Commissioning of Electrical Systems for Command, Control

TM 5-694
(5) Volumetric leakage current. The current that flows through the volume insulation itself is volumetric
leakage current. It is the current that is of primary interest in the evaluation of the condition of the insulation. In
shielded power cable systems (rated > 5 kV), humidity, condensation, and precipitation on the surface of a
termination can increase surface leakage current and volumetric leakage current. Surface leakage current and
volumetric current added together is the quotient of the applied dc voltage and insulation resistance. In large ac
rotating machines rated 10,000 kVA or greater and rated 6000 V or higher, moisture, on the end windings,
increases surface leakage current and volumetric leakage current especially when dirt is also on the winding.
Since the insulation may weaken over time, age may cause an increase in the various currents during testing.
d. Insulation-resistance (megger) testing. For equipment containing electronic components, megger
testing must not be performed. However, this equipment should be tested according to manufacturer
specifications. In an insulation-resistance test, an applied voltage, from 600 to 5000 volts, supplied from
a source of constant potential, is applied across the insulation. The usual potential source is a
megohmeter, also known as a megger, either hand or power operated that indicates the insulation
resistance directly on a scale calibrated in megohms. A megohmeter that is a hand cranked, a rectifiertype,
or battery-operated instrument is suitable for testing equipment rated up to 600 volts. For
equipment rated over 600 volts, use of a 1000-volt or 2500-volt motor-driven or rectifier-type
megohmeter is recommended for optimum test results. The quality of the insulation is evaluated based on
the level of the insulation resistance. The insulation resistance of many types of insulation is variable
with temperature, so the data obtained should be corrected to the standard temperature for the class of
equipment under test. Published charts similar to table 2-1 are available for this purpose. The megohm
value of insulation resistance obtained will be inversely proportional to the volume of insulation being
tested. For example, a cable 1000-ft (304.8 m) long would be expected to have one-tenth the insulation
resistance of a cable 100-ft (30.48 m) long if all other conditions were identical. The insulation-resistance
test is relatively easy to perform and is a useful test used on all types and classes of electrical equipment.
Its main value lies in the charting of data from periodic tests, corrected for temperature, over the life of
the equipment so that deteriorative trends might be detected.
2-10
Table 2-1. Recommended minimum insulation resistance values at 40 o C (all values in MΩ).
Minimum Insulation Resistance after 1 minute
Test specimen
(MΩ)
kV + 1
For most windings made before about 1970, all
(Example: 15 kV machine is 15 + 1 = 16 MΩ) field windings, and others not described below
For most dc armature and ac windings built after
100
about 1970 (form-wound coils)
For most machines with random-wound stator
5
coils and form-wound coils rated below 1 kV
Notes: kV is the rated machine terminal to terminal voltage, in rms (root mean square) kV.
Recommended minimum insulation resistance values above, in megohms, at 40 o C of the entire machine
winding for overvoltage testing or operation of ac and dc machine stator windings and rotor windings.
From IEEE Std. 43-2000. Copyright © 2000 IEEE. All rights reserved.
e. Dielectric absorption testing. In a dielectric-absorption test, a voltage supplied from a source of constant
potential is applied across the insulation. The test voltage used may have to be significantly higher than the
insulation-resistance test in order to obtain measurable current readings. The potential source can be either a
meg-ohmmeter, as described above or a high-voltage power supply with an ammeter indicating the current being
drawn by the specimen under test. The voltage is applied for an extended period of time, from 5 to 15 minutes,
and periodic readings are taken of the insulation resistance or leakage current. The test data is evaluated on the
basis that if insulation is in good condition, its apparent insulation resistance will increase as the test progresses.