Українська   Русский
DonNTU   Masters' portal

Abstract on the theme of master's work

Содержание

Introduction

Ensuring fire safety is one of the most important tasks of protection labor.

The concept of fire safety means a state of the object in which the possibility of a fire is excluded, and if it occurs, people are prevented from exposing themselves to dangerous fire factors and protection of material values.

For people, the following main fire factors are dangerous: open fire, sparks, increased temperature of air and surrounding objects, toxic products of combustion, smoke.

Achieving fire safety is facilitated by:

– Regulatory legal regulation and implementation of state measures in the field of fire security;

– development and implementation of fire safety measures;

– production of fire-technical products;

– accounting of fires and their consequences;

– Implementation of the State Fire Supervision (SFS) and other monitoring functions to ensure fire safety;

– scientific and technical support of fire safety.

1. Theme urgency

Existing normative documents, in particular, PUE, departmental norms and rules do not always allow an objective assessment of the danger of fire in electrical networks of residential and public buildings. Electrical products (cables, cords, wires, etc.) belong to the most fire-dangerous kinds of production as in their design are used combustible electroinsulating materials. If such products are damaged under voltage, there may be sources initiation of fires (sparks, electric arcs, splashing of molten metal particles).

According to the Ministry of Emergency Situations in the territory of Donetsk for 2016 there were 5513 fires, which is 12,6% more than in 2015 . Of the total number of fires, 11.1% of fires occurred as a result of violation of fire safety rules for device and operation of electrical installations. In this case, from the thermal manifestations of the electric current (short short circuits, attenuated and dangerously heated contact connections, overloads) 125 fires occurred, among them from damage to cable networks and wires - 65.

The given data testify that the estimation of a level of fire safety of premises at damage in the electrical network is an important task. This also requires improvement methods of accounting for fires and fires, resulting from damage to electrical equipment and wiring.

2. Description of the process of fire formation in the premises from damage in the electrical conductor network.

Postings of premises of residential and public buildings are carried out, as a rule, isolated aluminum installation wires of all sections, as well as non-armored power cables with rubber or plastic Isolation in metal, rubber or plastic sheath up to 16 mm2 inclusive. [1]

With short-circuiting at the point of contact of live conductors, a contact with a large transient resistance is formed among themselves. The process of ignition formation in the operation of electrical equipment is represented by the following scheme. There was a short circuit in the wiring; the protective switchgear closest to the fault protection point (circuit breaker, fuse), through which the through fault current passed, failed to operate, the damaged network section was switched off with a time delay by a backup protective switching device providing longitudinal backup, or in the place of a short circuit, the conductors burned out, this protection against short circuits did not work.

During the disconnection of the short-circuit current by the backup protection (or the burn-out of a part of the conductor at the fault location), the surface material was burned to the depth of the cable (conductor) tab. Out of the formed opening, a spattering of the molten metal from which the conductor was made to the outside occurred (in the room), and in most cases there was a fire of paper or cloth wallpaper, carpets, wooden panels, etc. The cause of the formation of metal particles in the short-circuit zone is the electric explosion of a liquid bridge, formed as a result of the melting of conductor materials under the action of an electric arc at the point of their contact. [2]

The destruction of the bridge leads to the formation of a shock wave that sprays the liquid metal, after which the resulting arc discharge with an arc temperature of 3000-6000С in the channel zone causes a gas-dynamic shock, which tells acceleration to the metal particles [3]. Inflammation of combustible materials can occur when they are located at a distance from the point of origin of the fault.

Protection of four-wire electrical networks from short-circuit and overload is performed by widely used circuit breakers, which are available in single-phase or three-phase design.

In some cases, arc short circuits are not disabled by protection devices. The causes of failure of existing protective devices is a significant reduction in short-circuit currents due to the limiting effect of the electric arc. The resistance of the arc in the apparatus can vary from 0.075 to 0.5 ohms, and for cables and wires 0.05-4.0 ohms. The magnitude of the current in the arc sometimes does not reach the value of the breaking current settings due to the intermittent nature of arc burning, at which the time of continuous current flow is 0.05-0.04 sec, that is, it is often less than required for the protection to be triggered and furthermore for fusing [4]

There are fire-dangerous non-stationary modes of the electrical network, which can not be eliminated with the help of known protection devices (circuit breakers, RCDs). Such modes include intense sparking in attenuated (oxidized) contact connections of the network, arc closure mode through a large transient resistance between the phase and zero conductors. The insensitivity of the RCD to these modes is due to the fact that the sum of the currents in the window of the differential current transformer will be zero.

The main part of electrical devices and devices are contact connections, the technical condition of which depends not only on their normal functioning, but also on the fire safety of the facility.

Analysis of fires from electrical sources showed that in 84% they are due to weak and dangerously heated power contacts in the wiring. Heating of electrical contacts is due to the existence of a transient resistance between the surfaces to be contacted. The amount of heat released in the contact connection depends on the state of the construction of its contacting surfaces and the reliability of their attachment. During operation, over time, an oxide film is formed in the contact junctions (sockets, plugs, shields, etc.) and the transitional resistance of the connection increases and, as a result, its overheating. Overheating, in turn, causes the accelerated growth of the film oxide, which further increases the transient resistance and overheating at the point of contact. In a relatively short time, the number of contact points in the connection decreases, the current density increases and arcing (arcing) arises on the contact connection, as a result of which it is heated up to the ignition temperature of the insulation, which leads to the ignition of the insulation.

Preventing fires and fires from heating contact connections is achieved by monitoring the voltage drop across the contacts, i.e. by choosing the optimal timing for the prevention of contact junctions in order to ensure a normalized level of fire security [6] or the development of a new type of protection that would react to appearances between the contacting surfaces of the arc discharges.

3.Modeling the process of ignition in residential and public premises.

In the process of forming ignitions in 380/220 V networks with a deadly neutral neutral supplying the room electric receivers, it is represented as the coincidence of two random events in space and time: damage to the protected network (short circuits in the conductor or inadmissible from the ignition point of insulation overload of the line ) in which the action of protection is mandatory; failure to operate the main safety switching device. [7]

Based on the Markov random processes, a mathematical model is developed that explains the processes of fire formation during operation of the electrical network of premises and the following characteristics are calculated for the case when the protective switching device is not serviced during operation

The average time until the first fire

– average time interval between occurrence of short circuits in the protected network;

– average time of existence of a short circuit (the average time of protection operation);

– the average time between failures in the operation of the protective switching device;

– the average time of the protective switching device in the undetected failed condition. [8]

Dispersion of time before the first fire :

The probability of safe operation of the apartment network during the time

[4]

In the case where , then

In this paper, we have obtained and used formulas for determining , and for the case when When – the time interval between inspections of the circuit breaker of the protective switching device, which provides a standardized level of fire safety, the intensity of the fires is determined as follows:

Using the mathematical model developed in the work, the intensity of fires in the considered room is determined from short circuits in wires, sockets, forks and switches. This value is compared with the confidence interval obtained by processing statistical data with a confidence probability of .

In order to find out the influence of the residual current device (RCD) on the fire safety of the building's power supply network, a mathematical model is proposed, according to which the probability of finding the system in each of the possible states during the time is determined from the following system of equations. [9]

(animation: 9 frames, always repeated, 61.7 kilobytes)

where

– the average time interval between failures of RCDs;

– the average time of finding the RCD in an undetected failed condition.

The system of equations must be solved under the initial conditions: , which follow from the assumptions made that at the initial moment of time the means of protection (maximum current protection and RCD) are in good order, in the network supplying the consumers of the room, there is no short circuit. The solution of the system of linear differential equations is found by numerical methods.

The fire in the system occurs at the moment it hits the absorbing state.

Systems of linear algebraic equations are obtained for a network of RCDs supplied, from which the mean time to the first fire is determined and the variance of this time is For the case when ( – time interval between RCD checks) is determined by formula:

In the case when the probability of fires is given by the formula:

Conclusion

This master's work is carried out in order to assess the fire safety level of the premises where the internal power supply system is calculated. The influence of the parameters of the selected electrical equipment on the intensity of the occurrence of ignition from damage in electric wiring will also be considered.

When writing this essay, the master's work is not yet complete, the completion date is – June 2018 The presented abstract is of an overview nature, the further work will be aimed at calculations and finalization of the results obtained.

List of sources.

  1. Rules for the installation of electrical installations/Ministry of Energy of the USSR. – Moscow: Energoatomizdat, 1986. – 640 p.
  2. Pashkevich IR, Deev G.F. Surface phenomena in welding processes. – Moscow: Metallurgy, 1974. – 122 p.
  3. A.A. Alexandrov, G.I. Smelkov, V.A. Pechotin, et al., "The nature of the formation of fire-hazardous metal particles in short-circuits," Industrial Energetika. – 1997. – № 2. – P. 45–46.
  4. Zabirov A.S. Fire hazard of short circuits. – Moscow: Stroiizdat, 1980. – 136 p.
  5. Smelkov GI Fire hazard of wiring during emergency operation. - M: Energoatomizdat, 1984. – 184 p. from.
  6. Kovalev AP On the problems of assessing the safety of electrical facilities / / Electricity. – 1991. – № 8. – С.50–55.
  7. Kovalev AP, Spivakovskiy AV, Chursinova AA, Shevchenko OA, Kolesnik LI, Nagorny MA Evaluation of the safety level of technological facilities // Explosion-proof electrical equipment: Sat. scientific. works of UkrNIIVE. - Donetsk, 1998. – P.177–180
  8. Kovalev AP, Shevchenko OA, Zhuravel Ye.A., Nagorny MA Choice of reliability of the switching device providing fire safety of the mine grid of electric power supply // Explosion-proof electrical equipment: Sat. sci. tr. UkrNIIVE. – Donetsk: OOO "Yugo-Vostok, LTD." – 2002. – P. 161–164
  9. Shevchenko OA On the influence of protective shutdown devices on the fire safety of the 380/220V network in residential apartments // Zb. sciences. prat DonNTU. Series: "Electrical engineering and power engineering". Vypusk 50. – Donetsk: DonNTU, 2002. – P. 141–143.