What is selectivity? Calculation of the selectivity of circuit breakers

By selectivity we mean a well-functioning mechanism for the operation of electrical circuit protection devices. As a result of the operation of fuses or circuit breakers, the combustion of the wiring and the failure of the loads connected to it are prevented in the event of short circuits and excess ratings in certain areas, when the rest of the circuit continues to operate.

Scheme of operation of automata

The idea of what selectivity is, you can get, if you consider the scheme of the work of a home electrical board.

In case of a short circuit in the kitchen or other room, only the protective device that belongs to this circuit should work. The automatic machine at the input will not be turned off and will conduct electricity to the rest of the sections. If for any reason the kitchen switch does not work, then the fault checks the input circuit breaker by turning off the power in all electrical circuits.

Classification

What is the selectivity of automata can be represented in the form of their collections and connection schemes.

1. Complete. When several devices are connected in series to overcurrent, the one that is closer to the emergency zone reacts.
2. Partial. Protection is similar to the full, but it only works up to a certain amount of overcurrent.
3. Temporary. When the series connected devices with the same current characteristics set a different time delay for triggering with its sequential increase from the area with a malfunction to the power source. The time selectivity of the machines is used to secure each other's shutdown speed. For example: the first is triggered after 0.1 second, the second after 0.5 seconds, the third after 1 second.
4. Current. Selectivity is analogous to time, only the parameter is the maximum current cutoff. The devices are selected in the direction of decreasing the set point from the power source to the objects of loading (for example, 25 A on the input and further, 16 A to the sockets and 10 A to the lighting).
5. Time-current. In machines there is a response to current, and also time. The automatons are divided into groups A, B, C, D. On them, it is difficult to organize time selectivity with short-circuit (short circuit), since the characteristics of the devices are superimposed on each other. The maximum protective effect is achieved in group A, which is used primarily for electronic circuits. The most common devices of type C, but thoughtlessly and where horrible to install them is not recommended. Group D is used for electric drive systems with large starting currents.
6. The zone. The operation of the power grid is monitored by measuring devices. When the setpoint threshold is reached (the set limit value), the data is transferred to the monitoring center, where an automatic switch-off device is selected. The method is used in industry because it is complex, expensive and requires separate power supplies. Electronic trip units are used here: when a fault is detected, the downstream machine sends a signal to the higher one and starts counting the time interval of about 50 msec. If the switch located down below does not work during this time, the one that is located higher up the chain is turned on.
7. Energy. Automatic machines have high speed, due to which the short-circuit current does not have time to reach the maximum.

Types of selectivity

The selectivity of protection is divided into absolute or relative, depending on which sections are disconnected. For the first case, the fuses on the damaged section of the circuit are most reliable. The second disables the above-located machines, if the protection below did not work for various reasons.

Selectivity tables

Selective protection works mainly when the circuit breaker I _{n is} exceeded, i.e. with small overloads. In case of short circuits, it is much more difficult to achieve. To this end, manufacturers sell products with selectivity tables, with which it is possible to create bundles with selectivity of operation. Here you can select groups of devices from only one manufacturer. The selectivity tables are presented below, they can also be found on the websites of enterprises.

To check the selectivity between the upstream and downstream devices, there is an intersection of the row and column, where "T" is the total selectivity, and the number is partial (if the fault current is less than the value indicated in the table).

Calculation of the selectivity of automata

Protective devices are mainly conventional switches, the selectivity of which must be ensured by the correct selection and settings. Their selective action for protection, established closer to the power source, is provided by fulfilling the following condition.

• I _s.o.last ≥ K _n.o. ∙ I _To.pr. , Where:
  - I _s.o.last - the current at which protection is triggered;
  - I _To.pr. - short-circuit current at the end of the protection zone located at a greater distance from the power source;
  - K _n.o. - coefficient of reliability, depending on the spread of parameters.

What is the selectivity in the regulation of automata in time, is seen from the relationship below.

• T _{soo later} ≥ t _to.pr. + Δt, where:
  - t _{s.o. result} and _t.c.pr. - time intervals through which cutoffs of automatic machines, respectively located close to and at a distance from the power source, are triggered;
  - Δt - time selectivity step, selected by catalog.

Graphical representation of selectivity

For a reliable current protection of the electrical wiring, a selectivity map is required. It is a diagram of the time characteristics of the apparatuses installed alternately in the circuit. The scale is chosen so that the protective properties of the apparatus can be seen at the boundary points. In practice, selectivity maps in projects are mostly not used, which is a big drawback and leads to power outages among users.

The nominal ratio should be at least 2.5 to ensure selectivity. But even they have common trigger zones, albeit small ones. Only when the ratio 3.2 is not observed their intersection. But in this case one of the values may turn out to be overestimated and you will have to install a larger cross-section after the machine.

In most cases, selectivity of protection is not required. It is needed only where serious consequences can arise.

If the calculation results in overestimated values of the nominal values of the machines, input switches or load switches are installed at the input.

You can also use special selective automata.

Selective automatic devices S750DR

ABB manufactures products of the S750DR brand, where the selectivity of the switches is provided by an additional current path that does not disconnect after the main contact has tripped during a short circuit.

If the lower emergency area is disconnected, a delayed response time is generated by the selective bimetallic contact. In this case, the main contact of the selective switch is returned to its place by the action of the spring. If the overcurrent continues to flow, after 20-200 msec the thermal protection in the main and auxiliary circuits is disconnected. In this case, the selective bimetallic plate blocks the disengaging mechanism, and the spring can no longer close the main contact back.

The current limit of the machine is provided by a selective resistor of 0.5 ohms and a large resistance of the electric arc inside the device.

Conclusion

What is selectivity is easy to understand when considering electrical circuits with serial connection of automata. It is not difficult to select them to ensure the selectivity of triggering overloads. Complexities appear at high short-circuit currents. For this, several methods are used, as well as ABB's special automatic machines, which create a time delay for the operation.