EK-C352C 15kV Insulation Testing Guide: PI, DAR, DD, Step & Ramp

High-voltage insulation resistance testing is not simply a matter of applying the highest available test voltage and reading a resistance value. Large motors, generators, transformers, long power cables, bus systems, and substation equipment contain significant capacitance, absorb test energy, and operate in electromagnetic environments that can destabilize weak measurement systems. A useful test must control voltage, time, discharge, interference, temperature, and interpretation.

This guide explains a practical workflow for the EK-C352C 15 kV high-voltage insulation resistance tester. It covers test planning, terminal connections, charging current, leakage and absorption behavior, polarization index (PI), dielectric absorption ratio (DAR), dielectric discharge (DD), step-voltage and ramp testing, capacitance, noise rejection, result trending, and safe discharge. It is intended for trained personnel working on de-energized, isolated equipment.

Critical safety note: An insulation tester can generate dangerous voltage and store hazardous energy in the test object. Follow the equipment manufacturer’s procedure, site electrical safety rules, lockout/tagout requirements, grounding practices, approach boundaries, and applicable standards. Prove the asset de-energized with an approved method before connection. Control access, use rated leads, never touch the circuit during a test, and confirm complete discharge before removing connections.

What an Insulation Resistance Test Measures

When DC voltage is applied across insulation, the measured current is initially a combination of capacitive charging current, dielectric absorption current, surface leakage, and conduction through the insulation. These components change at different rates. The resistance displayed by the tester is calculated from applied voltage and measured current, so the value normally rises during a timed test as charging and absorption currents decay.

Insulation resistance is therefore time-dependent. A value recorded after five seconds cannot be compared directly with a value recorded after one minute. Test voltage, duration, temperature, humidity, connection method, guard use, equipment condition, and residual charge must all be consistent.

A high resistance value is not by itself proof that equipment is fit for service. Local defects, voids, contamination, partial discharge, mechanical damage, or voltage-dependent breakdown may not be fully represented by one DC resistance point. Insulation resistance should be combined with applicable acceptance criteria, historical results, visual inspection, capacitance, dielectric loss or partial-discharge testing where required, and knowledge of the asset.

EK-C352C Capabilities for High-Voltage Assets

The EK-C352C provides selectable rated test voltages from 50 V to 15 kV and custom settings from 40 V to 15 kV. Its insulation resistance range extends from 0.005 MΩ to 30 TΩ, with short-circuit current of at least 7 mA. The instrument includes IR, PI, DAR, DD, STEP, RAMP, AC/DC voltage, capacitance, real-time current, voltage monitoring, automatic discharge, 1000 stored records, USB communication, Bluetooth communication, and a five-inch color touch screen.

Strong anti-interference design is valuable in substations and long-cable applications where induced voltage and distributed capacitance can destabilize readings. The instrument monitors terminal voltage and prohibits a test when more than 36 V is detected in the non-test state. Its closed case is rated IP65, and the instrument is designed for IEC 61010-1 and IEC 61326-1 requirements according to the product specification.

The wide range does not mean every asset should be tested at 15 kV. Test voltage must follow the asset manufacturer’s instructions, applicable standard, insulation class, condition, and test purpose. Excessive test voltage can stress or damage insulation. Low-voltage controls, surge devices, sensors, electronics, and connected accessories may require disconnection or a different procedure.

Planning the Test

Define whether the test is for commissioning, periodic maintenance, troubleshooting, repair verification, or condition trending. Commissioning may require acceptance limits and comparison with factory results. Maintenance emphasizes repeatability and trend. Troubleshooting may require sectional tests, guard connections, multiple voltages, or comparison between phases.

Collect asset data: manufacturer, model, serial number, rated voltage, insulation class, winding or cable configuration, length, temperature sensors, grounding method, prior results, repairs, storage history, operating temperature, and recent events. Obtain the required test voltage, duration, connections, and discharge time from the approved procedure.

Survey everything connected to the test object. Variable-frequency drives, protective relays, voltage transformers, surge arresters, electronic sensors, capacitors, neutral grounding equipment, and communication devices can affect the result or be damaged by the test. Isolate or protect them as specified. Do not improvise disconnections without an updated drawing and authorization.

Isolation, Grounding, and Proving Dead

Open and secure every energy source, including backfeeds, generators, UPS systems, control power, stored mechanical energy, and induced sources. Apply lockout/tagout. Prove the test instrument, test for absence of voltage at the work point, then prove the voltage detector again according to the site procedure.

Discharge and ground the asset before connecting the insulation tester. Long cables, capacitor banks, and large windings can retain charge. Nearby energized circuits may induce voltage after grounds are removed. Plan the order of connection so the test object is grounded for as long as practical.

Establish a controlled test area with warning signs and barriers. Assign responsibility for the test. Everyone involved should know when grounds will be removed, when voltage will be applied, and when the circuit can be approached again. Remote or automatic discharge does not remove the need for safe work control.

Connecting Line, Earth, and Guard

The line terminal applies the test voltage to the conductor or winding under evaluation. The earth terminal connects to the reference structure, shield, frame, or other side of the insulation. The exact polarity and connection must follow the instrument and asset procedure.

The guard terminal diverts selected surface leakage away from the measurement circuit. It is useful when contamination or surface moisture would otherwise mask the volume resistance of the insulation. Guarding does not “improve” the asset; it changes which current is measured. Record whether guard was used and where it was connected.

Keep leads separated, supported, clean, dry, and away from grounded metal where leakage could occur. Use only rated accessories. Route test leads to minimize movement and electromagnetic coupling. On long cable tests, maintain consistent shield and conductor treatment between phases.

Basic Timed Insulation Resistance Procedure

1. Confirm isolation, absence of voltage, grounding, and the approved test plan.
2. Inspect the EK-C352C, leads, test rod, clips, enclosure, battery, and calibration status.
3. Record asset temperature, ambient temperature, relative humidity, and surface condition.
4. Connect earth, guard if required, and line while the test object remains safely discharged.
5. Remove temporary grounds only according to the controlled sequence.
6. Select the prescribed voltage and duration; verify that custom settings are correct.
7. Start the test from a safe position and observe voltage, current, and resistance behavior.
8. Record values at defined times, commonly including 30 seconds, 60 seconds, and longer intervals.
9. Stop the test and allow the instrument to discharge the asset.
10. Confirm terminal voltage has fallen to a safe level, apply external grounding where required, and only then disconnect.

Never shorten the discharge interval simply because the display drops quickly. Dielectric absorption can cause voltage rebound after an initial discharge. Large equipment may require grounding for a defined multiple of the charging time or according to the manufacturer’s instruction.

Selecting Test Voltage

Test voltage depends on the rated system voltage, insulation design, asset condition, and governing procedure. Low-voltage electronic or control circuits may require 50, 100, 250, or 500 V. Industrial equipment may use 500 V, 1 kV, 2.5 kV, or 5 kV. High-voltage machines, cables, and apparatus may require 10 kV or 15 kV only when the applicable procedure permits.

Use the lowest voltage that satisfies the test objective. A maintenance trend should remain consistent with earlier tests unless a documented engineering decision changes the voltage. If voltage changes, do not compare raw resistance values as if the methods were identical because insulation can exhibit voltage-dependent behavior.

Before a high-voltage test, consider aged or suspect equipment. A step-voltage or ramp approach may provide more diagnostic control than immediately applying maximum voltage. Stop criteria should be defined in advance, including abnormal current rise, unstable voltage, audible discharge, odor, smoke, or other signs of distress.

Polarization Index

Polarization index is commonly calculated as the resistance at ten minutes divided by the resistance at one minute. It describes how resistance changes as absorption current decays. A rising curve and higher ratio can indicate clean, dry insulation, while a flat curve may indicate contamination, moisture, or dominant leakage. Interpretation depends on insulation type and applicable guidance.

PI is most meaningful when the tester maintains stable voltage throughout the test and the asset has sufficient absorption behavior. Very high resistance, small capacitance, modern insulation systems, temperature effects, or measurement noise can make the ratio less informative. Do not apply a universal PI threshold to every asset.

Record the full time curve, not only the final ratio. A curve can reveal voltage instability, sudden leakage changes, intermittent discharge, or early saturation that a single PI number hides. The EK-C352C stores real-time data and allows record review, supporting consistent trend analysis.

Dielectric Absorption Ratio

DAR uses resistance values at two earlier times, often 60 seconds divided by 30 seconds, although formulas vary. The EK-C352C allows selectable PI and DAR calculation formulas, so verify the configured times before comparing results with previous tests or external limits.

DAR is useful when a ten-minute PI test is impractical or when the asset response is most visible during the first minute. It remains sensitive to temperature, surface leakage, residual charge, and connection repeatability. Report the exact timing rather than recording only “DAR.”

Use DAR as one diagnostic indicator. Compare phases, similar assets, and historical results. A change in DAR accompanied by lower one-minute resistance and higher capacitance may deserve more investigation than a small ratio change alone.

Dielectric Discharge Testing

Dielectric discharge evaluates the current that flows after the charged insulation has been discharged for a defined time. It can provide information about multilayer insulation and retained polarization. The result depends on charging voltage, charging time, capacitance, discharge timing, and measured current.

Follow the approved formula and procedure. Do not compare DD values collected with different charge or discharge times. Large capacitive assets require strict safety control because the test intentionally stores energy before measuring discharge behavior.

Use DD together with IR, PI, DAR, capacitance, and trend. An unusual DD result is a reason to review insulation structure and history, not a standalone diagnosis of a specific failure mode.

Step-Voltage Testing

A step test applies a sequence of increasing voltage levels for defined intervals. Healthy insulation often shows predictable resistance behavior, while defects, contamination, or voltage-dependent conduction may produce disproportionate current increase as voltage rises.

Set each level and duration according to the asset procedure. Use consistent step ratios and allow enough time for a meaningful response. Plot applied voltage, resistance, and current. A resistance reduction with increasing voltage may be significant, but temperature, surface leakage, and incomplete stabilization must be considered.

Establish stop criteria before the test. If current increases abnormally, voltage cannot stabilize, or physical distress appears, terminate and discharge. Do not continue to the next step just to complete the programmed sequence.

Ramp Testing

Ramp testing raises voltage smoothly rather than in discrete steps. It can expose nonlinear leakage and offers a controlled view of current versus voltage. A smooth current increase may represent expected capacitive and conduction behavior; a sudden change can indicate a threshold effect or instability.

Choose ramp rate, maximum voltage, hold time, and stop conditions in advance. A rapid ramp emphasizes charging current and may hide subtle conduction behavior. A slow ramp increases test time and stored energy. Compare only tests performed with equivalent ramp parameters.

Save the curve and note any discontinuity. Confirm that apparent changes are not caused by lead movement, induced noise, range transitions, or poor contact. Follow with targeted inspection or another diagnostic method when the curve is abnormal.

Capacitance and Charging Current

The EK-C352C measures capacitance from 10 nF to 200 μF. Capacitance helps estimate charging behavior, test duration, and stored energy. For a given voltage, stored energy is one half of capacitance multiplied by voltage squared. This relationship shows why raising voltage greatly increases the energy that must be controlled and discharged.

Long cables and large windings may draw substantial charging current at the start. The tester’s short-circuit current capability influences how quickly voltage rises. If the asset capacitance is too large for the selected voltage and duration, the tester may take longer to reach the target, affecting timed ratios.

Record actual voltage rise and confirm that the test voltage was maintained during the timing interval. A ten-minute test that spent several minutes below target voltage should not be treated as equivalent to a stable test.

Working in High-Interference Substations

Nearby energized buswork can induce AC or DC voltage into isolated conductors. Check the terminal-voltage monitor before connecting and before every test. If voltage exceeds the permitted condition, do not force the test; investigate isolation, grounding, induction, and lead routing.

Use short, separated, supported leads where practical. Keep them away from energized conductors and switching equipment. Apply guard appropriately, close the instrument case as required, and avoid holding leads during measurement. Repeat the test with an approved change in lead routing if noise is suspected.

Strong anti-interference capability improves stability but cannot make an unsafe induced-voltage condition acceptable. Compare current and resistance trends rather than relying only on the final displayed resistance. Document nearby system operating state because induction can change when loading or switching changes.

Temperature Correction and Environmental Effects

Insulation resistance often changes substantially with temperature. Record winding or conductor temperature whenever possible, not only room temperature. Apply a correction method appropriate to the insulation system and governing procedure. Do not use a generic doubling rule without confirming that it applies.

Humidity and surface contamination affect leakage. Cleanliness, condensation, rain, salt, carbon dust, and industrial deposits can reduce surface resistance. Guarding may separate surface leakage from volume resistance, but the presence of contamination is itself important maintenance information.

Trend corrected values and preserve raw values, temperatures, and correction method. If historical records use different corrections, normalize carefully before drawing conclusions.

Application Examples

Long Power Cable

Isolate both ends, disconnect sensitive accessories, identify shield and conductor connections, and control induced voltage. Record capacitance and voltage rise. Test each conductor to shield and other grounded conductors according to the approved method. Allow extended discharge and watch for voltage rebound.

Large Motor or Generator

Record winding temperature and test each phase consistently. Use timed IR and PI where appropriate. Compare phases and previous results. Investigate a phase that differs materially even if every value exceeds a general minimum.

Transformer

Define which windings are connected together and which are grounded. Remove or protect connected devices as instructed. Record oil and winding temperature where relevant. Interpret IR with ratio and capacitance data, and consider additional dielectric tests for a complete assessment.

Result Table and Trend Review

Review absolute values, phase balance, curve shape, corrected trend, and rate of change. A sudden departure from the asset’s stable history is often more important than comparison with a broad generic threshold.

When Other Instruments Are Needed

Insulation resistance does not measure operating leakage current. Use a high-sensitivity clamp such as the EK-G668A to trend energized residual or earth current. Use a power quality analyzer such as the EK-F523A when voltage, current, harmonics, unbalance, sag, swell, flicker, and load trend must be evaluated together.

Partial-discharge, dielectric-loss, withstand, sheath, and very-low-frequency tests require dedicated methods and expertise. The phrase “IEC 60270 insulation resistance” combines different concepts: IEC 60270 concerns partial-discharge measurements, while insulation resistance is a DC measurement. Do not present a megohmmeter result as an IEC 60270 partial-discharge test.

Frequently Asked Questions

Should every high-voltage asset be tested at 15 kV?

No. Use the voltage specified by the asset manufacturer or governing procedure. The EK-C352C’s 15 kV capability provides range; it does not authorize maximum voltage for every asset.

Why does resistance keep rising?

Capacitive charging and dielectric absorption currents decay with time, so calculated resistance usually rises. Compare values at identical times and voltage.

Can PI be used on every insulation system?

PI may be less informative on small, very high-resistance, or low-absorption systems. Use asset-specific guidance and the complete curve.

How long must the asset be discharged?

Follow the manufacturer and site procedure. Large capacitive equipment can exhibit voltage rebound. Confirm voltage and apply grounding before approach.

What causes an unstable reading?

Possible causes include induced voltage, lead leakage, surface contamination, incomplete charging, intermittent discharge, poor connection, environmental changes, or actual insulation instability.

Calibration and Record Integrity

Before a critical campaign, confirm the tester’s calibration covers the voltage, current, and resistance ranges being used. Keep instrument serial number, lead identification, software version, and calibration status with the test file. If results are transferred through USB or Bluetooth, preserve an original read-only export and document any later calculation or temperature correction.

Final Field Checklist

Review all settings before energizing the test.

• Use an approved asset-specific voltage, duration, and acceptance method.
• Isolate every source, prove dead, discharge, and establish a controlled test area.
• Protect or disconnect sensitive connected devices.
• Record temperature, humidity, configuration, and guard connection.
• Confirm voltage rise and stable target voltage.
• Save timed IR, PI, DAR, DD, STEP, RAMP, current, and capacitance data as applicable.
• Control induced voltage and interference.
• Allow complete discharge and confirm safe voltage before disconnecting.
• Trend corrected results and investigate significant phase or historical changes.

When a test is repeated after cleaning, drying, repair, or cable termination work, preserve the original as-found result and repeat the same voltage, timing, guard, temperature treatment, and connection. A better number is meaningful only if the method remains comparable. Document the intervention and confirm that no sensitive device was unintentionally left disconnected before return to service.

High-voltage insulation testing creates value when voltage, time, temperature, current, capacitance, and discharge are treated as one controlled process. The EK-C352C provides the measurement modes and range needed for demanding assets; disciplined planning and interpretation turn those readings into defensible condition evidence.