DC hipot testing is used in shielded medium voltage cables to test the electrical safety of the insulators. This is to reduce the chances of someone being hurt or equipment getting damaged. 

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In DC hipot testing, a high voltage, usually more than 5 kilovolts, is applied to the test conductor then the leakage current is measured. Ideally, there is minimal leakage current due to the capacitance of the cable. A sudden rise in the leakage current while the test voltage is increased indicates a faulty insulation.

There are some things you should consider when performing the hipot testing to get an accurate reading:

• Make sure to ground all objects not to be tested.
• Ensure adequate clearance between tested terminals to prevent flash overs.
• Make sure terminals are free from dust, moisture and other contaminants.
• Ensure that the DC hipot tester has a clean filtered power supply. The reading may be impaired if the power line is fluctuating.
• Tests should be done on a cool dry weather. Humid or cold environments can cause an increase in leakage current, impairing your measurement.

Test and Measurement Methods

DC Hipot Test

This page describes DC hipot testing details, including types of DC hipot testing, iTIG measurement techniques, test conditions, and failure analysis. For a general description of DC hipot testing using the iTIG, see the DC hipot test summary.

What is a DC hipot test?  

The DC hipot (high potential) test, can provide important information about several conditions. The iTIG DC hipot test feature will not cause damage or degradation to the device under test (DUT), if used correctly. Read more about why DC hipot tests are not destructive.

Most common failure modes and weaknesses found with the DC hipot test

The IEEE 95 standard (for AC electric machinery V and above) states that the following insulation problems may be detected by a controlled direct-voltage test:

• Early warnings of weak groundwall insulation.
• Dielectric strength to ground.
• The dielectric strength of the phase to phase insulation.
• When the megohms measured in an IR test are lower than expected, a DC hipot step voltage test can indicate if the motor or device under test (DUT) is dirty and/or moist, or if the insulation is breaking down. Since insulation resistance tests usually are done at one voltage and do not provide this information, a hipot step voltage test can be very valuable.

Hipot Testing Prerequisites

Before Testing

Hipot Testing Low, Medium, and High Voltage Motors

For low voltage motors, the DC hipot test is normally a 1-minute test at a specified DC voltage that is higher than the peak lead to lead operating voltage. The test is referred to as an over-voltage test since the voltage is higher than what the motor normally sees.

Peak voltage =
line-to-line RMS × 1.414

For medium voltage and high voltage rotating equipment, a hipot step voltage test or ramp test is recommended.

Auxiliary Equipment Grounded or Disconnected During a Hipot Test

Before starting an over-voltage test like the DC high potential test, the following components should be shorted to ground:

• Stator resistance temperature detectors or thermocouples
• Other devices associated with the stator windings
• Current transformer secondary windings
• Rotor windings (both terminals) and the shaft
• Objects close enough to become charged
• Motor tester chassis ground

Surge Arresters and Surge Capacitors

These must be disconnected prior to any over-voltage tests. Surge arresters have resistive elements and surge capacitors have discharge resistors. They are in parallel with the winding under test and will invalidate the current measurements.

For single phase motors, start and run capacitors should be disconnected.

DC Hipot Test Process

Currents Measured in a Hipot Test

The currents measured during the hipot test are the same currents present in an insulation resistance test.

IC'Capacitive

Capacitive (or geometric capacitive) current is also called inrush current. The windings have capacitance. Current is required to elevate its voltage potential. Capacitive current typically drops to zero within seconds after the test voltage provided by the motor tester is stable.

IA'Absorption

Absorption current is present during atomic and any molecular polarization of the insulation, and is the current of interest during a PI test. This current will drop to zero, or near zero, over a period of time that varies by motor. The drop can happen in seconds or may take 10 minutes or more. 

IG'Volume Conduction

Volume conduction current is the current that flows through the entire volume of the insulation between ground and the conductors. In good windings, this current is usually zero or near zero, and depends on the composition and condition of the insulation system. People sometimes think of this current as 'leakage' current. The volume conduction current certainly leaks through the insulation, but the surface conduction current (IL) is usually the main leakage in a used motor.

IL'Surface Conduction

Surface conduction current is often referred to as surface leakage current. The surface conduction current runs over the end winding surfaces of the insulation.

• Surface conduction is a result of surface contamination, dirt and moisture on the windings that are connected to ground.
• As the contamination level increases, the resistance of the contamination drops, and the current increases.
• As the voltage increases, the current increases more or less proportionally with the voltage applied by the motor tester.
• For used, good motors, this current will dwarf the absorption and volume conduction currents because of the relatively lower resistance in the surface contamination.
• For new, totally clean, and dry motors this current should be zero or near zero.

Total Measured Current

Measured \hspace{1mm}current=

I_C+I_A+I_G+I_L=

\Bigg( C \left( \frac{dV}{dt} \right)+I_A+I_G+I_L \Bigg)

where

• IC is capacitive current, which is also known as geometric capacitive or inrush current.
• IA is absorption current dropping with time.
• IG' is the volume conduction current through the insulation.
• IL is the surface current, which depends on levels of surface contamination
• C (dV/dt') is the product of the capacitance and the rate of change of the voltage.

Definitions from IEEE 95 Recommended Practice for Insulation Testing of AC Electric Machinery (V and Above) With High Direct Voltage1

Breakdown Current1

The current discharged as a result of insulation failure. The peak value of this current may be very high, reflecting the energy stored in the capacitance of the winding. Normally, this current cannot be accurately measured.

Breakdown Voltage1

The voltage at which a disruptive discharge takes place through the volume or over the surface of the insulation.

High Direct Voltage (Also Referred to As Over-Voltage)1 

A unidirectional voltage whose magnitude is greater than the peak value of the nominal RMS line-to-ground rating of the insulation system under test.

1'IEEE Recommended Practice for Insulation Testing of AC Electric Machinery ( V and Above) With High Direct Voltage,' in IEEE Std 95- (Revision of IEEE Std 95-) , vol., no., pp.1-56, 12 April , doi: 10./IEEESTD...

DC Hipot Test Process

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The higher the total measured current is, the dirtier the motor is and/or the weaker the insulation is. Sometimes the question becomes whether the cause is dirt or weak insulation. Hipot step voltage test discusses test result analysis in more detail.

Measuring DC Voltage

DC voltage is applied to open (disconnected) windings by the iTIG hipot tester or motor analyzer. The DC voltage potential in the windings is rapidly raised to a predetermined level, or raised in steps up to this level, depending on what test method is used.

As the voltage is raised, several currents will flow into and possibly out of the windings to ground. The combined total of these currents are measured by the hipot tester. Currents present are the same as those in an insulation resistance test.

Insulation Failure

Electrical insulation failure or breakdown is usually indicated by an arc, a sharp capacitive discharge, at the failure location. There are times when failure or partial failure is indicated by a large abnormal change in the measured current or by erratic behavior of the measured current.

Warnings of insulation breakdown by accelerating measured current can start within as little as 5% of the breakdown voltage, however, it can also start much earlier.

Hipot Test Voltages

Standards and Industry Formulas

Proof test for new unused machines, test voltages per IEEE 95, IEC, and ANSI/EASA AR100 AC hipot 2E+V DC hipot (2E+V) x 1.7 = 3.4E + V Where E = line-to-line RMS rated voltage Test of used equipment, test voltages per IEEE 95, IEC, and ANSI/EASA AR100 AC hipot 125%'150% of RMS DC hipot (3.4E+V) x 65% DC hipot commonly used in the service and repair industry 2E+V Where E = line-to-line RMS rated voltage

1. 'IEEE Recommended Practice for Insulation Testing of AC Electric Machinery ( V and Above) With High Direct Voltage,' in IEEE Std 95- (Revision of IEEE Std 95-) , vol., no., pp.1-56, 12 April , doi: 10./IEEESTD...

2. IEC nnnn.

3. 'ANSI/EASA AR100'' in Recommended Practice for the Repair of Rotating Electrical Apparatus, Electrical Apparatus Service Association, .

Hipot Test Voltages

Hipot test voltage formulas used for both DC hipot and AC hipot tests are shown at left.  DC hipot tests are modeled after AC hipot tests.  Thus, a DC hipot test voltage is often derived from a typical AC hipot test voltage that is proven to work well.

Tests of New Unused Motors

For proof tests of new unused motors, the test voltages per IEEE 951, IEC2 and ANSI/EASA AR standards are presented in the upper left table.

Tests of Used Motors

For proof tests of used motors, the standards stipulate that an AC voltage hipot test ranging from 125%'150% of the rated RMS line-to-line voltage, equivalent to about 65%'75% of 2E+ V, has proven to be adequate. The formula in the standards for DC hipot test voltage with used motors is shown in the lower left table. 

Differences Between Testing Formulas

In the motor repair industry, the test voltage commonly used is 2E+V. This formula calculates a test voltage 10% lower than the formula used in the standards. For a 460V motor, the difference between the standards formula and the industry formula is about 200V. For a V motor, the difference between the standards formula and the industry formula is about 945V less. For a 13.8kV motor, the difference is about V less.

Deciding Which Formula to Use

For most applications, either the formula used in the standards or the industry formula will work fine. The formula used by the standards provides a little more information since the test voltage is a little higher. The iTIG motor analyzer can set either formula as the default formula for automatic calculation of the hipot test voltage.

DC Hipot Test Pass/Fail Analysis

A DC hipot test is done at a voltage higher than the peak operating voltage of the device under test (DUT), where Peak Voltage = RMS voltage x 1.41. Therefore, if a test of an installed motor in a plant fails above peak operating voltage, it does not mean that the DUT itself has failed and cannot continue to operate, or that it needs to be condemned.

For critical motors this information is important. If tracked over time, which the iTIG motor analyzer can do, planning for when to take the motor out of service is made much easier.

Failure Below Peak Operating Voltage

If the hipot test failed at or below the peak operating voltage, the chance of imminent failure during operation or at start-up is high. However, even in a case like this, a motor can continue to run under the right circumstances, especially if the test was done under moist conditions.

The Owner Decides

No standard provides specific guidelines on when a motor should be taken out of service because of a failed hipot test. Nor does a standard state which conditions are acceptable when tests are done in a motor shop or motor manufacturing facility. Standards only give recommendations for how to perform the tests.

Keep in mind that if a test fails, it is the test that failed, not necessarily the motor. The failure limits may be set by the test operator or other people for multiple reasons. A limit value may be set to trigger a red flag warning about the motor's state, as well as to collect data for planning use in a plant.

Whether issues are found above peak operating voltage or, in advance of catastrophic failures, below peak operating voltage, information from the test provides important input for planning purposes.

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DC Hipot Test Pass/Fail Analysis

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Various limits and automatic test shutoff mechanisms are used in modern motor testers and DC hipot testers like the iTIG.

Pass/Fail Limit Types That Can Be Set for Motor Analyzers

Leakage Current Limit

An over-current trip-out level that can be set in µA up to a couple of mA by the user. If the limit is exceeded, the test is
immediately shut off. This limit must be set high enough so the capacitive inrush current does not trip the limit. If the inrush current is high and the limit is to be set low, the test voltage may have to be raised manually and slowly which creates a lower inrush current. Or, the automatic voltage ramp controlled by the motor tester must be set to a lower voltage ramp rate if such an option is available.

Arc Detection

If an arc is detected, the test is immediately shut off regardless of what the leakage current and the voltage are.

Leakage Current Acceleration Limit

This is called a rate of change (ROC) limit in the iTIG. If the leakage current accelerates faster than the ROC limit in a hipot step voltage test, the test is shut off before the next voltage step. The ROC factor is adjustable.

Condemnation Methods

Different methods are used to determine what should be done with the DUT if the hipot test fails. Criteria used to decide whether to continue to run the DUT or take it out of service for reconditioning, repair or rewind varies.

Examples

• The test operator or DUT owner has determined the total leakage current levels above which the DUT will be considered too weak for continued operation and therefore must be repaired soon or replaced.
• The user has determined arc voltage levels below which the DUT will be considered too weak for continued or longer-term operation.
• The user has decided the DUT's performance during will be tracked so the trend in leakage current or arc voltage levels over time will determine when to take the DUT out of service.
• Evaluation of actual or potential insulation breakdown is assessed based on step voltage/leakage current graph(s) from hipot step voltage or ramp test graphs, which are compared over time.
• Data from assessments in addition to hipot testing will be considered. Additional assessments may include:

• other types of tests
• DUT operational information, such as voltage and current levels during operation
• operating temperature, and so forth are considered to make the determination

Hipot Step Voltage Test

Recommended Use

The hipot step voltage test is recommended for DUTs with an operating RMS voltage of V and higher. This test can also be done on motors with any operating voltage if more information is needed than what a 1-minute hipot test provides.

In a hipot step voltage test, the voltage is usually raised in equal steps. At each step the voltage and measured current are recorded after 1 minute intervals per IEEE 95. The number of steps for the test is determined by the test operator. The number of voltage steps can range from 5 to 30 or more. Using 10 steps is common for medium voltage motors. However, more steps may be used for high voltage motors. Profiles for step voltage tests are programmable in the iTIG D model. Profiles include the number of steps, dwell time at each step, and voltage ramp rate.

Step Tests Provide More Information Than 1-minute Tests

Hipot step voltage tests provide much more information than a 1-minute hipot test because the data is recorded as the voltage is raised. By charting a current vs. voltage curve, one can usually tell if the leakage current is mainly due to contaminated dirty windings, or due to a breakdown of the insulation as shown in the images below.

Arc Prevention

With a step voltage test, the motor tester may be able to shut down the test when it reaches the ROC (current acceleration) limit before an arc happens. The test operator can also manually abort the test if the leakage current appears to accelerate.

Unfortunately, an arc may happen shortly after the leakage current starts to accelerate. According to the IEEE 95 standard, the acceleration may start 5% or less below the arc voltage. In these cases of abrupt insulation breakdown, the current acceleration event may not be detectable.

Modern hipot and motor testers have arc detection that will shut off the test immediately when an arc is detected instead of continuing to ramp up the voltage with more and more severe arcing.

Hipot Step Voltage Test
Recommended Use

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Good Result for a 3,300V Motor at V Test Voltage 

The plot of the 5-point step voltage test at right is a straight line and the leakage is low, clearly indicating the motor passed the hipot test. 

Good windings will have a curve that is more or less a straight line as shown in the graph. The current depends on how contaminated the windings are. In this case the current and contamination level is low with a total leakage current less than 1.4µA.

Test Result Examples

OK Results for a V Motor at V Test Voltage

Four 10-point step voltage tests were done over time. At right, one curve was plotted for each test result. The two bottom curves (magenta and green) from the first two tests are very good. 

The red and blue curves from the next two tests show minor acceleration in the current. But, the max current is still relatively low at 11µA, and the acceleration starts above 7,000V, significantly above peak operating voltage.

The elevated total leakage is mainly due to increasing winding surface contamination (including moisture) judging from the previous tests.

If the acceleration in the current was significant and the blue line moved more towards vertical, it would indicate a breakdown of the insulation.

Concerning Results for a V Motor at 11,700V Test Voltage

The test represented by the blue curve did not shut the test down early because the max current at about 25µA did not reach the total leakage current limit, and the acceleration of the current was not high enough to exceed the chosen ROC (current rate of change between voltage steps) limit of 2.

However, the accelerating leakage current and the current level at the end of the test indicates weak insulation that is starting to break down. The current acceleration starts slowly at about 6,000V, so the motor condition is concerning, but still OK.

No arc was detected during the test since that would have shut off the test early.  With a lower ROC limit, the test would have been shut off before reaching full test voltage.

Problem Result for V Motor at V Test Voltage

In this test, there is a sudden breakdown of the insulation between about 5.6 and 6.5kV, and the test is shut down at the end of step 7 at about V because the acceleration in the measured current exceeded the ROC limit from step 6 to step 7.

With rapid acceleration of the current, the chance of an arc increases significantly, but that did not occur here. The breakdown is happening around the peak operating voltage of 5,882V. With this result, the motor may be able to operate for a while, but is on its way to a complete breakdown.

In general, any deviation from a smooth curve should be viewed as a potential warning. A very abrupt drop in conduction current is rarely found, but when it occurs above the peak operating voltage for the winding, it may indicate approaching insulation failure. Mechanical abrasion and cracking may cause abrupt and unexpected insulation breakdown.

Hipot Ramp Test

The ramp test is performed with a slowly rising voltage. The final voltage is the hipot test voltage. The voltage and measured current is recorded every second or few seconds during the ramp test.

The initial increase in the current is mainly due to capacitive inrush current, IC. This current quickly stabilizes and remains constant throughout the test as long as the voltage increase, dV/dt, is constant.

IC = C \left( \frac{dV}{dt} \right)

where C is winding capacitance.

At the end of the voltage ramp, the voltage is held steady for a short while. The capacitive inrush current rapidly drops to zero, and the remaining measured current is mainly due to surface currents and conduction leakage currents if the winding insulation is breaking down.

There may also be some absorption current throughout the ramp test.

The increase in the current as the test proceeds is due to surface leakage current. If the insulation is weak or breaking down, there will also be an accelerating volume conduction current through the insulation.

For a good motor, the slope of the curve is fixed (i.e. the curve is close to a straight line) since a higher voltage typically produces a proportionally higher measured current. The amount of surface leakage current indicates how contaminated and/or moist the windings are.

An acceleration in the total measured current after the initial current inrush has settled indicates a breakdown of the insulation.

Why DC Hipot Tests Are Not Destructive

One advantage of the DC hipot test is the following: Although the test voltage can be high, the energy available to be discharged as an arc is small. Thus, arcs from a DC hipot test are not destructive if the test is done properly

A good analogy is the static discharge one can get from a finger to a door handle, especially when the humidity is low. The voltage differential causing the arc can be between 10kV and 20kV. You feel the arc, but there are no burn marks. Even if this happens often, there is no harm done. The reason we do not die from the high voltage discharge is that the energy available is low. It is determined by the capacitance in your body which is low, typically a few hundred picofarad (pF).

Motor analyzers have relatively low output capacitance for DC hipot tests as well, typically 20 to 100nF. Consequently, no damage will be done if an arc happens, as long as it is confirmed that the megohms are higher than limits set by the standards beforehand. The motor analyzer will shut off the test if an arc is detected, so there will be no further arcs

If the insulation system is known to be weak, consider lowering the test voltage. If the insulation has cracks and fissures, contamination can be embedded, and an arc can cause carbon tracking in the crack. This may reduce the voltage at which arcs can occur.

Why DC Hipot Tests Are Not Destructive

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Electrom recently performed an interesting experiment in a case study that proved a properly run iTIG DC hipot test is non-destructive.

Case Study

A 5HP, 4-pole, 460V, used motor from the early s was tested to arcing repeatedly using DC hipot testing. The motor was last in operation in before sitting for another more than 10 years in storage. Before the DC hipot tests in t he case study, the motor was disassembled and cleaned out with high pressure air.

The motor was then surge tested to failure at 7.5kV' 8kV multiple times in short succession. After reaching failure, a megohm measurement at V detected no leakage current. The motor was then tested 18 times to failure in short succession with a DC hipot test. The graph at left shows the test result numbers and their corresponding arcing voltages.

As seen in the graph, the arc voltage recorded in the first tests stays relatively constant at about V for the first 12 tests. Normal DC hipot tests would be done at voltages between V'V. Furthermore, normally only one DC hipot test is done. In this case study, the tests shown in the graph were done in rapid succession causing additional stresses on the insulation.

The result serves as proof that the insulation does not degrade due to a single DC hipot test arc if the test is performed under the guidelines and test voltages set forth by the IEEE and ANSI/EASA standards.

There is a decline in the arc voltage after the 13th test, and there is little doubt that the insulation has been weakened. It is important to keep in mind that the tests were done at significantly higher voltages than what is recommended for a 460V motor, and that even after the 18th test the arc voltage was close to V. The motor would still pass a DC hipot test performed under conditions recommended by the standards.

The conclusion from this experiment is that the relatively low energy available in the DC hipot test will not cause damage to the insulation system as long as the test is performed in accordance with the standards.

Motors are designed to handle significantly higher voltages than normal DC hipot voltages. If an arc occurs at or below the recommended test voltage, one can be certain that the insulation is weak.