Absorption Ratio Insulation Resistance Testing: Principles, Methods, and Applications

Absorption ratio insulation resistance testing is a diagnostic technique that measures the dielectric absorption behavior of electrical insulation by comparing resistance readings taken at 15 seconds and 60 seconds under an applied DC voltage. Widely used for transformers, motors, generators, and cables, this test helps identify moisture ingress, contamination, and insulation aging before catastrophic failure occurs. This article explains the testing principles, applicable standards, sample types, procedures, and acceptance criteria used by accredited third-party laboratories and field service engineers.

Technical Overview

Absorption ratio insulation resistance testing is a non-destructive electrical test used to evaluate the condition of insulation systems in high-voltage equipment. When a DC test voltage is applied to an insulation structure, the total current consists of three components: the capacitive charging current, which decays rapidly; the absorption (polarization) current, which decays gradually over seconds to minutes; and the conduction (leakage) current, which remains essentially constant. In healthy, dry insulation, the absorption current dominates the early portion of the test, causing the measured resistance to rise steadily as charging progresses. In damp or contaminated insulation, the leakage current dominates, and the measured resistance stabilizes almost immediately.

The absorption ratio (K) is defined as the ratio of the insulation resistance measured at 60 seconds to that measured at 15 seconds after voltage application, expressed as K = R60s / R15s. A higher absorption ratio indicates good dielectric absorption characteristics and dry insulation, while a value approaching 1.0 indicates moisture, dirt, or degradation. The test is closely related to the polarization index (PI), defined as the ratio of the 10-minute to the 1-minute resistance value, and both are recognized in international and national standards such as IEC 60034, IEC 60076, IEEE Std 43, IEEE Std 95, ASTM D257, GB/T 50150, and DL/T 596.

• Absorption ratio (K): R60s / R15s, typically required to be no less than 1.3 for power transformers at 10–30°C
• Polarization index (PI): R10min / R1min, typically required to be no less than 2.0 for rotating machine windings per IEEE Std 43
• Test instrument: Insulation resistance tester (megohmmeter) capable of timer-controlled readings at 15 s and 60 s

Testing Samples

Power Transformers

Oil-immersed and dry-type power transformers are the most common subjects of absorption ratio testing. Measurements are performed between high-voltage winding and ground, between low-voltage winding and ground, and between windings, with non-tested windings and the tank grounded. The results are used to judge moisture in solid insulation and oil, particularly after transportation, storage, or before energizing.

Rotating Machines

Stator and rotor windings of AC motors, DC machines, turbo generators, hydro generators, and wind turbine generators are tested to assess winding dryness and insulation integrity. IEEE Std 43 provides minimum insulation resistance and polarization index criteria for rotating machine windings based on rated voltage and temperature.

Power Cables and Accessories

Medium- and high-voltage power cables, terminations, and joints are tested after installation or repair. Because cables have significant capacitance, long charging times and thorough discharge procedures are essential for valid absorption measurements.

Switchgear, Bushings, and Instrument Transformers

Metal-clad switchgear, circuit breakers, bushings, current transformers, and voltage transformers undergo absorption ratio testing during commissioning and periodic maintenance to verify the condition of their solid and oil-impregnated insulation structures.

Electrical Equipment and Components

Electrical appliances, solenoid coils, relay windings, insulating materials, and insulation components can also be evaluated for dielectric absorption characteristics as part of quality control and type testing programs.

Testing Items

Depending on the equipment class and the applicable standard, an absorption ratio test program typically includes the following measured and calculated quantities:

• Insulation resistance at 15 seconds (R15s)
• Insulation resistance at 60 seconds (R60s)
• Insulation resistance at 10 minutes (R600s) for polarization index determination
• Absorption ratio K = R60s / R15s
• Polarization index PI = R10min / R1min
• Temperature-corrected insulation resistance values (for example, converted to 40°C for oil-immersed transformers)
• Comparison of measured values with minimum acceptance criteria and with previous historical test records
• Dielectric absorption ratio trends for condition-based maintenance decisions

Testing Methods

Test Voltage Selection

The DC test voltage is selected according to the rated operating voltage of the equipment under test. Typical practice follows the guidance of IEEE Std 43, GB/T 50150, and DL/T 596:

• Rated voltage below 100 V: approximately 250 V DC
• Rated voltage 100 V to 1000 V: 500 V or 1000 V DC
• Rated voltage 1000 V to 5000 V: 1000 V to 2500 V DC
• Rated voltage above 5000 V: 2500 V or 5000 V DC

Test Procedure

• De-energize, isolate, and verify the equipment is dead, then discharge any residual capacitance to ground
• Clean bushings and terminals to prevent surface leakage from distorting the measurement
• Connect the line lead of the megohmmeter to the winding or conductor under test and the ground lead to the grounded component
• Apply the selected DC voltage and record resistance readings at exactly 15 seconds and 60 seconds (and at 1 minute and 10 minutes when the polarization index is required)
• Record the winding or insulation temperature at the time of testing
• Discharge the test object thoroughly to ground after the measurement, since capacitive components can retain a dangerous charge

Temperature Correction and Ambient Conditions

Insulation resistance changes significantly with temperature, decreasing by roughly half for every 10°C increase in many insulation systems. Measured values must therefore be corrected to a reference temperature, commonly 40°C for oil-immersed transformers as specified in IEC 60076 and GB/T 50150, or 40°C for rotating machine windings per IEEE Std 43. Tests should preferably be performed with the equipment at a stable temperature above the dew point, in relatively dry ambient conditions, because high humidity or condensation on bushing surfaces can produce misleadingly low readings.

Interpretation of Results

• K ≥ 1.3 (at 10–30°C for transformers): insulation is generally considered dry and acceptable
• K < 1.3: possible moisture ingress or surface contamination; drying or cleaning and retest are recommended
• PI ≥ 2.0: acceptable for most rotating machine winding insulation classes
• PI < 1.0 or a declining trend across maintenance cycles: indicates degradation, moisture, or contamination requiring further diagnosis
• Absorption ratio evaluation is most meaningful when compared with factory data, previous field records, and sister units tested under similar conditions

Applications

Absorption ratio insulation resistance testing is applied across the entire life cycle of electrical assets:

• Commissioning and handover: Verification of insulation condition before first energization of transformers, cables, and machines, as required by acceptance standards such as GB/T 50150 and DL/T 596
• Preventive and predictive maintenance: Periodic trending of absorption ratio and polarization index to detect moisture absorption and insulation aging early in power plants, substations, and industrial facilities
• Post-repair verification: Confirmation of successful drying, cleaning, or rewinding of transformers and rotating machines
• Condition assessment after abnormal events: Evaluation of water ingress after flooding, insulation damage after short circuits, or thermal stress after overloads
• Quality control in manufacturing: Routine factory testing of motors, generators, transformers, cables, and insulation components by manufacturers and accredited third-party laboratories
• Industry sectors: Power generation and transmission, petrochemical plants, rail transit, marine and offshore installations, mining, data centers, and general manufacturing

FAQ

What is the acceptance criterion for the absorption ratio?

For power transformers, Chinese national acceptance standard GB/T 50150 requires an absorption ratio of no less than 1.3 measured at 10–30°C. For rotating machine windings, IEEE Std 43 recommends a polarization index of at least 2.0, or a minimum insulation resistance value corrected to 40°C, whichever yields the higher assessment. Specific product standards may define additional criteria.

What is the difference between absorption ratio and polarization index?

Both quantities describe dielectric absorption, but they use different time scales. The absorption ratio compares the 60-second and 15-second readings (R60/R15), while the polarization index compares the 10-minute and 1-minute readings (R10min/R1min). Modern insulation materials with low absorption current may show a low absorption ratio despite being healthy, in which case the polarization index and the absolute resistance value provide a more reliable assessment.

Why is temperature correction necessary?

Insulation resistance and absorption characteristics vary strongly with temperature. Without correcting readings to a standard reference temperature, results from different test dates or different units cannot be compared meaningfully, and incorrect pass or fail decisions may result. Temperature, humidity, and surface cleanliness should always be recorded together with the resistance values.

Why did my absorption ratio come out low even though the insulation resistance is high?

Modern epoxy-mica and polyester insulation systems exhibit very fast polarization, so the resistance reaches its final value almost immediately, giving an absorption ratio close to 1.0. In such cases, standards recommend judging by the corrected insulation resistance magnitude and the polarization index rather than the absorption ratio alone.

How can a low absorption ratio caused by moisture be remedied?

Depending on the equipment type, remedies include hot oil circulation, vacuum drying, hot air circulation for dry-type units, controlled warm-up of machine windings, and cleaning of contaminated insulation surfaces. After treatment, the absorption ratio test should be repeated to confirm that the value meets the acceptance criterion before the equipment is returned to service.

Is absorption ratio testing destructive to the insulation?

No. The test applies a controlled DC voltage within the dielectric withstand capability of the insulation, making it non-destructive. However, the test object must be fully discharged afterward, because the capacitance of cables, windings, and transformers can store a hazardous electrical charge for an extended period.