A residual current device (RCD) is designed to open the electrical network in the event of leakage currents. These currents (they are also called differential) appear mainly as a result of a violation of the insulation of the elements of the electrical network. Since they only flow through the phase wire, and their values can be quite small (the insulation resistance does not necessarily immediately become zero, especially if this happens due to its aging), then in this case the current protection installed in the system does not work. The installation of an RCD makes it possible to protect users from the possibility of electric shock due to a violation of the insulation of electrical appliances.
Operating principle
The principle of operation of the RCD is based on the use of an instrument transformer, which has two inductively coupled windings. One of them is connected in series to the zero, and the other to the phase wire. Under normal operating conditions of the electrical network, the magnetic fluxes caused by these currents are mutually compensated, since their values are equal to each other.
In the event of a leakage current, the magnetic flux generated by the current of the phase wire exceeds the value of the magnetic flux in the neutral wire. As a result, a command is given to operate the relay, which opens the electrical circuit.
When designing and installing automatic protection, the features of the electrical network of the facility should be taken into account. If it is necessary to connect an RCD in an apartment, then, as a rule, it is installed on the input panel near the electricity meter.
In order to properly connect the RCD, it is necessary to understand the design features and the principle of operation of these devices.
There are two types of RCD according to the principle of operation:
- electromechanical;
- electronic.
For the operation of the first of them, there is no need to supply voltage to the control circuits of this device, in other words, the quality of functioning of such devices does not depend on the presence of voltage in the network. This greatly increases their reliability in relation to electronic models. An example of an electronic RCD is the ABB RCD F202 AC-80/0.03, and an electromechanical RCD is ABB FH202 AC-40/0.03.
In addition, depending on the number of phases of the network in which this device must be installed, it can be made in a two-pole (for example, ABB FH202 2P) or four-pole (ABB FH204 4P) version. The most common RCD connection scheme today is the installation of a two-pole device in a single-phase network. In turn, a four-pole RCD can be used in a three-phase network with an isolated or solidly grounded neutral, as well as in a single-phase network.
General RCD connection diagram
To answer the question of how to properly connect an RCD, you should carefully read the most common connection diagrams for these devices.
Understanding the general rules for installing protective automation and the criteria for choosing its parameters will help you correctly select and install an RCD with the necessary parameters in an electrical panel.
To ensure reliable operation of the RCD, it must be connected to the electrical network after the meter through a circuit breaker. This requirement is explained by the fact that in the event of shock currents of short circuits, the RCD does not ensure the timely operation of its breaking contacts. Thus, the result of such emergency situations can be not only the failure of the RCD, but also the destruction of the elements of the electrical network or even a fire.When choosing a circuit breaker, it should be taken into account that the value of its rated current cannot be higher than the rated current of the RCD.
The connection of the RCD can be carried out according to a scheme designed in such a way as to ensure its response to the leakage current flowing in any of the elements of the protected electrical network. To do this, such a device is connected immediately after the meter and the general circuit breaker on the input panel. Thus, simultaneous protection of all consumers receiving power through this machine is achieved. However, this installation method has a significant drawback - it does not ensure the selectivity of automatic protection, that is, its ability to turn off only those network elements in which emergency situations have arisen.
In order to ensure the required selectivity of the protection system, it is necessary to supply several separate devices that are connected after circuit breakersestablished to protect certain groups of consumers.
Thus, we can distinguish two basic principles for constructing a protection circuit using RCDs.
- One-level scheme. It implies the disconnection of all consumers from the network in the event of a leakage current in any of its elements. This scheme is used in small electrical networks with a limited number of low power consumers. The advantage of this method is the ability to simply calculate the values of the necessary elements, as well as the ease of installation in the electrical panel and maintenance of these devices. To implement it, it is enough to put only one RCD in the electrical panel after the meter and the general machine.
- Layered scheme. The correct choice of elements of such an automatic protection scheme is noticeably more complex than in the previous case. RCD parameters for each consumer group are determined based on their total power. When installing these devices, it is important to pay attention to the fact that the connection of the neutral wire is made in the cut of the conductor going to the protected group of consumers. If we take for these purposes, for example, a common zero immediately after the counter, then the RCD will trip when voltage is applied to the network. Otherwise, the protection parameters are selected taking into account the rated current of the circuit breakers, as well as their location in the tree structure. An RCD with a lower rated current must be placed after a similar device with a larger value of this parameter.
In principle, it is not so difficult to choose the required rating of protection devices. Compliance with the above rules will help you independently calculate the necessary parameters, as well as correctly install all the necessary protective automation devices.
When connecting wires to the contacts located on the RCD case, it is important not to confuse the phase and neutral terminals. Appropriate designations are marked on the device itself. As for the installation of wires, it is better to once again make sure that there is no phase on the wire connected to the zero contact of the RCD. A connection error can cause the device to fail after applying voltage to it.
In most apartment buildings, electrical wiring is represented by only two wires: zero and phase. Its design does not provide for the presence of a ground wire in the electrical energy meter shield. This creates numerous difficulties with the connection of modern household appliances, which must be grounded without fail.
However, based on the basic principle of protection, such an RCD connection scheme does not reduce its protective qualities, since in the event of a leakage, there is still a difference in the value of the currents in the neutral and phase wires (by the way, the electric meter “does not notice” the leakage current, which is actively used unscrupulous consumers for unauthorized connection of electrical appliances). Another thing is that if the body of the electrical installation does not have contact with the ground, then there is no circuit through which the leakage current could flow.A situation arises in which the path for the flow of this current is created only after a person touches the body of such a device (if the human body itself has contact with the ground).
The use of differential automata
In order to combine the functions of an RCD and a circuit breaker in one device, you can put instead of them differential machine.
The principle of its operation is the same as that of these devices, just all their elements are combined in one case. This combined device can respond to both short circuit current and leakage current. The undoubted advantage of such equipment is its compactness and ease of installation in the electrical meter panel. This makes it quite possible to carry out work on self-assembly of the protection circuit using difavtomatov.
However, since these machines have a more complex internal design, the failure of any of their elements will entail a costly replacement of the entire device. Therefore, it is advisable to install circuit breakers manufactured by reliable manufacturers of electrical equipment, such as ABB or Schneider Electric, to protect the network.
Only after a detailed study of the characteristics of protective devices and the parameters of consumers of the electrical network, as well as a clear understanding of how to properly connect the RCD, you can be sure that an unprepared person will be able to independently select and install automatic protection devices.
