Siemens SENTRON - Let-Through Energy Diagram

Methods of implementing selectivity using circuit breakers
3.6 Dynamic selectivity
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
43
3.6.2 Let-through energy diagram
The current-limiting capabilities of molded case circuit breakers are a feature that is
particularly utilized in dynamic selectivity systems.
According to IEC 60947-2, 2.3, the term "current-limiting circuit breaker" refers to a circuit
breaker "with a break-time short enough to prevent short-circuit current reaching its
otherwise attainable peak value". In other words, the let-through current and the other
variables dependent on the let-through current (such as the let-through energy) are
significantly reduced by the current-limiting capability of the circuit breaker.
It has become established practice to apply the integral of the square of the current over a
given time, otherwise known as the Joule integral Q, as a unit of measure of current-limiting
capability:
Q = ∫i
2
dt
The I
2
t characteristic of the circuit breaker is selected as a means of representing the let-
through energy of a circuit breaker as a function of the prospective current (I
p
) according to
DIN EN 60947-2, 2.18.
① Let-through energy of the upstream 630
A
circuit breaker
② Let-through energy of the downstream 250
A
circuit breaker
Figure 3-12 I
2
t diagram: Examples of let-through energy of 250 A and 630 A circuit breakers