Purpose of Miniature Circuit Breakers (MCBs)

Principle of operation and design

Miniature Circuit Breakers (MCBs) are primarily designed to protect cables and lines against overload ( thermal ) and short circuits ( electromagnetic ). They, therefore, take care to protect this electrical equipment against excessive temperature rises and destruction in the event of a short circuit.

Miniature circuit breakers are used in household distribution networks and industrial applications.

They meet the requirements of different applications with different designs and using a full range of accessories ( e.g. auxiliary and signaling contacts, etc. ).

The structural form of all line protection switches is similar. Some dimensions are defined by installation standards (sometimes national). The major differences are in the widths ( eg 12.5 and 17.5 mm ) or depths ( eg 68 and 92.5 mm ).

The breaking capacity is one of the factors that determine the size.

Standards, tripping characteristics, and nominal switching capacity

MCBs are subject to international and national standards. Design and test requirements are defined in IEC 60898.

For different applications, three travel characteristics B, C, and D are defined in IEC 60898 

The tripping characteristics B, C, and Dunder IEC 60898 are distinguished by the tripping level of the short-circuit release

  • Travel characteristic B is the standard characteristic of wall outlet circuits in domestic and utility buildings (I> ≥3… 5 * I e )
  • Travel characteristic C is advantageous when using electrical equipment with higher inrush currents, such as lamps and motors (I> ≥5… 10 * I e )
  • Travel characteristics d is suitable for electrical equipment capable of generating strong overcurrents such as transformers, electromagnetic valves, or capacitors (I> ≥10… 20 * I e )

Miniature AC circuit breakers are normally suitable for single-phase applications and three-phase supplies up to a rated voltage of 240 / 415V and MCB AC-DC for direct voltage supplies up to rated voltages of 1 25 V, 220 V, or 440 V as a function of the number of poles.

 Circuit Breakers
Circuit Breaker

In addition to the release quality according to addition to the tripping characteristic, a key characteristic of MCBs is their nominal switching capacity. They are assigned to switching capacity classes, indicating the maximum size of the short-circuit current that can be handled.

Standard values ​​under IEC 60898 are 1500, 3000, 4500, 6000, 10000, 20000 and 25000 A.

When choosing an MCB to protect cables and conductors, the permitted values 2 · T values ​​for the conductors must be observed. They should not be exceeded when resolving a short circuit.

Therefore, the 2 · T values ​​relating to the prospective short-circuit current is an important characteristic of MCBs.

In some countries, miniature circuit breakers are classified according to the permitted values 2 · T values. According to the ” Technical Connection Conditions,” ( LANGUETTE ) of the German utilities ( EVU ) For example, only MCBs with a nominal switching capacity of at least 6000 A and the energy limitation class 3 can be used for reasons selectivity in distribution panels in domestic and utility buildings behind the meter.

For industrial applications, a switching capacity of 10,000 A ( 10 kA ) is generally required.

Installation of miniature circuit breakers, safety clearances

MCBs as components of installation systems are usually designed in such a way that the safety distance requirements are met when their organization conforms to the structure of the system.

Circuit breakers can withstand very high currents at high voltages when breaking short circuits.

During the breaking process, contact systems and arcing chambers, therefore, convert large amounts of energy into thermal energy.

In addition to the high-temperature rises of components such as contacts, Deion plates, and walls of contact chambers, the energy converted into arcing results in heating of the air in the contact system to several thousand degrees Celsius and therefore to the formation of a conductive plasma. This plasma is generally emitted by discharge openings to the outside and must not reach any conductive part in order to avoid secondary short circuits.

For this season, safety distances are specified for circuit breakers, in which no conductive part – for example, metal walls or uninsulated conductors – can be located.

Often, additional insulation components ( bulkheads or phase covers; in some cases optional ) are used. With some products, additional insulation of the connected conductors is required according to the manufacturer’s specifications.

Failure to comply with safety authorizations can lead to accidents with the most serious consequences.

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