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Review of Published Data on the Humidity
Dependence of the Flashover Voltage
of Air Insulation
Jarle Sletbak,
Norwegian Research Institute of Electricity Supply
Fastän det allmänt anses att fuktighetskorrektioner
för överslagsspänningar i luft är nödvändiga,
existerar det inga allmänt accepterade värden för
korrektionsfaktorer. Vanligen använder man de vården
som finns angivna i EEI-NEMA-specifikationerna.
Syftet med uppsatsen är att ge en sammanställning
av publikationer beträffande fuktighetsberoendet
hos överslagsspänningar i luft. I uppsatsen anges alla
numeriska värden i överensstämmelse med den
praxis som används av IEC och i de flesta
europeiska nationella specifikationer, nämligen med
11 g/m’ som referensvärde på fuktigheten och med
korrektionsfaktorn definierad som uppmätt
överslagsspänning genom överslagsspänningen vid
referensfuktigheten.
Although it is generally agreed that a
humidity-correction for breakdown voltages in air is
necessary, there exists no generally accepted value for
this correction factor. Usually the values given in
the EEI-NEMA specifications are employed.
The aim of this report is to give a condensed
review of the published data on humidity
dependence of the flashover voltage in air.
In this report all numerical values are given in
accordance with the practice followed in IEC and
most national European standards using a
reference humidity of 11 g/m3 and the following
definition for the correction factor: Correction factor
= Measured flashover voltage/Flashover voltage at
reference humidity.
Table 1. Summary of numerical values of correction factor k.
621.317.333
Published data of humidity influence upon
flashover voltage
Based on observations it is usually assumed that
the flashover voltage for all types of air insulation
rises proportionally with the absolute humidity,
provided the relative humidity does not exceed 00 to
90 The value of this critical humidity is to a
certain degree dependent on the object being tested.
Accordingly, a constant percentage correction can
be defined as
k =
Eh - Eu
100 % per g/m3
£u(/i-ll)
Eh and E1X are the flashover voltages at h g/m3 and
11 g/m3 respectively. The numerical values of k are
summarized in table 1. In the third column of the
table the values given by EEI-NEMA are referred,
while in the fourth column the bands of values
found in the literature are stated.
These bands are determined by extracting typical
values from each publication. The extreme values
have not been taken into consideration, so it should
be noted that corrections lying well outside the band
quoted in the table are frequently observed,
especially on small test objects. These deviations can
not at the moment be taken into account in a test
specification, as their origins are still unknown.
By 50—60 c/s a.c. as well as by impulses the
correction is smaller at low flashover voltages. It
seems to be generally agreed that this can be taken
Humidity correction, % per g/m3
Type of test object Type of voltage used According to EEI-NEMA Band of typical published values
Rod gaps ...................... 50—60 c/s a.c. 1,18 0,6 —1,6
+ 1/50 or 1,5/40 imp. 0,96 0,8 —1,2
— 1/50 or 1,5/40 imp. 0,83 0,85—1,1
Suspension insulators .......... , , 50—60 c/s a.c. 1,18 0,9 —1,6
+ 1/50 or 1,5/40 imp. 0,93 0,7 —1,2
— 1/50 or 1,5/40 imp. 0,83 0 —9,5
Pin and apparatus insulators .... 50—60 c/s a.c. 1,53 0,5 —3,0
+ 1/50 or 1,5/40 imp. 0,83 0,5 —1,07
— 1/50 or 1,5/40 imp. 0,73 —
Bushings ...................... 50—60 c/s a.c. 1,18 0,9 —1,4
-h 1/50 or 1,5/40 imp. 0,96 1,07—1.0
-—-1/50 or 1,5/40 imp. 0,83 —
ELTEKNIK 1959 1 157
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