Protective measures in electrical installations. Protective measures against indirect contact

An important measure that ensures the electrical safety of the personnel serving electrical installations is protective grounding or grounding of metal non-current-carrying (structural) parts of electrical installations and electrical equipment that are not normally energized, but may be energized relative to the ground in emergency modes (in case of insulation damage).

Grounding is the deliberate electrical connection of any point in the network, electrical installation or equipment with a grounding device.

Grounding is subdivided into:

1. working grounding;
2. protective grounding.

PUE give the following basic definitions in relation to grounding:

Working ground is called the grounding of a point or points of current-carrying parts of an electrical installation, performed to ensure the operation of an electrical installation (to ensure the proper operation of the installation in normal and emergency modes).

Working grounding can be carried out directly or through special devices (resistances, arresters, reactors, etc.)

Protective grounding in electrical installations with voltages up to 1 kV, the deliberate connection of open conductive parts with deafly grounded neutral generator or transformer in three-phase current networks, with a solidly grounded outlet of a single-phase current source, with a grounded source point in networks direct currentperformed for electrical safety purposes.

Zero protective conductor - protective conductor in electrical installations up to 1 kV, designed to connect exposed conductive parts to a dead-grounded neutral of the power source.

Zero working (neutral) conductor (N) - a conductor in electrical installations up to 1 kV, intended for powering electrical receivers and connected to a dead-grounded neutral of a generator or transformer in three-phase current networks, with a dead-grounded outlet of a single-phase current source.

Grounding device - a set of grounding conductors and grounding conductors.

Grounding conductor - a conductor connecting the grounding point to the ground electrode.

Earthing switch - a conductive part or a set of interconnected conductive parts that are in electrical contact with the ground directly or through an intermediate conductive medium.

Voltage at grounding device - voltage arising when current flows from the ground electrode into the ground between the point of current input into the ground electrode and the zero potential zone.

Resistance of the grounding device - the ratio of the voltage on the grounding device to the current flowing from the ground electrode into the ground.

Grounding is used to convert a ground fault to a ground fault in order to reduce the voltage on the frame relative to earth to a safe value.

Protective earth

Main purpose protective earth:

1. elimination of the danger of electric shock in case of touching the case or other non-current-carrying metal parts of the electrical installation that are energized.

Protective grounding is used in 3-phase networks up to 1 kV with isolated neutral and in networks above 1 kV with any neutral mode. A schematic diagram of protective grounding is shown in Fig. 4.7.

Figure 4.7. Schematic diagrams of protective grounding (a) in a network with an isolated neutral and (b) in a network with a grounded neutral.
1 - protective equipment cases;
2 - protective grounding conductor;
3 - grounding conductor of the working grounding of the neutral of the current source; R3 and Ro - resistances of protective and working grounding.

The principle of operation of protective grounding is based on reducing the voltage between the case, which is energized, and the ground to a safe value.

Let us explain this by the example of a network up to 1 kV with an isolated neutral.

If the body of the electrical equipment is not grounded and it is in contact with the phase, then touching such a body of a person is equivalent to touching the phase wire. In this case, the current passing through a person can be determined by formula (2.5).

With low resistance of shoes, floor and insulation of wires to ground, this current can reach dangerous values.

If the case is grounded, then the current passing through a person at R about= R n \u003d0 can be determined from the following expression:

(4.1)

This expression is obtained in the following way:

from the grounded housing (Fig.4.8), the current flows to the ground through the ground electrode ( I s) and through a person ( I h). The total current is determined by the expression:

where:
R total - total resistance of parallel connected R sand R h:

Figure 4.8. To the question of the principle of action of protective grounding in a network with an isolated neutral.

From the diagram in fig. 4.8

I h × R h \u003d I s R s \u003d I total × R total, whence the current through the human body will be:

performing the simplest transformations, we obtain the expression (4.1).

For small R s compared with R h and R of this expression is simplified:

(4.2)

where:
R s- housing grounding resistance, Ohm

When R s\u003d 4 Ohm, R h\u003d 1000 Ohm, R of\u003d 4500 Ohm, the current through the human body will be:

Such a current is safe for humans.

The contact voltage in this case will also be insignificant:

U pr=I h × R h \u003d0.00058 × 1000 \u003d 0.58V

The less R s - the better the protective properties of protective grounding are used.

Functional earthing .. Protective earthing .. Sources of interference in earthing networks .. Methods for protecting equipment from interference .. Network with isolated neutral .. Power supply galvanic isolation .. Isolation transformer .. Equipment electromagnetic compatibility (EMC) .. Functional earthing options. Reconstruction of existing facilities .. Design of new facilities .. Independent functional grounding .. Main grounding bus (GZSh) .. Functional grounding bus (SHFZ) .. Zero potential zone .. Protective PE bus .. Functional bus FE .. Potential equalization bus .. Resistance of functional grounding .. Justification of design solutions .. Box of functional grounding ..

Functional (working) grounding is used for the normal functioning of an electrical installation or equipment, i.e. for their normal operation, not for electrical safety purposes, therefore, its use as the only grounding system is strictly prohibited.

This type of grounding can be combined with protective grounding or performed in addition to it based on the requirements of the equipment manufacturer, customer or regulatory documents.

Protective grounding is often a source of overvoltage and conducted noise in low-current automatic control systems, measuring, information or other equipment that is sensitive to interference, which prompts a search effective ways protection of such equipment from various kinds of interference and overvoltage.

Ways to Protect Information Equipment from Interference

1. Network with isolated neutral. A radical solution to the problems described above with interference on protective grounding is the use of galvanic isolation on the power supply (IT - network) with separate grounding of the power and measuring parts of the system, which excludes the flow of interference currents from the power ground.
The implementation of galvanic isolation can be performed using an isolation (isolation) transformer or using autonomous power sources: galvanic batteries and accumulators.

Basic idea of \u200b\u200bgalvanic isolation lies in the fact that in the electrical circuit, the path along which the transmission of conducted interference is possible is completely eliminated. Since in such a network there is no galvanic connection between ground, phase and neutral, a closed current loop with ground does not form and it is safe to touch any of the power outputs of the isolation transformer. Leakage currents to earth are microamperes, which is significantly less than the level of safety currents and does not pose a threat to humans.
An isolation transformer, in addition, is a good protection against impulse, lightning overvoltage, which ensures more reliable operation of the connected equipment.
In this way, high reliability, electrical safety and noise immunity of networks with isolated neutral is their indisputable advantage.
However, use of isolation transformers with insulation monitoring systems (SKI) requires rather large expenses and a legitimate question arises about the appropriateness of such costs. This topic deserves.

2. Electromagnetic compatibility of equipment (EMC).

In most cases of failures and failures in the operation of automation systems, computing and measuring equipment, it is possible to avoid compliance with the requirements for electromagnetic compatibility of equipment and the rules for performing grounding of such systems:

Use of equipment that meets the requirements of the relevant standards for electromagnetic compatibility (EMC);
Application of overvoltage protection devices in the supply feeder circuits;
Connection of metal sheaths of cables to the combined potential equalization system;
Separation of power and signal cables and correct implementation of their intersections;
Use of signal and data cables that meet the manufacturer's requirements for electromagnetic compatibility;
Power and signal cables must be separated from the down conductors of the lightning protection system minimum distance or by shielding in accordance with IEC 62305-3.
Power supply of low-current microprocessor devices must be made from uninterruptible power supplies (UPS) with noise suppression line filters.
External extended power supply networks must be laid with a cable with a shielding sheath connected to the existing protective grounding circuit.
The connection of functional and protective grounding conductors in order to equalize the potentials between them should be carried out at one point on the control system bus or GZSh - leakage currents along the PE conductor should not fall on the cable screens.

3. Correct grounding. This is one of the main and available methods of reducing impulse noise and overvoltage, which lead to malfunctions in the operation of low-current microprocessor equipment. Correct grounding usually solves b aboutmost of the issues of reducing overvoltage and interference.

4. Equipotential bonding between grounding devices of different purposes is the main condition for ensuring the electrical safety of personnel. In rooms intended for the operation of equipment sensitive to interference, a potential equalization system must be made. A ring connecting conductor connected to the main ground bus should be located along the inner perimeter of the building. Equipotential bonding ring conductors must also be located on each floor. An example of the internal contour of an equipotential bonding system along the perimeter of a building is shown in fig. 1.

Figure: 1

Functional grounding options

1. Reconstruction of existing facilities. In this case, according to the operating conditions of information equipment, a low-resistance earthing switch is often required, which is performed in addition to the existing protective grounding of the building's electrical installation.
According to PUE 1.7.55 " First of all, the requirements for protective grounding must be met". In other words, the first place should be to protect the life and health of people. Accordingly, the functional ground bus (FGB) must be connected to the protective ground on the main grounding bus (GZH) of the main potential equalization system of the electrical installation of the building, as shown in fig. 2.

This grounding schemeallows you to ensure electrical safety in accordance with the requirements GOST R 50571-4-44-2011 (IEC 60364-4-44), and PUE Ch. 1.7 provided that the available protective grounding is made in full compliance with the PUE.
The experience of reconstruction of existing facilities shows that almost all facilities, especially those that have been in operation for 10 years or more, have some kind of grounding deficiencies: corrosion of grounding devices, non-compliance with the requirements for the resistance of the ground electrode, non-compliance with the requirements of electromagnetic compatibility ...
Therefore, before installing information equipmentit is necessary to inspect the protective grounding devices. Inspection of grounding devices includes: external inspection, opening (if necessary) conductors in the ground, as well as a set of measurements of parameters of grounding devices.
Based on measurement results an appropriate amount of work must be performed to restore the parameters of protective grounding, which is advisable to combine with the installation of functional grounding and the transition (if necessary) to the TN-S or TN-C-S power supply system.

Low-impedance earthing switch for functional earthing in this case, it is desirable to perform according to the "beam" grounding scheme, which ensures stable operation of the equipment. In cramped conditions, it is possible to use a composite, deep ground electrode system.

Functional ground has its own requirements for grounding resistance that meet the requirements of the equipment manufacturer or departmental standards. For example, for computer technology and informatics according to SN 512-78 grounding resistance should be no more than 1 Ohm, for highly sensitive medical equipment in accordance with Design manual to SNiP 2.08.02-89 - no more than 2 ohms, etc.

2. Designing new facilities.

Figure: 3

When designing new objects, it becomes possible perform a grounding device for repeated protective grounding at the entrance to the electrical installation of the building on required functional earth resistance , which must be simultaneously used for all types of building equipment.
Grounding device diagram repeated protective earth to the required functional earth resistance is shown in fig. 3.
In a buiding the main grounding bus (GZSH) is installed, to which the following are connected: the grounding conductor of the repeated protective grounding, the PEN conductor, the potential equalization system conductor, the PE bus of the supply line in the TN system, the grounding device of the lightning protection system of the 2nd and 3rd categories, as well as the bus functional grounding (SHFZ).

Such a scheme recently it has become widespread in the design of new objects and corresponds to a high level of electrical safety.

3. Independent functional grounding. Sometimes the functional grounding electrode has to be placed separately, outside the zone of influence of natural and artificial grounding electrodes of the building's electrical installation.

Implementation of functional grounding, not associated with the protective grounding device and the main potential equalization system of the building,to be treated as a special case, in which special measures must be taken to protect people from electric shock, excluding the possibility of simultaneous contact with the parts connected to the equipotential bonding system of the electrical installation of the building and to the parts of the equipment connected to an independent grounding device for functional grounding.

There is always the possibility of a potential difference between separate grounding systems, if these grounding systems are within the zone of non-zero potential. A dangerous potential difference can arise, for example, in the event of a short circuit to the electrical equipment case in the TN-S network (before the protection system is triggered), when the lightning protection (step voltage) is triggered, when exposed to external electromagnetic fields, etc.
From the point of view of electrical safety option of independent functional grounding (not connected to the protective grounding device) permissible and m, if the equipment is powered by an isolation transformer or earthing switches of different purposes are located at such a distance that there is a zero potential zone between them. The distance between these two earthing switches must be ≥ 20 m.
More details about geographically close and independent grounding devices, see article An independent functional grounding diagram is shown in fig. 4.

The need for an independent functional grounding device can arise, for example, when the manufacturer of information equipment directly indicates the need for autonomous grounding (without a separate "functional ground" the equipment does not work). In this case, the manufacturer provides two grounding buses in the equipment cabinet:
protective PE;
functional FE.
FE function bus isolated from the cabinet body. Shields of signal (control) cables are connected to it. The FE bus is connected by an insulated copper cable (in order to avoid contact with the metal structures of the building) with a cross section of at least 1x25 mm2 with an earthing switch located at a distance of at least 20 m from the protective (or any other) earthing switch. potential equalization, connected to the main grounding bus. Note that this FE bus inside the cabinet is provided by the equipment manufacturer.

By way of illustration on fig. five shows a variant of an independent functional grounding that is not associated with the protective grounding device.

Figure: five

Justification of design solutions

So that there are no difficulties with the approval and delivery of the project, you need to be careful when receiving the technical specification for the design. If equipment that is sensitive to interference is used at the designed facility, then you must immediately request from the customer or from the manufacturer a passport for this equipment, where the need for an independent grounding device must be justified and the required functional grounding resistance is indicated. Passports (certificates) for the equipment used are attached to the project and serve as a justification for design decisions at all stages of project approval.
Independent functional grounding is carried out according to the diagram on fig. 4.

If an independent functional earthing switch is not provided by the equipment manufacturer, then in this case functional grounding must be performed according to one of the schemes ( fig. 2, 3) taking into account the requirements for electromagnetic compatibility. In this case, the insulated functional grounding bus can be installed in a separate grounding box, excluding simultaneous touching of parts that may be under a dangerous potential difference in case of insulation damage.
Example such a functional grounding box is shown in fig. 6.

Content:

During the operation of electrical equipment, it becomes necessary to use grounding devices. Depending on the purpose, protective and working grounding can be used. In the first case, the safety of personnel working at electrical installations is ensured, and in the second case, we are talking about the normal operation of devices in normal and emergency modes. Both grounds are different and cannot be used together. In order to better understand the purpose and principle of operation, you need to consider each of them in more detail.

What is called protective ground

Protective earthing devices are made by deliberately electrically bonding metal parts to the ground that are not supplied with electrical current and which may unexpectedly be energized.

The main function of protective grounding is considered reliable protection people from electric shock in case of contact with non-current-carrying metal parts that are energized for various reasons, mainly due to insulation damage.

Protective grounding should not be confused with, working and re-grounding, neutral protective conductor. Its action is primarily aimed at reducing to a safe value of the step and touch voltages, which are formed upon short circuit to the case. This is achieved by lowering the potential of the grounded equipment by reducing the resistance of the grounding device. At the same time, the potentials of the base, where the person is located, and the grounded equipment itself are equalized.

Protective grounding is used in the following areas:

• V, voltage up to 1 kV s.
• In single-phase two-wire AC networks, isolated from earth, with voltages up to 1 kV.
• In two-wire DC networks, in which the middle point of the current source windings is isolated.
• In AC and DC networks with any modes of the current source windings at a voltage of more than 1 kV.

Direct contact with the ground or its equivalent is carried out using earthing switches. They fall into two main types:

1. Artificial grounding conductors. They are used for grounding purposes only. They are made from various steel structures and must not be painted. For protection against corrosion, a galvanized coating, an increased number of grounding conductors, and special electrical protection can be used. In some cases, electrically conductive concrete can be used as a ground electrode.
2. Natural grounding conductors. For this purpose, electrically conductive parts of networks and communications in buildings and structures that are in contact with the ground are used. It is recommended to ground electrical installations primarily from natural earthing... Use pipes for water supply and heating systems, structures of buildings and structures made of metal and reinforced concrete, rail tracks, lead sheaths of cables, etc. Do not use pipelines carrying flammable liquids, gases or mixtures.

What is called a working ground

A working ground is the intentional connection to the ground of certain points in electrical circuits. First of all, these are the neutral points of the generator and transformer windings. Reliable conductors are used as connections, as well as special equipment in the form of breakdown fuses, arresters, resistors, etc.

The main purpose of the working grounding is to create obstacles to failures and short circuits, to maintain the system in the event of an emergency. Under its influence, there is a decrease electrical voltage in parts and parts of the mechanism that are directly energized. The measures taken contribute to the localization of electrical failures, their removal and prevention of further spread.

In accordance with safety regulations, it is forbidden to combine protective and working grounding. This is due to the fact that various interference currents, for example, atmospheric electrical discharges, can be superimposed on the currents flowing in single-wire circuits. This can lead to disruption of external communication of devices and even damage to the equipment. In addition, such alignments can render voltage protection ineffective. In case of emergency, it will work as a working one or will not function at all.

The resistance of the working ground should be no more than 4 ohms. This limitation is associated with the magnitude of the voltage that appears relative to the ground on the neutral wire, during the flow of the earth fault current through the working ground. This is especially true when a high voltage transformer winding is closed to a low voltage winding.