When operating existing power grids, it is important to take care of the safety of service personnel and consumers using their services. According to the requirements of the PUE, this applies to both single-phase and three-phase circuits, often equipped in private houses. To protect users from electric shock, devices called residual current devices (RCDs) are installed on the consumer side. At the same time, it is important to know how to connect an RCD with grounding in a private house, without violating the provisions of current standards.
Generalized view of protection
The safety of operating personnel and users of power grids is achieved through the following measures:
- grounding or grounding (connection to neutral) of all metal parts of the equipment;
- organization of re-grounding by arranging a separate circuit;
- installation in load circuits of especially dangerous rooms (bathrooms, for example) disconnecting devices such as RCD.
The latter option can be used in both grounded and ungrounded electrical circuits.
With a general approach to the assessment of protective equipment, it is noted that it is necessary to ground structures to reduce the potential that threatens a person to a safe level. In contrast, the RCD provides security due to the instantaneous disconnection of the network when the leakage currents reach their limit values. In the technical characteristics of these devices, this parameter refers to the main performance indicators.
What is an RCD
In decoding the abbreviation RCD, the main emphasis is on disconnection, which indicates the cardinal nature of protective measures. To understand how this device works in a hazardous situation, you should familiarize yourself with its design. The RCD device consists of the following main parts:
- a differential device that compares the inflowing and outflowing currents;
- an electronic circuit capable of responding to their imbalance;
- an executive module designed as a contactor that disconnects the power grid from the consumer.
The principle of the protective action of the RCD is based on the features of its design, which make it possible to assess the magnitude of earth leakages and instantly respond to them. Due to the high speed of breaking the connection with the existing network, the current in the load does not have time to reach critical values.
Traditional RCD connection schemes
In electrical networks for household use with sockets and lighting devices installed in them, RCDs without grounding are used, which is typical for the TN-C protection system. In accordance with the peculiarities of its functioning, a line is drawn from the station equipment to the consumer, in which only the combined PEN conductor is provided. As a rule, it is divided into a protective PE bus (the ground loop is connected to it) and a working N in apartment buildings.
Classic RCD circuit without grounding
Usually, RCD devices are connected to ungrounded networks of household consumers, the power supply of which is organized through a two-wire line. All they guarantee is to turn it off if the leakage current exceeds the permissible value (30 mA, for example).These devices are not able to provide such protective switching, such as disconnecting the mains supply in case of overload or short circuit. Therefore, the connection diagrams for RCDs in single-phase networks presuppose the obligatory presence of a circuit breaker against short-circuit and overload in them.
The range of currents for which the circuit breaker is calculated are selected individually for each specific load line. The joint operation of these two devices guarantees reliable protection of a person from high voltages in a bath, for example. At the same time, their use allows you to protect household appliances used in a modern apartment from failure. Quite often, a circuit breaker together with an RCD is replaced with a difavtomat, which contains both devices in a common case.
Group and multi-stage protection
With the so-called "group" switching on of the RCD, a separate device with an automatic switch or a difavtomat is placed on the dedicated line. In this case, each of the load groups connected to the network is served independently of the others, which increases the selectivity of the protective functions. As a result, the safety of using household appliances in each room increases markedly.
Greater security is provided by a stepped scheme, in which a group of loads is connected to the network through another similar device (it forms the second stage). The use of these systems makes it possible to increase the reliability of protection in comparison with the classical one. But due to the complexity of execution and technical redundancy in everyday life, they are rarely used.
RCD connections in a network with grounding
A typical circuit for connecting an RCD in a single-phase network with grounding is built according to the same rules, according to which it is mounted immediately after the energy meter. The difference lies in the presence of a separate bus in it, which is laid bypassing the set of protective devices. At the same time, the reliability of operation of each of the devices is significantly increased due to significant leaks in the "phase - equipment case - ground" circuit.
In this case, special operations for arranging the protection are not required. If there is a protective circuit in a private house, for example, it will not be difficult to ground an existing electrical network with an RCD. To do this, you should make a splitting on the main grounding bus (GZSH), and then arrange a tap from the PE conductor.
Which scheme is better
When evaluating the considered schemes, one proceeds from the level of safety provided by each of them. To resolve this issue, it will be necessary to compare them not only in terms of protection efficiency, but also in terms of implementation costs. After careful study, the following conclusions can be drawn:
- With a limited number of linear consumers, the simplest set of devices is used, consisting of one RCD and a linear machine behind it.
- In the case of a branched network of single or three-phase loads, group connection is preferable.
- With high safety requirements, it is possible to use staged connection of protective devices.
The latter method is optimal for a private house.
Before connecting an RCD without grounding in private houses, its switching diagram should be carefully studied. In this case, the most reliable option is to use multistage systems of several devices with different values of current leakage.
Modern country houses are distinguished by a developed power supply system with good protection against electric shock due to the presence of re-grounding. Therefore, they use simplified schemes involving the use of universal RCDs for leakage currents up to 30 mA (for separate protection of a water heater, for example). But most often, preference is given to typical differential devices designed for the appropriate cutoff for overloads.
Typical errors include violations in the choice of the RCD installation level, when it is included in circuits with incorrectly selected leakage currents. To avoid violations of the rules for connecting the inlet and outlet conductors, when switching them, they are guided by the diagram on the device case.
