Ratio Testing; Successful Insulation Resistance Testing - AEMC 6503 User Manual

4.3.3

Ratio Testing

In time resistance reading (Dielectric Absorption Ratio), readings
are taken at 30 seconds and 60 seconds to obtain the dielectric
absorption ratio.
Insulation resistance @ 60s
Insulation resistance @ 30s
This test is useful to increase the accuracy of spot testing. In gen-
eral, a ratio of 1.25:2 or better should be required. A ratio below this
indicates that repair is probably needed.
Remember that a DC insulation test may be used for acceptance
testing, but is more commonly used to check the gradual deteriora-
tion of equipment over its life. Consult your equipment manufacturer
for specific test or test voltage if not known.
Insulation resistance decreases with moisture, temperature and age
and should be recorded over time at a given temperature and cor-
rected.
4.4 Successful Insulation Resistance Testing
• Check with the equipment manufacturer for factory insula-
tion resistance readings.
• Do not rely on insulation resistance testing alone as proof
of winding conditions.
• Do not expect the same value for all parts of all machines.
• Observe consistent test time duration, recognizing that
total current through insulation under test will vary with
time.
• Correct all readings properly to a standard reference tem-
perature (see IEEE std #43-2000, Temperature Correction
Curve).
• Know what you are testing. Isolate the piece of equipment
from other circuitry.
• Watch trends rather than relying on single "spot" readings.
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