Uzo operating principle and connection. The principle of operation of an RCD: how to correctly connect an RCD

Introduction

Special electrical devices have been developed to protect people and animals. They are called a residual current device, abbreviated as RCD. The RCD protects against electric shock when touching live equipment. Protection occurs both through direct and indirect contact with live equipment. In addition to this task, the RCD is used to monitor the insulation condition of electrical wiring. This provides additional protection of the room from fire. Let's look at the functions of a residual current device (RCD) in more detail.

RCD functions

The RCD protects humans and animals from electric shock when touching the housings of electrical appliances that are energized.

Conductive housings and individual elements of equipment and devices may be live. This is definitely an emergency situation and it can arise in two cases.

1. If a phase wire of the electrical wiring is shorted to the body of the device, then, provided the body is grounded, a so-called short circuit occurs. Circuit breakers are designed to disconnect the network in the event of a short circuit. But the case may not be grounded or the resistance of the circuit is very high and the circuit breakers will not work. The problem of protection will be solved, in this case, by installing an RCD in the electrical circuit.
2. Or the contact of the phase wire with the equipment body is not complete. That is, the insulation on current-carrying wires can only be damaged, and then so-called leakage currents will appear. Leakage current can not only “bite” unpleasantly, but can be deadly, especially in damp rooms. A correctly selected and installed RCD will protect against leakage currents.

conclusions

There are two main functions of the RCD:

• Detect leakage current and automatically shut down the electrical circuit. The RCD circuit shutdown time is 200 milliseconds (1 millisecond = 0.001 seconds).
• Protect not only from indirect, but also from direct contact. Direct touch is contact by a person or an animal with live parts of live devices.

Additional RCD function

An RCD installed at the power supply entrance to the house provides additional fire safety for the premises. In some countries, installation of an RCD with a sensitivity of 500 mA is mandatory. In our country (in the Russian Federation), the installation of a 300 mA RCD at the entrance to the house is recommended for fire protection.

Let's look at how the RCD controls leakage currents and how it works in general.

Operating principle of a residual current device (RCD)

Let's consider the principle of operation of the RCD, explaining the principle of operation of the fault current relay (Scheme 1, Scheme 2)

The RCD body contains a magnetic circuit made of a circular core. The consumer INPUT current (I1) and the consumer OUTPUT current (I2) flow around the core. In normal operation, these currents are equal and the system is in equilibrium.

Scheme 1.

class="eliadunit">

When a leakage current occurs on the consumer side (Id), the balance of currents is disrupted and a current proportional to the leakage current begins to flow through the measuring winding of the RCD core. The relay in the RCD operates because the relay is powered by this measuring winding. “The relay is triggered” means that the circuit opens and no current flows to the damaged consumer and, as a result, the RCD protects a person from leakage current.

The difference in current is called differential current, so the RCD is said to respond to differential currents in the circuit.

A circuit breaker combined with an RCD is called a differential circuit breaker. That is, it responds to both short-circuit current and differential current arising from current leakage.

Scheme 2: Operating principle of a residual current device (RCD) in a circuit with a TN-S power system.

Scheme 2.

Legend:

• I 1 - current at the consumer INPUT
• I2 - current at the consumer OUTPUT
• Id - leakage current
• Ic - current through the body when touching the body under voltage
• RA - ground resistance

Read and watch a visual diagram of the RCD operation in the TN-S system . Diagram format 750×1120 pixels. Article with formulas and tables.

Electricity is one of the engineering systems that provides our comfort. But the same electricity carries a potential threat, so electrical networks must be as safe as possible. Ensures the safety of automatic protection devices. One of them is an RCD. What kind of device is this, what does it protect from, what is the principle of operation of the RCD - all this will be discussed in the article.

RCD is a residual current device(an alternative name is residual current switch, abbreviated as RCCB). Designed to turn off power in the event of an emergency that results in leakage current. This is possible in two cases: when the insulation breaks down to the ground and when a person touches live parts.

This picture will help to imagine the principle of operation of an RCD. The load is an incandescent lamp. The RCD compares the current before and after the load. If the difference exceeds the specified value, the device is triggered and opens the circuit

The principle of its operation can be compared to a scale with two bowls. The current in the circuit before and after the load is compared. As soon as one of the bowls outweighs, it means the current has found a “left” or bypass path. Most often, the workaround is through an insulation breakdown to the ground, or through the human body, also not to the ground. That is, part of the current “flowed” along this path. Hence the name - leakage current. The current did not flow through the laid wires, and this is dangerous. And the appearance of a leakage current is a signal to turn off the power. A relay in the RCD is triggered, breaking the contact and de-energizing the network. This is the principle of operation of an RCD described in simple words - for a better understanding of the purpose and principle of operation.

How to understand what leakage current is

A leakage current occurs when an insulation breakdown occurs on the housing (the wire is frayed, the heating element is “broken,” etc.). A leak is when you touch the body of a device that is energized. You touched with one hand and at the same time you are standing on a conductive floor without shoes or touching some other grounded object (central heating radiators, for example). Current will flow through your body, and it will “go” through the ground loop, since this is the path of least resistance. This will be the “workaround” path. As a result, the “returned” current will be less and the relay on the RCD will operate.

But, pay attention! Direct contact immediately with phase and zero is not our case. In this case, the body is perceived as a load rather than a leak. This is a normal situation and the protection will not work. Therefore, work with electricity with one hand, wearing dielectric shoes. And never touch zero and phase at once.

Connecting an RCD to the circuit increases safety. This is especially true for wet areas such as a bathroom.

Sometimes the protection reacts to unobvious things: neighbors are grounded in the wrong direction, a stove with piezo ignition is not grounded, a washing machine or dishwasher is connected with a metal braided hose to metal pipes. In general, there are many situations in which leakage current is generated. These are all also leakage currents, but they are the result of errors or violations. And the RCD also reacts to them. If outages occur for no apparent reason, they simply need to be identified. It’s not easy, but you shouldn’t ignore “false” shutdowns. The reason may be dangerous.

What does it look like

There is a switch on the front panel of the RCD that can be used to manually break the circuit or bring the device into working condition. There is also a “Test” button on the front panel, designed to test the functionality of the protection device. When it is pressed, a circuit containing a resistor is connected, which emits the occurrence of a leak. If the device is working properly, it will turn off the power - the “switch” will move down, opening the contact.

There are sockets for connecting wires at the top and bottom of the device. The wires supplying power are connected at the top, and the lines that go to the load or to downstream devices are connected at the bottom. Both phase wires and zero (neutral) pass through the RCD. That is, when triggered, the power is turned off completely.

There are inscriptions on the case that reflect the main parameters. The RCD is mounted on a DIN rail; for this purpose, there are special protrusions on the rear surface of the case. Fixation methods depend on the manufacturer. There are models that are simply hung, and others with fixation using a pressure valve.

How to ensure quality protection

Despite the obvious benefits of RCDs, you cannot do without a circuit breaker. The RCD does not respond to overcurrents (short circuits) or overload. It only monitors leakage current. So for the safety of the wiring, an automatic machine is also needed. This pair - an automatic machine and an RCD - is placed at the entrance. The machine is usually located before the meter, leakage protection is after.

Instead of a pair - RCD + automatic circuit breaker, you can use a differential circuit breaker. These are two devices in one case. immediately monitors leakage current, short circuit, and overload. It is installed if there is a need to save space in the panel. If there is no such need, they prefer to install separate devices. It is easier to determine damage, cheaper to replace if it fails.

Operating principle of RCD

The protective shutdown device consists of a transformer, a relay and a disconnecting mechanism. The main working element of the RCD is a differential transformer with two primary windings and one secondary. It is he who compares the currents. The primary windings of a differential transformer have exactly the same parameters, but are connected towards each other. The current that goes to the load passes through one winding, and the current that returns from the load passes through the second winding.

When the line is in good condition, the currents flowing through both primary windings are equal, but have opposite signs. As a result, the electromagnetic fields they create cancel out. In such a situation, there are no induced currents in the secondary winding, the contacts are closed, and there is power.

As soon as a leak appears on the monitored lines, an overweight appears in one of the primary windings (in the figure this is winding number 2). This leads to potential appearing on the secondary winding. When it reaches a threshold value (trip current), the relay is activated, cutting off the power. This is the principle of operation of an RCD.

In general, an RCD is a simple device, but very useful, since it is responsible for safety. For your safety and the safety of your children, we strongly recommend installing a residual current device in the switchboard.

Briefly about the parameters of the RCD

Despite the not too complicated device, there are many parameters by which it is necessary to select an RCD. This:

All these parameters are selected when drawing up the circuit, since the cross-section of the wire, the connected load and many other details are important for selection. So first you need to decide on the number and power of consumers (light bulbs, large and small household appliances, heaters, etc.).

What is a fire protection RCD

Clever minds have figured out how to use the operating principle of RCDs not only to protect people from electric shock when the insulation is damaged. The same device can be used to prevent fires. Structurally, they are no different, they are simply designed for high leakage currents.

How does the RCD work in this case? As you know, when current flows, the temperature of the conductors increases. If the current is sufficient, the heat can be so great that it can cause a fire. If you install a device with a leakage current of 100 mA or higher at the entrance to the house, it will not save a person from electric shock, but it can even prevent the occurrence of a fire. How? It may well happen that one of the protection devices turns out to be faulty. The phase insulation will be damaged, which will sooner or later lead to a fire. It may happen that the damage occurs on the unprotected part of the lines. In this case, the fire protection RCD will turn off the power. This will mean that there is too much leakage and it is necessary to inspect the wiring: measure the insulation, check the heating, etc.

A fire protection device is installed after the meter. If we talk about the parameters, the minimum shutdown current is 100 mA. The type is better selective, but choose the exposure time yourself. Selectivity will save you from false positives. Below, after the fire protection RCD, protection is installed on the line, selecting the disconnecting leakage current depending on the type of load.

If you follow GOST, then the installation of protective devices on lighting lines located in rooms with normal operating conditions is not necessary. That is, “personal” RCDs and automatic machines do not need to be installed on the lines that lead to lighting.

Manufacturers

There is no official rating of RCD manufacturers, so you should rely on reviews from practicing electricians. As a rule, when assembling a “sophisticated” shield, experts recommend using products from three European companies:

• ABB (Swedish-Swiss company);
• Legrand (France);
• Schneider Electric (France).

In the catalogs of the above manufacturers, alternative names for residual current protection devices will be more often found. RCD - residual current switch (RCB). Difavtomat is an automatic residual current switch (RCCB).

The Schneider Electric company has developed a line of Easy9 devices belonging to the middle price segment.

Differential switch EASY 9 (RCD) 2P 63A 30mA (article EZ9R34263). Easy9 devices belong to the middle price segment, but at the same time they are distinguished by the quality, reliability and ease of use characteristic of equipment in the upper price segment

Many electricians are not satisfied with the quality of products from companies such as IEK, TDM, DEKraft, EKF.

Imagine the following - you have a washing machine in your bathroom. No matter what well-known brand it is, devices from any manufacturer are subject to breakdown, and, let’s say, the most banal thing happens - the insulation on the power cord is damaged and the network potential is exposed to the machine’s body. Moreover, this is not even a breakdown, the machine continues to work, but is already becoming a source of increased danger. After all, if they touch both the body of the car and the water pipe at the same time, we will close the electrical circuit through ourselves. And in most cases it will end in death.

To avoid these terrible consequences they were invented RCD - residual current devices.

RCD- this is a high-speed protective switch that responds to differential current in the conductors supplying electricity to the protected electrical installation - this is the “official” definition. In more understandable terms, the device will disconnect the consumer from the power supply if there is a current leak to the PE grounding conductor (“ground”).

Let's look at the principle of operation of an RCD. For greater clarity, the figure shows its “internal” circuit diagram:

The main unit of the RCD is differential current transformer. It is otherwise called a zero-sequence current transformer. To make it easier for us and not get confused in terms, let’s just call this unit a current transformer.

As can be seen from the figure, in this case it has three windings. The primary and secondary windings are connected to the phase and neutral wires, respectively, and the third winding is connected to the starting element, which is carried out on sensitive relays or electronic components.

The starting element is connected to the executive control device, which includes a power contact group with a drive mechanism. The test button is used to check and monitor the serviceability of the RCD. Now imagine that a load is connected to the output of our circuit. Naturally, a current will immediately arise in the circuit, which will flow through windings I and II. To further consider the principle of operation of the RCD, let’s move on to a more visual diagram:

In normal mode, in the absence of leakage current, current flows in the circuit along the conductors passing through the window of the magnetic core of the current transformer. operating current loads. It is these conductors that form the back-to-back primary and secondary windings of the current transformer. These currents will be equal in magnitude and opposite in direction: I1 = I2. They induce equal but counter-directed magnetic fluxes F1 and F2 in the magnetic core of the current transformer. It turns out that the resulting magnetic flux is zero, the current in the third (executive) winding of the differential transformer is also zero and the starting element 2 is in this case at rest and the RCD operates in normal mode.

When a person touches open conductive parts or the body of an electrical device on which an insulation breakdown has occurred, along the phase (primary) winding of the current transformer, in addition to the load current I1, an additional current flows (indicated in the diagram IΔ), which is for the current transformer differential(difference: I1-I2= IΔ).

It turns out that our currents are unequal, therefore, the magnetic fluxes are also unequal, which no longer compensate each other. Because of this, a current arises in the third winding. If this current exceeds the set value, then the trigger is activated and affects the actuator 3.

The actuator, consisting of a spring drive, a trigger mechanism and a group of power contacts, opens the electrical circuit, as a result of which the installation is disconnected from the network. To carry out periodic monitoring of the serviceability (operability) of the RCD, a test button 4 is provided. It is connected in series with a resistor. The resistor value is selected in such a way that the difference current is equal to the rated leakage current of the RCD operation (we'll talk about the parameters of the RCD later). If when you press this button the RCD is triggered, then it is working properly. Typically this button is labeled "TEST".

Three-phase residual current devices They work on approximately the same principle as single-phase ones. In three-phase RCDs, four wires pass through the core window - three phase and neutral. The simplest three-phase RCD is shown in the figure:

The three-phase RCD includes a switch 1, which is controlled by element 2, which receives a shutdown signal from the secondary winding 3 of the current transformer 4, through the window of which the neutral working wire N and phase wires L1, L2 and L3 (5) pass.

If the load in the neutral and phase (or three phase) wires is equal, their geometric sum is zero (the current in the phase wire of a single-phase RCD flows in one direction, and the current in the neutral wire of exactly the same value flows in the opposite direction). Therefore, there is no current in the secondary winding of the current transformer.

If current leaks onto the grounded housing of the electrical receiver, as well as if a person standing on the ground or on a conductive floor accidentally touches the phase wire of the electrical network, the equality of currents in the primary winding of the current transformer will be violated, since in addition to the load current, a leakage current will pass through the phase wire, and a current will appear in its secondary winding - exactly as in the description of the operation of a single-phase RCD discussed above. The current flowing in the secondary winding of the transformer affects the control element 2, which, through switch 1, disconnects the consumer from the supply network. The appearance of a three-phase RCD is shown in the figure:

Let's consider practical circuits for switching on RCDs in distribution boards.
Circuit diagram for switching on an RCD for single-phase input. A switching circuit with separated zero (N) and ground (PE) buses is used here. As you can see in the figure, the RCD (5) is installed after the input circuit breaker, and after it there are circuit breakers installed for protecting and switching individual loops. Looking ahead, I would like to note that the presence of a connection between an automatic machine and an RCD is mandatory, since the RCD does not provide current protection, both thermal and against short circuits. Instead of this “combination” - automatic machine - RCD, you can use one universal device. However, more on this a little later.

Circuit diagram for switching on an RCD for three-phase input. Unlike the previous scheme, protection is provided here for both single-phase and three-phase consumers. In addition, a combination of input zero and ground buses (PEN) is used. An electricity meter - an electric meter - is connected between the input machine and the RCD. As you remember from the reviews on metering schemes, all switching devices that are installed before the metering device must be sealed by the energy supply organization. Therefore, the design of the input circuit breaker must provide for this possibility.

Before this, we only talked about electromechanical RCDs. But if you remember, I mentioned that sometimes there are electronic devices. In principle, an electronic RCD is built according to the same scheme as an electromechanical one.

Instead of a sensitive magnetoelectric element, a comparison device is used (for example, the most common example is a comparator). Such a circuit requires its own built-in power supply - after all, you need to power the electronic circuit with something.

The difference current has a very small value, therefore, it needs to be amplified and converted to a voltage level that is applied to. All this, of course, reduces the overall reliability of the device compared to an electromechanical one; this is exactly the case here - the simpler the better. And to be honest, I have not yet come across certified electronic RCDs at all. Therefore, I cannot say anything good or bad about them. Therefore, let’s leave electronic RCDs aside and focus on one of the main points in considering electromechanical residual current devices - their parameters:

RCDs have the following main parameters:

network type - single-phase (three-wire) or three-phase (five-wire)

rated voltage -220/230 - 380/400 V

rated load current - 16, 20, 25, 32, 40, 63, 80, 100 A

rated residual current - 10, 30, 100, 300 mA

type of differential current - AC (alternating sinusoidal current that occurs suddenly or slowly increasing), A (like AC, in addition - rectified pulsating current), B (alternating and direct), S (delayed response time, selective), G (like selective, only the delay time is shorter).

I would like to note one important point regarding the parameters of the RCD. Many are misled by the rated load current printed on the device body, and it is mistaken for the same parameter as in the circuit breaker. However, this parameter in an RCD characterizes only its “current carrying capacity.” This expression may not be entirely correct, but I introduced it to make the concept of the term “rated load current of the RCD” more accessible.

The RCD is not able to limit the load current and it must be protected from current overloads and short-circuit currents by automatic circuit breakers, which provide protection from both overcurrent and short-circuit currents. The load current of the RCD should be selected so that it is one step (rated range of currents) greater than the current rating of the circuit breaker of the protected line. That is, if there is a load protected by a circuit breaker with a current of 16 Amps, then the RCD should be selected for a load current of 25 Amps.

A logical question arises here - why not combine both a circuit breaker and an RCD in one housing, especially in the case when the RCD is used to protect only one power loop? After all, in this case they still work “in pairs”. This point was touched upon a little in a previous article. Well, the question is quite logical and such devices, of course, exist. They are called differential circuit breakers or simply differential circuit breakers.

In the figure you see just such a device. A three-phase differential circuit breaker is shown here. As in a three-phase RCD, it has four terminals each - phase and neutral and a “TEST” button. If one dwells on its internal structure, then it is difficult to say anything new here. This is a circuit breaker and an RCD in “one bottle”.

The cost of diffavtomats is quite high. For example, three-phase models from well-known foreign manufacturers cost about 100 Euros. Relatively expensive pleasure. However, a combination of AB + RCD will have approximately comparable cost, and instead of four standard 17.5 mm modules on a DIN rail (with a three-phase version), it will take eight. So in some cases, diffautomatic devices are still preferable, especially if there is a problem with free space in the distribution panel.

How to check the performance of an RCD or a circuit breaker? We have already mentioned the “TEST” button. However, such a check is very superficial and does not always reflect the real essence of things. Therefore, test circuits or specialized devices are used for objective verification.

Using a residual current device is a fairly relevant way to ensure the safety of the electrical network in the house. RCDs are usually used for human protection. This article will provide information about its purpose, operating principle and types of such equipment.

The main purpose of a residual current device is to protect a person from injury from alternating or direct current. If a faulty or damaged device is connected to the electrical network in a room, then if a person comes into contact with exposed conductors, he may be electrocuted. This equipment allows you to avoid electric shock due to accidental contact, as well as protect your home from ignition of wiring due to current leakage.

The device is a mechanical switching device for stopping the supply of differential current under certain conditions. The RCD de-energizes the electrical network when a person (animal) touches the conductors of household appliances and equipment or the grounding contact. Triggering occurs both in direct contact with parts of the electrical installation and in indirect contact.

Switching off the RCD can also occur if the external insulation or contact of the conductors with the grounded body is damaged, if the grounding conductor and the working neutral are incorrectly changed, or if the phase wire is changed with the neutral working while touching them under voltage. A disconnection can occur due to a break in the working neutral conductor, which according to the diagram can be placed before and after the protective device, and when a person touches live parts of the device with a bare part of the body.

Kinds

Like all protective devices for an electrical circuit, the RCD has its own types. Due to their design and mechanics, devices can be electromechanical or electronic.

Electromechanical. Connecting such a device allows you to provide the circuit with higher reliability. The action of the electromechanical type of devices allows it to be widely used in buildings for various purposes, due to the ability of mechanics to work at any load in the electrical network. The high cost is justified by the response speed and long service life.

The main design elements are a relay, a magnetoelectric latch, and a transformer for zero-sequence voltage. Electronic. The design of this type of equipment is very similar to electromechanical equipment, since it contains the same elements. However, the main magnetoelectric element is manufactured in the form of an electronic circuit.

The circuit itself consists of comparative elements, a filter, an amplifier and a rectifier. Thus, when an electronic type residual current device is connected to the network, its operation will depend on the line voltage. The residual current device can be connected according to a specific circuit to an electrical single-phase and three-phase power line.

Devices are classified according to their speed of reaction to a leak, according to the characteristics of the circuit break, according to the permissible load, according to the method of preventing short circuits and the type of housing installation. The choice of such a device should be based on its characteristics and the characteristics of the circuit.

Video “Operation principle and RCD diagram”

Principle of operation

The operating principle of protective equipment is based on comparing the potentials of the differential current that passes through it. To do this, the potential at its input and output is constantly measured. Ideally, after measurements, the sums of vector currents passing through the controlled conductors and should be equal to zero. So, with a single-phase electrical line circuit, the measurement is carried out on two conductors, and with a three-phase circuit, on three or more.

If the value of the flowing alternating current in the network differs from that entering the RCD, it will operate, de-energizing the room. Potential difference ranges can be standard for a certain type of equipment or have a range regulator to set an arbitrary value (of course, with a reasonable limitation). If a leak appears on the line (a person touches a phase wire or a decrease in the resistance of the external insulation of the cable), the vector sum of the incoming and outgoing current will be different. In this regard, the entire line will be de-energized.

So, when operating the equipment, we can identify the main element in its design - the differential current transformer. It is a core with a winding. The design also provides for preliminary testing. There is a special button on its body for testing the device. The operating principle of the test is an artificially created leakage charge. If everything is connected correctly and the residual current device itself is working correctly, then pressing the button will disconnect the power line branch.