8
2
Operating characteristics and performance
2-1 Overcurrent tripping characteristics
(3) Short-time delay trip
This type of tripping has a short-time delay to handle the
selective trip coordination of low-voltage circuits.
2-1-2 Factors affecting overcurrent trip
characteristics
There are basically three types of overcurrent tripping:
thermalmagnetic, hydraulic-magnetic and solid-state. The
effect of each varies with the principle involved.
Table 2-1 shows models organized by the type of trip device.
Table 2-1 Breaker trip devices
Trip device MCCB type ELCB type (Reference)
Thermal-
magnetic
The following models not
included.
The following models not
included.
Hydraulic-
magnetic
BW32AAG, BW32SAG
BW50AAG, BW50EAG
BW50SAG, BW50RAG
BW63EAG, BW63SAG
BW63RAG
BW100AAG
BW100EAG
EW32AAG, EW32EAG
EW32SAG
EW50AAG, EW50EAG
EW50SAG, EW50RAG
EW63EAG, EW63SAG
EW63RAG
EW100AAG
EW100EAG
Solid state BX100, BX160
BX250, BX400
BX630, BX800
BX1000
BX1250
BX1600
BW1000RAE
BW1250RAE
BW1600RAE
(1) Ambient temperature
If an MCCB is used at a temperature other than the
reference ambient temperature at which its overcurrent
trip characteristics are prescribed, the long-time delay trip
characteristic changes. Therefore, the choice of MCCB must
consider the cataloged temperature correction curve and
overcurrent trip characteristics. As Table 2-2 shows, the effects
of ambient temperature on the overcurrent trip characteristics
of an MCCB vary according to the type of trip device.
Table 2-2 Ambient temperature effects on overcurrent trip
Trip device Effect of ambient temperature change
Thermalmagnetic The minimum current for trip operation will
decrease when the ambient temperature exceeds
the reference ambient temperature, and vice
versa. This means that a lower overcurrent makes
the bimetal reach the operating temperature
as the ambient temperature rises, because the
bimetal’s operating temperature is constant.
Hydraulicmag-
netic
Although the minimum value of trip current
remains unchanged, the operating time varies
depending on the ambient temperature, as the
viscosity of silicon fluid in the oil dashpot varies.
2-1 Overcurrent tripping characteristics
2-1-1 Types of tripping
Overcurrent tripping in MCCBs occurs in three different ways
depending on the amount of overcurrent. For line protection
use in general, the breakers use an inverse-time delay trip and
instantaneous trip (dual trip-element characteristic).
Some breakers use a short-time delay in addition to the
inverse-time delay trip and the instantaneous trip mainly for
selective trip coordination. This is particularly true with larger
breakers (ternary trip-element characteristic).
Fig. 2-1 shows a dual trip-element characteristic curve while
Fig. 2-2 shows a ternary trip-element characteristic curve.
(1) Inverse-time delay trip (long-time delay)
This type of tripping delays the tripping time of the breaker at
a rate inversely proportional to the amount of overcurrent. It is
available as either a thermal-magnetic type that uses ordinary
bimetal elements or as a hydraulic-magnetic type that uses oil
dashpot damping. The trip is also referred to as a long-time
delay trip to distinguish it from the shorter tripping time of the
short-time delay trip.
(2) Instantaneous trip
This trips the circuit breaker immediately when there is
relatively significant overcurrent like short-circuit current.
Fig. 2-1 Dual trip-element characteristic
Inverse time-delay trip (Long-time delay trip)
Instantaneous trip
Operating time
Current
Fig. 2-2 Ternary trip-element characteristic
Inverse time-delay trip (Long-time delay trip)
Short-time delay trip
Instantaneous trip
Operating time
Current
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