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Typical tripping curves by circuit breakers


    A coil, placed on each conductor, continuously measures the current in each of them

    This information is processed by an electronic module which controls the tripping of the circuit breaker when the values of the settings are exceeded

    The curve of the release shows three operating zones

    • “Instantaneous” operating zone

    This provides protection against high intensity short circuits It is either set by construction at a fixed value (5 to 20 kA), or adjustable according to the device

    • “Short delay” operating zone

    This provides protection against lower intensity short circuits, which generally occur at the end of the line The trip threshold is generally adjustable The period of the delay may be increased by thresholds up to one second, to ensure discrimination with devices placed downstream

    • “Long delay” operating zone

    This is similar to the characteristic of a thermal release It protects conductors against overloads The electronic releases available on DMX and DPX improve the discrimination and make the devices able to communicate


    Electric arc

    The current is broken in the circuit breaker arc chamber, which is designed to control the electric arc produced when the contacts open (rather like electrodes). The arc energy can become considerable, up to 100 kilojoules and 20 000°C, and can cause the contacts to be eroded by vaporisation of the metal.


    It is therefore a good idea to extinguish the arc as quickly as possible, in order to limit its effects. The magnetic field produced by the arc (which is a conductor) is used to move it into an “arc chamber”

    and extend it until it has been extinguished.

    The circuit breaker mechanisms must combine very fast opening of the contacts (limiting erosion) and high contact pressure (opposition to the electrodynamic forces).


    Miniature Circuit-breakers

    (MCBs) to BS EN 60898 are suitable for operation by ordinary persons and have fixed protection settings, generally a two position on/off operating handle and a performance relative to the final circuits in an electrical installation. They would normally be the final overcurrent protection measure in the electrical system, for example before sockets or lighting circuits.

    Typical current ratings are from 0.5 A to 125 A. Short-circuit ratings may be up to 25 kA. Performance and testing is in accordance with BS EN 60898 for domestic and similar applications categorised by the trip characteristic types B, C & D. MCBs may also be available with application specific tripping characteristics.

    MCBs may also be rated in accordance with BS EN 60947-2 for industrial or similar applications


    Moulded Case Circuit-breakers

    (MCCBs) may have fixed or adjustable protection settings, normally a three position toggle operating handle giving on-off-tripped indication plus reset function, and a performance level relative to the incoming supply such that they can be installed at a point close to the supply transformer.

    Typical current ratings are from 16 A to 1600 A though ratings up to 3,200 A are available. Short-circuit ratings may be up to 100 kA. Performance and testing is in accordance with BS EN 60947-2.


    Air Circuit-breakers

    (ACBs) are normally used as the main incoming protection and have a spring-operated mechanism to open and close the device often charged by an internal motor. The protection settings will include time delays and the devices will have a short-time withstand value to give full discrimination under fault conditions with downstream protection devices.

    Typical current ratings are 630 to 6,300 A. Short-circuit ratings may be up to 150 kA. Performance and testing is in accordance with BS EN 60947-2.



    • Legrand
    • BEAMA

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