UZO: What is it? Purpose, application and technical characteristics

UZO: What is it? Purpose, application and technical characteristics

The requirement of reliable protection of a person from the damaging effects of electric current has always outstripped the capabilities of science and technology to create protective devices that meet this goal. Today, innovative developments in the electrical industry fully meet all the criteria for devices of this type. The article reveals the question of such a device as an RCD: what it is, its purpose, principle of operation, choice and application.

RCD stands for “protective shutdown device”

Means and methods of electrical protection: modern devices and features of their operation

As soon as the use of electric current entered our lives, there was immediately a need to protect against its damaging effects on human health. First of all, it is the isolation of the conductive parts of the wiring and parts of the current receivers.

Automatic switch manufactured by Interelectrocomplete

But complete isolation is impossible, since there are technological breaks and contact groups in any electrical circuit. There is always a possibility of violation (destruction) of the insulating layer of conductive elements and their mechanical damage, and most importantly – statistical regularity in violation of safety regulations, instructions and rules of operation of electrical equipment, both at the production and household level.

Electrical protection: insulation and grounding

One of the most effective ways to protect against the damaging effects of electric current is the organization of the grounding circuit. The grounding circuit is an artificial conductor connection with the “ground” (the so–called PE conductor) of neutral conductive housings or parts of electromechanisms, having a resistance of no more than 4 ohms. The listed elements of electrical equipment may be energized due to a short circuit to the housing of the phase wire or lightning current.

The main purpose of the grounding circuit device is to exclude the possibility of electric shock to a person or animal in case of touching the body or part of the mechanism of electrical equipment that is energized due to a phase electric current short circuit on them.

Pay attention! In AC networks with a grounded neutral and a voltage of up to 1 kV (this is the format of residential power supply), grounding is not used as the main protection against electric shock in case of indirect contact, since it is not effective.

The passage of electric current through the human body in the event of an impact in a system with grounding (right) and without grounding (left)

The problem of the most effective protection from the effects of electricity on a person was solved by the so–called differential current devices (UDT) – this is a large segment of control and protective devices for various purposes and design features. The classification of the UDT segment is quite extensive: from the control method, the type of installation and the number of poles, to the possibility of regulation and time delay of the disconnecting differential current.

Let’s consider what RCD is. The interpretation of this abbreviation is a protective shutdown device. Requirements for the installation and application of UDT are given in the expanded editions of the PUE – rules for the installation of electrical equipment and in the series of standards for electrical installations of buildings IEC 60364 and the effect of current on humans and livestock IEC 60479-1.

Historical background of RCD development

The innovator in the development of RCD was Germany. The first working sample of the protection device was designed and manufactured in the thirties of the last century. As a leakage current sensor, the smallest possible differential current transformer was used, and a polarized magnetic relay with a sensitivity of 100 milliamps (mA) and a speed of no more than 0.1 seconds was used as a control element.

The threshold for fixing the differential current in the prototype was about 80 mA. At that time, it was impossible to develop a control relay with a sensitivity of less than 80 mA due to the lack of materials with the necessary electromagnetic characteristics. And only in the middle of the twentieth century, a new constructive solution of the RCD was proposed. The design took into account mechanisms to eliminate false alarms from discharges during a thunderstorm and significantly increased the sensitivity of the differential current to 30 mA.

The first models of protection devices contained a differential current transformer and a polarized magnetic relay

The overall dimensions of the RCD have also undergone changes: from the size of a parcel box to a modern format capable of being mounted on a DIN rail in modern electrical cabinets.

Technical experts in the field of electrical and electronic developments are already making predictions for the future. They are firmly convinced that artificial intelligence will soon be in charge of such systems as protection against electric shock.

He will be able to perform not only measuring and control functions, but also by carrying out video and audio monitoring of the object given to him, make instant decisions on any random situations and, if necessary, notify the rescue services.

RCD: what is it and how it works

Among the most popular of the protective UDTs operating in domestic conditions are protective shutdown devices (RCD). RCD works as a human defender against electric shock and as a preventive mechanism to prevent accidental ignition of wiring cables and plug-in cords of electrical appliances.

Protective shutdown device Schnieder Electric

The functional idea of the device in question is based on the laws of electrical engineering, postulating the equality of incoming and outgoing current in closed electrical circuits with active loads.

This means that the current flowing through the phase wire must be equal to the current flowing through the zero wire – for single-phase current circuits with two-wire wiring and that the current in the neutral wire must be equal to the sum of the currents that flow in phases for a three-phase four-wire circuit.

When in such a circuit, due to the accidental touch of a person to the uninsulated parts of the conductive elements of the circuit or when the exposed part of the wiring (due to damage) comes into contact with other conductive objects forming a new electrical circuit, the so-called current leakage occurs – the equality of incoming and outgoing currents is violated.

This violation can be registered and used as a command to disconnect the entire electrical circuit. In this process, the RCD was designed. And the current of “leakage” in the framework of electrical engineering began to be called differential current.

RCD on the power and grounding wiring diagram

The RCD can register very small “leakage” currents and perform the functions of a switch mechanism. Theoretically, the principle of RCD operation looks like this (where Ivx is the input current of the zero wire, Ivx is the output current of the phase wire):

• Ivx = Ivy (system balance without disruption, RCD in standby state);
• Ivx > Ivy (the balance of the system is broken, the RCD registers the appearance of a differential current and disconnects the supply network).

The RCD will definitely protect

When an RCD is installed in the power supply network, it means that protection from:

• short circuit of the phase wire to the body of the electrical appliance. In a large number of cases, these are heating elements of washing machines, water heaters and heaters. Moreover, a breakdown can occur only when the thermal element heats up under the influence of current;
• incorrect wiring installation, when unscrupulous electricians brace up the “twist” of wires in plaster without using an assembly box. If the wall is wet, differential current will leak into the wall from this twist and the RCD will de–energize the line all the time until the plaster dries completely or the correct repair of the connections is made;

RCD protects in cases of phase wire closure and incorrect wiring installation

• incorrect installation in the electrical panel, when seemingly small but “useful” changes made to the circuit change the current distribution and lead to a loss of high efficiency of the device. This will be discussed in more detail later.

The RCD can be triggered for reasons that are not obvious from the first inspection of the connection scheme of household appliances. If you use a gas stove with electric gas ignition, or a washing machine is connected by a hose in a metal case to a water tap, or when neighbors have grounded the water supply or heating system, then a current leak will occur in the electrical circuit again, due to which the RCD will be triggered. In such cases, a thorough engineering analysis is required.

Boundary conditions of RCD operation

Rules very often have exceptions. This principle has not bypassed the universal qualities of the considered protective shutdown device.

The RCD will not react when a person or an animal comes under voltage, but the earth fault current will not occur. Such a case is possible when touching both the phase and zero conductor, which are under the control of the RCD, or when completely isolated with the floor. RCD protection in such cases is completely absent. The RCD cannot distinguish the electric current passing through the human or animal body from the current flowing in the load cell. In such cases, safety can be ensured by mechanical protection measures (complete insulation, dielectric casings, etc.) or complete de-energization of the electrical appliance before its technical inspection.

Legrand protective shutdown device

The RCD, which is completely dependent on the supply voltage of the network suitable for the object, is in working condition only if the specified network is fully operational. The situation can become dangerous when the zero wire breaks “above” the RCD, and the phase wire remains energized. Then the phase wire in the wiring can become a factor of electric shock, and the RCD, due to its own incapacity, will not be able to turn off the mains power.

The RCD can “hang” in the standby state if the main contact rod in the solenoid is jammed or if the secondary winding of the control device fails, and does not work at the right moment. In order to check the working condition of the RCD, there is a test mechanism. If you regularly perform a test check of the device (and for this it is enough just to press the “T” test button), the risk of RCD failure will have a minimal probability.

Application and how to connect the RCD

The main application in domestic conditions of the RCD was obtained when using a large number of connected devices and equipment in electrical groups of bathrooms, kitchens and outlet groups. This does not mean that it does not make sense to use RCD on a common incoming network. This selective scheme is dictated only by the efficiency of management and marketing expediency, since RCD for small currents is much cheaper at the price of devices with higher power.

RCD connection diagram

However, in some cases, if we consider dormitories, clubs, etc., it will be more reliable to use a general selective RCD due to the mass and simultaneous use of almost all elements of electrical equipment. The selective type RCD differs from the usual one by a long delay time of the switching-off differential current (i.e., the response time) and is one of the most used devices. When the usual local RCD is triggered in any circuit, the general selective RCD does not turn off all the wiring at once, but allows you to stop the power supply of only a separate group.

For example, if there is a breakdown in the insulation of the equipment at the disco and the housing (for example, the amplifier) is in contact with the phase wire, then at the moment the amplifier operator touches the local RCD is triggered and turns off only a group of amplifying equipment, and the selective general RCD will not turn off all power and groups such as general lights, toilets and cafes will work in standard mode.

The mechanism of connecting the RCD to the operating network is similar to connecting an automatic switch, with the only difference that when two terminals are required to tighten on a single–phase automatic machine, then four on the RCD.

If, when a person touches a bare section of wire or the housing of equipment that is under phase voltage, the electricity is instantly turned off, it means that the RCD has worked.

It is better to entrust the work on connecting the RCD to a specialist

Important! In AC systems, additional protection by means of RCD should be provided for outlet groups with a rated current of up to 20A (washing machines, boilers, stoves, etc.) and mobile (portable) equipment and power tools with a rated current of up to 32A, which are used outdoors.

The basic principles of the RCD mechanism and comparative analysis of analogues

The physical processes occurring in the mechanisms of operation of many modern electromechanical or electronic devices may be completely incomprehensible to us. Not everyone has knowledge of engineering and technical disciplines and, of course, is not able to understand and describe the physical basis of the principles of operation of a particular device. But the principle of use (rules of operation), built on safety elements, makes it possible to use the most complex inventions in our daily life.

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Each device has a technical passport, in which the purpose and the principle of operation are always described in an understandable language, and always, when required, measures for installation, connection and proper operation are prescribed in it. In our case, an attempt has been made to describe the principle of operation of the shutdown protection device (RCD) in the most accessible way and to give the reader the opportunity to independently make decisions in choosing one or another device if necessary.

The principle of RCD operation and design features

To perform its protection function, the device consists of a differential current transformer minimized in size, a control “tracking” magnetoelectric relay, a solenoid for controlling the main contact group and additional diagnostic elements – the “Test” button and elements of actuation mechanisms.

The RCD consists of a differential current transformer, a magnetoelectric relay, a solenoid and a Test button

The physical side of the work is as follows.

When the RCD is turned on (pressing the contact closure button), the solenoid turns on and holds the rod of the contact group in the same way as the electromagnet. Since at the same moment the terminals of the winding of the solenoid itself and the terminals of the supply wires come into contact. But in the power supply circuit of the solenoid, transit disconnection contacts are installed, which are controlled by a magnetoelectric relay and the relay is given the function of independently disconnecting the RCD.

The outgoing and incoming mains current, flowing in the corresponding windings of the transformer, due to the produced EMF (electromotive force) creates two equal, but multidirectional magnetic flows in the magnetic circuit (core).

Due to the complete compensation of magnetic fluxes, there is no EMF in the secondary winding wound on the core, feeding the control relay, and the relay is in a passive state.

At the moment when a person or an animal touches the bare part of the phase wire or the housing of any household appliance on which a phase breakdown has occurred, an additional differential current will flow through the incoming transformer winding.

Violation of the equality of incoming and outgoing currents instantly creates an uncompensated magnetic flux in the transformer core. And as a consequence, the instantaneous appearance of EMF in the secondary winding connected to the relay as its power source.

Design features of the protective shutdown device

The relay, having received the power supply, immediately triggers and turns off the power of the solenoid (the transit terminals open), which holds the main contacts in the closed position.

The contacts open, the solenoid de-energizes and releases the spring-loaded rod of the contact group, and the power supply of the network is interrupted. The more sensitive the control relay is to small values of differential current, the more effective the protective function of the RCD.

Pay attention! Such protection functions as power outage in case of short circuit and current overload phenomena are not provided in the RCD. In practice, the installation of RCD usually involves the joint use of an automatic switch (“automatic”), directly designed for the possibility of short circuit and current overload.

The correct connection scheme of the RCD and the machine. Installation errors

Both devices have the same mounting design for installation in the control panels of electricity metering and distribution. The task is reduced only to the correct connection to the power supply network and to each other:

Correct connection scheme of RCD and automatic machines

In this case, the recommended connection sequence is shown, but it is also necessary to take into account the correctness of the connection scheme itself:

• in any case, do not connect the zero wire to the ground terminal after it exits the RCD. In this case, periodic differential leakage current may occur, leading to false positives;
• incomplete phase connection of the RCD. If the zero wire from the supply network transits past the RCD, then the resulting current in the zero wire will be perceived as differential, which will lead to a constant operation of the device;
• do not allow the connection of the zero wires of sockets controlled by the RCD with the grounding wire (terminal). At the same time, even an outlet that is not connected to the consumer will create a differential current;
• when using RCD in a group, zero wire jumpers on the incoming terminals are not allowed. This will trigger all the RCD at the same time.

Useful advice! When connecting a four-pole, i.e. three-phase RCD to a similar network, strict compliance of the phase marking with the marking of the device terminals is required. Otherwise, the test mode will not be objective.

When connecting the RCD, it is not allowed to connect the zero wires of the sockets to the grounding terminal

Analogs of RCD with advanced functions

The UDT (Differential current devices) market is very diverse. It should be distinguished from a number of analogs competing with RCD, the so–called differential automaton belonging to the class of circuit breakers controlled by differential current – AVDT.

To answer the question in an accessible form: difautomat, what is it? – it is necessary to remember that its main feature is the combination of the main function of the RCD and the circuit breaker. Also, the difference between the RCD and the differential automaton is that the RCD itself requires protection against short circuits in the network and overcurrent (of course, for this purpose an automatic switch is installed in a pair), and the difautomat is able to protect itself.

It should be noted that new models of AVDT – electronic and with an auxiliary power source – have entered the market. They differ from electromechanical designs by the presence of an electronic circuit board with a differential current amplifier, which makes it possible to fix leaks of the order of 10 mA and are triggered even when the zero wire of the incoming network is broken when the phase wire remains energized. The usual RCD or AVDT in such a situation, when a person comes into contact with an open phase area, will not work.

Another novelty in the line of differential current devices is the so–called multifunctional protection device. What is UZM becomes clear from familiarization with its purpose. This device is used to completely turn off the equipment when the voltage parameters in the network go beyond the operating limits (less than 180V and more than 260V), as well as to protect the operating equipment from “burning” windings and electronic elements of devices voltage surges. These surges can be caused by electromagnetic pulses or phase-to-zero short circuits in a three-phase network.

Differential Current Circuit Breaker (AVDT)

RCD or differential automaton: how to distinguish and what to choose

There is no unambiguous algorithm that allows you to give preference to a particular device. The reason is the multivariate feature of the choice. Let’s consider the main factors that influence the choice of RCD or AVDT.

Is it possible to place this or that device in the main panel. In practice, the overall total size of the RCD and the circuit breaker is larger than the overall size of the difautomat.

What is the purpose pursued when making changes to the electrical circuit. If there is a need for individual protection of high-power equipment (kitchen stove, boiler, washing machine, etc.) from a possible “shock” by electric current, a differential automatic machine that clearly monitors the load current is optimally suited.

If it is necessary to protect against electric shock for some group of sockets or lighting line, in which the power may be increased over time, it is advisable to use RCD. The RCD has a large power reserve, and the differential automaton will need to be replaced with a more powerful one due to overload.

When using high-power equipment, it is better to install a difautomat

Qualitative assessment. Practice has proven that devices that combine many functions of various devices are very often inferior in quality to single devices. This also applies to such a multifunctional device as a differential automaton, which is inferior in quality and service life to RCD and circuit breaker.

The breakdown situation. In a situation when the RCD or circuit breaker stops working, either one or the other device needs to be replaced. But when the differential automaton does not work, even due to failure of one function, it is necessary to replace it with a new one. In this case, the costs are much higher.

Stability of power supply. If the RCD fails, it is enough to install jumpers between the circuit breaker and the electrically powered network (bypass the RCD) and the power supply is restored. But if the difautomat breaks down, either a spare difautomat or a spare circuit breaker will be required. So the prompt resumption of power supply may be questionable.

Useful advice! If it is necessary to choose the right differential current device (RCD or AVDT) correctly, it is necessary to use an engineering approach and an economic assessment even when one or another type of device is already at hand.

Differential Current Switch TDM Electric

There was a question about the external difference between the RCD and the AVDT.

Marking of the title side of the device. Example 1: “ABV 16A 30 mA” – we have an ABV RCD (manufacturer “ABV”) with a nominal current of 16 amperes and a lower differential current of 30 milliamps. Example 2: “CHNT C16 0.03 A” – in front of us is a difautomat, the manufacturer is CHNT with a rated current of 16 amperes and a characteristic of an electromagnetic and thermal interrupter of class “C” with a differential current of 30 milliamps.

The specified wiring diagram is on the title side. For RCD, the diagram shows a differential transformer (oval loop), a control relay (square) with a loop on an oval contour and a test contour in the form of a dashed line. For a diffautomat, the scheme is very similar to the RCD scheme, only there are additional figures in the form of a small arc and a stepped line – these are designations other than RCD, electromagnetic and thermal interrupter.

Application and installation of RCD: designations on electrical circuits

Most control and control devices installed in the power supply network have a small list of parameters necessary for their correct selection in the electrical circuit.

Wiring diagram in the apartment using RCD and automatic machines

The choice of RCD is made according to the rated load current and the threshold for fixing the differential leakage current. Practice recommends a value not higher than 30 mA. The installation of RCD in the electrical network is carried out on the basis of an engineering analysis of the elements existing in the network and installation possibilities. The RCD connection scheme to the network should take into account all possible switching errors and exclude them. Only when properly connected to the power supply circuit, the RCD will ensure maximum efficiency in triggering the protective mechanisms of the device.

Selection parameters and RCD connection diagram without grounding

Knowing the principle of RCD operation, with a standard two-wire power grid, represented only by phase and zero wires, which does not have a grounding circuit, it is possible and necessary to install RCD in accordance with the protection requirements. The correctness and installation schemes of the RCD were considered earlier.

The answer to the question of which RCD to put in the apartment is with a calculator in your hands. It is necessary to sum up the capacities of the units of equipment and equipment installed in the apartment, and divide the amount by the number 220. Thus, in a rough approximation, we calculate the rated current by which the choice of RCD will be made. This calculation is based on the mathematical dependence of electrical power on the mains voltage (220V) and the current strength that occurs when the load devices are powered:

M = U x I,

where M is power, U is voltage, I is current.

The scheme of checking and testing the RCD using a multimeter

Example: you need to select an RCD to protect a group of electrical appliances in a kitchen unit. There are such household appliances on this line:

Summing up the power consumption: 2000 + 1200 + 700 + 800 = 5300 Tue. We calculate the current according to the formula: I = M / U = 5300/220 = 24.09 A. We choose the closest RCD with a large value – 25A at face value.

For in-depth calculation of currents in wiring lines, knowledge of the basics of higher electrical engineering is required.

In addition to the rated load current and the threshold of differential current sensitivity, in some cases, when choosing an RCD, it is necessary to pay attention to another criterion – the category of leakage current. This in most cases concerns alternating and pulsed current in the network.

Connection diagram of RCD and automatic machines on the example of an apartment electricity meter

The AC category assumes the operation of the RCD in an alternating current environment of differential leakage. This category is the most common and can be used in all types of AC networks. In which cases the RCD is triggered – it was discussed above.

Category A has the lowest sensitivity threshold (about 10 mA) for differential current and is capable of recording a separate component of the current amplitude (the so-called half-wave). An RCD with such a leakage current category reacts not only to a variable current configuration, but also to a pulse one. Such RCD are becoming a priority application, as more and more household appliances, especially lighting elements, are being transferred to pulsed current power supplies.

The main trend of the European market is the expansion of the pulse equipment segment. This, of course, will lead to an increase in the number of pulse current RCD used. But since active current receivers (fully alternating) will remain in domestic use for a long time, the RCD of the AC category will occupy a fairly wide space on market shelves.

Returning to the question of the absence or presence of a grounding circuit in the power grid, it is necessary to emphasize that even in the presence of grounding, the organization of protection against electric shock is even more required due to the installation of an RCD in the network.

Connection of 2P or 1P+N automatic machines to a group RCD

The basic principles of the RCD connection scheme to a single-phase network have already been discussed earlier. The RCD connection scheme with grounding is no different from the scheme without grounding.

Useful advice! If the power grid has a grounding circuit, it is necessary to check and ensure the correct circuit when connecting the RCD, when no zero wire in the wiring should be mated to the wire (terminal) of the grounding circuit.

Graphic designation of the RCD on the power supply scheme

The main directive provisions included in GOST 2.755-87 ESKD “Conventional graphic designations in electrical circuits of switching and contact connection devices” and GOST 2.710-81 ESKD “Alphanumeric designations in electrical circuits” prescribe graphic and letter designations of such devices as RCD. But there are no strict prescriptions for different designations of differential current devices.

As we already know, all differential current devices are represented by the mechanism of the interrupter and the control element – the differential current transformer. Therefore, the RCD designation in the diagram is represented by two standard graphical designations – a circuit breaker and a transformer registering differential current. You can see the graphic designation of the RCD on single-line diagrams and other drawings.

AVDT Schnieder Electric in the switchboard

Three-phase RCD connection diagram

This type of device is usually called a four-pole and the specifics of its connection to a three-phase network are completely similar to connecting a two-pole RCD. The terminals for connecting the phase wires and the zero wire are indicated on the device body. A passport is also attached to the device, which presents standard wiring diagrams for a four-pole RCD to a three-phase network.

Different manufacturers sometimes have differences in the location of the zero terminal on the device body – on the right or on the left, and the connection of phase wires requires only matching the designation at the input and output.

Four-pole three-phase RCD are used for large differential leakage currents and their main purpose is only to protect against ignition of electrical wiring. To organize the protection of people from electric shock, it is necessary to install two-pole single-phase RCD on each separate group of equipment with a leakage current adjustment of no more than 30 mA.

A difautomat in a three-phase electrical network

Model range, manufacturers and prices of RCD

The market segment of UDT products is represented by a number of foreign brand companies, as well as domestic manufacturers. To date, preference is given to trademarks from Italy, Poland, Germany and Spain, as their products have received the best consumer rating according to the criteria of quality, reliability and price-quality ratio. The existing market of differential current devices UDT allows you to produce a wide selection of various types of devices, providing a diverse range of products both in price and quality.

The table shows the products of the most common manufacturers of UDT and shows the market prices they offer:

As can be seen from the comparative table, the price of RCD 25A 30 mA (the most popular on the market) depends on the manufacturer. So the price of RCD ABB 25A 30 mA is higher than Chinese analogues, but lower than that of manufacturers such as Legrand or Schneider Electric. Taking into account such criteria as quality and cost, it is preferable to buy a 25A 30 mA RCD from ABB, and the necessary circuit breaker can be purchased from Chinese production or Legrand.

Useful advice! Having made the decision to install the RCD in the home network, but without having experience in the electrical installation of similar devices, use the services of a qualified electrician.

Summing up this excursion into the world of differential current devices, in particular, the protective shutdown device (RCD), we will focus on the important points considered.

The range of RCD and automatic machines of the manufacturer ABB

One of the most effective means of protecting humans and animals from the damaging effects of electric current is the installation of protective disconnection devices – RCD in the power supply network.

The RCD has the function of reacting to the differential leakage current that appears when a person comes into contact with a bare part of the wiring or the housing of any electrical equipment. It may be under phase voltage due to damage to the insulation of the phase wire and its contact with the housing. Also, the RCD reacts to current leakage in places where the insulation of the wiring is damaged, when this can lead to heating and fire.

However, the RCD does not react to the phenomena of a short circuit in the wiring circuit and to excess power in the current circuit. In this regard, it is necessary to install the device in conjunction with an automatic switch (“automatic”), which reacts to a short circuit and overload in power.

The most important thing is to always observe safety regulations and caution when working with electrical appliances and appliances. As often as possible, make a visual inspection of open current-carrying elements of electrical wiring and connected elements of current collectors.