
10
2
Operating characteristics and performance
2-1 Overcurrent tripping characteristics
(c) High frequency
For operation at higher frequencies, such as 400 or 750Hz,
the current rating of a thermal-magnetic MCCB has to be
derated due to the heat generated by the skin effect in the
conductors or the eddy current in the iron core. The rate of
reduction slightly depends on the ampere-frame size and the
rated current. The available current rating at 400Hz decreases
to 70–80% of the rated current.
As the iron loss lowers the attractive force of the trip device,
the instantaneous trip current will increase. Hydraulic-magnetic
MCCBs cannot be used in a high-frequency circuit because
the operating characteristics will change greatly due to the
temperature rise of the moving iron core and the reduced
attractive force by the high frequency.
Table 2-5 Operating characteristic changes for DC circuit application
Trip device
Inverse time-delay trip characteristic
Instantaneous trip characteristics
Operating characteristic curve
Thermal-magnetic
None
The instantaneous trip current is
higher than that for an AC circuit.
The rate of variation depends
on the ampere-frame size, rated
current and model. The trip current
can be as high as 140% of the AC
value.
AC
DC
Current
Oper
ating time
Hydraulic-magnetic
The minimum operating current at DC
is about 110–140% of that for AC.
AC
DC
Current
Oper
ating time
Solid state
Not applicable
(5) Frequencies
(a) Commercial frequencies (50Hz, 60Hz)
The characteristics of breakers are generally the same at 50
and 60Hz.
(b) Direct current (DC)
If an MCCB designed for operation in an AC circuit were used
in a DC circuit, its operating characteristics would change as
shown in Table 2-5. Hence, an MCCB exclusively designed for
operation in a DC circuit has to be used on this occasion.