37
3
Selection and application
3-1 Selection check points
3-1-4 Protective coordination
When an overcurrent fault occurs, an overcurrent (overload
current or short-circuit current) flows from the power source
to the fault point. In this situation it is essential to not only
safeguard the system against the fault current but ensure
system reliability and economics while keeping other systems
least affected by the fault. A scheme of overcurrent protection
encompassing all of these considerations is called overcurrent
protective coordination.
Generally, overcurrent protective coordination allows for the
following (Table 3-2):
s #OORDINATIONBETWEENTHEPROTECTIVEDEVICEANDPROTECTED
equipment
s 3ELECTIVETRIPCOORDINATIONBETWEENPROTECTIVEDEVICES
s #ASCADEBACKUP TRIPCOORDINATIONBETWEENPROTECTIVE
devices
(1) Methods of coordination
It is important that the operating characteristics of the
protective device (such as a circuit breaker or fuse) span
the whole overcurrent range to safeguard the cable and load
equipment. Reviews of both the overload current (overcurrent
closer to the rated current) and the short-circuit current region
are required.
Factors for consideration to ensure positive overcurrent
protection should include:
s 3HORTCIRCUITCURRENTATTHEPOINTATWHICHAN-##"ISTOBE
installed
s $AMAGECHARACTERISTICSOFTHEWIREINTHEOVERLOADREGION
s !LLOWABLECURRENTANDALLOWABLE)
2
t value of the wire at the
short-circuit time
s #URRENTTIMECHARACTERISTICSINTHE-##"OVERLOADREGION
s 2ATEDSHORTCIRCUITBREAKINGCAPACITYOFTHE-##"
s -AXBREAKING)
2
t value at the time of MCCB breaking
s &REEDOMFROM-##"MALFUNCTIONINGCAUSEDBYAMBIENT
conditions, starting characteristics of the load equipment,
etc.
3-1-3 Overcurrent protection principle
Fig. 3-2 is a schematic diagram of a typical low-voltage
distribution system. The aim of overcurrent protection is to
safeguard the system against overcurrent faults, to ensure
high power-feeding reliability, and to establish an economical
protecting system.
In the overload or intermediate overcurrent region, the
combination of a protective device and load equipment
to be protected, such as motors including cables, must
be determined carefully. Generally, the combination is
determined by considering the protection characteristic curve
of the protection device (MCCB) and the thermal damage
characteristics of the equipment to be protected. As shown
in Fig. 3-3, overcurrent protection is available in the region
where the operating curve of the circuit breaker lies below
the thermal damage characteristics of the equipment to be
protected.
Fig. 3-2 Typical low-voltage distribution system
Primary circuit
breaker
Main power distribution board
Main circuit
breaker
Main circuit
breaker
Power receiving and
distribution room
Bus bar
Bus bar
Feeder circuit
breaker
Branch circuit
breaker
Branch circuit
Feeder
Distribution
board
Fig. 3-3 Wiring and load protection using MCCBs (Overload and
intermediate overcurrent region)
a
b
c
MCCB operating characteristics
Time
Current
a, b, c:
a, b:
No and partial protection
c:
Complete protection
Protected region
Thermal damage characteristics
of wire and load equipment
