Standards and description of circuit-breakers

Standards

Industrial circuit-breakers must comply with IEC 60947-1 and 60947-2 or other equivalent standards. Domestic-type circuit-breakers must comply with IEC standard 60898, or an equivalent national standard

For industrial LV installations the relevant IEC standards are, or are due to be:

• 60947-1: general rules
• 60947-2: part 2: circuit-breakers
• 60947-3: part 3: switches, disconnectors, switch-disconnectors and fuse combination units
• 60947-4: part 4: contactors and motor starters
• 60947-5: part 5: control-circuit devices and switching elements
• 60947-6: part 6: multiple function switching devices
• 60947-7: part 7: ancillary equipment

For domestic and similar LV installations, the appropriate standard is IEC 60898, or an equivalent national standard.

Description

Figure H24 shows schematically the main parts of a LV circuit-breaker and its four essential functions:

• The circuit-breaking components, comprising the fixed and moving contacts and the arc-dividing chamber
• The latching mechanism which becomes unlatched by the tripping device on detection of abnormal current conditions

This mechanism is also linked to the operation handle of the breaker.

• A trip-mechanism actuating device:

  - Either: a thermal-magnetic device, in which a thermally-operated bi-metal strip detects an overload condition, while an electromagnetic
    striker pin operates at current levels reached in short-circuit conditions, or 
  - An electronic relay operated from current transformers, one of which is installed on each phase

• A space allocated to the several types of terminal currently used for the main power circuit conductors
  

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Fig. H24: Main parts of a circuit-breaker

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Domestic circuit-breakers (see Fig. H25) complying with IEC 60898 and similar national standards perform the basic functions of:

• Isolation
• Protection against overcurrent
  

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Fig. H25: Domestic-type circuit-breaker providing overcurrent protection and circuit isolation features

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Some models can be adapted to provide sensitive detection (30 mA) of earth-leakage current with CB tripping, by the addition of a modular block, while other models (RCBOs, complying with IEC 61009 and CBRs complying with IEC 60947-2 Annex B) have this residual current feature incorporated as shown in Figure H26.

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Fig. H26: Domestic-type circuit-breaker as above (Fig. H25) with incorparated protection against electric shocks

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Apart from the above-mentioned functions further features can be associated with the basic circuit-breaker by means of additional modules, as shown in Figure H27; notably remote control and indication (on-off-fault).

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Fig. H27: “Multi 9” system of LV modular switchgear components

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Moulded-case circuit-breakers complying with IEC 60947-2 are available from 100 to 630 A and provide a similar range of auxiliary functions to those described above (see Figure H28).

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Fig. H28: Example of a Compact NSX industrial type of circuit-breaker capable of numerous auxiliary functions

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Air circuit-breakers of large current ratings, complying with IEC 60947-2, are generally used in the main switch board and provide protector for currents from 630 A to 6300 A, typically.(see Figure H29).
In addition to the protection functions, the Micrologic unit provides optimized functions such as measurement (including power quality functions), diagnosis, communication, control and monitoring.

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Fig. H29: Example of air circuit-breakers. Masterpact provides many control features in its “Micrologic” tripping unit

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