Different Short-Circuit Currents At Different Fault Locations - Siemens SENTRON Configuration Manual

Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
3.3.3

Different short-circuit currents at different fault locations

In order to demonstrate the principle of current selectivity, three further scenarios with
different fault locations and short-circuit currents are presented below.
Figure 3-5
Fault location within protection zone of circuit breaker S2
Circuit breaker S1 is closer to the infeed. The impedances of the cables, etc. between the
infeed and circuit breaker S1 are lower than those between the infeed and circuit breaker S2.
A higher short-circuit current therefore flows through circuit breaker S1 than circuit
breaker S2.
● Scenario 1:
The short-circuit current at circuit breaker S2 reaches 1500 A.
Circuit breaker S2 trips, but circuit breaker S1 does not. The circuit breakers operate
selectively.
● Scenario 2:
The short-circuit current at circuit breaker S2 reaches 2600 A.
Both circuit breakers trip. The circuit breakers do not operate selectively.
Fault location within protection zone of circuit breaker S1
The short-circuit current at circuit breaker S1 reaches 3000 A. Circuit breaker S1 trips, but
circuit breaker S2 does not. Since S1 is the upstream circuit breaker, the question of
selectivity is irrelevant.
22
Examples of short-circuit currents at different fault locations
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01