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SEFELEC1500M Instructions
149
V1.06
16.3.
INFLUENCE OF THE ENVIRONMENTAL CONDITIONS
Temperature, pressure and humidity affect the results of dielectric tests.
16.3.1.
TEMPERATURE
As temperature affects the density of gases, it will modify the insulation properties of these
gas and liquid insulators. The oils often used as insulators are never pure and the amount of
dissolved gas increases with temperature which will adversely affect the insulating properties
of the oil. So many different types of material are used as solid insulators that it is impossible
to define a general law on their behaviour with temperature (with the insulating properties
tending to degrade as the temperature increases).
16.3.2.
PRESSURE
The voltage resistance of gases changes according to the pressure according to the
PASCHEN law. This law presents a minimum breakdown voltage at a particular value of the
product of the pressure by the distance, otherwise, the higher the pressure, the higher the
breakdown voltage. The liquids used as dielectric insulators are influenced by pressure, the
rigidity increases monotonically with pressure. Solid insulators are, in theory, unaffected by
pressure as this does not substantially change either their thickness or their internal composi-
tion.
16.3.3.
HUMIDITY
The voltage resistance of gases changes according to the presence of water vapour. For air,
for example, and for relative humidity values greater than 80%, the dielectric strength in-
creases slightly with increasing the humidity (the water molecules are denser than the gas
and inhibit the avalanche effect). Water in a liquid insulator such as an oil degrades the die-
lectric strength by electrolysis of the water (forming gases leading to partial discharges induc-
ing a breakdown). Under the combined effect of high humidity (> 95%) and temperature (>
100°C) most polymer insulators are degraded. Water can make the insulators swell and fis-
sure, providing paths for electric arcs.