A spot reading tells you the insulation resistance at one voltage. A PI test tells you how resistance changes over time. But neither one tells you how the insulation responds to increasing electrical stress.
That’s what the step voltage test does. It applies voltage in defined steps — 500V, then 1000V, then 2500V, then 5000V — and measures the insulation resistance at each level. If the resistance stays stable across all steps, the insulation is healthy. If it drops at higher voltages, there are hidden defects that lower-voltage tests can’t detect.
This guide covers the test principle, how to perform it, how to interpret the results, and when it adds real value over simpler methods.
Table of Contents
What the Step Voltage Test Detects
The step voltage test finds insulation defects that only appear under higher electrical stress. These are problems that a standard 500V or 1000V spot reading will miss:
Pinholes and voids — Tiny holes or air pockets within the insulation. At low voltage, leakage current flows around them. At higher voltage, the electrical field concentrates at these weak points and current flows through them.
Cracks in aged insulation — Thermal cycling, vibration, and chemical exposure create micro-cracks in insulation over time. At low voltage, these cracks have high resistance. At higher voltage, they begin to conduct.
Dry, brittle insulation — Old varnish-based insulation that has lost its flexibility. It may still have high resistance at operating voltage, but breaks down under modest overvoltage.
Contamination trapped inside insulation layers — Moisture, dust, or chemical residue that has penetrated between insulation layers. At low voltage, the contamination path has high resistance. At higher voltage, it becomes conductive.
Tracking paths — Carbon traces left by previous partial discharges or flashovers. These paths have lower breakdown voltage than the surrounding insulation.
The common thread: these defects are voltage-dependent. They don’t show up at normal test voltage, but they reveal themselves when you increase the stress.
How It Works
The principle
Good insulation follows Ohm’s law. If you double the voltage, the current doubles — and the calculated resistance stays the same. The insulation resistance should be independent of the applied voltage.
Damaged insulation doesn’t follow Ohm’s law at higher voltages. Weak spots begin to conduct disproportionately more current, and the calculated resistance drops.
The test
You apply test voltage in equal steps, holding each step for a fixed duration (usually 60 seconds). At the end of each step, you record the insulation resistance. Then you increase to the next step.
Typical step sequence for a motor rated below 1 kV:
| Step | Test Voltage | Duration |
|---|---|---|
| 1 | 500V | 60 seconds |
| 2 | 1,000V | 60 seconds |
| 3 | 1,500V | 60 seconds |
| 4 | 2,000V | 60 seconds |
| 5 | 2,500V | 60 seconds |
For medium-voltage equipment (1 kV – 5 kV):
| Step | Test Voltage | Duration |
|---|---|---|
| 1 | 1,000V | 60 seconds |
| 2 | 2,500V | 60 seconds |
| 3 | 5,000V | 60 seconds |
The number of steps and the maximum voltage depend on the equipment’s voltage rating. A common guideline is to test up to approximately 2× the rated voltage, but never exceed the cable or equipment manufacturer’s maximum test voltage.
What you’re looking for
Plot the resistance values against the voltage steps. For healthy insulation, you get a flat line — resistance stays the same or even increases slightly at each step. For damaged insulation, resistance drops at higher steps.
A resistance drop of 25% or more between any two consecutive voltage steps is generally considered significant and warrants investigation.
When to Use This Test
The step voltage test adds value in specific situations. It’s not a routine test for every piece of equipment — it’s a diagnostic test for when you need deeper insight.
Good candidates for step voltage testing
Aging motors with acceptable spot readings — A motor that has been in service for 15+ years and still shows acceptable IR at 500V or 1000V. The step voltage test can reveal hidden deterioration that the standard test misses.
Equipment after a suspected overvoltage event — After a lightning strike, switching surge, or VFD-related transient, insulation may have been weakened internally even though the equipment still runs. Step voltage testing reveals stress damage.
Motors in harsh environments — High temperature, high humidity, chemical exposure, or heavy vibration. These environments create the types of defects (cracks, voids, contamination) that step voltage testing is designed to find.
Before returning critical equipment to service after repair — After a rewind or insulation repair, step voltage testing provides deeper verification than a spot reading alone.
When trending shows a decline but spot readings still pass — If IR at 500V has been trending downward but is still above minimum, a step voltage test can determine whether the decline is caused by surface contamination (which won’t get worse at higher voltage) or internal insulation degradation (which will).
When NOT to use step voltage testing
New equipment — New insulation in good condition will show flat resistance across all steps. The test confirms what you already know.
Equipment with known low IR — If the insulation already reads below minimum at normal test voltage, there’s no need to apply higher voltage. You already know the insulation is compromised.
Equipment not rated for the higher test voltages — Some control cables, instrument circuits, and electronic equipment are only rated for 500V test voltage. Applying 2,500V will damage them.
Step-by-Step Procedure
Preparation
- De-energize, isolate, lockout/tagout.
- Disconnect the equipment from all external circuits.
- Disconnect VFDs, PLCs, surge protection, and all electronic devices.
- Discharge all terminals to ground for at least 60 seconds.
- Record ambient temperature, equipment temperature, and humidity.
Running the test
- Connect the megger: LINE lead to the winding or conductor under test, EARTH lead to ground / frame.
- Step 1: Apply the lowest test voltage (e.g., 500V DC). Hold for 60 seconds. Record the IR reading at 60 seconds. Do not remove the voltage.
- Step 2: Increase to the next voltage level (e.g., 1,000V DC). Hold for 60 seconds. Record the reading at 60 seconds. Continue holding.
- Steps 3–5: Continue stepping up through each voltage level, holding 60 seconds at each step and recording the reading.
- After the final step, remove the test voltage.
Critical detail: Many modern megohmmeters have a built-in step voltage function that automates this process. You program the voltage steps and duration, and the instrument runs the entire sequence and logs all readings. If your instrument doesn’t have this function, you can perform the test manually by adjusting the voltage between steps — but you must keep the voltage applied continuously throughout the test (don’t remove and reapply between steps).
After the test
- Discharge the equipment to ground for at least 4 times the total test duration. For a 5-step test (5 minutes total), discharge for at least 20 minutes. The higher voltages used in step testing store more energy in the insulation.
- Record all readings and plot resistance vs voltage.
How to Interpret the Results
The decision rule
Good insulation: Resistance stays within ±10% across all voltage steps. The line is flat or slopes slightly upward.
Marginal insulation: Resistance drops 10%–25% at one or more higher voltage steps. The insulation has minor defects that are beginning to conduct. Schedule maintenance and increase testing frequency.
Bad insulation: Resistance drops more than 25% at a higher voltage step, or drops progressively across multiple steps. The insulation has significant internal defects. Investigate further before operating.
Critical: Resistance collapses (drops by 50%+ or approaches zero) at a higher step. The insulation is on the verge of failure. Do not energize.
Interpreting the shape of the curve
Flat line — Resistance is essentially the same at every voltage. The insulation is healthy and follows Ohm’s law. No internal defects.
Slight upward slope — Resistance increases slightly at higher voltages. This can happen with some insulation materials and is normal. The insulation is healthy.
Downward slope starting from a specific step — Resistance is stable at lower voltages but drops at a certain voltage threshold. This means the insulation has a defect that only conducts above that threshold. The higher the threshold, the less severe the problem — but it still needs investigation.
Steady downward slope from step 1 — Resistance drops at every step. Multiple defects at various severity levels. The insulation is extensively degraded.
Sudden collapse — Resistance drops sharply at one step. A major defect (crack, void, or contamination pocket) just reached its breakdown voltage. Stop the test. Do not energize.
Real-World Examples
Example 1: Motor with hidden crack damage
A 200 kW, 400V motor. In service for 18 years. Spot reading at 500V DC: 85 MΩ. PI: 2.3. Both values are acceptable.
Step voltage test results:
| Voltage | IR at 60s |
|---|---|
| 500V | 85 MΩ |
| 1,000V | 82 MΩ |
| 1,500V | 78 MΩ |
| 2,000V | 45 MΩ |
| 2,500V | 22 MΩ |
Interpretation: Resistance is stable through 1,500V but drops sharply at 2,000V (42% decrease from the previous step). The insulation has an internal defect — likely a crack or void — that begins conducting at around 1,500–2,000V. The standard spot reading at 500V completely missed it.
Action: Schedule rewind or at minimum run the motor on reduced load while planning the repair. This motor will likely fault during the next voltage transient.
Example 2: Healthy motor
Same rating. In service for 10 years.
| Voltage | IR at 60s |
|---|---|
| 500V | 220 MΩ |
| 1,000V | 215 MΩ |
| 1,500V | 218 MΩ |
| 2,000V | 210 MΩ |
| 2,500V | 208 MΩ |
Interpretation: Resistance is stable across all steps (within 5% variation). The insulation follows Ohm’s law. No internal defects. The motor is in excellent condition.
Example 3: Surface contamination vs internal damage
Motor A reads 30 MΩ at 500V. Step test: 30 → 28 → 27 → 26 → 25 MΩ. Slight decline but essentially flat. Diagnosis: Low IR caused by surface contamination. Clean the motor windings and retest.
Motor B reads 30 MΩ at 500V. Step test: 30 → 25 → 15 → 8 → 3 MΩ. Progressive collapse. Diagnosis: Internal insulation degradation. Cleaning won’t fix this. The motor needs repair.
Both motors have the same spot reading. The step voltage test reveals completely different conditions.
Step Voltage vs Other Insulation Tests
| Test | What It Measures | Duration | Detects |
|---|---|---|---|
| Spot reading | IR at one voltage, one time | 60 seconds | Gross insulation failures |
| DAR | Absorption at one voltage over 60s | 60 seconds | Moisture, contamination |
| PI | Absorption at one voltage over 10 min | 10 minutes | Moisture, contamination, general degradation |
| Step voltage | IR vs voltage stress | 5–10 minutes | Cracks, voids, brittle insulation, tracking |
| Hi-pot | Withstand at high voltage | 1 minute at test V | Breakdown voltage of weakest point |
The step voltage test fills the gap between the PI test (which stresses insulation over time at one voltage) and the hi-pot test (which applies a single high voltage). It reveals voltage-dependent defects without pushing the insulation all the way to breakdown.
Why not just do a hi-pot test?
A hi-pot test applies a very high voltage (typically 2× rated + 1,000V) and checks for breakdown. It’s pass/fail. If the insulation survives, it passes. If it breaks down, it fails — and the insulation is permanently damaged.
The step voltage test is gentler. You increase voltage gradually and monitor the response. If resistance starts to drop, you stop before breakdown occurs. You get diagnostic information without destroying the insulation. This makes it ideal for maintenance testing where you want to assess condition without damaging the equipment.
Equipment Requirements
What you need
A megohmmeter with multiple selectable test voltages. Most modern instruments support at least 500V, 1,000V, 2,500V, and 5,000V. Better instruments offer a built-in step voltage (SV) test mode that automates the sequence.
Recommended features for step voltage testing:
- At least 4–5 selectable voltage levels up to 5 kV
- Built-in timer with automatic readings at each step
- Data logging (to store the complete step sequence)
- Graph display (some instruments show IR vs voltage in real time)
Popular instruments with SV capability include models from Megger (MIT series, S1 series), Fluke (1550C), and others.
What you don’t need
You don’t need a hi-pot tester or impulse generator for the step voltage test. This is a DC insulation resistance test at elevated voltages — your megohmmeter handles it.
Limitations
Not suitable for all equipment. Some equipment is only rated for 500V test voltage. Applying 2,500V will damage it. Always check the equipment and cable manufacturer’s maximum test voltage.
Not a replacement for the PI test. The step voltage test reveals voltage-dependent defects. The PI test reveals moisture and contamination. Use both for a complete picture on critical equipment.
Requires interpretation. Unlike a spot reading (which has clear pass/fail thresholds from standards), the step voltage test requires judgment. A 15% drop might be acceptable on one machine and concerning on another, depending on history and application.
Temperature still matters for absolute values. While the relative comparison between steps is somewhat temperature-independent, the absolute IR values at each step are still affected by temperature. Record the temperature and correct if comparing to historical data.
Risk of damage at high test voltages. Although gentler than a hi-pot test, applying 5 kV to insulation that is already severely degraded can push it over the edge. If the first two steps show declining resistance, consider stopping rather than continuing to higher voltage.
FAQ
How many voltage steps should I use?
Five steps is typical and provides a good balance between diagnostic depth and test duration. Three steps is the practical minimum for meaningful interpretation. More than five gives diminishing returns unless you’re doing detailed research testing.
What’s the maximum voltage I should apply?
A common guideline is approximately 2× the rated voltage of the equipment. For a 400V motor, that’s about 800V — but most technicians test up to 2,500V for diagnostic purposes. Never exceed the insulation’s rated test voltage from the manufacturer’s data sheet. For cables, check the maximum DC test voltage specified by the cable manufacturer.
How long should I hold each step?
60 seconds is standard. This allows the absorption current to settle and gives a stable reading. Some testing protocols use 30 seconds per step for faster screening, but 60 seconds gives more reliable data.
What does it mean if resistance increases at higher voltage?
A slight increase (5–10%) can be normal for some insulation materials and is not concerning. The key diagnostic signal is a decrease, not an increase.
Can I combine the step voltage test with a PI test?
Yes. You can run a PI test at the first voltage step (record at 1 minute and 10 minutes), then continue with the step voltage sequence. This gives you both time-dependent (PI) and voltage-dependent (SV) diagnostics in a single session. Many advanced megohmmeters support this combined test mode.
Is the step voltage test destructive?
No — if done correctly. The test voltages are below the equipment’s withstand level, and you monitor resistance at each step. If resistance starts dropping significantly, you stop before damage occurs. However, applying high voltage to already severely degraded insulation carries some risk of triggering a breakdown.
Key Takeaways
- The step voltage test applies increasing DC voltage in steps and measures IR at each level.
- Healthy insulation shows flat resistance across all steps. Damaged insulation shows decreasing resistance at higher voltages.
- A resistance drop of 25% or more between steps is a significant warning sign.
- This test detects voltage-dependent defects — cracks, voids, brittle insulation — that spot readings and PI tests miss.
- It’s not a routine test. Use it on aging equipment, after overvoltage events, or when trending shows decline.
- It’s gentler than a hi-pot test — you get diagnostic information without pushing insulation to breakdown.
- Always check the equipment’s maximum rated test voltage before testing. Never exceed it.
- Combine with PI testing for a complete picture: PI catches moisture and contamination, step voltage catches structural defects.
Standards Referenced in This Article
| Standard | Key Content |
|---|---|
| IEEE 43-2013 | Test voltage selection for rotating machinery (Table 1), general insulation testing methodology |
| IEEE 95-2002 | Recommended Practice for Insulation Testing of AC Electric Machinery (2300V and Above) With High Direct Voltage — defines step voltage methodology with 1-minute intervals |
| IEC 60204-1 | Machinery insulation: 500V DC, ≥1 MΩ (Clause 18.3) |
| IEC 60364-6 | LV installation test voltages: 250/500/1000V DC (Clause 6.4.3.3) |