Description
2.4 Selectivity
3VA molded case circuit breakers with IEC certificate
54
Manual, 03/2019, A5E03603177010-03
The selective behavior of circuit breakers can be implemented technically by a variety of
selectivity concepts:
●
Current selectivity
The selectivity can be calculated in the overload range by comparing the time/current
characteristics. In the short-circuit range, this comparison leads to values that are too low.
The reason for this is that the trip unit behaves differently in the case of short-circuit
currents compared with its long-term behavior, e.g. in the case of overload.
If the short-circuit currents differ sufficiently at the installation points of two molded case
circuit breakers, the instantaneous short-circuit releases can normally be set such that if a
short-circuit occurs behind the downstream circuit breaker, only this downstream breaker
trips.
If the short-circuit currents are approximately the same at the installation points of the
molded case circuit breakers, the grading of the tripping currents of the short-circuit
releases only enables selectivity up to a specific short-circuit current .
This current is referred to as the ultimate selectivity value I
s
.
●
Time selectivity
Selectivity can be achieved by time selectivity up to the threshold values of the
instantaneous short-circuit releases. To achieve this, the upstream circuit breaker
requires delayed short-circuit releases, so that in the event of a fault, only the
downstream circuit breaker will disconnect the part of the electrical installation affected by
the fault from the supply.
Both the tripping delays and the tripping currents of the short-circuit releases are graded.
●
Zone selective interlocking - ZSI
Selective behavior is achieved by installing parallel control cables between the molded
case circuit breakers. The electronic trip units then use a fast signal link to determine
priorities in the tripping sequence.
Zone selective interlocking (ZSI) was developed by SIEMENS to prevent undesirably long
tripping times when several molded case circuit breakers are connected in series.
ZSI enables the tripping delay to be reduced to 50 ms for the circuit breaker upstream
from the location of the short circuit.
●
Dynamic selectivity
This method is based on evaluation of the arc power which is generated in the arc chute
when the contacts open dynamically in response to a short circuit. During this process, a
smaller sized downstream molded case circuit breaker converts more energy in the arc
chute than the larger, upstream molded case circuit breaker. A selective trip unit
evaluates the energy conversion in both molded case circuit breakers. The downstream
molded case circuit breaker trips, while the contacts of the upstream circuit breaker close
again. Since both molded case circuit breakers perform a current limiting function, the
residual current limit imposed in practice is higher than the limiting action specified for the
individual molded case circuit breakers.
