. Phase voltage \u003d 220 V is used for the lighting installation - connect lamps between phase and zero wires. Linear voltage is always more phase 1.73 times.

By security conditions It is advisable to apply networks with an isolated neutral, when it is possible to maintain a high level of network insulation, providing a minor wire capacity relative to the Earth. These may be low-consumable networks that are not exposed to the aggressive environment and under constant supervision of qualified personnel.

When touched to one of the phases of the network with an isolated neutral (Fig.) Sequentially with the resistance of a person turn out to be included insulation resistance and capacity relative to the Earth of two other intact phases.

Fig. Single-pole touch to the network with a neutral insulated during normal operation

In normal operation of the mains, the power supply voltage in relation to the Earth is zero. The phase voltage relative to the Earth is the same and equal to the phase voltages of the power supply.

Wire insulation resistance is never equal to an infinitely large value, necessarily have a leakage currents.

Wires and land in this case are as if plated capacitor, between which an electric field occurs. The longer electrical network, the greater its container.

According to technological requirements, preference is given to the four-wire network, since this network is characterized by two voltages - linear and phase (380/220 V). The linear voltage of 380 V feeds the power load - include electric motors of production equipment between phase wires. Phase voltage \u003d 220 V is used for the lighting installation - connect lamps between phase and zero wires. Linear voltage is always more phase 1.73 times.

Under the safety conditions of the network with an isolated neutral, it is advisable to apply when it is possible to maintain a high level of network insulation, providing a slight capacity of wires relative to the Earth. These may be low-consumable networks that are not exposed to the aggressive environment and under constant supervision of qualified personnel.

Networks with a grounded neutral are used where it is impossible to provide a high level of insulation of the electrical installation or cannot be quickly found and eliminate its damage.

By virtue of the specifics and minor production capacity compared to other food industry enterprises in catering establishments, single and two-phase networks with a grounded neutral can be used, and during the operation of small mechanization, the electrical network with an isolated neutral network is recommended. The degree of electrical safety in such networks increases due to the large resistance of the insulation of electrical conductors relative to the Earth.

Man damage to electric current can be caused by single-pole (single-phase) or two-pole (two-phase) touch to the current-carrying part of the installation.

With an increase in insulation resistance, the danger of electric shock decreases.

In enterprises where networks are branched and have a significant length, and therefore, a large capacity, the system with an isolated neutral loses its advantage, as the leakage current increases, the resistance of the phase-ground section is reduced. From the point of view of electrical safety in such cases, the preference is given to a network with a grounded neutral (Fig.).

Scheme of human touch to one network phase with grounded neutral

The resistance of the Earth, as in the case of an electrical network with an isolated neutral, can be neglected.

Examples indicate that, with other things being equal, a single-phase connection of a person into a network with an isolated neutral is less dangerous than the network with a grounded neutral.

Cases of two-phase touch occur rarely and mainly in electrical installations up to 1000 V when working on panels and assemblies, during the operation of equipment with uninsulated current-friendly parts, etc.

There are various inclusion schemes in the electric circuit chain:

Single-phase touch - touching the conductor of the same phase of the active electrical installation;

Two-phase touch - simultaneous touch to the conductors of the two phases of the active electrical installation;

Touching the inactive parts of electrical installations that were under voltage as a result of insulation damage;

The inclusion of the step is the inclusion between the two points of the Earth (soil), which are under different potentials.

Consider the most characteristic schemes for inclusion of a person in an electrical circuit of the current.

Single-phase touch on a network with a deaf-free neutral.The current flowing through the human body ( I H.) With single-phase touch (Fig. 6) closes the chain: phase L. 3 - human body - base (floor) - neutral earthing - neutral (zero point).

Fig. 6. Scheme of single-phase touch in the network

with deaf-andled neutral

According to the law of Ohm:

Where R. o - neutral grounding resistance

R. OSN - base resistance.

If the base (floor) is conductive, then R. Osn ≈ 0

Given the fact that R. about " R H.T.

U H. = U. F.

Such a touch is extremely dangerous.

Single-phase touch in a network with an isolated neutral. The current flowing through the human body (Fig. 7) will be closed by chains: phase L. 3 - human body - the floor and further comes to the network through the insulation of the phases L. 2 I. L. 1, i.e. Next, the current follows the chains: phase insulation L. 2 - Phase L. 2 - neutral (zero point) and phase insulation L. 1 - Phase L. 1 - neutral (zero point). Thus, in the circuit of the current flowing through the human body, the insulation of the phases is consequely included with it. L. 2 I. L. 1 .

Fig. 7. Scheme of single-phase touch in the network

with isolated neutral

Phase insulation resistance Z. has active ( R.) and capacitive components ( FROM).

R. - characterizes the imperfection of insulation, i.e. The ability of isolation to carry out current, although much worse than metals;

FROM - The phase capacity relative to the Earth is determined by the geometric dimensions of the imaginary condenser, the "plates" of which are phases and land.

For R. 1 = R. 2 = R. 3 = R. F I. FROM 1 = FROM 2 = FROM 3 = FROM F current flowing through the human body:

where Z. - Complete resistance of the isolation of the phase wire relative to the Earth.

If the capacitance of the phases must neglect FROM F \u003d 0 (air networks of a small length), then:

from where it follows that the current value depends not only on the resistance of a person, but also on the resistance of the isolation of the phase wire relative to the Earth.

If, for example, R. 1 = R. 2 = R. 3 \u003d 3000 ohms, then

; U H. \u003d 0.0111000 \u003d 110 V

Two-phase touch.With a two-phase touch (Fig. 8), regardless of the neutral mode, a person will be under linear network voltage U. l and according to the law of Ohm:

for U. l \u003d 380 V: I. \u003d 380/1000 \u003d 0.38 A \u003d 380 mA.

Fig. 8. Scheme of two-phase touches of man

A two-phase touch is extremely dangerous, such cases are relatively rare and are, as a rule, the result of working under voltage in electrical installations up to 1000 B, which is a violation of the rules and instructions.

Touching a metal housing that produced under voltage.Touching the electrical installation case (Fig. 9), in which the phase ( L. 3) closed on the housing, tantamount to the phase itself. Therefore, the analysis and conclusions for cases of single-phase touch, discussed earlier, are fully used for the case of closure on the body.

Fig. 9. Scheme of human touch to metallic

corpus

A large percentage of injuries caused by the exposure to electric current occurs when a person touching metal parts or electrical installation cases, randomly produced by voltage due to the malfunction of isolation.

The severity of the electrician depends on the current flowing through the human body, the frequency of the current, the physiological state of the body, the duration of the current exposure, the current path in the body and production conditions.

At the same time, the person turns out to be tensioned - with a voltage between the two points of the circuit circuit to the ground (on the housing) while simultaneously touching them

where - the current flowing through the human body, and;

- resistant human body, Ohm.

The maximum allowable values \u200b\u200bof the voltage of touch and currents flowing through the human body, intended for the design of methods and means of protecting people, when they interact with electrical installations, it is normalized / 2 / and during emergency mode of production electrical installations with a voltage to 1000 V alternating 50 Hz current with duration of exposure over 1 C must not exceed
\u003d 20 V and \u003d 6 mA.

The values \u200b\u200bof the tap of the touch and current flowing through the human body depend on a number of factors: a person's inclusion circuit into an electrical network, network voltage, network diagrams, the mode of its neutral, the degree of isolation of current parts from the ground, as well as the capacity of the current-carrying parts relative to the Earth and T .P. This dependence must be known when evaluating a network under the conditions of safety, choosing and calculating the relevant protection measures, etc.

At the same time, we assume that the resistance of the foundation on which the person is standing (soil, gender, etc.), as well as the resistance of its shoes is insignificant, and zero is equal.

The human body resistance varies widely (from 400 to 100,000 ohms) depending on the skin condition (dry, wet, clean, damaged, etc.), contact density, contact area, current flowing through the human body and tensions, as well as on the time of exposure to the current per person.

At voltage up to 1000 V in our country, they are used mainly two schemes of three-phase current networks - four-wire with a grounded neutral voltage 220/127, 380/220 and 660/380 V and three-wire with an isolated neutral voltage of 36, 42, 127, 220, 380 and 660 V.

Let's analyze the danger of damage to the current in the normal operation of the networks.

2.1. Three-phase four-wire network with deaf-free neutral

Consider the network with a voltage of 380/220 V (Fig. 1).

P man's ricking towards the electrical installation case, produced by voltage, in a four-wire network

Under normal operation of the network, the resistance of the isolation of phase and zero wires relative to the Earth compared to the neutral ground resistance is very large values \u200b\u200band with some assumption can be equal to infinity, i.e.
.

In this case, the current flowing through the human body

where \u003d 220 V - phase voltage, i.e. In this case, the voltage between the beginning and the end of one transformer winding.

- Resistance to the grounding device to which the neutral of the transformer is attached, Ohm.

In accordance with PUE / 1 / the greatest value is 66 ohms; Human body resistance , does not fall below a few hundredthm. Consequently, without a big mistake, you can neglected the value .

Thus, the electrical installation that produced in a voltage in the network with a deaf-free neutral is processed to the housing, the person turns out to be practically under phase voltage, i.e. In this case, the voltage between the phase and zero wire.