
39
3
Selection and application
3-1 Selection check points
(3) Cascade (backup) trip coordination
Selective trip coordination requires that each protective
device have a sufficient short-circuit breaking capacity (fully-
rated system). A fully-rated system, however, would not
be economical to implement in large-capacity low-voltage
systems. An economic solution is cascade (backup) trip
coordination.
In cascade (backup) trip coordination, if a short-circuit current
higher than the rated short-circuit breaking capacity of the load
side protection device flows, the line side protection devices
connected in series break the current to protect the load side
protection device against thermal or mechanical damage.
Either a current-limiting fuse or current-limiting circuit breaker
is used as the line side protection device.
Fig. 3-5 shows typical cascade trip coordination with a
distribution board circuit breaker backed up by a current-
limiting fuse.
The cascade (backup) system combines non-selective tripping
systems for short-circuit current tripping, but short-circuit
failures that actually require backup occur only once every few
years. Because the initial cost of installing the system is high,
however, an economical protective system designed using
backup coordination is a more sensible approach for general
circuits.
Fig. 3-5 Cascade trip coordination
10
100
1000
Current (A)
2
4
8
2
4
8
2
4
8
Region protected by
main circuit breaker
Region protected by branch
circuit breaker
100
10
1
10
1.0
0.1
0.01
0.001
(min
)
(sec
)
Operating characteristic
of main circuit breaker
Operating characteristic
of branch circuit breaker
Oper
ating time
(2) Selective trip coordination
In the main circuit of facilities having a large power receiving
capacity or in systems containing an important load, selective
trip coordination should be used to provide improved power
feeding reliability.
Selective trip coordination between protection devices ensure
that only the protection device located closest to a fault point
trips, and the line side protection devices remain closed.
In Fig. 3-4, for example, when a short-circuit fault occurs
at point F, only protection device PB is tripped. Line side
protection device PA is not unactuated thus allowing an
uninterrupted supply of power to the normal circuits L
1
, L
2
, and
L
3
.
The device system configuration for selective trip coordination
must be such that the load side protection device completes
the breaking of the fault current over the entire overcurrent
range before the line side protection device is tripped, or
before starting irreversible trip operation.
This condition must be met in both the overload current and
the short-circuit current regions.
Selective trip coordination should be designed based on the
overall system, but it is more commonly used in critical circuits
or on key lines near the power supply. It is particularly
important to coordinate between take-off circuit breakers
and branch MCCBs in spot network systems. It ensures the
take-off line will not be cut off when there is a terminal system
failure.
Fig. 3-4 Low-voltage power receiving system
L
1
L
2
L
3
L
X
P
A
P
B
F