Insulation resistance (IR) testing and hipot (dielectric withstand) testing answer different questions. IR testing applies a moderate DC voltage and measures the resistance of the insulation, producing a condition indicator that can be trended over time. Hipot testing applies a higher voltage to prove that the insulation can hold its withstand level without breakdown, producing a pass or fail result.

IR testing is the non-destructive screening and trending tool; hipot testing is the proof tool. The choice between them depends on the question being asked, the asset condition and the governing standard.

What Each Test Measures: Resistance Trend vs Withstand Proof

An insulation resistance test measures the current that leaks through and across the insulation under a moderate DC voltage and expresses it as a resistance in megohms. The value is a snapshot of the insulation condition at that voltage, temperature and moisture level, and its power comes from comparison: the same measurement repeated over time and across phases reveals trends that no single value can show. IR testing also provides the timed readings from which polarization index and dielectric absorption ratio are calculated, giving a deeper view of the insulation’s behaviour.

A hipot test applies a voltage above the normal operating stress, usually for a defined duration, and watches for breakdown. The result is binary in character: the insulation held the voltage, or it failed. That binary outcome is exactly what an acceptance or proof decision needs, but it says little about the margin before failure or the condition of insulation that passed. The two tests are therefore complementary: IR describes condition, hipot proves capability.

Voltage Stress and the Risk of Damage

The voltage difference between the two tests is the source of their different risk profiles. IR testing uses a moderate voltage chosen for the insulation class, and in normal use it does not damage healthy insulation; it is considered a non-destructive test. Hipot testing deliberately applies a voltage high enough to prove the withstand margin, and if the insulation is weak, the test will find it by breaking it down. That is the purpose of the test, but it is also the risk: a hipot test on aged or weakened insulation can convert a repairable condition into a failure that the test itself caused.

This is why the sequence matters. IR testing is performed first, as the screening step, and its results inform whether a hipot test is appropriate. If IR testing indicates significant deterioration, moisture or a developing fault, the correct response is usually investigation, not an immediate hipot proof. The hipot test is reserved for insulation whose condition supports the application of the full withstand voltage, or where the governing specification requires the proof regardless and the risk has been accepted in writing.

Reading the Results: Condition Indicator vs Pass/Fail

An IR result is read in context: compared with the asset’s own history, across phases under the same conditions, and with temperature and moisture accounted for. A single megohm value means little without that context, which is why IR interpretation is built around trends and ratios rather than universal thresholds. The interpretation of IR, PI and DAR results is developed in the transformer insulation resistance interpretation article in this series for transformer insulation; the same logic of context and trend applies across assets.

A hipot result is read against the specified test level: the insulation held the voltage for the duration, or it failed. The pass criterion is defined by the governing standard or specification, including the voltage, duration and the conditions under which the test is performed. A hipot pass does not certify the insulation as healthy, and a hipot fail does not, by itself, identify the cause; it establishes that the insulation could not hold the applied stress and triggers the investigation.

Where Each Test Belongs in the Asset Lifecycle

IR testing belongs throughout the asset life: at commissioning to establish the baseline, during routine maintenance to build the trend, after events to check for change, and before other tests to screen the condition. It is the workhorse of condition assessment because it is portable, non-destructive and repeatable. Hipot testing belongs at the decision points where a proof is required: factory acceptance, installation acceptance, after major repair or modification, and at the intervals defined by the maintenance specification for the equipment class.

The two tests often appear in the same programme in a defined order. The maintenance specification for many equipment classes lists the IR test as a routine item and the hipot test as an acceptance or periodic proof item, with the IR results reviewed before the hipot level is applied. Reading the specification in this order, rather than treating both as interchangeable checks, is what protects the asset from an unnecessary withstand stress on weakened insulation.

Standards Context for IR and Hipot Limits

Both tests are governed by standards and manufacturer guidance that define the voltage, duration and acceptance logic for the equipment class. IR testing guidance is found in equipment-specific standards and maintenance specifications, and the interpretation of the readings is supported by recommended practice documents. Hipot levels are defined by the equipment standard for the voltage class and the test objective, with different levels for factory, site and maintenance tests in some specifications.

The discipline is identical for both: verify the governing document, its edition and its scope before applying a voltage or quoting an acceptance value. A hipot level intended for a factory test on new insulation is not automatically the right level for a maintenance test on an aged asset, and an IR threshold quoted from one source may not apply to another insulation system. The document, not the habit, sets the level.

Choosing the Right Test for the Question

Use the table to frame the decision, then read the governing document for the voltage, duration and acceptance logic. The question defines the test; the standard defines the level; the asset condition defines whether the test is appropriate.

A combined example shows how the two tests work together. A motor is returned to service after a period in storage, and the maintenance specification requires both a condition check and a proof. The test sequence begins with IR testing on each winding and the calculation of PI where the readings support it; the results are compared with the unit’s baseline and with the other phases. If the IR trend is stable and the PI indicates healthy insulation, the specification allows the hipot proof at the defined level, and the motor is tested and returned to service with a complete record. If the IR testing shows a low, deteriorating reading, the hipot proof is postponed and the insulation is investigated first, because applying the full proof voltage to weakened insulation would risk converting a repairable condition into a failure. The same logic applies to cables, transformers and switchgear: the screening test protects the proof test from being applied to an asset that cannot safely take it.

Frequently Asked Questions

Which test is non-destructive?

Insulation resistance testing is considered non-destructive in normal use because it applies a moderate voltage that does not damage healthy insulation. Hipot testing applies a higher voltage to prove the withstand level, and on weakened insulation it can cause breakdown, which is why IR screening should come first.

When should I use a hipot test instead of an IR test?

Use a hipot test when the decision requires a proof: acceptance after installation, verification after repair, or the periodic withstand proof defined by the maintenance specification. Use IR testing when the decision requires a condition trend, and use it before a hipot test to confirm the insulation can safely take the higher stress.

Do both tests need discharging afterward?

Both apply DC voltage in their basic forms, so both can leave charge on the test object that must be discharged and verified before connections are touched. A DC hipot in particular stores charge on the capacitance, and the discharge and re-verification are part of the safety procedure.

For insulation and withstand test equipment, see the transformer maintenance test equipment range and the high voltage insulation and withstand testing page.