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MISCELLANEOUS DATA
49-3
EFFECT OF ALTITUDE ON
BREAKDOWN VOLTAGES
Pressure-Altitude Graph
Design of electrical equipment for aircraft is some-
what complicated by the requirement of additional
insulation for high voltages as a result of the decrease in
atmospheric pressure. The extent of this effect may be
determined from Figs.
1
and
2
and Table
1.
(1
inch
mercury
=
25.4
millimeters mercury
=
0.4912
pound/inch2
=
3.38
X
IO3
pascals.)
Spark-Gap Breakdown
Voltages
Fig.
2
is for
a
voltage that is continuous or at a
frequency low enough
to
permit complete deionization
between cycles, between needle points or clean smooth
spherical surfaces (electrodes ungrounded) in dust-free
dry air. Temperature is
25
degrees Celsius and pressure
is
760
millimeters
(29.9
inches) of mercury. Peak
kilovolts shown in the figure should be multiplied by the
factors given in Table
1
for atmospheric conditions
other than the above.
An approximate rule for uniform fields at all frequen-
cies up to at least
300
megahertz is that the breakdown
gradient of air is
30
peak kilovoltskentimeter or
75
peak kilovolts/inch at sea level
(760
millimeters of
mercury) and
normal
temperature
(25
degrees Celsius).
The breakdown voltage is approximately proportional
to pressure and inversely proportional to absolute (Kel-
vin) temperature.
Certain synthetic gases have higher dielectric
strengths than air. Two such gases that appear to be
useful for electrical insulation are sulfur hexafluoride
(SF,) and Freon
12
(CCI,F;),
which both have about
100
50
v)
t;
20
9
4
2
10
3
L5
x
2
1
GAP LENGTH IN INCHES
Fig 1. Pressure as a function
of
altitude.
-
60
v)
5
2
3
8
-50
y
W
0
r
v)
Ez
3
-40
2
K
0
:
W
-30
z
s
5
K
-20
c>
z
<
z
W
0%
n
B
0
z
Ez
d
ob
n
0)
V
W
K
-
10
9
W
Ez
ln
c
C
0)
Q
z
2
0
10
20
30
40
50
60 70
ln
ALTITUDE IN THOUSANDS
OF
FEET
ABOVE SEA LEVEL
Fig. 2. Spark-gap breakdown voltages.
2.5
times the dielectric strength of air. Mixtures of
sulfur hexafluoride with helium and of perfluoromethyl-
cyclohexane
(C,
F14) with nitrogen have good dielectric
strength as well as other desirable properties.
WEATHER DATA*
Temperature Extremes
United States (contiguous):
Lowest temperature:
Rodgers Pass, Montana (January
20,
1954)--57
"C
(-70
"F).
Highest temperature:
Greenland Ranch, Death Valley, California (July
10,
1913)-57
"C
(134
OF).
Alaska:
Lowest temperature:
Prospect Creek Camp (January
23, 1971)-
-62
"C
(-79.8
OF).
Highest temperature:
Fort Yukon (June
27, 1915)-38
"C
(100
"F).
*
Compiled in part from "Climate and Man,"
Yearbook
of
Agriculture,
US
Dept. of Agriculture. Obtainable from
Su-
perintendent of Documents, Government Printing Office,
Washington, DC 20402.
See
also "Weather Extremes
Around the World," a world map compiled (1984) by the
Geographic Sciences Laboratory,
US
Army
Engineer
Topo-
graphic Laboratories, Belvoir, Virginia 22060.
For a comprehensive summary of available climatological
information, refer
to
Selective Guide
to
Climatic Data Sources
(No.
4.11), by Warren L. Hatch, July 1983, available from
the
National Climatic Data Center, Asheville, North Caroli-
na 28801-2696; phone:
(704)
259-0682.