Content
Introduction
In electrical installations and electrical networks of residential and public buildings with a voltage of 0,4 kV, abnormal modes associated with an increase in current (overcurrent), which leads to overload, short circuit (short circuit), self-starting of electric motors are possible. Such abnormal conditions can lead to damage to electrical networks and equipment, as well as creating situations dangerous to personnel.
Short circuit refers to emergency modes and can be one-, two- and three-phase. The most severe is three-phase short-circuit, but it occurs much less frequently than single-phase or two-phase short-circuit. Causes of fault are: a breakdown of insulation; ceiling insulation; improper Assembly diagrams; errors of staff.
Short-circuit currents, which are many times higher than the nominal currents of the connected electrical receivers and the permissible currents of conductors, have a thermal and dynamic effect on the current-carrying parts, causing their failure. Therefore, the fault should be localized and quickly disable the damaged area network.
1. The relevance of the topic
Electrical networks of residential and public buildings should have overload protection and short-circuit protection, which ensures the shortest shutdown time and compliance with the requirements of selectivity (selectivity) of action.
Usually, the protection of a person from electric shock when indirectly touching a damaged installation is carried out by disconnecting its fuses or circuit breakers. But these protections do not react to the small leakage currents arising at the beginning of development of damage in a network, and also at break of the zero conductor. In these cases, the only means of protecting a person from indirect contact is RCD [1], which provides a quick (in a fraction of a second) disconnection of the unit from the network.
2. Residual current device
One of the effective ways to improve electrical safety in the operation of electrical installations and devices in residential and public buildings is the use of residual current controlled devices (RCD-D). The principle of operation of RCD is shown in figure 1.
Figure 1 – The principle of RCD operation
(animation: 11 frames, 117 kilobytes)
In a properly functioning power supply network how much current entered the apartment on the phase wire, exactly so much should come out. For example, when a person touches the washing machine with his hand, in which the wire with broken insulation comes into contact with the body. Appears the chain of home network – building – man – land. In this situation, there is a significant difference between these points. It serves as a signal to the RCD that it is necessary to stop the supply of electricity.
A residual current device (RCD) can thus be defined as a high-speed safety switch that responds to differential current in electrical conductors supplying a protected electrical installation or network. The principle of operation of the RCD of differential type is based on the use of electromagnetic vector current totalizer – differential current transformer.
RCD is designed to protect against electric shock or fire due to sliding discharge currents when leaking to the ground. The device is used in electrical networks for domestic and industrial use (220/380V). Figure 2 shows the kinematic scheme of RCD.
Figure 2 – Kinematic scheme of RCD: 1 – current transformer, 2 – magnetoelectric latch,3 – the mechanism of the release
As long as there is no leakage, the currents going in the forward and reverse (phase and zero wires) directions are equal and direct in the magnetic core of the current transformer (1) RCD equal, but counter – directed magnetic flows, resulting in a current I2 in the secondary winding is zero and does not cause the operation of the sensing element – the magnetoelectric latch (2).
In the event of leakage current Iyt – for example, when a person touches the phase conductor, the balance of the passing currents
and magnetic fluxes is violated in the secondary winding current unbalance I2, which causes the operation of the magnetoelectric
latch (2), acting in turn on the mechanism of the uncoupling with the contact system (3). Electromechanical system RCD is calculated to operate at certain values
of leakage current: 10, 30, 300 mA. A test circuit is provided to monitor the operability of the residual current device. When you click Test
artificially
created residual current. The operation of RCD means that it is generally correct.
RCD is installed [2]:
• in the introductory switchgear, located in areas without increased risk of electric shock, in places available for maintenance;
• in group circuits of electroinstallations of buildings where there is the greatest probability of electroporation of people at a touch to current-carrying or open conducting parts of electric equipment which may be due to damage to the insulation to be energized (outlet groups, bathrooms, showers, washing machines, etc.);
• at the main input of the object for fire protection;
• in apartment buildings in the group, including in the housing panel; it allowed them to install in the floor distribution panels; in private houses – in the introductory switchgear and floor distribution panels.
In schemes of power supply of radial type with a significant number of outgoing groups, it is recommended to install a common at the input and a separate RCD for each group, provided that the appropriate choice of RCD parameters ensuring the selectivity of their action [3].
RCD can significantly increase the safety of the power supply, but it cannot completely eliminate the risk of electric shock or fire. RCD does not react to emergency situations if they are not accompanied by leakage from the protected circuit. In particular, RCD does not react to short circuits between phases and neutral.
RCD also does not work if the person was energized, but the leakage did not occur, for example, when you touch your finger at the same time and to the phase, and to the zero conductors. It is impossible in principle to provide electrical protection against such touches, since it is impossible to distinguish the flow of current through the human body from the flow of current in the load.
Conclusion
In conclusion, it should be taken into account that in a reliably designed electrical installation there is always a possibility of emergency mode, this formed the basis for the development of various protection of electrical installations in residential and public buildings.
References
- УЗО – устройство защитного отключения. Учебно-справочное пособие – М.: ЗАО
Энергосервис
, 2003 – 56 с. - Монаков В. К. Смирнов В. В.
Особенности применения и эксплуатации устройств защитного отключения.
Электронный источник: [Электронный ресурс]. – Режим доступа: https://cyberleninka.ru/... - Расчет заземляющих устройств защитного отключения. Обеспечение селективности при применении УЗО. // Студопедия. [Электронный ресурс]. – Режим доступа: https://studopedia.ru/....