This
interrupting
current
is
,
of
course,
lower
than
the
momentary current and depend on the speed of the breaker,
such as 8,5,3 or 2 cycles, which is a measure of the time from
the occurrence of the fault to the extinction of the arc.
Professor Arturo Bretas

EEL 4251  PSA
The Selection of Circuit Breakers
60
Breakers are identified by nominalvoltage class, such as 69 kV.
Among other factors specified are
rated continuous current
,
rated
maximum voltage
,
voltage range factor K
, and
rated shortcircuit
current
at rated maximum kilovolts.
Professor Arturo Bretas

EEL 4251  PSA
The Selection of Circuit Breakers
61
The rated maximum voltage of a circuit breaker is the highest
rms voltage for which the circuit breaker is designed.
The rated voltage range factor K is the ratio (rated maximum
voltage
÷
the lower limit of the range of operating voltage). K
determines the range of voltage over which the product (rated
shortcircuit current
×
operating voltage) is constant.
Professor Arturo Bretas

EEL 4251  PSA
The Selection of Circuit Breakers
62
In the application of circuit breakers it is important not to exceed
the shortcircuit capabilities of the breakers.
A breaker
is
required
to
have
a
maximum
symmetrical
interrupting capability equal to K
×
rated shortcircuit current.
Professor Arturo Bretas

EEL 4251  PSA
Example 3:
63
A 69kV circuit breaker having a voltage range factor K of 1.21 and a continuous
current rating of 1200 A has a rated shortcircuit current of 19.000 A at the maximum
rated voltage of 72.5 kV. Determine the maximum symmetrical interrupting capability
of the breaker and explain its significance at lower operating voltages.
SOLUTION
:
The maximum symmetrical interrupting capability is given by:
This value of symmetrical interrupting current must not be exceeded. From the
definition of K we have:
Professor Arturo Bretas

EEL 4251  PSA
Example 3:
64
Hence, in the operating voltage range 72.5 60 kV, the symmetrical interrupting current
may exceed the rated shortcircuit current of 19.000 A, but it is limited to 22,990 A .
For example, at 66 kV the interrupting current can be:
Breakers of the 115kV class and higher have a K of 1.0.