Megohmmeters, sometimes referred to as insulation testers or, informally, as meggers, are electrical meters used to determine the condition of the insulation
on wire and motor windings. Megohmmeters introduce a high voltage, low current DC (direct current) charge and measure the resistance to determine current
leakage and identify faulty or damaged insulation which can lead to arc faults, blown circuits, and risk of electrical shock and/or fire. Routinely using a
megohmmeter to check insulation in both new installations and as part of a maintenance program is a prudent way to ensure your circuits are safe.
The insulation found on wires, cable, and motor windings serve to protect the wire and keep it separate from other wires. Accidental touching of two conducting
wires can result in an arc fault. Insulation, however, begins to degrade from the moment it’s made and as it ages, there is a reduction in its insulating performance.
Exposure to extreme environmental conditions and/or chemical contamination, accelerates this process. Megohmmeters provide a way to quickly and easily test to
identify the deterioration of insulation before it results in conditions that can damage expensive equipment, lead to an unplanned shutdown, or threaten personal safety.
How Megohmmeters Work
Megohmmeters are simply large capacity ohmmeters capable of creating a DC voltage from an internal battery. The level of resistance required to test insulation and
motor windings is far higher than normally found on multimeters or standard ohmmeters. Depending upon the standards referenced, acceptable insulator resistance
values are typically between 1 and 10 megohms (millions of ohms).
Megohmmeters must be able to generate voltages ranging from 50 to 15,000 volts to accurately measure such high resistances. A small internal generator, either
hand-cranked or with an internal motor, is used to produce that voltage. The voltage is supplied at a very low current so as not to damage sensitive equipment
or be dangerous to the tester.
When testing with a megohmmeter, low resistance values indicate current leakage which is indicative of compromised insulation.
Though valuable tools, megohmmeters also have limitations. When using megohmmeters it is important to remember the following:
- The high voltages produced by these instruments should be kept in mind at all times when testing electrical equipment.
- Megohmeter test voltages should not exceed the operating voltage of the equipment being tested by too large a margin as this may cause irreversible damage.
- While they identify problems with insulation, megohmmeters do not pinpoint the location of the current leak.
- Never use an insulation tester if the motor windings are under a vacuum.
Using a megohmmeter
Testing insulation resistance provides a numerical value to represent the condition of conductor insulation and the internal insulation of electrical equipment.
But how do we arrive at that value and what does that number mean?
During testing, the high DC voltage generated by the megohmmeter will cause a small current to flow through the conductor and insulation. The amount of current
depends on the amount of voltage applied, the system’s capacitance, the total resistance, and the temperature of the material. In general, the higher the current,
the lower the resistance. The value of insulation resistance displayed on the meter is a function of following three independent sub-currents.
1. Conductive leakage current:Conductive current is a small amount of current that normally flows through insulation, between conductors or from
a conductor to ground. This current increases as insulation deteriorates and becomes predominant after the absorption current vanishes. Because it is fairly steady
and time independent, this is the most important current for measuring insulation resistance.
2. Capacitive charging leakage current: When two or more conductors run parallel to each other, they act as a capacitor. Because of this capacitive
effect, a leakage current flows through conductor insulation. This current lasts only for a few seconds as the DC voltage is applied and drops out after the
insulation has been charged to its full test voltage. In low-capacitance equipment, the capacitive current is higher than conductive leakage current, but it
dissipates very quickly. With high capacitance equipment the capacitive charging leakage current can last for a very long time. For this reason, it’s important
to let the reading settle out before recording it.
3. Polarization absorption leakage current: Absorption current is caused by the polarization of molecules within dielectric material. In
low-capacitance equipment, the current is high for the first few seconds and decreases slowly to nearly zero. When dealing with high capacitance equipment or wet
and contaminated insulation, there will be no decrease in the absorption current for a long time
Megohmmeter Tests
Megohmmeters are routinely used for testing both following installation as well as part of a predictive maintenance program. Proof tests are conducted for new
installations to ensure proper installation and integrity of conductors. This is a quick and simple test, often called a “go/no go test” since it tests cable
systems for maintenance errors, incorrect installation, serious degradation, or contamination. The installation passes the test if no breakdown occurs.
Proof tests involve applying a single voltage, generally 500 to 5000 volts, for about one minute. The idea is to stress the insulation above normal working
voltages in order to detect subtle weaknesses in the insulation. This is usually about 60 to 80% of the manufacturer’s factory test voltage. Proof tests can
be performed on equipment of any capacitance.
Predictive maintenance tests are performed on existing equipment and provide important information about the present and future state of conductors, generators,
transformers, and motors. As with any predictive maintenance regime, comparing results gathered over time will help with scheduling diagnostic and repair work,
which will reduce downtime from unexpected failures.
The following are the most commonly applied predictive maintenance tests performed with a megohmmeter:
Insulation Resistance (IR) Test
The Insulation Resistance test is the simplest test conducted with a megohmmeter. It is a short-duration test in which a test voltage is applied for about one
minute. The amount of voltage applied is calculated from DC test voltage formulas.
When interpreting test results, equipment rated at or below 1000 volts should have a reading of 1 MΩ or greater. For equipment rated above 1000 volts, the expected
resistance should increase to one megohm per 1000 volts applied. Please consult the manufacturer of equipment for acceptable values and test procedures.
When compared to past test results, it is expected that insulation resistance will be a bit lower than previously recorded values. This is a normal sign of
insulation aging. Sharper lower values would indicate an insulation failure or a warning of troubles ahead. Any values below standard minimums or sudden
departures from previous values should be investigated.
It is important to note that the Insulation Resistance test is temperature sensitive. When the temperature goes up, IR goes down, and vice versa. To compare
new readings with previous readings, they must be corrected to a base temperature, usually, 20°C or 40°C. Tables are available for temperature correction. A
common rule of thumb is that IR changes by a factor of two for each 10°C change.
Step Voltage Test
The step voltage test involves resistance testing at various voltage settings. Test voltage is applied for a length of time, around a minute, in increasing
steps and the test value is recorded. If the insulation is in good shape the resistance value should remain approximately constant as the voltage is increased.
If the insulation is degraded and pinholes, cracks or other physical damage or contamination, it will experience increased current flow, especially at higher
voltages. This will exhibit itself in decreasing insulation resistance. If testing finds a significant drop in resistance values, say above 25%, age
deterioration or damaged insulation should be suspected.
Step voltage tests are independent of insulation material, equipment capacitance, and temperature effect. The test is ideal to pinpoint problems that have been
identified by an insulation resistance test.
Dielectric-Absorption / Time-Resistance Test
The dielectric-absorption test, also called the time-resistance test, compares the absorption characteristics of good insulation with those of contaminated
insulation. The test consists of applying test voltage over a ten minute period and recoding results at frequent intervals. When the results are plotted onto a
graph they can be interpreted to determine the condition of the insulation. A continuous increase in graphed resistance indicates good insulation. A flat or
downward curve indicates cracked or contaminated insulation.
Things to Consider When Purchasing a Megohmmeter
- What test voltages are required?
- Are and specific insulation tests required? Which models support those tests?
- What experience level does the technician possess?
- Do you prefer battery powered or hand–cranked?
- What datalogging or communications options are needed?
- Are any accessories (test leads, etc.) needed?
If you have any questions regarding megohmmeters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
