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Selectivity techniques

The different selectivity techniques, also from the operational point of view

Selectivity (discrimination) is achieved by automatic protective devices when a fault is cleared by the protection device installed immediately upstream of it, while all the other protection devices remain unaffected.

Two areas can be defined, depending on the current ranges and circuit-breaker trip curves:

In < I < 8÷10 x In
Overload zone

In which the thermal protection for thermal-magnetic trip units and protection L for electronic trip units are normally called upon to trip.

I ≥ 8÷10 x In
Short-circuit zone

In which the magnetic protection for thermal-magnetic trip units or protections S, D and I for solid-state trip units are normally called upon to trip.

Different selectivity techniques are associated with these two zones.

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