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Standard IEC 60947-2

Standard IEC 60947-2 | Circuit Breakers

This standard applies to circuit-breakers, the main contacts of which are intended to be connected to circuits, the rated voltage of which does not exceed 1000 VAC or 1500 VDC.; it also contains additional requirements for integrally fused circuit-breakers.

It applies whatever the rated currents, the method of construction or the proposed applications of the circuit-breakers may be.

Changes in dependability needs and technologies have led to a marked increase in standard requirements for industrial circuit-breakers.

Conformity with standard IEC 947-2, renamed IEC 60947-2 in 1997, can be considered as an ‘all-risk’ insurance for use of circuit-breakers. This standard has been approved by all countries.

The principles

Standard IEC 60947-2 is part of a series of standards defining the specifications for LV electrical switchgear:

  • the general rules IEC 60947-1, that group the definitions, specifications and tests common to all LV industrial switchgear.
  • the product standards IEC 60947-2 to 7, that deal with specifications and tests specific to the product concerned. Standard IEC 60947-2 applies to circuit-breakers and their associated trip units. Circuit-breaker operating data depend on the trip units or relays that control their opening in specific conditions.

This standard defines the main data of industrial circuit-breakers:

  • their classification: utilisation category, suitability for isolation, etc.
  • the electrical setting data
  • the information useful for operation
  • the design measures
  • coordination of protection devices

The standard also draws up series of conformity tests to be undergone by the circuitbreakers. These tests, which are very complete, are very close to real operating conditions. Conformity of these tests with standard IEC 60947-2 is verified by accredited laboratories.

Table of main data (appendix K IEC 60947-2):

Table of main data (appendix K IEC 60947-2)

Circuit-breaker category

Category IEC 60947-2 defines two circuit-breaker categories:

  • category A circuit-breakers, for which no tripping delay is provided. This is normally the case of moulded case circuit-breakers. These circuit-breakers can provide current discrimination.
  • category B circuit-breakers, for which, in order to provide time discrimination, tripping can be delayed (up to 1 s) for all short-circuits of value less than the current Icw.

This is normally the case of power or moulded case circuit-breakers with high ratings. For circuit-breakers installed in the MSBs, it is important to have an lcw equal to lcu in order to naturally provide discrimination up to full ultimate breaking capacity Icu.

Reminders of standard-related electrical data

The setting data are given by the tripping curves. These curves contain some areas limited by the following currents.

The setting data are given by the tripping curves.

  • Rated operational current (In)
    In (in A rms) = maximum uninterrupted current withstood at a given ambient temperature without abnormal temperature rise.
    E.g. 125 A at 40 °C
  • Adjustable overload setting current (lr)
    Ir (in A rms) is a function of ln. lr characterises overload protection. For operation in overload, the conventional non-tripping currents lnd and tripping currents ld are:

    • Ind = 1.05 Ir
    • Id = 1.30 Ir

    Id is given for a conventional tripping time. For a current greater than ld, tripping by thermal effect will take place according to an inverse time curve. Ir is known as Long Time Protection (LTP).

  • Short time tripping setting current (Isd)
    Isd
    (in kA rms) is a function of Ir. lsd characterises short-circuit protection. The circuit breaker opens according to the short time tripping curve:

    • either with a time delay tsd,
    • or with constant I2t,
    • or instantaneously (similar to instantaneous protection).

    Isd is known as Short Time Protection or lm.

  • Instantaneous tripping setting current (Ii)
    Ii (in kA) is given as a function of ln. It characterises the instantaneous short-circuit protection for all circuit-breaker categories. For high overcurrents (short-circuits) greater than the li threshold, the circuit-breaker must immediately break the fault current.
    .
    This protection device can be disabled according to the technology and type of circuit-breaker (particularly B category circuit-breakers).

Rated short time withstand current

Table for calculation of asymmetrical short-circuits (IEC 60947.2 para. 4.3.5.3.)

Table for calculation of asymmetrical short-circuits

  • Rated short-circuit making capacity(*) (Icm)
    Icm (peak kA) is the maximum value of the asymmetrical short-circuit current that the circuit-breaker can make and break. For a circuit-breaker, the stress to be managed is greatest on closing on a short-circuit.
  • Rated ultimate breaking capacity(*) (Icu)
    Icu (kA rms) is the maximum short-circuit current value that the circuit-breaker can break. It is verified according to a sequence of standardised tests. After this sequence, the circuit-breaker must not be dangerous. This characteristic is defined for a specific voltage rating Ue.
  • Rated service breaking capacity(*) (Ics)
    Ics (kA rms) is given by the manufacturer and is expressed as a % of Icu. This performance is very important as it gives the ability of a circuit-breaker to provide totally normal operation once it has broken this short-circuit current three times. The higher Ics, the more effective the circuit-breaker.
  • Rated short time withstand current(*) (Icw)
    Defined for B category circuit-breakers
    Icw (kA rms) is the maximum short-circuit current that the circuit-breaker can withstand for a short period of time (0.05 to 1 s) without its properties being affected. This performance is verified during the standardised test sequence.
    .
    (*) These data are defined for a specific voltage rating Ue.
Circuit-breaker coordination

The term coordination concerns the behaviour of two devices placed in series in electrical power distribution in the presence of a short-circuit.

Cascading and discrimination

  • Cascading or back-up protection
    This consists of installing an upstream circuit-breaker D1 to help a downstream circuit-breaker D2 to break short-circuit currents greater than its ultimate breaking capacity IcuD2. This value is marked IcuD2+D1.
    IEC 60947-2 recognises cascading between two circuit-breakers. For critical points, where tripping curves overlap, cascading must be verified by tests.
  • Discrimination
    This consists of providing coordination between the operating characteristics of circuit-breakers placed in series so that should a downstream fault occur, only the circuit-breaker placed immediately upstream of the fault will trip.
    IEC 60947-2 defines a current value ls known as the discrimination limit such that:

    • if the fault current is less than this value ls, only the downstream circuit-breaker D2 trips,
    • if the fault current is greater than this value ls, both circuit-breakers D1 and D2 trip.

    Just as for cascading, discrimination must be verified by tests for critical points.

Discrimination and cascading can only be guaranteed by the manufacturer who will record his tests in tables.

IEC 60947-2 Summary

Standard IEC 60947.2 specifies the main data of Industrial Circuit-Breakers:

  • the utilisation category
  • the setting data
  • the design measures
  • etc.

It draws up a series of very complete tests representative of circuit-breaker real operating conditions.

SOURCE: Schneider Electric

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What is cascading?

Cascading is the use of the current limiting capacity of circuit breakers at a given point to permit installation of lower-rated and therefore lower-cost circuit breakers downstream. The upstream circuit breakers acts as a barrier against short-circuit currents. In this way, downstream circuit breakers with lower breaking capacities than the prospective short-circuit (at their point of installation) operate under their normal breaking conditions. Since the current is limited throughout the circuit controlled by the limiting circuit breaker, ascading applies to all switchgear downstream. It is not restricted to two consecutive devices.

General use of cascadingcom

With cascading, the devices can be installed in different switchboards. Thus, in general, cascading refers to any combination of circuit breakers where a circuit breaker with a breaking capacity less than the prospective Isc at its point of installation can be used. Of course, the breaking capacity of the upstream circuit breaker must be greater than or equal to the prospective short-circuit current at its point of installation.
The combination of two circuit breakers in cascading configuration is covered by the following standards:

  • IEC 60947-2 (construction)
  • NF C 15-100, § 434.3.1 (installation)
Coordination between circuit breakers

The use of a protective device possessing a breaking capacity less than the prospective short-circuit current at its installation point is permitted as long as another device is installed upstream with at least the necessary breaking capacity. In this case, the characteristics of the two devices must be coordinated in such a way that the energy let through by the upstream device is not more than that which can be withstood by the downstream device and the cables protected by these devices without damage.
Cascading can only be checked by laboratory tests and the possible combinations can be specified only by the circuit breaker manufacturer.

Cascading and protection discrimination

In cascading configurations, due to the Roto-active breaking technique, discrimination is maintained and in some cases, even enhanced.

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