GRBTU 3BSE013175R1 ABB distributed control system industrial DCS module card

Description

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Selectivity is normally actuated for tripping overcurrent protection devices in civil and industrial installations to isolate the
part affected by a fault from the system, causing only the circuit-breaker immediately on the supply side of the fault to trip.
The example in the figure highlights the need to coordinate
tripping between the two circuit-breakers A and B so that only
circuit-breaker B is tripped in the event of a fault in C, ensuring
continuity of service for the rest of the system supplied by
circuit-breaker A.
Whereas natural selectivity within the overload current range is
normally found due to the difference between the rated currents
of the load protection circuit-breaker and the main circuit-breaker
on the supply side, selectivity can be obtained in the shortcircuit current range by differentiating the current values and, if
necessary, the trip times.
Selectivity can be total or partial:
– total selectivity: only circuit-breaker B opens for all current
values lesser than or equal to the maximum short-circuit current in C;
– partial selectivity: only circuit-breaker B opens for fault currents below a certain value; A and B are both tripped for
greater or equal values.
Example of current-type selectivity
Example of time-type selectivity
Circuit diagram with selective
coordination of protections
Time selectivity, obtained by
intentionally incorporating increasing time-delays in the trip
times of the circuit-breakers furthest to the supply side in the
chain.
In principle, the following types of selectivity are possible:
Current selectivity, obtained
by setting the instantaneous
trip currents of the circuitbreaker chain to different
values (higher settings for the
circuit-breakers on the supply
side). This often results in
partial selectivity.