Minimum ambient
temperature:
The minimum
ambient (air) temperature specifies the lowest temperature at which the circuit
breaker shall be able to operate, at specified ratings.
Important
standard values are -30 °C and -40 °C
Maximum ambient
temperature:
The maximum
ambient (air) temperature specifies the highest temperature at which the
circuit breaker shall be able to operate, at specified ratings.
The maximum ambient
temperature can affect continuous current-carrying capability.
The standard
value is +40 °C.
Altitude:
The external
dielectric strength becomes reduced at higher altitudes due to the lower the density of air.
Standard
dielectric type tests are valid for installations up to 1000 masl.
For
verification of the suitability of installation at higher altitudes, the test
voltages have to be corrected.
Creepage
distance:
The creepage
distance is defined as the shortest distance along the surface of an insulator
between two conductive parts.
The required
creepage distance is specified by the user in:
- mm (total creepage distance)
- mm/kV (creepage distance in relation to the phase to ground voltage).
Creepage
distance voltage used to be phase to phase voltage. To avoid confusion check
which voltage reference that is used.
These are the values suggested in IEC- 815, 1986, which is applicable to
System Voltage of up to 525kV.
As the pollution withstand characteristics is known to be non-linear for
higher voltages, requiring increased creepage distance in proportion to the
non-linearity factor.
Studies by ENEL, EPRI, CIGRE suggest that this factor may be as high as 20%
for 800 kV System
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