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Let-through energy

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Tripping of an installation circuit by circuit-breaker when there is a short-circuit requires a certain amount of time depending on the characteristics of the circuit-breaker and the entity of the short-circuit current.

During this period of time, some or all of the short-circuit current flows into the installation; the parameter I2t defines the specific let-through energy, ie. the specific energy that the breaker allows through when there is a short-circuit current Icc during the tripping time t.

In this way, we can determine the capacity of a circuit-breaker to limit, ie. break high currents up to the rated breaking power of the device, by reducing the peak value of the above-mentioned currents to a value which is considerably lower than the estimated current.

This can be achieved using mechanisms which open very rapidly and have the following advantages:

  • they limit the thermal and dynamic effects both on the circuit-breaker and on the protected circuit;
  • they reduce the dimensions of the current-limiting circuit-breaker without reducing breaking capacity;
  • they considerably reduce ionized gases and sparklers emitted during the short-circuit and therefore they avoid the danger of ignition and fires.

Irms = perspective symmetrical short-circuit current