Siemens SENTRON - Page 24

Methods of implementing selectivity using circuit breakers
3.4 Time selectivity
Selectivity for 3VA molded case circuit breakers
24 Configuration Manual, 08/2016, A5E03603181010-01
Advantages and limitations
Advantages of time selectivity:
● Simple to implement
● High selectivity limit current values (I
s
) or total selectivity can be achieved if the short-
circuit current strength of the upstream circuit breakers is sufficiently high. Total selectivity
can be achieved if the instantaneous short-circuit release of the upstream protective
device can be disabled.
Limitations of time selectivity:
● Tripping is severely delayed when a fault occurs close to the infeed in power distribution
systems with several grading levels. In the event of short-circuits, this delay can result in
extreme thermal and mechanical loading of electrical equipment.
In such cases, combining time selectivity with zone selective interlocking (see chapter
"Zone selective interlocking (Page 30)") can shorten trip times and significantly reduce
the thermal a
nd mechanical loads on components.
● Voltage dips in the electrical network can occur if the short-circuit persists for a
comparatively long period and causes the voltage to drop in the incoming cables and
equipment. This might cause electrical equipment to malfunction and undervoltage
protective devices to respond.
Circuit breakers suitable for this application
Time selectivity implemented with molded case circuit breakers is often only partial
selectivity.
The reason for this is that a short-time delay with molded case circuit breakers can only be
applied in the current zone in which the contacts cannot open by themselves as a result of
forces caused by current flow. Since the upper limit of this zone is very low by comparison
with the rated ultimate short-circuit breaking capacity (I
cu
), the selectivity limit current (I
s
) is
also very low.
Time selectivity can be effectively implemented in most cases with air circuit breakers
because the contact system of an air circuit breaker can conduct high short-circuit currents
for a specific time period. Air circuit breakers of utilization category B in particular are
characterized by a high rated short-time withstand current (I
cw
).
Total selectivity can be achieved if the values for rated ultimate short-circuit breaking
capacity (I
cu
) and the high rated short-time withstand current (I
cw
) are very similar.
Circuit breakers must have an electronic trip unit if they are to be used to implement time
selectivity. As a general rule, the short-time delay on circuit breakers with a thermal-
magnetic trip unit cannot be varied.
DIN EN 60947-2 Part 4.4 distinguishes between two utilization categories in relation to time
selectivity in order to determine which circuit breakers fulfill the requirements for rated short-
time withstand current I
cw
with a short-time delay.