In a perfect world, electrical insulation would allow no current to flow through it.
Unfortunately, a number of factors can over time result in the deterioration and ultimate
failure of electrical insulation.
Excessive heat or cold, moisture, dirt, corrosive vapors, oil, vibration, aging, and damaged
wiring can all compromise an insulation system.
Faulty insulation can result in equipment underperformance and downtime, and be a serious
danger to personnel.
To assess and monitor insulation integrity, several tests have been developed.
These typically involve injecting a test voltage and then measuring resistance.
This “stress tests” the insulation, similar to applying high water pressure to plumbing
to test for leaks.
A regular program of resistance testing can detect insulation deterioration so it can
be identified and addressed before it becomes a major problem.
Insulation resistance testing helps ensure personnel safety and optimal operation of
equipment.
It also helps evaluate the quality of repairs that may be required before equipment is put
back into operation.
In this video, we discuss three commonly used methods for testing insulation resistance:
the Spot Reading test, the Time Resistance test, and the Step Voltage test.
These three tests are used primarily to test motor, generator, cable, and transformer insulation.
To perform these tests, you will need a megohmmeter with a timed test function.
AEMC Instruments offers a complete line of megohmmeters designed for insulation testing,
ranging from 100 volt handheld instruments to heavy-duty models providing test voltages
up to 15,000 volts.
And with some models, you can download and analyze the results on a computer running
AEMC’s DataView software.
You will also need a thermometer or similar temperature measuring instrument.
And if the equipment temperature is below the dew point, a humidity measuring instrument
will be necessary, especially when performing a spot test.
Before performing any insulation resistance test, be sure to observe the following safety
measures, as well as any additional instructions specified in the documentation that comes
with your test instrument.
Insulation resistance testing involves the application of high DC voltages.
Therefore properly preparing the system under test, and the instrument used to conduct the
test, is crucial to your safety and helps prevent damage to your wiring and machinery.
First, take the equipment under test out of service.
Shut down the apparatus, open all switches, and de-energize the unit.
Disconnect from all other equipment and circuits, including neutral and protective ground connections.
Be sure to follow proper lock-out/tag-out procedures during this step.
Next perform a thorough inspection of the system under test.
In general, the more equipment included in a test, the lower the resistance reading.
Therefore it's critical to inspect the system and understand exactly what you're including
in the test.
Make note of any equipment that might be damaged by high test voltages, and either adjust the
test voltage accordingly or exclude these components from the test.
Also, be sure to discharge capacitance before and after conducting an insulation resistance
test.
Note that AEMC megohmmeters automatically discharge capacitance when not running a test.
Finally, check current leakage at switches and other connections.
When performing the test, restrict personnel access to the test site.
Also, be sure to use personal protective gear such as gloves where appropriate.
And after the test is complete, make sure the system under test is fully discharged.
A minimum discharge time of four to five times the duration of the applied test voltage is
recommended.
Some insulation resistance test instruments feature a built-in circuit to ensure a safe
discharge after the test.
Instruments with this capability ensure devices are safely discharged after every test.
To properly interpret test results, it’s important to understand that the total current
flowing through the insulation consists of three components: capacitance changing current,
absorption current, and conduction or leakage current.
When two conductors are in close proximity separated by an insulator – for example,
a length of common two-wire electrical cord -- they can act as a capacitor.
When test voltage is first applied, this capacitive charging effect results in current briefly
flowing through the conductors until the voltage across the insulation reaches the test voltage.
Consequently the initial resistance measurement will be relatively low and then quickly rise
as the capacitance becomes fully charged.
This effect is usually brief; often lasting less than a second (although on very long
cables or large motors this can last much longer, up to 30 minutes or more).
Capacitive charging current is not an indicator of insulation quality; but it needs to be
accounted for to ensure your measurements are meaningful and relevant.
Absorption current, also called polarization absorption current, is caused by the insulating
material becoming polarized by the electricity flowing through the conductor.
As the polarization level increases, the absorption current decreases.
This gradual change reflects the storage of potential energy in and along the insulation.
As a result, resistance is initially lower and then rises.
This produces a measurement profile similar to capacitive charging current, but at a much
slower rate; the effect can last several seconds up to a minute or more.
The length of time it takes for absorption current to fall off can be affected by moisture
or other contaminants in the insulation material.
Therefore absorption current is an important indicator of insulation integrity.
Conduction current, often called leakage current, is the steady current present both through
and over the insulation.
This is a critical measurement, since an increase in conduction current over time is likely
an indication of deteriorating or damaged insulation.
Combining these three components produces a total insulation current profile similar
to the illustration shown on the screen.
To summarize: for a typical test, the initial measurement primarily reflects capacitance
charging current.
After a period of time, absorption current is dominant.
And beyond one to ten minutes, the measurement is mainly composed of conduction current,
the primary value used to calculate the quality of insulation resistance.
Understanding how these individual currents contribute to the total insulator current
can help you correctly interpret the results you receive when performing a test.
It’s also important to be mindful of how environmental factors can affect resistance.
For example, oil or soot on the equipment’s surface can lower insulation resistance.
And if the equipment’s surface temperature is at or below the dew point of the ambient
air, a film of moisture forms.
This can significantly lower the equipment’s resistance value.
Temperature is also a critical consideration.
Insulation resistance can vary with temperature, with different materials exhibiting different
rates of change.
Ideally, all resistance testing should be done at the same temperature.
If this is not possible, temperature should be carefully recorded so correction factors
can be applied to the resistance measurements.
The first type of insulation resistance test we’ll look at is the Spot Reading test.
This is relatively straightforward: simply connect the megohmmeter leads across the insulation
to be tested, apply test voltage for a fixed period of time (typically one to ten minutes),
then take a resistance reading.
Spot testing is suitable for a system with small or negligible capacitance effect, for
example a short wiring run.
A single Spot Reading test is of limited value, but the results become meaningful when a series
of tests, all featuring the same test voltage and duration, are performed over time and
the results compared.
This comparison can help predict a potential insulation failure in time to take corrective
action.
For example, suppose you perform a spot test every six months.
Plotting the results on a graph, you observe a slow and gradual downward trend, as would
be expected by the normal aging of the insulation.
However, your latest measurement reveals a sudden drop in resistance.
This is likely indication that the insulation has begun to deteriorate at an accelerated
rate.
To address this, you can schedule downtime for the system and take appropriate measures
such as cleaning, upgrading, or replacing the insulation.
To ensure your results are valid, spot testing should ideally only be performed on systems
with temperatures exceeding the dew point.
If tests are performed at different temperatures, carefully record the temperature of each test
and apply the appropriate correction to determine what the resistance would be if the test were
performed at 20 degrees Celsius, or 68 degrees Fahrenheit.
Another insulation resistance measurement method is the Time Resistance test, also referred
to as the dielectric absorption test.
It involves conducting a 10 minute test.
For the first minute, during which absorption current will have the highest effect on resistance,
measurements are taken every 10 seconds.
After the first minute, measurements are taken once per minute.
When you plot the results, you should see a curve that rises relatively rapidly at first,
and then continues to gradually rise throughout the testing period.
If instead the curve is relatively flat or begins to turn down as the test progresses,
moisture, dirt, or other factors may be compromising your insulation.
Time Resistance tests on large rotating electrical machinery, especially systems with high operating
voltage, require high insulation resistance ranges and a very constant test voltage.
Since this test provides meaningful results within a single 10-minute duration, it is
relatively independent of temperature.
It is also independent of the size of the system under test.
The Time Resistance test is sometimes associated with two values, the Polarization Index, or
PI, and the Dielectric Absorption Ratio, or DAR.
The Polarization Index is derived by dividing the 10 minute resistance measurement by the
1 minute measurement.
The Dielectric Absorption Ratio is calculated by dividing the 1 minute measurement by the
30 second measurement.
Although DAR is no longer commonly used with newer insulation systems, it still may have
applicability when testing older insulation materials.
A third method is the Step Voltage test.
This involves testing at least two or more test voltages and comparing the results.
The test begins at an initial test voltage.
At a specified interval, typically one minute, a measurement is recorded, after which the
test voltage is increased.
This increase is usually to five times the initial voltage.
This process may be repeated through several steps, with measurements taken after one minute
and the test voltage increased at a five to one ratio over the previous voltage.
A common practice is to test at five voltage steps.
The Step Voltage test is designed to create electrical stresses on internal insulation
cracks, identifying potential problems that may not be revealed by testing at lower voltages.
Insulation that is thoroughly dry, clean, and in good physical condition should provide
roughly the same resistance measurements across the voltage range.
If instead you observe a significant decrease in resistance at higher voltage, your insulation
may be contaminated or deteriorating.
Step Voltage testing is also often used as a way to dry wet cables or equipment.
Gradual voltage steps, applied for increasingly longer durations, can facilitate drying through
heating.
This concludes our quick introduction to insulation resistance testing.
AEMC offers a complete line of megohmmeters designed for quick and accurate testing of
electrical insulation in a variety of devices, environments, and applications.
Consult the AEMC web site for more information about these instruments.
And be sure to check our YouTube channel for instructional videos on other topics in electronics.