Modern human life, its comfort and providing with everything necessary, are inextricably linked with electricity. Thanks to him, man has a livelihood and the ability to influence the forces of nature in order to maximize the benefits for his life. But to the many pluses that electricity has, there is one huge minus - appliances and equipment that consume and generate electricity pose a threat to human life if you do not adhere to the rules for 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;
- neutral grounding type;
- amount of current locked to ground;
- isolation of parts along which current moves;
- resistance of the human body;
- resistance of the earth (soil) in the area of electric current.
Based on these main sources, in the current "Electrical Installation Rules" (PUE), all installations are divided into four categories.
The first is made up of installations with dead-grounded neutral of 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 due to which the earth fault current is limited, it can contain various types of resistances (active, non-linear and reactive), as well as a non-grounded neutral.
The second includes installations where isolated neutral or its resonant grounding is used with the help of arc suppressors and reactors operating in networks whose voltage is from 3 to 35 kV.
The third is represented by electrical installations that use a network with a grounded neutral and operating at voltages from 110 to 600 V. In these installations, earth fault currents are large.
The fourth category consists of installations with isolated neutral operating in networks up to 1 kV. Such installations have a low earth fault current.
Safe operation of electrical installations
It is impossible to completely eliminate factors that threaten the health and life of people working in electrical installations, 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. For this, all maintenance and operation of electrical installations are regulated in a single collection of rules and norms: “Electrical Installation Rules” (PUE). One of the most important requirements of the PUE is the protective grounding of electrical installations. It is this requirement that will be considered in more detail in this article.
Protective grounding is designed to protect the personnel working and servicing these plants and networks, as well as consumers of electricity using it in household appliances and devices. What provides protective grounding? Human safety in case of accidental contact with metal parts of electrical installations that are not live but energized due to breakdown of the insulation of conductors under current.
What is grounded in electrical installations?
Requirements and rules when using protective grounding are summarized in a single document that regulates and defines the standardization of the entire process - GOST. Grounding, which protects personnel and consumers from electric shock, is carried out strictly in accordance with the requirements of the PUE and the relevant GOST. Protective grounding of electrical installations provides for electrical connection of metal parts of electrical installations with earth, and in the absence of it, with a conductor that replaces the earth. It should also be noted that those parts of installations that do not have any other protection are grounded.
Thus, the metal enclosures of electrical units, apparatuses, machines, cable couplings, lamps, sockets and switches, as well as armor of the cable and wires are grounded.
Existing grounding systems for electrical installations
Protective grounding systems of electrical installations are determined on the basis of such characteristics of a power source as dead-grounded neutral, isolated neutral. There are three main systems developed by the International Electrotechnical Commission (IEC): TN, IT and TT. Let's consider them in more detail.
TN system and its subsystems
Systems with dead-grounded neutral, in which the metal parts of the electrical system are connected to the neutral using zero grounding conductors, belong to the TN group. In turn, this group has subgroups formed by the method of using zero working and protective conductors. So, if these conductors are combined in one 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 wires are combined only on a section of the circuit starting from the power source (transformer substation), then this is the TN-CS subsystem. Well, in the case when the zero working and protective wires are separated by separate wires throughout the network, this subsystem is denoted by TN-S. It is considered more preferable for the complete safety of the electrical installation.
IT and TT systems
A system in which there is no neutral grounding or is made through resonant grounding is referred to as IT. In such a system, the metal parts of the electrical equipment are grounded by separate conductors connected to the grounding devices.
A system with a 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 supply, is designated TT and is used exclusively for mobile rooms. In other cases, such a system requires the use of residual current circuit breakers.
Grounding devices
According to the PUE, to protect a person from dangerous voltages , a grounding circuit is used, mounted by electrically connecting parts of the installation made of conductive materials and isolated from current-carrying parts, with an earthing switch. In turn, the ground electrode is a conductor made of metal, which has good electrical conductivity and a large area of contact with the soil. All together - an earthing switch and wires that electrically connect 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, grounding circuits with a grounded neutral or isolated (alternating current), grounded or isolated midpoint (direct current) are used. The neutral of a power source (generator or transformer) is called dead-earthed if it is connected directly to the grounding device, and the neutral that is not connected to it or connected through devices with high resistance is considered isolated.
Types of grounding devices
Earthing switches 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 (dug) in the ground. They are interconnected by strips of steel or pieces of metal wire - wire rod - of large diameter (at least 8–10 mm) by welding. Grounding conductors can be both metal and copper busbars, and copper wire harnesses connected to parts of electrical equipment either by welding or by bolting.
The second type of grounding structures involves the use of building structures made of metal and securely connected to the ground as an earthing switch. All reinforced concrete structures must have metal mortgages for attaching grounding conductors. In this case, the grounding conductors are no different from the conductors used in artificial grounding conductors.
Another type of grounding device is grounding. This type of protective grounding consists in connecting parts of electrical installations isolated from the current to a grounded neutral via a neutral wire. Zeroing provides the occurrence of short-circuit in case of any phase fault on the device case and allows the protective disconnecting equipment to operate more efficiently.
Grounding Requirements
All devices used for grounding must comply with the standards approved by the state, building codes and PUE. Their task is to ensure the safety of people, the protection of electrical installations and the modes of their operation.
In no case is the series connection of several parts of the electrical installation with grounding conductors allowed - each part must correspond to only one grounding cable having a cross-sectional diameter not less than that specified in the PUE. Grounding conductors placed openly are protected from the effects of an aggressive environment by painting them black.
The technical condition of grounding devices and grounding verification is carried out by inspection with the naked eye of the visible part of the device, inspection with partial opening of the soil and measuring the parameters of the grounding device. The visible part of the device is inspected once every six months.
Requirements for connections of protective and grounding conductors
All connections of the ground electrode and grounding conductors are made by welding. The cases of electrical installations, machines and apparatuses, the main grounding contact on the ground loop and the supports of high-voltage lines are connected using a grounding conductor with bolt fastening. Grounding conductors are made of steel or copper busbars, as well as copper harnesses. Also, grounding cable can be used as grounding conductors. For these purposes, both multicore and single-core copper cables are used, the cross-section of which allows low-resistance connections.
Earth resistance measurement
To make sure that the resistance of the current grounding device meets the requirements of the rules and instructions, measurements of the existing resistance are carried out. The objective of this measurement is to determine the resistance of the grounding system to the current passing through it to the ground - the so-called spreading current.
Measurements are carried out in accordance with the required safety standards: prevention of single-phase short circuit and the use of personal protective equipment, including dielectric gloves and bots, as well as an insulating tool.
Equipment and means for measuring grounding resistance
The main device used to measure resistance to spreading currents is the IS-10 grounding meter. This device operates in five measuring ranges, which explains its widespread use. The minimum range is resistance from 0.01 to 9.99 Ohm, then ranges of 0.1–99.9 Ohm, 1–999 Ohm, 0.01–9.99 kOhm. The maximum resistance determined by this device is in the range of 1 to 999 mOhm. In combination with a measuring device, remote current and potential electrodes are used.
It should be noted that the measuring grounding 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 ground electrode.
Grounding Test Methods
The magnitude of the resistance to spreading current for different grounding devices is not the same and depends on many factors, such as the type of electrical installation, the state of the soil at the installation site of this installation, and the type of device used.
The measurement procedure contains two methods that are displayed in the rule applicable to the IS-10 devices when they measure grounding resistance. If the resistance of the device indicated in its passport is above 5 Ohms, a three-wire circuit is used. If the values are less than this value, a four-wire circuit is used.