Before a major overhaul or installation of home wiring, the user will need to think about the principle of selectivity of the protective devices used. Its elaboration and competent implementation will improve the reliability and ease of operation (maintenance) of the equipment used. But first you need to understand what selective protection is and why it is taken into account when installing electrical wiring.
What is selectivity
Selectivity is a method of selecting a circuit for switching on protective devices, which allows only emergency sections of the power grid to be disconnected in a timely manner.
At the same time, the rest of it remains in working order. This technical solution allows you not to waste time searching for faults throughout the apartment, but to begin to find out the cause in a separate wiring line, where the protection worked.
On the other hand, selectivity is understood as the consistency of action of means of protection for very specific parameters. Its main task is reliable operation of a protective device in the presence of disturbances in the operation of electrical circuits. All other electrical equipment in this case continues to function normally.
Selectivity types
Several types of equipment selectivity are known, among which they are most often used:
- Full and partial circuit protection.
- Time selectivity.
- Current and time-current.
In the "full" version, at least two circuit breakers are used, connected in series. The first to operate is the device that is located closer to the place of failure (short circuit, for example). Partial protection differs from full protection only in the magnitude of the pickup currents (given settings).
In the third option, several devices connected in series with the same current settings, but differing in response time, are used. Each stage reacts to a fault (short circuit) after a delay provided in it. If for some reason the first automatic machine, calculated for 0.1 seconds, does not work, for example, then the next device should turn off (0.3 seconds) and so on.
The overcurrent protection is organized according to the principle of selection of the magnitude of the emergency current. It is she who is most often used in domestic conditions. The next type is a combination of the two previous ones, which makes it possible to increase the effectiveness of the protective equipment. There is also the so-called "directional" protection used at transformer substations and intended for sections of circuits.
Practical example
An example of the simplest selectivity is the distribution of functions in a home electrical cabinet, where a common input automatic machine and its linear counterpart are installed (photo below).
Here, 16 Amp devices protect the lines of power outlets and turn off when they are short-circuited or over-current. The introductory automaton represents the second stage in this scheme, which is triggered if the first device for some reason did not fulfill its function.