# The selectivity of the circuit breakers: it is, principle of organization, types

Selectivity or selectivity breakers - the key point to ensure reliable operation of the circuit. This feature helps to prevent accidents, rises to a higher level of security.

In the event of an overload line, short-circuit protection comes into operation only link with the damage, the rest of the power plant is operational. Why is this happening we will analyze in detail in this article, consider the basic problem of selective protection, wiring diagrams and their features.

Also be given to the calculation of selectivity and regulations create a map by providing the material with visual charts, tables and photos. And supplementing article detailed explanations in the videos.

The content of the article:

• The meaning and the basic tasks of selective protection
• Absolute and relative selectivity of protection
• Types of selective circuit connection
• View # 1 - full and partial protection
• View # 2 - current type selectivity
• View # 3 - time and time-current version
• View # 4 - energy selectivity machines
• View # 5 - zone protection scheme
• Calculating machines selectivity
• Map selectivity and rules of its creation
• Conclusions and useful videos on the topic

## The meaning and the basic tasks of selective protection

Safe operation and stable operation of electrical installations - these are the tasks that are assigned to the selective protection. It instantly calculates and cuts off the damaged area without stopping the power supply to the serviceable areas. Selectivity reduces the load on the installation, reduces the effects of short-circuit.

When debugged the circuit breakers to the maximum grants the request with regard to ensuring uninterrupted power supply and as a result of the process.

When automated equipment performing the opening, as a result of short-circuit is defective, due to the selectivity of consumers will have a normal diet.

Typically, stating that the amount of current passing through raspredvyklyuchateli set of introductory machine, less than indicated by the last current is the basis of the selective protection.

In sum, these ratings there may be more, but each individual is required to at least one step below the opening. For example, if you enter a set of 50-amp machine, then followed by a set switch, rated current of 40 A.

The circuit breaker consists of the following elements: the lever (1), terminal screws (2) and the movable contacts the fixed (3, 4), the bimetallic plate (5), the adjusting screw (6), a solenoid (7), arc lattice (8), a latch (9)

By means of a lever like include so on and off a current intake terminals. The terminals fed and fixed contacts. The movable contact spring serves for quick disconnection and connection circuit with it is formed through the stationary contact.

Decoupling, in the case of overlapping of its current threshold value occurs due to the heating and bending of the bimetallic plate, and also a solenoid.

Currents pickup is adapted by means of the adjusting screw. In order to prevent the occurrence of electric arc during the contact opening, introduced into the circuit element such as a grille interrupter. there is a latch to lock the machine body.

Selectivity, as a feature of relay protection - is the ability to detect the faulty node of the system and cut it from the operating part of the EPS.

Here is a board diagram shows how the load is distributed around the apartment. Before installing the machine need to perform a calculation of the total power of the equipment to be connected to it

The selectivity of the machines - it is their property to work alternately. If this principle is violated, it will be heated and circuit breakers, and wiring.

The result may be a fault on the line, the fuse contacts, isolation. All this will lead to failure of the electrical and fire.

Let's say on a long power line originated emergency. According to the first main selectivity rule triggered automatic closest to the fault.

If in an apartment in the socket there is a short circuit in the instrument should trigger line protection, of which this is the outlet. If it does not, the turn circuit breaker on the panel, and only for them - opening.

## Absolute and relative selectivity of protection

The concept of selectivity is determined GOSTotm IEC 60947-1-2014. There are two types of selectivity - absolute and relative. If the security work is coordinated in such a way that it operates entirely within the protected area, it is an indication of its absolute selectivity.

In these circumstances, the maximum current selectivity becomes the same as the maximum breaking capacity of the underlying machine.

Triggering a backup when the trip happened on the problem areas, referred to as a relatively selective protection. Thus there is arranged above the disconnection switches.

In case of exceeding a predetermined value the switch current machine, i.e. in the absence of large overload, selective protection operates almost perfectly. Much more difficult to do so at short circuits.

Data on manufactured products, the company is placed on the device and on their websites. It is important to read marking machines - switches bundle is formed only on the tables of a particular manufacturer. Note that the group arranged by the principle of relativity, have a large number of functions.

Simplify the task of selectivity tables that manufacturers are making their products. Applying them, create a group with a selectivity of tripping

The letter "T" in the table denotes the total selectivity couple devices, and the number - partial. When border expected short-circuit current value is less than the number shown in the table, selectivity is ensured

To test the selectivity between the machine upstream and downstream, are crossing vertically and horizontally. Selectivity - a very important task in the diet of consumers belonging to a particular category.

In its absence stop the production process may occur damage to lines, disabling air conditioning systems, and other smoke.

## Types of selective circuit connection

In addition to the absolute and relative selectivity there are 7 types of selective protection:

• band;
• time-current;
• energy;
• time;
• complete;
• partial;
• current.

To provide the desired selectivity in Auto power with circuit breakers use different methods. But in any case it is important to correctly set the switchFollowing the chosen scheme and installation rules.

### View # 1 - full and partial protection

Full protection means that if the series connected pair of machines, the appearance of an overcurrent causes tripping, located near fault zone.

Partial protection operates on the same principle as complete, but only after the current reaches the set threshold value.

Selectivity shutdown provides automatic (A and B), is that the fault, no matter what point in the installation it may arise, will cut off the nearest switch, located above this point. The remaining devices are not shut off

If the selectivity is provided to the lesser of the current two-AB, have occasion to speak of total selectivity between them. In this case, the limit value of the prospective short circuit current at any installation circumstances will equal or lesser amount of current two AB.

### View # 2 - current type selectivity

In the current selectivity of the main indicator - limiting current mark. From the object to the input values ​​build on the basis of the increase. The action of this selective protection is based on the same base as that of time-based discrimination.

Only difference is that the shutter is made of the current value - with the approach of fault point to the input, increasing evidence the fault current. Temporary off indicator can be the same.

Damaged due to short-circuit zone is determined by actuation setpoint for different current values. Full selectivity can only be in an environment where low fault current, and in the gap between the two guns have equipment, characterized considerable electric resistance. In this scenario, short-circuit currents will vary considerably.

Apply this kind of selectivity is mainly in the final switchboards. It combines the rated current and low value fault current with a high impedance connecting cables.

This option selectivity is economical, simple and lasts for a moment. Yet often the selectivity may be specified as a partial max current is usually small.

The photo current selectivity with AB. In this type of selectivity exists for the current axis offset current characteristics are arranged one behind the other automata

When the values ​​Isd1 and Isd2 same or very close, then Is - the maximum current is equal to the selectivity Isd2. If these values ​​are much different, Is = Isd1.

Condition of selectivity is to comply with current inequalities: Ir1 / Ir2> 2 and Isd1 / Isd2> 2. In this case, the maximum selectivity - Is = Isd1.

The drawbacks include the rapid increase in the level and setting of protection from a large level of current. It is impossible to quickly disable a damaged chain, if one of the machines fails.

When calculating the current protection settings should take into consideration the real currents passing through the switches operating in automatic mode.

### View # 3 - time and time-current version

When the circuit has a number of circuit breakers having identical current characteristics, but with different exposure times, when a fault occurs, they insure each other. The one that is in close proximity to the site of injury, will be released immediately, the next - after some time, etc.

On this two-level scheme "A" switch has such a holding time, which provides complete selectivity characteristics with AB "B"

In the case of time-current selectivity of protective devices respond not only to current but also on the duration of the reaction. At a certain value of the current after a time delay protection is activated, the distance from which to the fault place less. That the remaining settings are not disabled.

The photo schedule time selectivity using AB. Time-current characteristics of circuit breakers B and A do not intersect. They staggered

The combination of the current and time selectivity increases off efficiency. When Isc B

### View # 4 - energy selectivity machines

When energy selectivity tripping taking place within the machine body. The processing time is so small that the fault current does not have time to approach its limiting value.

Time-current protection system is considered to be difficult. Here it's not just a reaction to the current, but also the time during which this occurs.

With the current increase in the machine drops the response time. The basis for this kind of selectivity is to adjust the protection in such a way that the part protected object it works faster when all the current threshold, compared with a gun on entering.

### View # 5 - zone protection scheme

The zone method complex and expensive, so it is used mainly in industry. As soon as the threshold current performance peak in the control center receives data, and the chosen machine is activated. The electrical network with this type of selectivity includes special electronic releases.

When an infringement of the circuit breaker disposed below is supplied to the signal device that is above. The first machine should respond within seconds. If he does not react, the second is triggered.

Comparing this type of discrimination with the Provisional Election, you can see that the response time in this case is much lower - sometimes in the hundreds of milliseconds. Decreased as a percentage of the intervention in the system, and the percentage of damage. Reduced thermal and dynamic effects on parts of the plant. It increases the number of selectivity levels.

When currents flowing through the protective device reaches a greater value than its own setpoints, interlock signal transmitted by each switch to protection of higher level

In case of zone selectivity triggered protection on the part of the power supply, if we take as a starting point the short circuit. Until the response of the automaton is monitored to ensure the security device with the loaded side not submitted the same signal.

But such selectivity requires the presence of an additional power supply. Therefore, efficient use of this type of selectivity - system parameters with high short-circuit current and the current of considerable magnitude. These are the switching and distribution devices, which are from the loading of generators, transformers.

## Calculating machines selectivity

Literate machine selection and the correct setting - the basic principle of respect for the selectivity of circuit breakers. Selectivity for the switch, located near the source, making the requirement: ≥ Is.o.posled Kn.o. ∙ I k.pred.

Here Is.o afterbirth. - this current value, followed by the operation of the protection. I k.pred. - short-circuit current at the end point of an area which is subject to the machine, located far from the power source. Kn.o. - a safety factor. Its value is dependent on the scatter parameters.

Denomination automaton chain selected not only by the calculation, but also by such a table, guided on cable section in Scheme

Alignment ts.o.posled ≥ tk.pred. + Δt demonstrates the selectivity in the case of adjusting the AV time. ts.o.posled, tk.pred. - intervals actuation switch located at a long distance from the power source and arranged side by side. Δt - a parameter that is taken from the catalog and indicating a temporary degree of selectivity.

## Map selectivity and rules of its creation

Time-current characteristics of all devices included in the electric circuit network portrayed on the map selectivity. The purpose of it is made is to ensure maximum protection of machines. Framework for the protection switches - principle on which switches are connected each other strictly sequentially.

There are a number of rules required when creating a selectivity card:

1. Installation must have one source of supply.
2. All important design point must be clearly visible. In view of this requirement, it is necessary to choose the scale.
3. On the map indicate the protective properties, minimum, maximum short-circuit parameters in the points system.

Often, the design rules are violated, and no card of selectivity projects. This may lead to interruptions in the electricity supply to consumers.

Applied to map characteristics of machines connected in series to each other. Scheme itself constructed in axes

The card gives a complete picture on the harmonization of the setting. It provides the ability to compare the performance of machines for such a characteristic selectivity.

Time-current varieties axes are the basis not only for the construction of selectivity cards for overcurrent protection of circuit breakers, but also for its other forms: Fuse, relay. Usually one card contains characteristics 2-3 AB. The abscissa point current value in kW, and the ordinate axis - time in seconds.

## Conclusions and useful videos on the topic

Problems with circuit breakers and their solutions:

Drawing selectivity card through a special program:

Reliable, safe use of the electrical wiring is not possible without taking into account the selectivity of machines. Knowing about the main points of creating selective protection can competently perform the selection of equipment for your technical project.

You are professionally involved in electrical works and want to supplement the above material? Or noticed the discrepancy or error in this article? Or maybe you want to ask our experts? Please write your comments in the box below.

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