Manufacturers warn against installing circuit- breakers of one manufacturer as replacements for cbs of another manufacturer, without the necessary verification of performance. Two requirements that ensure reliability of circuit- breakers are the ultimate breaking capacity (I cu) and service breaking capacity (I cs).

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault. Circuit-breakers to BS EN 60947-2 are intended for operation by skilled users and they must be maintained. They are suitable for Pollution Level 3, which includes unheated rooms, boiler rooms, industrial and farming areas. Typical domestic panel circuit breakers are rated to interrupt 6 kA ( 6000A) short-circuit current.

The latest articles and guides from NBS subject specialists and construction industry contributors. What is an MCB and how does it work?". Fusebox Shop. 16 September 2016. Archived from the original on 11 July 2018 . Retrieved 11 July 2018.

Types [ edit ] Front panel of a 1250 A air circuit breaker manufactured by ABB. This low-voltage power circuit breaker can be withdrawn from its housing for servicing. Trip characteristics are configurable via DIP switches on the front panel. Miniature circuit breaker (MCB)—rated current up to 125 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category. Stotz miniature circuit breaker and domestic appliances", ABB, 2006-01-09, accessed 4 July 2011". Archived from the original on 2013-10-29 . Retrieved 2011-07-04. Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panel board; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers.Assemblies such as distribution boards are validated with specific circuit-breakers installed and these cbs are usually from the same manufacturer as the distribution board. Where cbs made by a different manufacturer are to be installed, verification will have to be undertaken by the manufacturer of the distribution board to BS EN 61439-2 or BS EN 61439-3. Fitting unverified devices will invalidate any verification and the warranty. Weedy, B. M. (1972). Electric Power Systems (Seconded.). London: John Wiley and Sons. pp. 428–430. ISBN 0-471-92445-8. Interactive training courses and educational material, to help you get the most from NBS software tools

Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt. This latter figure is the ampere interrupting capacity ( AIC) of the breaker. The characteristics of low-voltage circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear. A cb designed for Pollution Level 2 conditions would not be suitable for harsh outdoor or humid applications that require Pollution Level 3. Circuit-breakers to BS EN 60898-1 may look identical to those complying with BS EN 60947-2 but they are not necessarily inter-changeable.The circuit breaker must first detect a fault condition. In small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating or magnetic effects of electric current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition and to operate the opening mechanism. These typically require a separate power source, such as a battery, although some high-voltage circuit breakers are self-contained with current transformers, protective relays, and an internal control power source. Recloser — A type of circuit breaker that closes automatically after a delay. These are used on overhead electric power distribution systems, to prevent short duration faults from causing sustained outages.

Residual-current device (RCD), or residual-current circuit breaker (RCCB) — detects current imbalance, but does not provide over-current protection. In the United States and Canada, these are called ground fault circuit interrupters (GFCI). Due to environmental and cost concerns over insulating oil spills, most new breakers use SF 6 gas to quench the arc. Any installer who plans to substitute a different cb must obtain authority from the assembly manufacturer to do so in compliance with Regulation 536.4.203. Otherwise, responsibility for the testing and integrity of the compatibility between devices would be down to the contractor. When this is not carried out, there is a probability that, in the event of an accident, fire or other damage, the installer would be accountable under Health and Safety Legislation. In the US, where split phase supplies are common, in branch circuits with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two-pole common trip breakers are common on 120/240-volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

Secure PDF Files

Many classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features. I cs is expressed as a percentage ratio of I cu and gives the maximum short-circuit current a cb can break three times and still function in normal service. Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.