By selectivity is understood the debugged mechanism of operation of electrical circuit protection devices . As a result of the action of fuses or circuit breakers, the wiring is prevented from burning and the connected loads fail during short circuits and exceeding the ratings in individual sections when the rest of the circuit continues to work.
The scheme of the machines
The idea of what selectivity can be obtained by considering the operation scheme of a home electrical switchboard.
In the event of a short circuit in the kitchen or in another room, only that protective device that applies to this circuit should work. The input machine will not turn off and will conduct electricity to the remaining sections. If for some reason the switch for the kitchen did not work, then the malfunction will be controlled by the input circuit breaker by turning off the power in all electrical circuits.
Classification
What is the selectivity of machines, can be represented in the form of their selections and wiring diagrams.
- Complete. When several devices are connected in series, one that is closer to the emergency zone reacts to overcurrents.
- Partial Protection is similar to full, but is valid only up to a certain amount of overcurrent.
- Temporary. When series-connected devices with the same current characteristics have a different time delay for operation with its sequential increase from the area with the malfunction to the power source. The time selectivity of the machines is used to secure each other in terms of shutdown speed. For example: the first works after 0.1 seconds, the second - after 0.5 seconds, the third - after 1 second.
- Current. Selectivity is similar to time, only the parameter is the maximum current cutoff. The devices are selected in the direction of decreasing the setpoint from the power source to the load objects (for example, 25 A at the input and further, 16 A to sockets and 10 A to lighting).
- Vremyatokovaya. The machines have a reaction to the current, as well as time. Automata are divided into groups A, B, C, D. It is difficult to organize temporary selectivity during short circuit (short circuit) on them, since the characteristics of the devices are superimposed on each other. The maximum protective effect is achieved in group A, which is mainly used for electronic circuits. Type C devices are most common, but mindlessly and wherever it is not recommended to install them. Group D is used for drive systems with high inrush currents.
- Zone. The operation of the mains is monitored by measuring devices. When the setpoint threshold (set limit value) is reached, the data is transmitted to the monitoring center, where the machine for shutdown is selected. The method is used in industry, because it is complex, expensive and requires separate power sources. Electronic trip units are used here: when a malfunction is detected, the downstream machine sends a signal to the upstream one and it starts to count down a time interval of about 50 ms. If the switch located below does not work during this time, the one located upstream is turned on.
- Energy. Automata have high speed, due to which the short-circuit current does not have time to reach a maximum.
Types of selectivity
The selectivity of protection is divided into absolute or relative, depending on which areas are disabled. For the first case, fuses in the damaged section of the circuit are most reliably triggered. In the second, the above located machines are switched off if the protection below did not work for various reasons.
Selectivity tables
Selective protection works mainly when exceeding the rating I n of the circuit breaker, i.e., with small overloads. With short circuits, it is much more difficult to achieve. To do this, manufacturers sell products with selectivity tables, with which you can create links with selectivity of operation. Here you can select groups of devices from only one manufacturer. 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 the intersection of the row and column, where "T" is the total selectivity and the number is partial (if the short-circuit current is less than the value indicated in the table).
Calculation of the selectivity of automata
Protective devices are mainly conventional circuit breakers, the selectivity of which must be ensured by proper selection and settings. Their selective action for protection installed closer to the power source is ensured by the following conditions.
- I s.o. last ≥ K n.o. ∙ I candidate where:
- I sec. Last - current at which the protection is triggered;
- I candidate - short circuit current at the end of the protection zone located at a greater distance from the power source;
- K n.o. - reliability coefficient, depending on the spread of parameters.
What is the selectivity in the regulation of time machines, it can be seen from the ratio below.
- t s.o. last ≥ t + ∆t, where:
- t So. last and t to. - the time intervals at which the cutoffs of the machines located adjacent and at a distance from the power source are triggered;
- ∆t is the temporal selectivity selected from the catalog.
Selectivity graphic
For reliable current protection of electrical wiring, a selectivity map is required. It is a diagram of the time-current characteristics of devices installed alternately in a circuit. The scale is chosen so that the protective properties of the devices are visible at the boundary points. In practice, selectivity cards are not predominantly used in projects, which is a big drawback and leads to power outages among users.
The ratio of the ratings should be at least 2.5 to ensure selectivity. But even they have common triggering zones, albeit small ones. Only with a ratio of 3.2 their intersection is not observed. But in this case, one of the ratings may turn out to be overestimated and you will have to install a larger cross-section wiring after the machine.
In most cases, selectivity of protection is not required. It is needed only where serious consequences may arise.
If, in the calculation, overestimated values of the ratings of the machines are obtained, on the input, knife switches or load switches are installed.
You can also use special selective machines.
Selective machines S750DR
ABB Company manufactures S750DR brand products, where the selectivity of the circuit breakers is ensured by an additional current path that does not disconnect after the main contact is activated during a short circuit.
When the downstream emergency section is disconnected by a selective bimetallic contact, a delay in the response time is created. In this case, the main contact of the selective switch returns to its place under the action of a spring. If the overcurrent continues to flow, after 20-200 ms the thermal protection in the main and additional circuits is switched off. At the same time, a selective bimetallic plate blocks the release mechanism, and the spring will no longer be able to close the main contact again.
The current limit of the machine is ensured 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 machines. They are not difficult to pick up in order to provide selectivity of operation on overloads. Difficulties appear at high short circuit currents. For this, several methods are used, as well as special automatic machines of the ABB company, which create a delay in response to time.