There are many concepts in electrical and power engineering. Each of them plays a specific role. Selectivity is a protective mechanism that protects equipment from damage. Its presence allows you to extend the service life of devices and devices and prevent the occurrence of malfunctions. Selectivity involves the use of specific equipment.
Content
- Main characteristic
- Two types of protection
- Main types
- Selectivity map
Main characteristic
Fuses, difavtomats, RCDs and other devices are necessary to prevent the combustion of devices. A correctly connected instrumentation circuit allows you to disconnect only certain sections of the circuit, without disrupting the operation of the rest of the system. Selectivity of electrical network protection - this is a well-functioning equipment.
Its main tasks:
- ensuring the safety of electrical appliances;
- timely shutdown of the power zone where the breakdown occurred;
- reducing the likelihood of negative consequences for other mechanisms;
- continuity of the work process;
- frugality;
- simple operation.
For selectivity to work properly, you need to establish consistency across all devices. For a better understanding of what it is, it is enough to consider the principle of operation on the electrical panel. If a short circuit occurs in the bathroom or kitchen, only the machine that is connected to this circuit is triggered. All other sections continue to operate and supply energy. If the shutdown does not occur, then the automatic input will stop the operation of the entire panel.
Such measures help prevent the occurrence of fires and preserve equipment.
Two types of protection
Selectivity is defined in GOST IEC 60947-1-2014. According to him, there are two types: absolute and relative. The first includes systems with protection, which operates only within the protected area. The fuses in the damaged area blow.
Relative selectivity is back-up protection. It turns on when, for some reason, the damaged area is not blocked. Then the higher-level automata completely shut off the power supply.
However, relative selectivity works at high overloads. With short circuits, this rarely happens. All devices must be connected in a circuit in a certain sequence. Each specific manufacturer produces tables of a bundle of devices.
Main types
There are several types of selectivity. When fully connected, two devices with a serial connection are connected. If a malfunction occurs, the area that is closest to the problem is turned off. Partial protection works in a similar way, but with a slight difference: the protection works only up to a certain overcurrent value.
Time selectivity includes several machines with the same current characteristics. But they all differ in the shutdown time delay. As a result, the machine closest to the malfunction is triggered first. Then the circuit turns on:
- after 0.2 s;
- after 0.5;
- after 1 s;
- after 2 s.
Such a system of switches allows the machines to insure each other and, if necessary, gradually turn off the system, preventing it from overloading. Current protection works in a similar way, but the shutter speed is set not in time, but in accordance with the increase in current. For machines, indicators are set at 25 A - 15 A - 10 A.
There is also time-current protection, which combines the reaction of mechanisms to current and turn-off time.
Zone is aimed at identifying the faulty zone. When it is detected, the system turns off the area with a breakdown. This allows the rest of the machines to remain operational.
With energy protection, all failures occur in the molded case of the circuit breaker. The current does not have time to reach the maximum indicators, since the system instantly turns off the power supply.
Selectivity map
All characteristics of current devices are entered into a specific circuit. It allows you to create maximum protection for machines. Its main principle is the sequence of connecting devices.
When creating a map, certain rules are taken into account:
-
one voltage source for all installations; - the correct scale is needed for a good view of the calculated points;
- the minimum and maximum short-circuit and protective properties are noted.
The lack of a well-constructed card leads to power outages. A visual diagram allows you to see the consistency of settings and compare the operation of the machines. The circuit itself consists of two axes:
- the abscissa is the current value in kV;
- the ordinate axis is the time in seconds.
Do not neglect its manufacture, since the lack of accuracy in the calculations will lead to incorrect operation of the protective system. The map is easy to draw in a special program.
