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Selectivity

Switching devices connected in series, e.g. molded case circuit breakers and fuses, work in a coordinated manner to ensure that switching devices are tripped successively. The closest, upstream switching device before the location of the short-circuit must trip. The other switching devices on the same current run do not trip. The purpose of selectivity is to minimize the effects of a fault in terms of its duration and the area affected by the fault.

Selectivity is achieved when the circuit breakers are matched to each other by means of selection, configuring and trip settings in such a way that, in the event of a fault, only the breaker closest to the location of the fault trips.

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Short-circuit location

Selective behavior

The selective behavior of molded case circuit breakers is mainly influenced by the following factors:

  • Tripping value settings of the trip unit

  • Tripping and break times

  • Let-through current values

  • Modes of switching of the relevant circuit breakers

    • Zero-current interrupter

    • Current limiter

The selective behavior of circuit breakers can be implemented technically by a variety of selectivity concepts:

  • Current selectivity

    The selectivity can be calculated in the overload range by comparing the time/current characteristics. In the short-circuit range, this comparison leads to values that are too low. The reason for this is that the trip unit behaves differently in the case of short-circuit currents compared to its long-term behavior, e.g. in the case of overload.

    If the short-circuit currents differ sufficiently at the installation points of two molded case circuit breakers, the instantaneous short-circuit releases can normally be set such that if a short-circuit occurs behind the downstream circuit breaker, only this downstream breaker trips.

    If the short-circuit currents are approximately the same at the installation points of the molded case circuit breakers, the grading of the tripping currents of the short-circuit releases only enables selectivity up to a specific short-circuit current .

    This current is referred to as the ultimate selectivity value Is.

  • Time selectivity

    Selectivity can be achieved by time selectivity up to the threshold values of the instantaneous short-circuit releases. To achieve this, the upstream circuit breaker requires delayed short-circuit releases, so that in the event of a fault, only the downstream circuit breaker will disconnect the part of the electrical installation affected by the fault from the supply.

    Both the tripping delays and the tripping currents of the short-circuit releases are graded.

  • Zone-selective interlocking - ZSI

    Selective behavior is achieved by installing parallel control cables between the molded case circuit breakers. The electronic trip units then use a fast signal link to determine priorities in the tripping sequence.

    Zone Selective Interlocking (ZSI) has been developed by SIEMENS in order to prevent unacceptably long tripping times when several molded case circuit breakers are connected in series.

    ZSI enables the tripping delay to be reduced to 50 ms for the circuit breaker upstream from the location of the short circuit.

  • Dynamic selectivity

    This method is based on evaluation of the arc power which is generated in the arc chute when the contacts open dynamically in response to a short circuit. During this process, a smaller sized downstream molded case circuit breaker converts more energy in the arc chute than the larger, upstream molded case circuit breaker. A selective trip unit evaluates the energy conversion in both molded case circuit breakers. The downstream molded case circuit breaker trips, while the contacts of the upstream circuit breaker close again. Since both molded case circuit breakers perform a current limiting function, the residual current limit imposed in practice is higher than the limiting action specified for the individual molded case circuit breakers.

Full selectivity

There is an increasing demand for full selectivity in order to safeguard continuity of service by power distribution systems. A power system is said to be fully selective if only the protective device located upstream of the fault location when viewed in the direction of energy flow, i.e. from the infeed to the load, trips in the event of a fault.

Full selectivity always refers to the short-circuit current occurring at the installation point.

Partial selectivity

A system is said to be partially selective when selective tripping in response to a system fault is not ensured up to the maximum ultimate short-circuit breaking capacity Icu of the switching devices. Selectivity is then ensured only up to a certain Is current value (ultimate selectivity value). If the calculated prospective short-circuit current at the location of installation of the downstream protective device is lower than the ultimate selectivity value specified for the switching devices, then it is still possible to describe the system as fully selective.

If the values determined by the short-circuit current calculation (e.g. according to IEC/EN 60909, DIN VDE 0102) at the installation point of the downstream circuit breaker lie below the ultimate selectivity value listed in the respective table for the selected combination, selectivity is assured for all possible short circuits at the installation point.

If the calculated short-circuit current at the installation point is higher than the ultimate selectivity value, selective tripping by the downstream circuit breaker is only assured up to the value listed in the table. A judgment must be made as to whether the value can be considered to be sufficient because the probability of the maximum short-circuit occurring is low, for example. Otherwise, a circuit breaker combination should be chosen whose selectivity limit lies above the maximum short-circuit current.

Selectivity with 3VA2 molded case circuit breakers

Series 3VA2 circuit breakers are designed to deliver excellent selective tripping combined with optimum current limiting and outstanding breaking capacity.

3VA2 molded case circuit breakers have been specifically designed to meet the following requirements:

  • System-wide, high selectivity with a rated operational current differential of 1 : 2.5 up to the miniature circuit breaker

  • Selectivity in combination with high current limiting and high breaking capacity

  • Cost-effective design / configuring of selective power distribution systems

These molded case circuit breaker requirements are achieved in engineering terms as follows:

  • Rotary, double-break contact system for highly dynamic opening response

  • Coordinated electronic trip units

  • Dynamic selectivity

Depending on use of molded case circuit breakers with a rated operational current differential in a ratio of at least 1 : 2.5 and selection of suitable breaking capacity classes, you can achieve selective tripping of the area of the installation directly affected by the fault up to the maximum ultimate short-circuit breaking capacity.

You can find information on selectivity values for 3VA2 molded case circuit breakers on the Internet under the link for 3VA documentation.

Electronic trip units and fast trip units

As a protective device, the molded case circuit breaker is required to clear electrical faults in the system. For this purpose, series 3VA2 circuit breakers are equipped with intelligent electronic trip units which can be combined with metering functions.

The tripping characteristic of the electronic trip units can be finely and flexibly adjusted. In the event of short circuits, a fast trip unit also responds according to the arc power from the arc chute. This selective trip unit ensures that major short circuits are cleared more quickly, while at the same time ensuring that medium short circuits are interrupted selectively.