modern life, its comfort and providing all necessary inextricably linked with electricity.Thanks to him, has the livelihoods and the ability to influence the forces of nature in order to derive maximum benefit for their lives.But to a lot of advantages, which has electricity, there is a huge minus - apparatus and equipment, consume and produce electricity, pose a threat to human life, if you do not adhere to the rules of their use.
Electrical and classification of safety
main factors influencing the degree of danger for human life in all types of electrical installations are:
- type of neutral grounding;
- value is shorted to ground current;
- insulation parts, which moves the current;
- resistance of the human body;
- resistance to earth (ground) in the area of the electric current.
Proceeding from these basic sources, operating the "Rules for Electrical Installation" (PUE) all settings are divided into four categories.
first setting up of transformers with earthed neutral, runnin
The second consists of the installation, which uses an isolated neutral or resonant grounding it via resistors and arc suppression reactors operating in networks, the voltage of which is from 3 to 35 kV.
third are electrical, using a network with earthed neutral, and the voltage is 110 to 600 V. In these plants, earth fault currents have large quantities.
The fourth category consists of systems with isolated neutral, working in networks up to 1 kV.Such systems have a small earth fault current.
Safe operation of electrical
completely eliminate the factors that threaten the health and lives of people working on electrical installations, it is impossible, because they have a natural underpinnings.However, to minimize them and get the job done in the most secure installations is not only possible, but necessary.To do this, all the maintenance and operation of electrical equipment are regulated in the same collection of rules and regulations, "Rules for Electrical Installation" (PUE).One of the most important requirements of the SAE is a protective earthing of electrical installations.It is this requirement and will be discussed in this article, in more detail.
Protective earth is intended to protect personnel working and maintenance of these installations and the network, and electricity consumers using it in household appliances and devices.Which provides protective earth?Human security in case of accidental contact with the metal parts of the electrical non-conductive, but have come under strain as a result of the breakdown of insulation under the shock.
that the ground in the electrical?
requirements and regulations when using the protective earth consolidated into a single document, which regulates and defines the whole process of standardization - Standard.Grounding protects personnel and consumers against electric shock is performed strictly in accordance with the requirements of the EMP and the relevant state standard.Protective earth Electrical provides electrical connection of the metal parts of electrical installations to the ground, and in the absence of it - to the conductor, substituting ground.It should also be noted that the parts are grounded installations which have no longer any other protection.
thus grounded metal housings of electrical equipment, vehicles, machinery, cable boxes, lighting fixtures, switches and sockets, as well as the armor of the cable and wires.
existing electrical grounding system
protective earthing system are based on electrical characteristics such as the power supply earthed neutral, isolated neutral.There are three basic systems developed by the International Electrotechnical Commission (IEC): TN, IT and TT.Let us examine them in detail.
TN system and its subsystems
systems with earthed neutral, in which the metal parts of the installation are connected to neutral via zero grounding conductors, are a group of TN.In turn, this group is a subgroup, shaping the way the use of the neutral and protective conductors.Thus, if the conductors are aligned in a single length of wire across the network subsystem denoted TN-C.This is the old Soviet system.If the protective worker and neutral wire are combined only at the portion of the chain, starting from the power supply (substation), then it is a subsystem of TN-CS.But in the case where the neutral and protective conductor are spaced on separate wires all over the network, this subsystem is designated S-TN.It is considered preferable for the total security installation.
IT systems and TT
system in which the neutral grounding is missing or it is made through the resonant grounding, referred to as IT.In this system, the metal parts of electrical equipment are grounded individual conductors connected to the grounding device.
system with earthed neutral, in which metal parts are grounded using electrical devices, in any way unconnected with neutral power supply, is indicated by TT and is used exclusively for the mobile space.In other cases, such a system requires the use of protective devices.
According to the EMP, to protect people from hazardous voltages grounding scheme is used, mounted by an electrical connection system parts made of conductive materials and isolated from live parts, with grounding.In turn, the earth electrode is made of metal wire having good electrical conductivity and a large area of contact with soil.All together - the earthing wire and electrically connecting with the electrical parts and have grounding device.
Depending on the type of current used in electrical installations up to 1000 V, applied to the circuit ground isolated or earthed neutral (AC), isolated or earthed mid-point (direct current).Neutral power source (generator or transformer) is earthed when it is connected directly to the ground connection and the isolated neutral is the one that does not have with him connections or connected via a high-resistance device.
Types of grounding devices
grounders divided into two types: natural and artificial.The first kind of grounding structures involves various metal objects.They may be corners, rods and tubes having a length of not less than two and half meters and buried (transfixed) to the ground.They are connected with each other steel strips or segments of metal wire - rod - large diameter (at least 8-10 mm) by welding.The grounding conductor can be both metal and copper bars and copper wire harnesses are connected to the electrical parts or welding or bolting.
second type involves the use of ground structures as earthing building structures made of metal and is securely connected to the ground.All concrete structures have to have metal embedded connection to grounding conductors.In this case, the grounding wires are no different from the conductors that are used in artificial grounders.
Another type of grounding device is vanishing.This type of protective grounding is to connect isolated parts of the current installations with earthed neutral through the neutral wire.The vanishing provides the appearance of any fault in the closure phase the device, and allows you to work more effectively breaking the protective equipment.
requirements for the grounding devices
All devices used for grounding must comply with the standards approved by the state building regulations and codes.Their task - to ensure the safety of people, the protection of electrical installations and modes of operation.
In no case shall the series connection of several parts of the electrical grounding conductors - each part must correspond to only one ground cable having a diameter section of not less than specified in SAE.Grounding conductors, location is open, protected from corrosion by painting them black.
technical condition of equipment grounding and ground verification is carried out by examination with the naked eye visible part of the device, with the partial opening of the inspection and measurement of ground grounding device.The visible part of the device is inspected once every six months.
connection requirements and protective grounding conductors
All connections earthing and grounding conductors are carried out by welding.Cases of electrical machinery and apparatus, the main ground contact on the ground loop and supports high-voltage lines are connected by a ground conductor bolt.Ground wires are made of steel or copper bars and copper cables.Also, as a grounding conductor grounding cable can be used.For these purposes, it is used as a multi-core and single-core copper cable, the section which allows low-resistance connection.
Resistance measurement of grounding devices
To ensure compliance of the current resistance grounding device requirements of rules and regulations, the measurements of the existing resistance.The objective of this measure is to determine the resistance value of the grounding system passing through it to the ground current - the so-called current spreading.
measurements carried out in accordance with the required safety standards: prevention of a single-phase circuit and the use of personal protective equipment, including insulating gloves and boots, as well as isolating tool.
equipment and tools for the measurement of earth resistance
main device that produces currents spread resistance measurement is measuring the earth IP-10.This device operates in five measuring ranges, which explains its widespread use.The minimum range is a resistance of 0.01 to 9.99 ohms, then the following range 0.1-99.9 ohms, 1-999 Ohm 0,01-9,99 coma.Maximum resistance, determined by this instrument, is in the range from 1 to 999 milliohms.In conjunction with the instrument used to measure the outrigger current and potential electrodes.
should be noted that the measuring circuit ground is going according to strict rules - connection conductors of the device, first of all, to the current and potential electrodes, then to the instrument and the last - to the earth electrode.
Methods of testing ground
resistance value of the current spreads for various grounding devices varies and depends on many factors such as the type of installation, soil conditions at the installation site of the installation and use of this type of device.
measurement procedure contains two methods that appear in the regulations applicable to ICs 10 in the measurement of earth resistance.If the resistance of the device, indicated in his passport than 5 ohms, use three-wire.If the values are less than this value - used four-wire circuit.