DET-882
SecoGear Medium-voltage Switchgear Application and Technical Guide
Circuit Breaker Selection
©2017 General Electric All Rights Reserved
13
air as a heat transfer and a dielectric medium; however, at
high altitudes it may result in excessive corona at
operating voltages, which could cause an inability to
operate due to dielectric breakdown of the air insulation
due to the reduced air density.
SecoVac VB2+ circuit breakers and SecoGear switchgear
use air for an insulating and cooling medium. At altitudes
above 1000 meters [3300 ft] the air is thinner, causing a
higher temperature rise and lower dielectric withstand
capability. If we intend to use a SecoGear switchgear at
high altitudes we need to use the correction factors (listed
in Table 2-3) and apply them to the continuous current
rating, the impulse withstand voltage and the power-
frequency withstand voltage (1 minute), to obtain derated
ratings.
If the Voltage Correction Factor is applied to the maximum
designed voltage rating of 15 kV, 8.25 kV or 4.76 kV for
metal-clad switchgear, it may not be possible to install the
equipment at altitudes above 1000 meters [3300 ft] using
their typical nominal system voltages. Another accepted
option is to apply the voltage correction factors to the BIL
rating of the switchgear using the equipment at their usual
rated nominal voltage, providing a surge protection device
on the load side of the switchgear (see ANSI C37.010, 4.2.2).
The Current Correction Factor is applied to the continuous
current rating of the equipment only. It is necessary to
derate the continuous current rating, because switchgear
assemblies depend on the air for cooling and will have a
higher temperature rise when operated at altitudes above
1000 meters. The short-time and interrupting current
ratings on vacuum breakers are not affected by altitude
variations. Since the Current Correction Factor is small and
the actual continuous current duty is less than the
equipment rating, current correction is typically not as
serious a consideration as the voltage correction. An
additional consideration is that often, at higher altitude, the
ambient is reduced, which can offset the higher altitude
continuous current derating effect.
NOTICE
The recommendations are subject to modification,
depending on the actual system conditions.
Correction factors are different for switchgear and for
power transformers. Any application to use metal-clad
switchgear above 1000 meters [3300 ft] should be referred
to GE for evaluation.
Table 2-3: Altitude Correction Factors* for SecoVac VB2+
Circuit Breakers and SecoGear Switchgear
Altitude
Rated Continuous
Current Correction
Factor
Rated Voltage
Correction
Factor
1000 m [3300 ft]
1.00
1.00
1200 m [4000 ft]
0.995
0.98
1500 m [5000 ft]
0.991
0.95
1500 m [6000 ft]
0.987
0.92
2100 m [7000 ft]
0.985
0.89
1500 m [8000 ft]
0.970
0.86
1500 m [9000 ft]
0.965
0.83
3000 m [10000 ft]
0.960
0.80
3600 m [12000 ft]
0.950
0.75
4000 m [13000 ft]
0.940
0.72
4300 m [14000 ft]
0.935
0.70
* From ANSI C37.20.2 – 2015, Table 8.
“Unusual service conditions” include other factors that
require special attention, such as the exposure to corrosive
atmosphere, explosive fumes, excessive dust, steam,
dripping water, or salt spray; also, exposure to abnormal
vibration, shock, unusual transportation or special storage
conditions. GE should also evaluate installations accessible
to general public and applications with special equipment
requirements.
BREAKER ACCESSORIES
Each circuit breaker shall be provided with an auxiliary (S5)
switch with two “a” and two “b” contacts to indicate if
breaker is open or closed;
Optionally, each circuit breaker can be provided with a
four-stage MOC switch with four “a” and four “b” contacts
to indicate breaker open or closed status. In addition, each
circuit breaker can be provided with a TOC (position) switch
with four “a” and four “b” contacts indicating whether the
circuit breaker is in the “Connect / Service” or “Disconnect /
Test” position.
An option for an eight stage MOC switch or TOC position
switch is available, providing eight “a” and eight “b”
contacts.
Another optional feature for SecoVac VB2+ circuit breakers
is an undervoltage trip device that monitors the circuit
control voltage and trips the breaker if the control power
drops below a preset value. Refer to SECTION 3. “Control
Power Equipment” for more information.
BREAKER LIFT TRUCK
With every order, GE provides one standard SecoVac VB2+
lift truck, which is required to reach the upper
compartment rollout. (See Figure 2-2.)
