Voltech AT3600 User Manual page 325

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1.26. IR - I
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An Insulation Resistance check is recommended as good practice for most
transformers to check the integrity of the insulation between separate windings, or
between a winding and a screen.
It is used where winding isolation is a part of the functionality of the transformer, but
not part of a safety requirement, for which a Hi-Pot (EHT) test is preferred.
It would therefore be used, for example, between two primary windings, or between
two secondary windings.
M
To measure insulation resistance, the tester applies a dc voltage between two groups
of windings, with the windings in each group being shorted together. Each group
may contain as many windings as you wish.
The voltage and current are measured; dividing the voltage by the current gives the
insulation resistance.
You can also command the tester to perform a compensation measurement on the
empty fixture at the start of a test run. This is recommended as it will allow the tester
to subtract the errors due to the fixture from subsequent measurements, to give more
accurate results.
The test voltage may be chosen in the range from 100V to 7kV. (500V AT1600)
Normally however, you would perform the test with a voltage just greater than twice
the highest possible peak working voltage on the winding. Therefore, for example, to
check the isolation between the two primary windings of a line frequency
transformer, which could be connected in series and operated at 240V, 800V would
be a suitable test signal.
S
Although the tester is capable of measuring insulation resistance values greater than
1GΩ, there is no need to specify so high a value to check that windings are isolated.
In many cases, specifying a lower value will speed up the execution of the test. A
value of 100MΩ is generally suitable for most applications.
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