DSRF197K01 ABB distributed control system industrial DCS module card

Description

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Limitation of specific let-through energy
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.