What is an RCD: device, principle of operation, existing types and labeling of RCDs

Vasily Borutsky
Checked by a specialist: Vasily Borutsky
Author: Alena Slepakova
Last update: May 2019

Any electrical network must have a protection device, but not everyone knows what an RCD is and what principle of its operation. Decoding of the abbreviation looks like this - protective shutdown device.

This electric low-voltage device is designed to turn off the protected section of the circuit when creating a differential current exceeding the rated value for this device.

In our article we will try to analyze in detail the device and the principle of operation of the RCD, consider the existing varieties and figure out what information the labeling of the residual current devices contains.

The purpose of the protection device

The grounding device of the RCD is a PE conductor of neutral conductive housings or parts of electrical mechanisms with a resistance of not more than 4 ohms.

If a leakage current occurs, these items of equipment may be energized, which poses a danger to human and animal life in contact with them, as well as property as a whole.

To save from receiving electrical injuries is the calling of survey devices. If a leakage current is detected, they turn off the voltage.

The greatest danger lies in the fact that such disturbances in the circuit are invisible and, in rare cases, noticeable, when you touch the device you can feel a slight electric shock.

The main reason for this phenomenon is a violation of the insulating layer of the wiring. Uncontrolled processes can cause great harm, so protective equipment is gaining great popularity in the domestic environment.

RCD device
The impact of conductive networks on the human body can turn into disastrous consequences. This problem was solved with the help of RCD control devices related to the protective segment.Basic installation and use requirements are described in IEC 60364

The use of RCDs is most common in single phase networks with alternating current and grounding of the neutral line, as well as with voltage indicators up to 1 kW in the format of domestic power supply.

Design RCD

The optional features of the protective mechanism will help to understand the principle of operation of the RCD, namely the reproducible reaction of the device to current leakage.

Key work nodes include:

  • transformer differential sensor;
  • trigger element - a mechanism that breaks an incorrectly functioning electrical circuit;
  • electromagnetic relay;
  • control unit.

Counter windings are connected to the sensor - phase and zero. Under normal network operation, these semiconductor elements form magnetic fluxes in the core that have the opposite direction with respect to each other. Due to this, the magnetic flux is zero.

RCD transformer
The transformer consists of a closed steel core, on which two coils are worn: primary - connected to an alternating current source, secondary - connected to the load. How many times the transformer increases the AC voltage, the number of times the current decreases

An electromagnetic relay is connected to the secondary winding wound on the transformer magnetic circuit. If the network meets standard operating conditions, it is not involved.

When a current leak occurs, all work changes dramatically. Phase and neutral conductors begin to pass different amounts of current. Now the force value and the direction of magnetic fluxes on the transformer core will also have different parameters.

In the secondary turns, current appears and when the set values ​​are reached, the operation of the electromagnetic relay is reproduced. It is paired with a trip mechanism. This bundle at the right time reacts and disengages the electrical network.

Differential protection device
According to fire safety requirements, control checks of the differential protection device are carried out regularly, at least once a month. To do this, the device has a special button "TEST"

The test unit is represented by a resistance mechanism - a certain load connected bypassing the differential sensor. This element simulates a leakage of current and thus checks the operability of the device. We talked about verification methods in more detail. in this article.

The principle of operation / operation of the RCD is as follows: supplying current from the phase line to the control resistance and then to the neutral wire, bypassing the sensor.

Thus, the conditions of different current indicators at the input and output of the device are created. This imbalance should lead to the start of the shutdown node.

Depending on the developers, the circuitry device may vary, however, the principle used in the operation of the RCD will be identical for all models.

The principle of operation of the protective mechanism

Consider why you need to use an RCD. The operation of the safety device is based on the measuring method.

The input and output parameters of the currents flowing through the transformer are recorded. If the first value is greater than the second, this means that a current leakage occurs in the circuit and the device reproduces a trip. If the parameters are identical, the device does not work.

RCD trip
In a two-wire system, the differential device does not work if a current of the same strength passes through the phase and neutral wires. If there is a difference in these values, then an insulating breakdown and a protective mechanism in the network will disable the damaged area

For a better understanding, consider how an RCD will work in a household switchboard with a bipolar system.

An input two-wire wire (phase and zero) is connected to the upper terminal blocks.The phase and zero are connected to the lower terminal blocks, laid to the load site, for example, to the power outlet of the boiler or electric kettle. Protective grounding of the device will be performed by cable, bypassing the RCD.

In standard operating mode, the movement of electrons is performed along the line-phase from the incoming cable to the electric heater of the boiler / kettle, flowing through the differential protection device. Back they move to the ground again through the RCD, however, along the neutral line.

The principle of operation of the protective device
If a person touches the case of a conductive device, on which potential has appeared due to a breakdown, in this situation a current leak will pass through the human body, to which the device responds almost instantly, turning off the power system

For example, in the heating element of the device, the insulation was damaged. Thus, through the water inside, the current will partially be conducted by the housing, and then go into the ground by wiring a protective device.

The residual current will return in a neutral line through the RCD. However, its strength will become less by the amount of leakage compared to incoming.

The difference of indicators is calculated by the differential transformer. If the number is larger than the allowed, the device instantly reacts and breaks the circuit.

In our other article, we made recommendations for choosing and connecting correctly. RCD for the boiler.

The feasibility of using an RCD

Consider why it is necessary to use an RCD and from what negative factors of influence the device provides protection.

First of all - phase closure on the body of electrical engineering. Basically, the problem areas include heating elements of heaters and washing machines. It is worth noting that a breakdown is formed only when the heat-generating part is heated by current.

Also, with incorrect connection of wires. For example, if twists are used without a terminal box, which are subsequently recessed into the wall and covered with a layer of plaster. Since the surface has high humidity, this twist will be a breakdown that will leak into the wall.

In this case, the differential protective mechanism will constantly de-energize the line until the section is completely dry or until the connection unit is redone.

RCD connection
Automatic protection is effectively used in everyday life: in electrical groups for the bathroom, kitchen and sockets, with a large number of powered appliances. Ideal when such devices are installed on each group of outlets

The scope of survey devices is quite diverse - from public buildings to large-scale enterprises. They complete electrical structures and circuits intended for reception and distribution: shields in residential buildings, power supply systems for individual consumption, etc. The main thing with this is right choose RCD by power.

Types of devices and their classification

Developing companies give their products diverse capabilities that must be taken into account when determining the desired type of RCD, based on the specific operating conditions of the electrical network.

In order for an ordinary consumer to be able to choose the necessary protective shutdown device among the variety of models offered, a classification system was created based on the following characteristics:

  • trigger principle;
  • kind of differential current;
  • time delay of the tripping differential current;
  • number of poles;
  • installation method.

Next, we consider in more detail each of these classifications.

Classification # 1 - inclusion method

There are only two switching methods - electromechanical and electronic. In the first case, the machine will turn off the power on the damaged line regardless of the mains voltage. The main working body is a toroidal core with windings.

When a leak is formed, voltage is generated in the secondary circuit to activate the polarization relay, which leads to the activation of the shutdown mechanism.

Electromechanical RCD
Electromechanical devices do not require external voltage. The source for their operation is the differential current on the fault line.

The functioning of an apparatus with electronic filling depends entirely on the additional voltage, i.e. external power required. Here, the working body is an electronic board with an amplifier.

Inside such a mechanism there are no additional sources that accumulate energy, therefore, external circuit electricity is used to operate the circuit and, if there is no voltage, the device will not break the circuit.

Uzo Type Check
Determining the type of device: solder two wires to the terminals of the AA battery. Turn on the RCD and connect to the input of the protective unit, and the next to the output. Lines are connected at one pole. If the device turns off - this means that the electromechanical type is presented, if not - electronic

An example of the operation of an electronic RCD installed on the line with the socket from where the microwave oven is fed: a zero phase has been interrupted, in addition to this, a microwave circuit wiring malfunction is formed in the same period and the phase is shorted to the case, i.e. dangerous potential appears on it.

If you touch the stove, the electronic type of protection will not be activated, because no mains supply. It is precisely because of the unreliability in comparison with the electromechanical analog that this device received less distribution.

Classification # 2 - by type of leakage current

All models of manufactured safety automats are additionally divided by the current of the load passing through the device. They handle the voltage of a given waveform format.

On the case of all devices and in the passport the nominal value of the operating voltage is registered. This parameter should correspond to the range of the rated current of electrical engineering.

Speaker type will be activated when an instantaneous leakage voltage occurs in a controlled circuit or when it is wave-wise. These devices are marked with the inscription "AC" or the symbolic sign "~".

RCD type AC
The most suitable form factor for domestic use is UZO-AS. The model is the cheapest of devices of a similar action. In the passport to electrical engineering, manufacturers often indicate a specific model of the circuit breaker suitable for this product

Type A is triggered by the instantaneous formation of an alternating or ripple breakdown current in a controlled circuit, or when they slowly increase.

Such a mechanism can be used in any presented situations. The abbreviation "A" or a symbol is applied to the machine case, as in a graphic image in a rectangle Special symbol for RCD.

Most often, the A-type is connected to a circuit where load regulation is reproduced by cutting off the top of the sinusoid, for example, adjusting the speed of the motor's reverse movements with a thyristor converter.

RCD type A
The cost of model A is several times more expensive compared to speakers, because here, the DC voltage arising in the power converter technology is additionally monitored

The RCD of subspecies B is effective for reproducing the reaction in a slave electrical circuit of a constant, alternating or converted (rectified) leakage current.

This is an expensive equipment designed for industrial facilities. In domestic conditions, they do not apply.

The presented shutdown protection devices of type A, B and AC are designed for an activation time of 0.02-0.03 s.

Classification # 3 - by type of time delay

This classification suggests a difference in two types: S and G. Automatic protection of type S can be characterized by a selective format reaction. The exposure time delay corresponds to a range of 0.15-0.5 s.It is advisable to choose it in the case of a group connection of an RCD.

Types of RCD
Diagram of a dashboard with two load groups, where two different types of protective devices are connected: AC or A, and S

According to the scheme, two load groups are placed in the shield in the form of sockets No. 1 and No. 2, to which RCDs of type A are connected, and a second circuit breaker is connected to the entrance of the room.

If a breakdown occurs in one beam, the input device is activated only when the collective device does not fulfill its function and does not disconnect the defective section.

The selectivity of activating an open circuit can be performed using another method - by means of the settings of the leakage current. This method has received the greatest distribution.

RCD type according to current setting
The layout of the dashboard with two load groups, where two different types of protective devices are connected: speakers with a breakdown setting, and second A, but with a large value

Let us take a similar previous circuit and modify it in such a way: we select an AC type automatic machine only with a differential setpoint of 0.03 A, and at the input there will be a similar device only 0.1 A.

There are situations when the differential current in the fault circuit exceeds the nominal settings of the two protection devices. For the first circuit, the selectivity will not be violated, and in the second, the cut-off current can be supplied by any of the connected devices.

The apparatus of the form factor G is also represented by the selective principle of operation and has a shutter speed of 0.06-0.08 s. All described selective species are designed for the influence of extreme currents - up to 15 kA.

RCD marking
Some RCD models have a system for regulating the setpoint of the diforg, others do not have this capability. However, for domestic purposes, the second embodiment is suitable.

The current limit is an important parameter of choice, because It is precisely because of this that safety is ensured.

For example, in rooms with high humidity, power supply of electrical appliances is carried out by connecting disconnection devices to the circuit with a setting of 0.01 A. For standard living conditions - 0.03 A.

For the organization of fire safety of buildings - 0.1-0.3 A. We recommend that you read the tips on selection of fire protection RCD and the intricacies of its installation.

Classification # 4 - by the number of poles

Due to the fact that the automatic device operates on the principle of comparing the values ​​of the current passing through it, the number of poles at the machine will be identical to the number of conductive lines.

Bipolar RCD is designated as 2P. It is included in a single-phase circuit to provide human protection and prevent possible causes of fire.

Marking of four-pole UZO - 4P. They are designed to work in a network with three phases. A combination of the installation is also possible, for example, a four-pole device is inserted into a two-wire network.

However, this will not realize the full potential of the device, which is economically disadvantageous.

Poles RCD
When installing a circuit breaker, it is worth considering the probability that the load current may exceed the maximum operating values ​​of the device. Therefore, an additional circuit breaker with a rated voltage of no more than the operating current of the security system is additionally installed

Classification # 5 - by the method of installation

Since differential protective devices are implemented in various cases, they can be used as stationary or portable.

In the second case, the apparatus is equipped with an extension wire. Din rail mount devices mounted in an electrical panelthat is located either in the corridor or in the apartment.

There are also options RCD socket and an RCD plug. In both the first and second cases, any electrical appliance connected by such a mechanism does not pose a danger to humans if it breaks.

Full decoding of marking values

Without fail, the name of the developer company is present on the device case. The following is a standardized marking with a serial number.

To decrypt the abbreviation we will use such an example [F] [X] 00 [X] - [XX]:

  • [F] - protective shutdown device;
  • [X] - execution format;
  • 00 - digital or alphanumeric designations of a series;
  • [X] - number of poles: 2 or 4;
  • [XX] - characteristics according to the type of leakage current: AC, A and B.

Also, the nominal parameters of the device will be indicated here, to which special attention should be paid when choosing.

Decoding of the RCD marking
Decoding of abbreviation: 1 - brand; 2 - device type; 3 - selective view; 4 - compliance with European standards; 5 - rated operating current and setting; 6 - maximum alternating operating voltage; 7 - rated current that the device can withstand; 8 - differential enable and disable ability; 9 - electrical circuit; 10 - manual performance check; 11 - marking the position of the switch

The maximum parameters for which the devices are designed include: voltage Uncurrent In, the differential value of the open circuit current IΔnon and off ability Im circuit switching capacity Icn.

The main marking values ​​must be positioned so as to remain visible after installation. Some parameters can be applied on the side or on the rear panel, visible only before the installation of the product.

The outputs intended only for connecting the neutral wire are indicated by the Latin symbol "N". The disconnected RCD mode is indicated by the symbol "ABOUT"(Circle), included - short vertical bar"I».

Not every product has the optimum ambient temperature. In those models where there is a symbolSpecial symbol for RCD - this means that the range of the operating mode is from -25 to + 40 ° C, if there are no indications - we mean standard indicators from -5 to +40 ° C.

Conclusions and useful video on the topic

Video material with a detailed overview of all the constituent elements of the overview protection mechanisms, their purpose and the principle of interaction with each other:

Description of all types of circuit breakers, as well as tips on how to make your choice:

The answer to the eternal question of what to choose - on a differential machine, or on an RCD + installation secrets:

The use of RCDs is a profitable and correct solution not only from the side of economy, but, from the point of view of fire safety, and human protection.

It is recommended to maximize its potential in domestic conditions, installing on all groups of electrical engineering to ensure complete isolation from the effects of electricity.

Do you have questions about the principle of operation or classification of residual current devices? Or do you want to supplement the material with useful information? Please write your clarifications in the comments section, ask questions - experts and competent visitors to our site will try to answer you as detailed as possible.

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Visitors Comments
  1. Michael

    In my apartment there were ordinary traffic jams, and when there were voltage drops, they did not work. At first I did not pay attention to this, but when I found out that it was unsafe, I decided to install an RCD. I bought an RCD certified in order to protect my apartment completely.After that, during power surges, the RCD trips, thereby protecting the apartment from a short circuit and subsequent fire.

    • Expert
      Vasily Borutsky

      Good afternoon, Michael. It turns out that the RCD was installed, but the plugs, which are an anachronism, were left? You did not specify which device is mounted, so I want to warn that the PUE imposes a number of restrictions on the installation of RCDs as the only protection - this is covered in clause 7.1.6. In order not to quote, I attached a screenshot of a fragment of the Rules.

      Attached photos: