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
• 60947-8: Part 8: Control units for built-in thermal protection (PTC) for rotating electrical machines.

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
  

Fig. H24: Main parts of a circuit-breaker

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

• Isolation
• Protection against overcurrent

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

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.

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

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).

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

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).

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

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.

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