Under selectivity is meant a well-functioning mechanism of operation of electrical circuit protection devices. As a result of the action of fuses or circuit breakers, the burning of electrical wiring and the failure of the loads connected to it during short circuits and exceeding the ratings in certain sections are prevented, while the rest of the circuit continues to work.
Scheme of operation of machines
An idea of what selectivity is can be obtained by looking at the operation of a home electrical panel.
In the event of a short circuit in the kitchen or in another room, only the protective device that belongs to this circuit should work. The machine at the input will not turn off and will conduct electricity to the rest of the sections. If for some reason the switch for the kitchen did not work, then the malfunction will be controlled by the automatic input, turning off the power in allelectrical circuits.
Classification
What is the selectivity of automata can be represented in the form of their selections and connection diagrams.
- Full. When several devices are connected in series, the one located closer to the emergency zone reacts to overcurrents.
- Partial. Protection is similar to full protection, but it only works up to a certain amount of overcurrent.
- Temporary. When devices connected in series with the same current characteristics have a different time delay for operation with its sequential increase from the section with a malfunction to the power source. The time selectivity of the automata is used to secure each other in terms of shutdown speed. For example: the first one fires after 0.1 sec, the second one - after 0.5 sec, the third one - after 1 sec.
- Current. Selectivity is similar to time selectivity, only the maximum current cutoff is a parameter. The devices are selected in the direction of decreasing the setting from the power source to the load objects (for example, 25 A at the input and beyond, 16 A to sockets and 10 A to lighting).
- Time current. The machines provide a reaction to the current, as well as time. Automata are divided into groups A, B, C, D. It is difficult to organize time selectivity in case of short circuit (short circuit) on them, since the characteristics of the devices overlap each other. The maximum protective effect is achieved in group A, which is mainly used for electronic circuits. Type C devices are the most common, but it is not recommended to install them thoughtlessly and anywhere. Group D is used for drive systems with high starting currents.
- Zone. Measuring devices monitor the operation of the electrical network. When the setpoint threshold (set limit value) is reached, the data is transmitted to the control center, where the machine is selected for shutdown. The method is used in industry because it is complex, expensive and requires separate power supplies. Electronic releases are used here: when a malfunction is detected, the downstream machine gives a signal to the upstream one and it starts counting the time interval of about 50 ms. If the downstream switch fails to operate within this time, the upstream switch will switch on.
- Energy. The machines have high speed, due to which the short-circuit current does not have time to reach the maximum.
Types of selectivity
Protection selectivity is divided into absolute or relative, depending on which sections are disabled. For the first case, the fuses in the damaged section of the circuit are most reliable. In the second, the machines located above are disabled if the protection below has not worked for various reasons.
Selectivity tables
Selective protection works mainly when the rating Iof the circuit breaker is exceeded, i.e. with small overloads. With short circuits, it is much more difficult to achieve it. To do this, manufacturers sell products with selectivity tables, with which you can create links withselectivity of operation. Here you can select device groups 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, the intersection of the row and column is found, where "T" is full selectivity, and the number is partial (if the short-circuit current is less than the value indicated in the table).
Calculation of selectivity of automata
Protective devices are mainly conventional switches, the selectivity of which must be ensured by proper selection and settings. Their selective action for protection installed closer to the power supply is ensured by fulfilling the following condition.
-
Is.o.last ≧ Kn.o.∙ I to.prev., where:
- Is.o.last - current, at which the protection is triggered;
- I k.prev. - short-circuit current at the end of the protection zone located on the larger distance from the power source;
- Kn.o. - reliability factor depending on the spread of parameters.
What is selectivity in the regulation of automatic machines in time, can be seen from the ratio below.
ts.o.last ≧ tto.prev.+ ∆t, where:
- t s.o.last and tto.prev. - time intervals through which cutoffs of machines, respectively located nearby and at a distance from the power source;- ∆t - time stepselectivity, selected by catalogue.
Graphic representation of selectivity
For reliable current protection of electrical wiring, a selectivity card is needed. It is a diagram of the time-current characteristics of devices installed alternately in the circuit. The scale is chosen so that the protective properties of the devices can be seen from the boundary points. In practice, selectivity maps are mostly not used in projects, which is a big disadvantage and leads to power outages for users.
Ratio should be at least 2.5 to ensure selectivity. But even they have common trigger zones, albeit small ones. Only at a ratio of 3, 2, their intersection is not observed. But in this case, one of the ratings may turn out to be too high and you will have to install a larger section after the machine.
In most cases selectivity is not required. It is needed only where serious consequences can arise.
If the calculation results in overestimated values of the ratings of the machines, knife switches or load switches are installed at the input.
You can also use special selective machines.
S750DR selective automata
ABB produces products of the S750DR brand, where the selectivity of the circuit breakers is provided by an additional current path that does not disconnect after the main contact trips during a short circuit.
When the downstream fault section is switched off, a selective bimetal contact creates a delay in response time. 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 ms, the thermal protection in the main and additional circuits is turned off. In this case, the selective bimetallic plate blocks the release mechanism, and the spring will no longer be able to close the main contact back.
The current limit of the machine is provided by a 0.5 ohm selective resistor and a high resistance of the electric arc inside the machine.
Conclusion
What is selectivity is easy to understand when considering electrical circuits with serial connection of automata. They are easy to pick up to ensure the selectivity of operation for overloads. Difficulties arise at high short-circuit currents. For this, several methods are used, as well as special automatic machines from ABB, which create a time delay for operation.
