Unitronics electric UM2B, User Manual

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3.- UM2B EQUIPMENT
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For the measurement to be performed under optimum conditions, there are certain
details that should be taken into account:
Conditions of the surfaces . Any dust accumulating on the surface of the sample tested may
alter insulation resistance measurements if there is associated humidity, for example in the
case of rainfall.
Temperature . The resistance of insulating materials changes with temperature.
Consequently, the result of a test will be comparable to that of another only if both are
performed at the same temperature. For this purpose, it is habitual to refer tests to certain
reference temperatures, with the appropriate correction parameters, in order to allow for
comparison. It is of interest that the tested machine has his temperature stabilized (let settle
after switched off from service) and his temperature be measured from the inside with
appropriate accuracy. Temperature has a large influence in dielectric evaluation as insulation
resistance is directly related to temperature variations. To figure out, each 10ºC of thermal
increment for the same increment to the same dielectric, his resistance is halved.
Test voltage . Insulation measurements will be performed at test voltage values agree with
the working voltage of the machines to be tested, in order not to cause degradation to their
insulations.
Previous charge effect . A factor that affects insulation and dielectric absorption
measurements is the preliminary presence of a previous charge in the insulation. This charge
may come from the normal operation of a generator with its neutral not grounded or from
previous insulation resistance measurements. A lot of time may be saved if the generator
winding is grounded until such time as the test is to be performed. The duration of this
grounding should be around four times the charge period of the previous test.
Measuring cables . In view of the weak currents involved in the measurement and its special
characteristics, it is important to take into account the following as regards the cables:
–
Do not tread on cables or knock or move them during testing.
–
Locate the cables extended, without bending or folding, as close as possible and in
parallel throughout their entire length.
–
Should be in perfect condition and checked for use.