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Precautions When Making DC Insulation Tests
• The equipment should be taken off the line sufficiently in advance to
permit it to cool to ambient temperature.
• When you are testing windings, they should be clean and dry; let
solvents and cleaners evaporate. Should foreign matter or wet
surfaces be present, erroneous readings may result. (A clean, dry
sample's resistance will rise for 10 to 15 minutes, whereas a wet,
dirty one will stabilize quickly.)
• Make sure that the equipment tested is properly discharged and
grounded before testing.
• When testing individual windings, connect all other windings (not
under test) together and ground to motor frame.
• When testing phases, be sure they are open to test each individually.
• After applying a test voltage, allow sufficient discharge time. As a
rule, discharge twice as long as tested.
Note: The instrument voltmeter will indicate the discharge voltage at the
terminals.
Insulation Measurement - Connections
Figure 12 shows the connections to measure the insulation of one
conductor to the other conductors. The cable should be disconnected at
both ends to avoid leakage through switchboards and panels.
Megohmmeter Models 5000N/5100/5110
Conductor under test
- LINE
GUARD
MEGOHMMETER
+ EARTH
Figure 12
- 27 -
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