DonNTU   Masters' portal

Sasha Ivzhenko

Electtrical engineering faculty

Industrial enterprises and sities electrical supply department

SpecialityElectrotechnical systems of electrical consumption

Safety conditions in the four-wire systems: TT without zeroing and IT

Scientific adviser: Ph.D., Professor Kourennyi Eduard

Resume Abstract

Summary on the theme of master's work

Contents

• Introduction
• 1. relevance of the topic
• 2. purpose of the study
• 3. Electrical Safety
• 4. PUE requirements for a four-wire systems electrical unit
• 5. protective ground
• 6. Calculation of short-circuit
• Conclusion
• References

Introduction

The widespread use of electricity has led to a significant expansion of the circle of persons associated with the operation of electrical equipment. In connection with this issue of workplace safety in the operation of electrical equipment are of particular importance. Ensure safe working conditions in the production in Ukraine received much attention.

Possible sources of electric shock to persons in the workplace can be a fault in the power networks, in electrical machinery, lack of knowledge or failure to comply with electrical safety.

1. relevance of the topic

In this paper, we consider the risk of electric shock. it will be calculated load of residential homes as well as resistance of the earth, that the earth is laid in a single layer. It is proposed to allow the system to use the TT without zeroing in the presence of residual current devices and standards for grounding resistance and duration of shutdown buildings damaged electrical equipment. Theme is very relevant and important in our time.

2. purpose of the study

The purpose of my work: analysis of conditions electrical and fire safety systems, IT and TT in compliance with modern requirements PUE.

3. Electrical Safety

Electrical Safety - a system of organizational and technical measures, means and methods that provide protection from the harmful and dangerous effects of electric current, electric arc, electromagnetic fields and static electricity.

Scientific and industrial achievements, application of new insulating materials, circuit switching and protection devices, improvement of the rules, regulations, instructions, significantly increased the level of electrical current electrical and reduced risk. As for the separate electrical systems, then its electrical we can say this:

- Electrical installation - lack of the electrical installation of the threat to life, health and property of the people, animals, plants and the environment, which exceeds the acceptable risk.

The analysis of accidents in industry shows that of the total number of accidents leading to death of a man in the production of 20-40% (energy up to 60%) is due to an electric shock, as a rule, more than for any other reason, with 75-80% of fatal electric shock occurs in the electrical voltage of 1000 V.

Effects of electric current on the human variety. It depends on many factors. The nature of the distinction: thermal, biological, electrolytic, chemical and mechanical damage.

The thermal effect of the current shows some parts of the body burns blackening and charring of the skin and soft tissue by heating to a high temperature of organs located in the path of the electric current, blood vessels and nerve fibers, causing them to functional disorders.

The electrolytic action of the current shown in the decomposition of various ions in body fluids, breaks their properties.

Chemical action of the current expressed in the occurrence of chemical reactions in the blood, lymph, nerve fibers to produce new substances, no peculiar body.

Biological effects of current manifests itself in irritation and excitation of body tissues, seizures occur in respiratory arrest, changes of cardiac activity.

The thermal effect of the current shows some parts of the body burns blackening and charring of the skin and soft tissue by heating to a high temperature of organs located in the path of the electric current, blood vessels and nerve fibers, causing them to functional disorders.

The mechanical effect of the current leads to a strong muscle contractions, until they rupture, tear skin, blood vessels, bone fractures, dislocations of the joints, tissue bundle.

By type of lesion distinguish electrocution and electric shocks.

Electric shock - it is local damage (burns, skin plating, mechanical damage, elektrooftalmiya).

Electric shocks - it's common lesions associated with the excitation current of tissues (malfunction of the central nervous system, respiratory system and blood circulation, loss of consciousness, speech disorder, convulsions, respiratory failure, right up to the stop, instant death) passing through them.

– environmental conditions (temperature, humidity, the presence of current-carrying dust and others)..

In the CT system for end circuits operating current to 32 A, and distribution circuits such off time is allowed, which does not exceed 1 s. Alternating current is more dangerous than a constant, but at a high voltage (500 V) is more dangerous than DC. The most dangerous frequency range from 20 VAC to 100 Hz. The bulk of industrial equipment operates at a frequency of 50 Hz (which is included in the dangerous range). High-frequency currents are less dangerous. High frequency currents can cause a superficial burns, since they extend only to the human body surface.

The total electrical resistance of the human body resistance of the stack of each of the body portions arranged on the current path. They have different electrical resistance. The greatest resistivity is the upper stratum corneum, which has no nerve endings and blood vessels. In wet or damaged skin electrical resistance is about 1000 ohms. When dry, no damage, skin electrical resistance increases dramatically. Between the current flowing through the human body, and the application of a voltage there is a nonlinear relationship - with increasing voltage DC power is growing rapidly. This is due mainly to non-linear electrical resistance of the human body. Thus, when the voltage across the electrodes 40 ... 45, in the outer skin layer, considerable electric field strength at which all or part of the sample is the outer layer of the skin, reduces the impedance of the human body. At voltage 127 ... 220, it falls practically to the internal resistance of the body. The greater the current flow, the resistance of the skin to fall faster. The total resistance of the internal environment of the human body does not exceed a few hundred ohms. As the calculated values of the resistance of the human body taken at a voltage of 1000 ohms and 50 above.

Low electrical resistance of the organism contributes to serious consequences of electric shock. Reduce the electrical resistance of the human body such indicators as the physiological and psychological state (fatigue, drunkenness, hunger, disease, emotional arousal).

4. PUE requirements for a four-wire systems

It is known that the most common four-wire 380/220 V. In previous PUE [5] in the 380/220 neutral to earth ground buildings without zeroing is not permitted, and in earlier editions used tough language "strictly forbidden." At the present time PUE CT system capable of implementing both the vanishing or without it. Also under the new rules [2] changed and the requirements for electrical safety in the IT. Now, in this four-wire grounded neutral is not allowed. In this regard, the research paper analyzes the modes of operation of IT and TT in terms of electro-compliance with modern fire safety requirements PUE.

electrical unit:

The strength of the current passing through the human body is the main factor that determines the consequences of defeat. Different sizes and currents have different effects on the human body. There are three basic threshold current: On measures against electric shock, electrical divided into:

– electrical voltage up to 1 kV electric networks with earthed neutral;

–electrical voltage up to 1 kV electric networks with isolated neutral;

–electrical voltages above 1 kV electric networks with isolated or compensated (i) the neutral grounded through a resistor;

–electrical voltages above 1 kV electric networks with effectively earthed or grounded neutral[2].

Depending on the type of earthing system distinguish the network with grounded and isolated neutral (mid-point). This system ТТ, IT, TN-C, TN-S, TN-C-S Three-phase AC or DC. Each of them is characterized by its degree of shock.

– electrical voltage up to 1 kV electric networks with earthed neutral;

the four-wire systems: TT and IT

Go to the consumer electric power is supplied through lines from the distribution network to originate at the transformer substation (TS). By TA electricity supplied in most cases by a three-wire three-phase current high-voltage lines with a voltage of 6 to 35 kV and discharged the following schemes: a three-phase three-wire or four-wire network with isolated neutral – IT system, three-phase five-wire or four-wire network with earthed neutral, the system TT or TN.

according to the PUE[2] TT system (Pic. 1) – system, one point of the current-carrying parts of the power supply is grounded, and the exposed conductive parts of electrical equipment connected to the PE conductor connected to the earthing switch is electrically independent of the earth electrode, which is connected to the point of live parts supply and the IT system (Pic. 2) – system in which the supply network is isolated from ground or connected to earth through the appliances or (i) a device having a large resistance, and the exposed conductive parts are connected to a grounded electrical conductor PE.

PE conductor[2] – protective conductor in electrical voltage up to 1 kV, designed to protect against electric shock.

– electrical voltage up to 1 kV electric networks with earthed neutral;

5. protective ground

When servicing the electrical danger is not only non-insulated live parts under voltage, but also the design of the electrical equipment is not normally energized, but may be energized when damaged insulation (building electrical appliances, metal frames and panels, etc.). To protect people from electric shock if damaged insulation PUE regulates perform alone or in combination following protective measures:

– protective ground;

– automatic power-off;

– bonding;

– Class II equipment or equivalent insulation;

– Electrical safety department;

– insulating (non-conductive) areas, zones, playgrounds;

– of ultra-low (low) voltage SELV, PELV, FSNN;

– potential equalization.

protective ground [2] – a grounding point or points of the system, or hardware umtanovki to provide electrical safety. In other words, the purpose of the protective grounding is to create a housing between the device and the electrical connection of the ground sufficiently small resistance to fault in the case body touching it a person can not cause the current through its body is of such magnitude that would it is dangerous to life or health (Figure 4 - supports loop phase-zero not shown).

Distinguish between artificial and natural grounding. Artificial earthing - is earthing, specially made ??for the purpose of grounding and earthing natural - the leading part, in addition to its direct functions at the same time can serve as a grounding (eg, reinforcement of foundations and utilities buildings and structures, the underground portion of the metal and concrete supports of overhead lines, and so etc.). First of all it is necessary to use natural grounding, because it offers significant cost savings, and in those cases where they are not available or resistance exceeds the desired value, arranged artificial grounders.

6. Calculation of short-circuit

The results of the calculation of short-circuit (SC) are used to select electrical power supply circuit of TP and the distribution network, the calculation of relay protection, if the proper safety and communication equipment.

Calculation of short-circuit currents in the network voltage of 1 kV is performed usually named units. This should be taken into account resistive and inductive impedance of all the elements of a short-range, and active resistance of transient contacts (on the buses, input and output switching and protection devices, plug contacts and arc resistance in short circuit).

At high power supply grid (xc <0.1 xt), the current single-phase short-circuit the metal is given by:

where - phase power supply line;

– loop impedance phase-zero TA from the transformer to the point of fault;

-Impedance transformer TP-phase short-circuit currents.

Described unified synthesis process can be successfully implemented automated and provide for the management onlyUser functions: a description of Moore FSM and the choice of the algorithms and structures. Moreover, the approach presentedcan be applied to a number of other contemporary bases (CPLD, ASIC) and to determine the general direction of the optimization work in the synthesis ofposlidovnistnih logic circuits.

Conclusion

According to current regulations, the distribution network in homes of old building refers to the system power supply TN-C. In this network is implemented vanishing electrical enclosures. But very often in a building, you may find that the supply of electrical receivers laid-wire single-phase, that is, the protective conductor, zeroing, no. At present, when a lot of high-power electrical appliances, in order to electro-body electric fire to vanish. But you need to upgrade the network in apartment shields. If we assume that instead of TN-C system is implemented TT, it is allowed to use the grounding of electrical enclosures without zeroing. The main protection against indirect contact in such a system is the power supply to power off with the obligatory application of the RCD

In writing this essay master's work is not yet complete. Final completion: December 2013.

References

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