Circuit breakers
Circuit breaker excerpt from Chapter H - LV switchgear
This is an excerpt from the LV switchgear: functions & selection chapter in The Electrical Installation Guide. You can download this excerpt which looks at circuit breakers by simply downloading the LV switchgear: functions and selection chapter by clicking here.
Circuit breakers
The circuit-breaker/disconnector fulfills all of the basic switchgear functions, while, by means of accessories, numerous other possibilities exist.
As shown in Figure H23 the circuit-breaker/ disconnector is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation.
Moreover, it can, by means of auxiliary units, provide a wide range of other functions, for example: indication (on-off - tripped on fault); undervoltage tripping; remote control… etc. These features make a circuit-breaker/ disconnector the basic unit of switchgear for any electrical installation.
Standards and description
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.
Standards
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
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
Domestic circuit-breakers (see Fig. H25 next page) complying with IEC 60898 and similar national standards perform the basic functions of:
- Isolation
- Protection against overcurrent
Some models can be adapted to provide sensitive detection (30 mA) of earthleakage 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.
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).
Moulded-case type industrial circuit-breakers complying with IEC 60947-2 are now available, which, by means of associated adaptable blocks provide a similar range of auxiliary functions to those described above (see Figure H28).
Heavy-duty industrial circuit-breakers of large current ratings, complying with IEC 60947-2, have numerous built-in communication and electronic functions (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.
