40
3
Selection and application
3-2 Cascade trip applications
3-2 Cascade trip applications
3-2-1
Conditions for cascade (backup) trip
coordination
A cascade (backup) system established between overcurrent
circuit breakers can yield a very economical system as
described in 3-1-4.
When the main circuit breaker in a cascade (backup) system
has sufficient breaking capacity and trips quickly in the event
of a short-circuit fault, it can minimize the amount of energy
passing through the branch MCCB. This depends on the
following conditions, however, which the main breaker alone
or a branch MCCB connected in series with the main breaker
must satisfy:
(a) The peak let-through current must be kept below the
allowable mechanical strength limit of the branch MCCB.
I
PL
< I
PA
(b) The let-through I
2
t must be kept below the allowable
thermal strength limit of the branch MCCB.
³
i
L
2
dt <
³
i
A
2
dt
(c) The arc energy generated in the branch breaker must be
kept below allowable limits for the branch MCCB.
³
e
L
i
L
dt <
³
e
A
i
A
dt
where
I
PL
:
Peak let-through current (A)
³
i
L
2
dt: Let-through
I
2
t (A
2
s)
³
e
L
i
L
dt:
Arc energy generated in the branch MCCB
I
PA
:
Allowable through current peak value for the
branch MCCB
³
i
A
2
dt:
Allowable I
2
t for the branch MCCB
³
e
A
i
A
dt: Allowable arc energy for the branch MCCB
Condition (a) shows the effect the backup breaker has
on controlling current and it suggests that current-limiting
coordination of breakers is easier.
Condition (b) suggests that coordination is easier at each
current level as the time the current is on gets shorter. The
main breaker must trip at high speed in this case.
Condition (c) suggests that coordination is easier with less arc
energy passing through the branch MCCB
The amount of arc energy present with a short circuit is
determined by the short-circuit capacity of the system. If the
amount of arc energy present when the backup MCCB trips is
³
e
S
i
S
dt, it yields the following equation.
³
e
S
i
S
dt +
³
e
L
i
L
dt = C (a constant)
It follows that
³
e
S
i
S
dt should be as high as possible for easier
coordination. This suggests that a system with backup MCCBs
that have a faster contact opening time and higher arc voltage
is better. It also suggests that either a current-limiting fuse or
current-limiting circuit breaker is most appropriate for backup
coordination.
3-2-2
Criteria for cascade (backup) trip
coordination
Various breaker-based breaker-breaker or breaker-fuse
combinations suitable for backup have been reported.
However, testing and other standards are not well defined
for backup protection at this point. Protective equipment
combinations will have to be defined through uniform testing
methods and criteria in order to ensure proper backup
protection with minimal confusion.
Appendix A of IEC60947-2 stipulates protection coordination
standards for cascade (backup) systems. Table 3-4 shows
criteria from that appendix.
Table 3-4 Criteria for cascade (backup) systems (Appendix A of
IEC60947-2)
Item
Items tested after the shorting test
Criteria
1
Withstand voltage and insulation resistance
Good
2
Contact welding
Not welded
3
250% current tripping
Good