XS series operating manual
XS user manual v0.55 - 133 -
Environmental influences
Temperature, pressure and humidity conditions have an influence on the dielectric tests results.
TEMPERATURE : The temperature having an influence on the gas density, this one is altering the
performances of the liquid or gaseous insulating material. The oils, often used as insulation are never
pure, the dissolved quantity of gas increases with the temperature and is decreasing the insulation
quality of the oil. The large variety of materials used as solid insulation doesn’t allow to deduce a
general rule on their behavior with the temperature (the insulation specifications having a tendency to
dissipate when the temperature increases).
PRESSURE : The withstand voltage in gas changes with the pressure following the Paschen’s law.
This law shows a minimum of the breakdown voltage for a particular value of the pressure by distance
product, otherwise the more the pressure increases, the more the breakdown voltage is high. The
liquids used as dielectric insulation are influenced by the pressure, the dielectric strength increasing
with the pressure. In theory, the solid insulations are a little influenced by the pressure because this
one doesn’t modify a lot their thickness and their internal composition.
HUMIDITY : The withstand voltage of gas changes with humidity. In example for the air and for values
of relative humidity < 80%,the dielectric strength increases a little with the humidity increasing (the
water molecules, more dense than the gas, slows down the avalanche phenomena). The water
presence in an insulating liquid such as oil degrades the dielectric strength by water electrolysis,
development of gas producing partial discharges bringing to breakdown. Under combined effect of the
humidity (>95%) and the temperature (>100°C) most of the polymers dissipate. The water can
produce inflating in the insulation and create cracks which will facilitate the electric arcs advance.
Insulation resistance measurement
The insulation resistance measurement is intended to verify that the various components and sub-
assemblies of electrical equipment have an insulation resistance such as the leakage currents don’t
reached inadmissible values.
The principle is to apply a DC voltage, stable and specified (selected among the standard values)
between defined points and after a prescribed time, to measure the current flowing through the tested
material. By using the Ohm’s law (resistance = voltage / current), the result is given by the value of the
insulation resistance. Then this value is compared to the minimum threshold specified by the standard
used for the test.
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