Network with a deadly grounded neutral. Grounding cable. Earthing of electrical installations

Modern life of man, its comfort and provision with all necessary, are inextricably linked with electricity. Thanks to him, a person has a livelihood and the opportunity to influence the forces of nature in order to maximize the benefits for his life. But to the many advantages that electricity has, there is one huge minus - appliances and equipment that consume and produce electricity, are a threat to human life, if not adhere to the rules of their use.

Electrical installations and their classification according to safety requirements

The main factors affecting the degree of danger to human life in electrical installations of any type are:

• voltage;
• Type of neutral ground;
• The magnitude of the current to be closed to ground;
• Isolation of parts along which the current moves;
• Resistance of the human body;
• Resistance of the earth (ground) in the area of the electric current.

Proceeding from these main sources, in the current "Rules for the installation of electrical installations" (PUE), all installations are divided into four categories.

The first is the installation with deadly neutral transformers, operating from 220 kV and higher, and with effectively-grounded neutral - installations from 110 to 220 kV. An effectively grounded neutral is a circuit that limits the earth fault current, it can contain different types of resistances (active, non-linear and reactive), as well as non-earthed neutral.

The second includes installations where isolated neutral or resonant grounding is used with arc suppression resistors and reactors operating in networks whose voltage ranges from 3 to 35 kV.

The third is represented by electrical installations using a network with a deadly grounded neutral and operating at a voltage of 110 to 600 V. In these installations, the earth fault currents have large values.

The fourth category consists of installations with isolated neutral operating in networks up to 1 kV. Such installations have a small earth fault current.

Safe operation of electrical installations

Completely eliminate the factors that threaten the health and lives of people working on electrical installations is impossible, because they have a natural background. But to minimize them and make work in installations as safe as possible is not only possible, but also necessary. To do this, all the maintenance and operation of electrical installations are regulated in a single collection of rules and norms: "Rules for the installation of electrical installations" (PUE). One of the most important requirements of the electrical safety system is the protective earthing of electrical installations. It is this requirement that will be considered in more detail in this article.

Protective earthing is designed to protect personnel working and serving these installations and networks, as well as consumers of electricity that use it in household appliances and devices. What does protective grounding provide? Safety of the person at casual contact with metal parts of electroinstallations which are not current-carrying, but appeared under pressure owing to breakdown of insulation of the conductors which are under a current.

What is grounded in electrical installations?

The requirements and rules for the use of protective earthing are summarized in a single document regulating and determining the standardization of the entire process - GOST. Grounding ensuring the protection of personnel and consumers from electric shock is carried out strictly in accordance with the requirements of the Electrical Code and the corresponding GOST. The protective earthing of electrical installations provides electrical connection of metal parts of electrical installations to the ground, and in the absence of it - with a conductor replacing the earth. Also it should be noted that those parts of the installations that have no other protection are grounded.

Thus, metal enclosures of electrical assemblies, apparatuses, machines, cable couplings, lamps, sockets and switches are grounded, as well as armor of cables and wires.

Existing earthing systems for electrical installations

The protective earthing systems of electrical installations are determined on the basis of such characteristics of the power source as a grounded neutral, an isolated neutral. There are three main systems developed by the International Electrotechnical Commission (IEC): TN, IT and TT. Let us consider them in more detail.

The TN system and its subsystems

Systems with deadly grounded neutral, in which the metal parts of the electrical installation are connected to the neutral with zero grounding conductors, belong to the TN group. In turn, this group has subgroups formed by the way of using the zero working and protective conductors. So, if these conductors are combined in the same wire along the entire length of the network, the subsystem is designated TN-C. This is an old Soviet system. If the protective and working neutral wire are combined only in the circuit segment starting from the power supply (transformer substation), then this is the TN-CS subsystem. Well, in the case when the zero working and protective conductor are separated by separate wires all over the network, this subsystem is denoted by TN-S. It is considered preferable for complete safety of the electrical installation.

IT and TT systems

A system in which neutral grounding is absent or it is performed through a resonant ground is denoted as IT. In such a system, the metal parts of the electrical equipment are grounded by separate conductors connected to grounding devices.

A system with a deadly grounded neutral, in which the metal parts of the electrical equipment are grounded using devices that are not connected in any way to the neutral of the power source, is designated TT and is used exclusively for mobile premises. In other cases, such a system requires the use of residual current devices.

Earthing devices

According to the PUE, a grounding scheme is used to protect a person from dangerous voltages, mounted by electrically connecting parts of the installation made of conductive materials and isolated from live parts, with a grounding electrode. In turn, the earth electrode is a conductor made of metal, which has good electrical conductivity and a large area of contact with the soil. All together - a grounding electrode and wires, electrically connecting it with parts of electrical installations and there is a grounding device.

Depending on the type of current used in electrical installations up to 1000 V, earthing schemes with a grounded neutral or isolated (alternating current), deadly grounded or isolated middle point (direct current) are used. Neutral of a power source (generator or transformer) is called dead-ground if it is connected directly to the grounding device, and an isolated one is considered to be a neutral that does not have a connection with it or is connected through devices with greater resistance.

Types of grounding devices

Earthing devices are divided into two types: artificial and natural. The first type of grounding structures involves the use of various metal objects. They can be corners, rods and pipes, having a length of at least two and a half meters and buried (buried) in the ground. Between them, they are connected by steel strips or pieces of metal wire - wire rod - of a large diameter (not less than 8-10 mm) by welding. Earthing conductors can be both metal and copper buses, and copper wire bundles, connected to parts of electrical equipment or welding, or bolted connection.

The second type of grounding structures provides for the use as a ground plane of structures of buildings made of metal and reliably connected to the ground. All reinforced concrete structures must have metal fences for attaching earthing conductors. In this case, the grounding conductors are no different from the conductors used in artificial earthing.

Another type of grounding device is zeroing. This type of protective earthing consists in connecting isolated from the current parts of electrical installations with a deadly grounded neutral through a neutral wire. Zeroing ensures the occurrence of short-circuiting in any phase closure on the body of the device and allows more efficient operation of the protective shutdown equipment.

Requirements for earthing devices

All devices used for grounding must comply with standards approved by the state, building codes and electrical codes. Their task is to ensure people's safety, protection of electrical installations and their operation modes.

In no event shall the series connection of several parts of the electrical installation be connected by grounding conductors - only one ground cable having a cross-sectional diameter not less than the one specified in the PUE must correspond to each part. Earthing conductors, placed openly, are protected from the influence of an aggressive environment by coloring them in black.

The technical condition of the grounding devices and the grounding check are performed by the method of inspection with the naked eye of the visible part of the device, inspection with partial opening of the ground and measuring the parameters of the grounding device. The visible part of the device is inspected once every six months.

Requirements for the connection of protective and grounding conductors

All connections of the earthing conductor and grounding conductors are performed by welding. The casing of electrical installations, machines and apparatus, the main grounding contact on the ground loop and the supports of the high-voltage lines are connected by means of a grounding conductor with bolt fastening. Earthing conductors are made of steel or copper busbars, as well as copper harnesses. A grounding cable can also be used as grounding conductors. For these purposes, both multi-core and single-core copper cables are used, the cross-section of which allows for low-impedance connections.

Measuring the resistance of earthing devices

To verify that the resistance of the operating earthing device is in accordance with the rules and instructions, measurements of the existing resistance are carried out. The task of this measurement is to determine the magnitude of the resistance of the grounding system passing through it to ground current - the so-called flowing current.

The measurements are carried out in accordance with the required safety standards: prevention of single-phase closure and use of personal protective equipment, including dielectric gloves and bots, as well as an insulating tool.

Equipment and means for measuring earth resistance

The main instrument for measuring resistance to spreading currents is the IS-10 grounding meter. This device operates in five measurement ranges, which explains its wide application. The minimum range is the resistance from 0.01 to 9.99 Ohm, followed by the ranges of 0.1-99.9 Ohm, 1-999 Ohm, 0.01-9.99 kΩ. The maximum resistance determined by this device is from 1 to 999 mOhm. In combination with the instrument for measurements, remote current and potential electrodes are used.

It should be noted that the grounding measuring circuit is assembled according to strict rules - the connecting conductors of the device, first of all, to current and potential electrodes, then to the device and lastly to the earth electrode.

Earthing check methods

The magnitude of the resistance of the flowing current for different earthing devices is not the same and depends on many factors, such as the kind of electrical installation, the condition of the ground at the installation site of this installation, and the type of such device used.

The measurement procedure contains two methods that are displayed in the rule that applies to the IS-10 devices when measuring ground resistance. If the resistance of the device specified in its passport is greater than 5 ohms, a three-wire circuit is used. If the values are less than this value, a four-wire circuit is used.