Difavtomats and RCDs: what is the difference (photo, video)

To ensure the safe operation of the electrical wiring of modern apartments, protective shutdown devices (RCDs) or differential machines are increasingly being chosen today. The use of each of them provides an early shutdown of the section of the electrical circuit in which there is a violation of insulation. In addition, with the proper organization of automatic protection of the power grid using such devices provides reliable disconnection of consumers in the event of overloads or short closures. In this case, the main difference between the RCD from the difavtomat is the need to put and use an additional circuit breaker together with such a device.

It should be noted that for the proper functioning of the protection against high differential currents, a three-wire single-phase system is required, which includes a grounding wire. Such a wiring system is now universally installed in new buildings, but it is extremely rare in old-built houses.

In order to answer the question, what is the difference between RCDs from difavtomat and which device is more preferable to choose for use in your apartment, you need to familiarize yourself with their basic technical characteristics, operating principles, as well as design features and operation.

RCD application

Residual Current Device It is used for switching in a network supplying consumer groups when currents flow under normal operating conditions.

The main task of the RCD is to disconnect a section of the electrical system if a differential current occurs in it that exceeds the permissible value.

The occurrence of leakage current is due to the presence of some insulation resistance of the wiring and electrical consumers. Since this resistance cannot be infinitely large, it will always flow through it called normal leakage current, the value of which must be in certain permissible limits.

In order to better understand what undesirable processes occurring in the power grid, protects difavtomat or RCD, consider the following schemes.

The first of them depicts a case of a person being struck by electric shock, which occurs as a result of touching an ungrounded case of an electrical appliance with broken insulation. This circuit has a circuit breaker that disconnects its contacts in the event of an overload current or short circuit, but this protection does not work when the phase is shorted to ground.

The second figure shows the path of the leakage current in case of violation of the insulation of the grounded case of the electrical appliance. Since the resistance of human skin is much higher than the resistance of the ground loop, in this case, electric shock does not occur. However, the metal parts of the body have a certain potential relative to the ground.

The danger of such a situation lies in the fact that when using conventional circuit breakers, in a significant reduction in the insulation resistance of electrical appliances does not automatically disconnect the consumer from the network.

The leakage current causes heating of the ground connection points to the housing, which increases their resistance. In turn, air humidity, the condition of human skin, the material of shoes and floor indoors, as well as many additional factors affect the value of the resistance of the circuit body - man - land. If you take into account the features of the operation of electrical appliances in places with high humidity (kitchen or bathroom), then the danger of electric shock remains quite high.

In addition, the flow of current through the damaged insulation causes its heating and even greater destruction. In certain cases, this may cause a fire.

The principle of operation of the RCD is based on a constant measurement of the magnitude of the differential current. While it lies within the permissible limits, no action takes place, but as soon as this value exceeds the permissible value, the RCD disconnects the consumers from the mains.

The rated values ​​of the leakage currents, which are designed for most modern residual current circuit breakers, are 30 and 100 mA. The increase in differential currents can be caused by various reasons, the most common of which is the deterioration of insulation between the grounded casing of the appliance and the phase conductor electric network. High leakage currents appear in those cases when during the installation of electrical wiring there were violations associated with improper connection of the neutral and ground wires.

The third diagram shows an electrical circuit in which, in addition to a circuit breaker, an RCD is used. In the event of a leakage current, the value of which exceeds the nominal, the automation breaks the circuit.

If you use such protection in electrical networks, consumers of which do not have grounding, then for its operation, the occurrence of a closed circuit between the metal housing of the device and ground. As a rule, such a circuit closes if a person touches the body of the electrical installation.

Thus, the use of RCD allows you to open the electric circuit in the following cases:

1. If a person touches an ungrounded electrical installation enclosure that has become energized due to insulation damage.
2. If a leakage current occurs through the ground loop due to a violation of the insulation of live parts, the value of such current must exceed the permissible value.
3. In case of an erroneous connection of the neutral and ground wires in the electrical installation.
4. When the neutral wire breaks.

RCD connection

It should be noted that the RCD does not provide protection for short circuit and overload currents. Therefore, such devices must be connected together with a circuit breaker, the permissible current of which must be less than the same value for RCDs. This power supply circuit is shown in the third figure.

As can be seen from the diagram, the phase wire is connected to the corresponding contact of the device through a circuit breaker. The neutral wire must also be connected to the consumer via an RCD.

If you choose to use three-phase protection devices, their connection is performed in the same way: three phase and neutral wires are inserted into the connectors that are marked accordingly.

In order to check the functionality of the RCD, just click on the TEST button located on its body. A healthy device will immediately shut down. However, some models are not equipped with such a button. You can check their performance by creating an artificial circuit between the phase conductor and the protective earth, in this case, a leakage current immediately occurs, to which the RCD responds. You can make such a closure using any metal object, but it is better to choose a cartridge with a bulb.

The test leakage current method is the most reliable, since it allows not only to check the operation of the RCD, but also to evaluate the correctness of its connection.

To monitor the health of the RCD, it is impossible to short circuit the phase and neutral wires, since this will cause the circuit breaker to trip as a result of a short circuit.

Selection of residual current device

When choosing an RCD for your apartment, you should pay attention to such its characteristics:

1. Rated current. This value is selected based on the total capacity of consumers connected to a section of the electric network, at the input of which an RCD is installed. In any case, the rated current of the device cannot be greater than the corresponding current value of the circuit breaker.
2. Rated voltage. There are no problems with choosing this value, 230 V devices are used in single-phase networks, and 400 V in three-phase networks.
3. Manufacturer. With a certain degree of distrust, one must relate to the products of Chinese companies. Brands such as Legrand, Schneider Electric, and EATON have proven themselves in the market of relay protection and automation devices. One of the commonly used single-phase RCDs is VAD2. As for domestic manufacturers of such devices, it is quite safe to choose them, so as given products in many cases are not inferior to Western counterparts, while it costs significantly cheaper.

Differential machine

Difavtomat - a device that combines an RCD and an ordinary circuit breaker.

The characteristics of each model are printed on its body in the form of special markings. The main ones are:

1. Rated current and type of time-current characteristic, for example, C63. This means that the rated current is 63 A. The time-current characteristic is the dependence of the trip time of the circuit breaker contacts on the current flowing through them. For circuit breakers used in power supply systems of various objects, these characteristics are different. In apartments and residential buildings, machines with a C-type characteristic are used.
2. Leakage current (0.03 A, 0.1 A) at which the part of the circuit breaker that responds to the differential current is tripped.
3. Rated voltage (230 or 400 V).
4. Type of machine (for working with alternating or rectified direct current).
5. Schematic diagram of the connection.

To answer the question of how to distinguish RCDs from a difavtomat, just look at their appearance. Although at first glance the differences are not very noticeable, but for a knowledgeable person they, as they say, are obvious:

1. On the RCD, the type of time-current characteristic is not indicated.
2. In the difavtomat, in a circuit diagram deposited on its body, two additional switches are installed, indicating thermal and electromagnetic trip units.
3. The name of the device (VD or AVDT).

A residual current controlled circuit breaker has the following advantages:

• When installing the difavtomat, there is no need to install any additional protective equipment.
• Almost all such devices are equipped with a special display that allows accurate determine what triggered the device: from the appearance of a large leakage current, short circuit, or overload.

In answering the question posed, what is the difference between RCDs from a difavtomat, you need to consider the following. Judging by the quantity and quality of the functions performed, there is no particular difference in which device to choose: a difavtomat or an RCD. At the same time, the cost of a combined device is even higher than the combined price of an RCD and an ordinary machine. In addition, if one of the separate devices fails, it can be repaired or replaced without removing the second, it is much cheaper than repairing a difavtomat.