Ir - Insulation Resistance - Voltech AT5600 User Manual

C H A P T E R 7 – T E S T S
7.27.

IR - Insulation Resistance

Where Used
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.
Measurement Conditions
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.
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.
Specifying the Test Limits
Although the tester is capable of measuring insulation resistance values greater
than 1G, 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 100M is generally suitable for most applications.
Please also see
14.6.23 for specific advice on programming the test, and
10.2.11 for test accuracy specification
AT5600 User Manual 98-119 issue 14
A N D T E S T C O N D I T I O N S
Page 136