Insulation and dielectric
The polarization index and dielectric absorption ratio compare insulation resistance readings taken at different times under one steady test voltage. They show whether windings in reactors, regulators, transformers and machines are dry and clean, independent of equipment size and less sensitive to temperature than a single reading.
The polarization index and the dielectric absorption ratio are two ways of summarizing a timed insulation resistance test. A direct test voltage is applied to a winding and held, the resistance is read at set moments, and the later reading is divided by an earlier one. The dielectric absorption ratio compares two readings taken fairly close together early in the test, while the polarization index compares readings separated by a longer span. Both are pure ratios, so they have no units and do not depend directly on how big the winding is. They are used on windings of liquid-filled reactors, step and induction voltage regulators, power and distribution transformers, motors and generators, and on other large insulation systems where a single resistance value could be misleading. The measurement is made with the same megohmmeter used for ordinary insulation resistance, provided it can hold a steady voltage and log readings over a longer period. The question the ratios answer is not how high the resistance is, but how the resistance behaves as the voltage stays on, which is a direct signal of how dry and clean the insulation is.
A single resistance reading can be high on equipment that is wet or contaminated, if the equipment is small or cold, and low on sound equipment that is large or warm. Moisture ingress, surface contamination from oil, dust or salt, thermal aging, and mechanical damage all weaken the insulation, and they change the way the reading climbs over time before they change the final value much. A timed ratio is far less dependent on the size of the winding and on temperature, which makes it a more reliable screen for the question that matters before energizing or returning a machine to service: is this insulation dry enough? Finding a wet reactor, a regulator that absorbed moisture during storage or a motor winding with surface contamination in a planned outage is far cheaper than a ground fault in service. The ratio also helps decide whether a unit should be dried out, cleaned or baked before any higher-voltage test, because applying a strong test to wet insulation risks damage. Run on a schedule, the ratios build a history that shows slow deterioration.
When a direct voltage is applied to insulation, the current has three parts. The charging current fades within moments as the geometric capacitance charges. The absorption current comes from the slow alignment of molecules and the trapping of charge in the insulating materials, and it decays over minutes. The leakage current is the small steady flow through and across the insulation. As the first two parts die away, the total current falls and the calculated resistance rises. In clean, dry insulation the absorption component is significant and decays slowly, so the resistance keeps climbing during the whole test and the later reading is clearly larger than the earlier one. In wet or contaminated insulation the leakage component dominates from the start, so the current barely changes and the resistance stays almost flat, giving a ratio near unity. The ratio therefore tells how much of the current is leakage and how much is absorption. Because the temperature changes both the absorption and the leakage in a similar way, it affects the ratio much less than a single value, although it is still recorded. Older insulation systems and modern synthetic ones behave differently, and some synthetic windings reach very high resistance so quickly that the ratio can look unusually high or noisy.
The equipment is de-energized, isolated from every source, proven dead and grounded under the owner's lockout and permit procedures, and is allowed to discharge fully before leads are attached, because residual charge from an earlier test or from operation distorts the first readings. Bushings and terminations are cleaned and dried, connected apparatus and electronics are removed or protected, and the winding under test is separated from the others, which are grounded or connected to the guard. The megohmmeter is connected between the winding and ground, or between windings, and its voltage is chosen from the governing standard's table for the equipment rating, not by habit. The voltage is applied and kept steady while readings are taken at the specified moments. Instruments with automatic timing log the values and compute the ratios, and a manual operator watches a clock and reads the dial at set times. The start of the test is critical, since readings depend on the voltage being applied promptly, and a drifting supply or a loose connection ruins a ratio. At the end the insulation is discharged through the instrument and then with a grounding hook, and it remains grounded for a period related to how long it was charged. Winding temperature, ambient temperature and humidity are recorded, and the same instrument, voltage and connections are used on later tests so the ratios can be trended.
The record holds the resistance reading at each timed moment, the two ratios derived from them where used, the test voltage applied and the connections that were made, and the temperature of the winding and of the surroundings with the relative humidity. It also notes the identity and calibration status of the instrument, the winding or insulation path tested, the as-found condition such as freshly removed from service or visibly contaminated, and any connections that were lifted. A resistance ratio has meaning only alongside the way it was measured, so these items allow a later reader to compare a new result with the stored one on equal terms.
No single number decides whether insulation is acceptable, and this page does not give one. Published criteria for these ratios take several shapes. The first is a minimum ratio that is judged against the published guidance for the class of equipment, with ANSI/NETA ATS-2025 setting expectations for acceptance testing and ANSI/NETA MTS-2023 for maintenance testing; the guidance and the tables of test voltage differ by equipment type. The second is trending against earlier results on the same winding, where a falling ratio matters even if it is still within the published guidance. The third is comparison between similar windings or phases, where an outlier points to a local problem. The fourth is reading the ratio alongside the corrected resistance from a single reading, because a high ratio with a very low resistance does not mean the insulation is sound, and a low ratio with a very high resistance can simply reflect insulation so good that the leakage is negligible and the absorption has already finished. Manufacturer data also come into play. For the actual values and tables, refer to your own licensed copy of the standards named above, which remain the authority for acceptance.
Acceptance and maintenance tests serve different goals. At acceptance, the ratio screens new or repaired equipment for moisture picked up in transit, storage or installation and starts the history. In maintenance it follows the condition over time, and consistent conditions matter more than any single result. The two ratios are used in different places. The dielectric absorption ratio, with its short span, suits smaller equipment and quick checks, while the polarization index, with its longer span, is used where the size of the equipment and the speed of its absorption make a longer test meaningful. On rotating machines the choice depends on machine size and the standard's table. On liquid-filled reactors and transformers, the winding-to-ground reading and the polarization index are taken with the liquid at a stable temperature, and the oil condition affects the result. Regulators with their windings and tap changer follow the same logic. Very high-resistance insulation can give a low ratio without being poor, and the standards allow for that, so the ratio is never read alone.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Both divide a later insulation resistance reading by an earlier one under the same steady voltage. The dielectric absorption ratio uses two readings close together early in the test, while the polarization index uses readings separated by a longer span, which makes it a better indicator on large equipment.
Temperature changes the absorption and leakage parts of the current in a similar way, so dividing one timed reading by another largely cancels the effect. A single resistance value shifts strongly with temperature, which is why it must be corrected before comparison, while the ratio is still recorded with the temperature.
A ratio close to unity means the resistance barely rises during the test, which points to leakage dominating, as in wet, dirty or aged insulation. It is a reason to inspect, clean or dry the equipment and to retest, not a diagnosis on its own, because very high-resistance insulation can also give a low ratio.
It can be taken on most winding insulation systems with a suitable megohmmeter, but the standards name where it is expected, for example on larger machines and on certain liquid-filled equipment. Where the equipment or its insulation makes the test unsuitable, the ordinary timed reading and other tests are used instead.
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