
29
2
Operating characteristics and performance
2-5 Durability
Table 2-13 Rated ultimate short-circuit breaking
Circuit conditions
Number of breaks
Single-pole
breaking*
Maximum rated
operating voltage
25% of (Icu) rated
ultimate short-circuit
breaking capacity
O-t-CO at each pole: 1 time
O: Breaker opens when a
short-circuit occurs in a
closed circuit.
CO: Breaker closes and
opens in a shorted
circuit.
t:
Time interval between
O and CO
Three minutes or the
time it takes to reset
the MCCB, whichever
is longer.
Single or
three-phase
breaking
Maximum rated
operating voltage
Rated ultimate
short- circuit breaking
capacity (Icu)
O-t-CO: 1 time
Notes: * Applies to multi-circuit breakers in voltage phase grounding-type
distribution systems.
2-5-3
Rated ultimate short-circuit breaking
performance
See Table 2-13.
The operating duty for breaking current corresponding to
the rated ultimate short-circuit breaking capacity (Icu) of the
MCCB can be broken twice with O-t-CO. However, CO after
interval t was implemented in case you need to restart when
it is not clear what caused the breaker to trip before you have
eliminated the cause of the fault. This does not mean that the
breaker can function for long after it has tripped, however, and
you should replace it with a new one after short-circuit current
breaking has occurred.
s&ORBREAKERSEQUIPPEDWITHASHUNTTRIPDEVICEORANUNDERVOLTAGE
trip device, 5% of the total number of switching operations should be
allocated at the beginning and the end of the test for operation with
those tripping devices.
Notes: * An operating cycle constitutes one making and breaking. It should be
closed for 1.5 to 2 seconds.
s4HEBREAKERMUSTCLOSETHECIRCUITLONGENOUGHTOALLOWCURRENTTOREACH
maximum levels. This must not take longer than 2 seconds however.
Table 2-12 MCCB switching durability
Circuit conditions
Rated current
(A)
Operations
per hour*
No. of operations
Voltage
Current
Power factor/
time constant
Frequency
With current
Without
current
Total
Rated
operating
voltage (Ue)
Rated current
(In)
Power factor:
0.8
Time constant:
2ms
45 to 62
100 or less
120
1500
8500
10000
More than 100,
but 315 or less
120
1000
7000
8000
More than 315,
but 630 or less
60
1000
4000
5000
More than 630,
but 2500 or less
20
500
2500
3000
More than 2500 10
500
1500
2000
2-5-2 Trip switching durability
There are two types MCCB trip action: trip actuated by the
overcurrent trip device, and trip actuated by accessories such
as a shunt trip or undervoltage trip device. Trip switching
durability is defined as 10% of the total number of switching
operations both with and without current as given in Table
2-12. This value, however, assumes mechanical switching,
or the breaking of the rated current by a shunt trip device. If
the trip is caused by an overcurrent, the durability is lowered
depending on the magnitude of the overcurrent because of
the resultant contact wear and arc quencher thermal damage.
According to IEC 60947-2, trip switching durability is defined
as the current and the number of switching operations for an
overload test as given in Table 2-12.
2-5 Durability
2-5-1 Switching durability
MCCBs do not require the high-frequency switching
capability needed by magnetic motor starters because their
primary purpose is to protect cables or equipment against
overcurrents. Further, longer durability would detract from the
economy of the MCCBs because they are furnished with a
switch mechanism and a trip mechanism. IEC specifies the
switching performance and durability requirements listed in
Table 2-12.