Selectivity and undervoltage protection
Selectivity for 3VA molded case circuit breakers
70 Configuration Manual, 08/2016, A5E03603181010-01
Implementation of selectivity
Assuming that the operating time of the undervoltage releases t
Δu’
= 20 ms, instantaneous
over-current protective tripping must be implemented in t
a
≤ 20 ms. This applies both to near-
to-infeed and far-from-infeed short-circuits. This instantaneous protective tripping ensures
that the fault location is selectively disconnected from the network before the undervoltage
releases can respond. If instantaneous protective tripping in t
a
≤ 20 ms cannot be
implemented for all the relevant fault locations, the feasibility of using undervoltage releases
with drop-out delay must be assessed.
Contactors
In the event that the voltage dips as a result of short-circuits, it is probable that contactors
will drop out. Contactors can be expected to drop out if the control supply voltage falls below
80 % of its rated value for longer than 5 - 30 ms.
If instantaneous protective tripping in t
a
≤ 20 ms cannot be implemented, the feasibility of
using contactors with drop-out delay must be assessed.
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