12
2
Operating characteristics and performance
2-2 Breaking performance
2-2-2 Breaking characteristics
(1) Breaking performance
The characteristics that define MCCB breaking performance
are the rated short-circuit breaking capacity, peak let-through
current, and maximum let-through I
2
t. The rated short-circuit
breaking capacity is defined by the rated ultimate short-circuit
breaking capacity (Icu), and the rated service short-circuit
breaking capacity (Ics).
(2) Rated short-circuit breaking capacity
Fig. 2-6 is a typical oscillogram of a short-circuit current. In the
figure, t=0 denotes the time the short-circuit fault occurred.
The rated load current was flowing at the supply voltage before
the short-circuit fault occurred. The current by several factors
of ten flows after the occurrence of the short circuit. Because
the load current immediately after the short-circuit fault
contains a DC component, the current flow is asymmetrical
with respect to the zero-current line, with the DC component
being attenuated rapidly. The curve C-C’ represents the DC
component of the asymmetrical short-circuit current, and is
indicates the current that would flow if a short circuit occurred.
This current is called the available short-circuit current.
i
S
=i
AC
+i
DC
=I
m
[sin (
Z
t+ø−
M
)−e
L
R
t
−
sin (ø−
M
)]
ø:
Making phase angle
cos
M
: Short-circuit power factor
The value of the above equation reaches
its maximum when (ø−
M
) = ± .
I
m
=
R
2
+(
Z
L)
2
√
E
m
2
π
Fig. 2-6 Short-circuit current oscillogram
1/2 cycle
i
A
B
C
X
Y
P
i
S
P'
A'
C'
B'
t
0
i
i
S
:
Short-circuit current
C-C': Intermediate line between the envelopes A-A' and B-B'
P-P': 1/2 cycle after occurrence of short-circuit fault
X:
AC component of short-circuit current
Y:
DC component of short-circuit current
The rated breaking current of an MCCB is represented as
X/
√2
, the effective value of the AC component 1/2 cycle after
the occurrence of the short-circuit fault. For a three-phase
circuit, the rated breaking current is represented the average
of
the three phases.
For DC circuits, the maximum available short-circuit current is
used.
(3) Operating duty
Under conditions where the displayed rated breaking capacity
is specified, breakers will break properly at an operating duty
of “O” -t- “CO” for Icu and “O” -t- “CO” -t- “CO” for Ics (where
t is three minutes or the time it takes to reset the breaker,
whichever is longer). All this is done at the rated voltage and
frequency. After the breaker trips, however, the rated current
may or may not flow, but the breaking capacity, durability,
letthrough current and overload switching capacity will be
diminished. Therefore, replace the breaker with a spare as
quickly as possible. If current must be supplied with the same
breaker, conduct a maintenance inspection that looks closely
at the operating conditions prior to the breaker tripping, the
amount of short-circuit current, as well as future operating
conditions. Special attention must be paid to temperature rise
as well.