
38
3
Selection and application
3-1 Selection check points
Table 3-3 Low-voltage overcurrent protection systems
Protection system
Purpose
Features
Protective device
Fully-rated system
Selective tripping
s#UTSOFFOVERCURRENT
s4HERMALLYAND
mechanically protects
wiring and load
equipment across the
entire overcurrent range.
Improves the system's
power feeding reliability.
Combination
Non-selective tripping
–
Single or combination
Cascade (backup) system
Non-selective tripping
Provides an economical
protection system.
Combination
The breaker away from the shorting point on the power supply
side must trip whenever short-circuit current occurs, but it must
protect equipment from the thermal and mechanical stresses
generated as short-circuit current passes through the circuit as
well. This means the current peak value i
pb
and the let-through
current
³
i
b
2
dt at the time of MCCB breaking must be below the
allowable current peak value i
pa
of the protected equipment
as well as the
³
i
a
2
dt in the breaking characteristics of the
overcurrent protective device. In short, the following must be
true.
i
pa
> i
pb
,
³
i
a
2
dt >
³
i
b
2
dt
This point is especially important because breakers with
relatively low rated currents and higher short-circuit braking
capacity are used more offen in today’s branch circuits.
Overcurrent protection method:
An overcurrent breaker operates on the principle that one
protective device alone will cut off short-circuit current passing
through it. This is called a fully-rated system. When a single
protective device is insufficient, then another breaker is
installed at the power supply side. This is called a cascade
(backup) system, and it is often used to take advantage of
more economical breaking method.
In an effort to ensure a more reliable power supply, only the
breaker on the power supply side that is closest to the fault
point will trip when a short-circuit fault occurs at a branching
circuit such as a distribution system terminal. The operating
times must be coordinated between the breakers as a result
so they will not track the breaker on the power supply side.
This is known as selective trip coordination as opposed to the
fully-rated system. (Table 3-3)
Table 3-2 Low-voltage overcurrent protective coordination
Kind of coordination
Coordination between the protective
device and equipment to be protected
Coordination between protective devices
Selective trip coordination
Cascade (backup) trip coordination
Objective
Protecting equipment
Improved power supply reliability
Economical protective coordination
Description
A protective device protects the wiring
and load equipment against thermal
and mechanical damage due to
overcurrents.
Protective devices on the line
side and the load side working in
coordination prevent the short-
circuit fault from extending from
the fault circuit to other cables and
also minimize the scope of power
failure.
An economical circuit breaker with a small-
short-circuit breaking capacity is used,
with the short-circuit breaking of short-
circuit currents higher than the rated short-
circuit breaking capacity being undertaken
by protective devices connected in series
on the source line side.
Coordination condition
s3AFEBREAKINGOFFAULTCURRENTS
s0ROTECTIONOFWIRINGANDLOAD
equipment against thermal or
mechanical damage.
The load side protection device
completes current breaking over
the entire fault current range
before the line side protection
device is tripped, or before starting
irreversible trip operation.
If a short-circuit current higher than the Icu
of the load side protection device flows,
line side protection devices connected
in series break the current, protecting
the load side protection device against
expected thermal and mechanical
damages.
Protective device state
Single or combined
Combined
Combined
Typical system
(indicating the
relationship of
coordination)
Motor
(Protected equipment)
Motor starter
(Protective device)
MCCB
(Protective device)
Wiring
(Protected equipment)
M
Solid-state trip
type MCCB
MCCB
MCCB
MCCB
MCCB
MCCB
Fuse
Fuse