13
2
Operating characteristics and performance
2-2 Breaking performance
(5) Maximum let-through current and maximum
breaking I
2
t
The current that would flow through a short circuit without a
circuit breaker is called the available short-circuit current. It is
the short-circuit current that is determined from the impedance
map at circuit breaker selection, not the current that is actually
interrupted by the circuit breaker. The current that actually
flows through the circuit is smaller than the available short-
circuit current. As the trip device in the circuit breaker is
actuated to open the contact immediately on occurrence of a
short circuit, the arc voltage is increased to inhibit current flow.
This is equivalent in effect to having a variable resistor, called
an arc resistor, connected in series in the circuit.
Current-limiting circuit breakers that take advantage of the
magnetic repulsion force represent an application of this
principle; current-limiting breaking is done before the short-
circuit current reaches its peak value.
The maximum current that can flow through the circuit breaker
is called the maximum let-through current, which is expressed
as a peak value. The smaller the maximum let-through current,
the less mechanical stress is imposed on the cable and load
equipment. With a high short-circuit current having a low
power factor, the transient peak value is more than twice the
symmetrical effective value. In mechanical stress studies,
therefore, a choice must be made between the current-
limiting type and the non-current-limiting type, along with full
allowance for electromotive force.
The squared product of the let-through current, or
∫
t
2
t
1
i
2
dt from
t1 short-circuit occurrence time t1 to completion of breaking
time t2, is called the maximum breaking I
2
t. The smaller this
value, the smaller the thermal effects on the cable and load
equipment. Fig. 2-8 to 2-9 and Fig. 2-10 to 2-11 give the
maximum let-through I
2
t or maximum let-through current, and
available short-circuit current of MCCBs.
2-2-3 Arc space
When a short-circuit current is broken, an ionized gas is
emitted from the breaker’s line side exhaust vent and, because
this gas is conductive, it could induce an interphase short
circuit or ground fault if it bridges adjacent bare live parts or
a bare live part and an adjacent grounded metallic surface.
Because this is potentially hazardous, an arc space (insulation
space) is required for safety.
Table 2-7 lists the arc spaces required for specific conditions.
When wiring is done, live parts should be either taped or
protected by insulating barriers in the ranges specified in
Table 2-7 to allow for conditions that could be encountered
while the MCCB is in service. Improved insulation may be
needed outside the arc space depending on the service
conditions of the MCCB.
Fig. 2-7 Current breaking process
Load current
Total opening time
Arcing time
Arc voltage
Recovery voltage
Available short-circuit current
Short-circuit occurrence
Time
Restrike voltage
Current
Contact
opening
time
Breaking
current
Maximum peak
let-through current
Maximum peak available
short-circuit current
Zero-crossing point of
short-circuit current
Short-circuit current
breaking point
(4) Breaking characteristics
When the magnitude of an overcurrent exceeds a certain
limit, the instantaneous trip device is actuated to open the
pole immediately. The minimum current that can actuate the
instantaneous trip device is called the instantaneous trip
current, which is expressed as a symmetrical effective value.
With thermal-magnetic MCCBs, the instantaneous trip current
setting is adjustable because their instantaneous trip device
is installed separately from the inverse time-delay trip device.
This eases coordination with other devices. Fig. 2-7 shows
the progress of time after the start of short-circuit current
flow until completion of breaking. The time interval between
the occurrence of a short-circuit fault and the opening of the
contacts is called the contact opening time. The time interval
between completion of breaking and quenching of the arc
generated by contact opening is called the arcing time. The
sum of the contact opening time and the arcing time, or the
period of time from the occurrence of a short-circuit fault to
completion of breaking, is called the total opening time. Table
2-7 lists the contact opening times, arcing times, and total
opening times of MCCBs at breaking of the rated breaking
current.
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