Siemens SENTRON - Page 45

Methods of implementing selectivity using circuit breakers
3.6 Dynamic selectivity
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
45
The characteristic curves shown in the diagram indicate the let-through energy of the
relevant circuit breaker if it is limiting and interrupting the short-circuit current on its own.
If the 630 A circuit breaker is located upstream of the 250 A circuit breaker, however, the
current-limiting capabilities of both circuit breakers act cumulatively in the event of a short-
circuit. The common let-through energy is reduced by an amount that is determined by the
630 A circuit breaker.
① Tripping energy of the 630
A
circuit breaker
② Let-through energy for the combination of 250
A
circuit breaker and 630
A
circuit breaker
③ Tripping energy of the 250
A
circuit breaker
Figure 3-14 Examples of let-through energy values for the combination of 250 A circuit breaker and
630 A circuit breaker
The I
2
t diagram shows that the let-through energy of the circuit breaker combination for the
prospective current (I
p
) zone in this example is higher than the tripping energy of the
downstream 250 A circuit breaker. Even at short-circuit currents of 100 kA, the downstream
250 A circuit breaker trips.
The circuit breaker combination performs selectively across the entire short-circuit zone.