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Abstract

Contents

Introduction

The increasing use of electricity in all areas of human activity, the steady increase in the energy intensity of labor, the sharp increase in the number of electrical appliances in everyday life and in production naturally led to an increase in the risk of injury to people by electric current.

Electric current does not have any physical signs or properties on which a person could feel his senses, compounding the risk to humans.

Electrical injury accounts for a significant proportion of the total number of accidents. Skilled electricians and ordinary users known to a large number of cases of death or serious injury to persons by electric shock or fire, and fires caused by faulty electrical equipment and electrical wiring [2].

Protection of people's life, health and property is of paramount importance, which determines the requirements for electrical equipment. Safety in the operation of electrical installations and devices is achieved by using a set of protective measures. One of the ways to improve electrical safety is the use of protective disconnect devices.

Theme urgency

Protective devices responsive to differential current protection functions have complex and in this sense has no analogues. Safety shut-off devices provide a high degree of protection for people against electric shock caused by direct and indirect contact, and in addition, safety shut-off devices ensure the reduction of the fire hazard of electrical installations [5]. It should be noted that in the case of deliberate contact with live parts, the use of protective disconnect devices is the only possible way to provide protection, as in the case of failure of the main types of protection.

The use of protective disconnect devices is expedient and justified for social and economic reasons in electrical installations of all possible types and of various purposes.

The cost of installing protective disconnect devices is disproportionately less possible damage – the death and injuries of people from electric shocks, fires, fires and their consequences caused by faults in electrical wiring and electrical equipment. If we consider that the cost of a single protective switch-off device does not exceed the cost of a simple household appliance, and the possible damage is calculated in huge amounts, it becomes obvious that it is necessary to implement protective shut-off devices in all electrical installations as soon as possible.

Operating principle

What is the principle of the protective shutdown device?

The safety shutdown device works as follows(Figure 1):

Structure of protective devices

Figure 1 – Structure of protective devices

During normal operation of the power supply system and, therefore, no leakage, the operating current flowing through the primary windings of the transformer switched in opposite (which are connected to the forward and return conductors leading to the load) induces counter-directed magnetic fluxes of the same magnitude. Their interaction leads to the fact that the current of the secondary winding is almost zero and the threshold element does not work.

When an emergency situation – occurrence of current leakage or the touch human live parts during leakage current (essentially, the occurrence of leakage of the same through the human body) the balance of currents in the primary windings of the transformer is broken, causing the appearance of current in the secondary winding.

In turn, the induced current in the secondary winding will trip threshold element and actuation of the actuator. This mechanism is controlled by de-energizing circuit.

However, it must be remembered that the safety device is not a panacea for all possible problems, although it significantly increases the safety of electrical equipment operation.

If there are no leakage currents in the circuit to be protected, the protective shutdown device does not react (for example, if there is a short circuit between the phase and neutral wires). Does not work such devices, and in cases when a person touches at the same time to the phase and neutral conductor. This is due to the fact that in terms of the flow of current human body, it is the same ohmic resistance, as well as any other load.

In this regard, in order to exclude such cases, mechanical protection measures are applied: electrical installation of additional conductor insulation, installation of housings and ducts, restriction of access for unskilled personnel. In some cases, the installation of safety shutdown devices is prohibited, as disabling vital equipment may affect the safety of a person’s life support.

Classification of device of protective shutdown

Knowing the purpose and principle of operation of a protective device, it is necessary to become familiar with the characteristics and types of protective devices.

Residual current devices are classified according to operating conditions and their technical implementation. Under the terms of their use in electrical installations, the protective shutdown devices are divided into types AC, A, B, S, G [1].

Of fundamental importance when considering the design of protective cutout devices is the separation of devices according to the method of technical implementation into the following two types:

1. The protective device operatively independent from the supply voltage (electromechanical) (Figure 2). The source of energy required for the operation – perform security functions including a disconnect operation, the device is the signal itself – the differential current for which it reacts;

Electromechanical type of protective devices

Figure 2 – Electromechanical type of protective devices

2. Protective disconnect devices, functionally dependent on the supply voltage (electronic) (Figure 3). Their mechanism to perform the tripping operation needs energy received or from the monitored network or from an external source. The use of devices that are functionally dependent on the supply voltage is more limited due to their lower reliability, exposure to external factors, etc. However, the main reason for the smaller distribution of such devices is their inoperability when the electrical installation is often encountered and is the most dangerous under the terms of the probability of electro-fire, namely when the neutral conductor in the circuit is broken before the safety shut-off device towards the power source. In this case, the electronic safety shutdown device, having no power, does not function, and the potential dangerous for human life is carried out to the electrical installation using a phase conductor.

Electronic protective cutout device with network shutdown function

Figure 3 – Electronic protective cutout device with network shutdown function

1 – differential current transformer; 2 – electronic amplifier; 3 – test chain; 4 – holding relay; 5 – control unit; H – load; T – button Test

There is a class of devices – a protective shutdown device with built-in overcurrent protection (RCBO), the so-called combined protective shutdown devices (Figure 4).

A device with built-in protection against over currents

Figure 4 – A device with built-in protection against over currents

1 – coil current cutoff; 2 – bimetallic plate; 3 – differential current transformer; 4 – magnetoelectric release; 5 – test resistor; 6 – power contacts; H – load; T – button Test

Practically all manufacturers of protective devices have in their production program protective devices with built-in overcurrent protection. As a rule, their share in the total volume of manufactured safety devices does not exceed one or two percent. This is due to the rather limited scope of their application – a slight, unchanged load, an autonomous electric receiver, etc.

An illustrative example is the lighting of billboards installed on the street pavilions of public transport stops, where two or three fluorescent lamps are powered through a combined residual current device with a rated operating current of 6 A and a rated tripping differential current of 30 mA.

The design feature of protective devices with built-in overcurrent protection is that the opening mechanism of the power contacts is triggered when it is exposed to any of the three elements – a coil with a current cutoff core that reacts to short-circuit current, a bimetallic plate that reacts to overload currents and a magnetoelectric release reacting to differential current.

The use of protective disconnect devices with built-in protection against overcurrents, is advisable only in justified cases, for example, for single consumers of electricity [3].

Thus, having considered all the factors that influence consumer safety, for use in electrical installations and electrical boards, first of all, an electromechanical safety device should be recommended as devices providing more reliable protection.

Application area

The use of protective devices has become quite widespread. The protective disconnecting device is used to complete the input-distribution devices, switchboards, group panels (apartment and storey), installed in public buildings – preschool institutions, schools, vocational, secondary, special and higher educational institutions, hotels, sanatoriums, motels libraries, indoor sports and fitness facilities, swimming pools, saunas, theaters, clubs, cinemas, shops, catering establishments, enterprises x consumer services, trade pavilions, kiosks, etc., residential buildings – individual and multi-family, summer houses, garden houses, dormitories, household premises, etc., in office buildings, production rooms – workshops, workshops, gas stations, car washes , hangars, garages, warehouses, etc., as well as to protect individual consumers of electricity.[6]

To date, most electrical installations in our country work with a grounding system similar to the TN-C.

TN-C System

Figure 5 – TN-C System (animation: 5 frames, always repeat, 44 kilobytes)

It is necessary to examine in greater detail the functioning of protective devices in such electrical installations.

In such an electrical installation, during the breakdown of insulation on the case of an electric receiver if this case is not grounded (for example, a refrigerator or a washing machine on an insulating base), the protective cutout device included in the power supply of the electric receiver will not work because there is no leakage circuit no differential (differential) current. At the same time on the case of an electrical receiver will be a dangerous potential relative to the earth.

In this case, when a person touches the body of an electric receiver and a current flows to earth through its body, exceeding the nominal disconnecting differential current of protective disconnect devices (set current), the protective disconnect device reacts and disconnects the electrical installation from the network, as a result, the person’s life will be saved.

This means that in the present case since the insulation failure and the occurrence of the housing electroreceivers electric potential until the shutdown of the defective circuit from the network there is a period of potential danger of defeat.

From the above it follows that in installations with grounding system TN-C application tripping device also justified because the device and in such electrical provides effective protection against electrocution [4].

Electrical installations with earthing systems TN-S, TN-C-S, TT in this aspect have a significant advantage: in a similar situation – in case of insulation breakdown on the case, the protective shutdown device instantly turns off the power supply, since all the buildings have a reliable connection to the protective conductor.

Devices breakers used in residential buildings should not disable the user in case of disappearance or reduction of mains voltage. They must remain operable when the supply voltage drops to 50% of the nominal for at least 5 s (the response time of the automatic switching on reserve). It is permitted to use protective disconnecting devices of both type A (for sinusoidal and pulsating voltage) and type of AC (for sinusoidal voltage) [8]. When choosing a protective shutdown device, it should be noted that the source of pulsating voltage can be washing machines, light source controllers, televisions, video recorders, personal computers.

Protective disconnect devices in residential buildings are recommended to be installed in apartment or floor panels. In group electric circuits that supply power sockets, it is necessary to install protective disconnect devices with a differential response current not exceeding 30 mA. It does not require the installation of protective devices in the circuit supplying stationary lighting equipment. It is allowed to connect several power supply circuits to one protective shutdown device through separate circuit breakers installed in each circuit. Installation of protective disconnect devices is mandatory in group circuits supplying power sockets located outdoors, in rooms with increased danger and in especially dangerous rooms (in bathrooms and showers). In this case, it is recommended to use protective devices with a differential response current not exceeding 30 mA, and for bathrooms and showers (if a separate line is used for them) – 10 mA [7].

Conclusion

In conclusion, we can say that all protective devices are used for one important purpose – to protect a person from electric shock in the event of a malfunction of the electrical equipment and to turn off the power supply if the person inadvertently contacts open conductive parts of electrical equipment during a leakage current. As well as protection and from the ignition of electrical wiring when a circuit to the body or to the ground.

References

  1. Устройство защитного отключения: теория и практика использования Монаков В.К. УЗО. Теория и практика Энергосервис, 2007. – 356 с.
  2. Ф. Штепан Устройства защитного отключения, управляемые дифференциальным током /Ф. Штепан Прага, 2000. – 90  с; с ил.
  3. ГОСТ Р 51326.1-99 Выключатели автоматические, управляемые дифференциальным током, бытового и аналогичного назначения без встроенной защиты от сверхтоков. Часть 1. Общие требования и методы испытаний.
  4. ГОСТ Р 51327.1-99 Выключатели автоматические, управляемые дифференциальным током, бытового и аналогичного назначения со встроенной защитой от сверхтоков. Часть 1. Общие требования и методы испытаний.
  5. Правила устройства электроустановок Изд. 7-е, 1999 г.
  6. ГОСТ Р 50807-95 (МЭК 755-83). Устройства защитные, управляемые дифференциальным (остаточным) током.
  7. МЭК 1200-53. Электроустановки зданий. Глава 53. Выбор и монтаж электрооборудования. Коммутационная аппаратура и аппаратура управления. Требования к устройству электроустановок зданий.
  8. МЭК 364-5-53. Электроустановки зданий. Часть 5. Выбор и монтаж электрооборудования. Коммутационная аппаратура и аппаратура управления.