Choosing An Insulation Tester
Many times we get confused about for which Insulation Tester we should go for which will get our job done when there are multiple numbers of Brands and models are available. This blog will help you to understand how to choose an insulation tester which can suits to your application.
Insulation testers aren’t really application specific; they can all get the job done! Instrument selection is based on specifications rather than application. Examine closely what is actually required of the tester. These, obviously, will be the prime considerations. But simple user preference, what the operator likes and doesn’t like, is almost as important. Indeed, for instruments that are to be used in a broad, general manner, user preferences may be the prime considerations. And then there’s price.
Your prime consideration will probably be the voltage or voltages at which you need to test. Range of measurement values may also be a leading consideration. If proof testing is the application, there may be more latitude as to range, since an “infinity” reading is likely to be an acceptable standard for a “passed” test. If long-term maintenance is the testing goal, then greater range is desirable, as extremely high values will want to be recorded and tracked while equipment under test is new. Power source is also a major consideration, as experience, preference, and specific job situations may make demands between battery, hand-cranked, and line-powered models.
After a few of these principle considerations have been decided upon, eligible brands or models can be selected largely on the basis of preference. Such considerations as analog versus digital, computer capability, automated tests, and others, are not specific to an application, but rest on the likes and dislikes of the ultimate user.
Let us first understand some basics of Insulation Resistance testing and then we will see how to choose a appropriate one.
What Is Insulation Testing
What is insulation resistance testing? Basically, you’re applying a voltage (specifically a highly regulated, stabilized DC voltage) across a dielectric, measuring the amount of current flowing through that dielectric, and then calculating (using Ohm’s Law) a resistance measurement. Let’s clarify our use of the term “current.” We’re talking about leakage current. The resistance measurement is in megohms. You use this resistance measurement to evaluate insulation integrity.
Current flow through a dielectric may seem somewhat contradictory, but remember, no electrical insulation is perfect. So, some current will flow.
Dielectric Testing & Insulation Resistance Measurement
Dielectric strength testing, also called “breakdown testing”, measures an insulation’s ability to withstand a medium-duration voltage surge without sparkover occurring. In reality, this voltage surge may be due to lightning or the induction caused by a fault on a power transmission line. The main purpose of this test is to ensure that the construction rules concerning leakage paths and clearances have been followed. This test is often performed by applying an AC voltage but can also be done with a DC voltage. This type of measurement requires a hipot tester. The result obtained is a voltage value usually expressed in kilovolts (kV). Dielectric testing may be destructive in the event of a fault, depending on the test levels and the available energy in the instrument. For this reason, it is reserved for type tests on new or reconditioned equipment.
Insulation resistance measurement, however, is nondestructive under normal test conditions. Carried out by applying a DC voltage with a smaller amplitude than for dielectric testing, it yields a result expressed in kW, MW, GW or TW. This resistance indicates the quality of the insulation between two conductors. Because it is non-destructive, it is particularly useful for monitoring insulation aging during the operating life of electrical equipment or installations. This measurement is performed using an insulation tester, also called a megohmmeter.
Causes Of Insulation Failure:
1) Electrical Stresses
2) Mechanical Stresses
3) Chemical Stresses
4) Stresses linked to temperature variations
5) Environmental Contamination
Types Of Insulation Resistance Tests:
1) Short-time or spot-reading measurement
This is the simplest method. It involves applying the test voltage for a short time (30 or 60 seconds) and noting the insulation resistance reading at that moment. As indicated previously, this direct measurement of the insulation resistance is significantly affected by the temperature and humidity, so the measurement should be standardized at a reference temperature and the level of humidity should be noted for comparison with the previous measurements. With this method, it is possible to analyze insulation quality by comparing the current measured value with several previous test results. This trend, over time, is more representative of the evolution of the insulation characteristics on the installation or equipment being tested than a single test. If the measurement conditions remain identical (same test voltage, same measurement time, etc.), it is possible to obtain a clear assessment of the insulation by monitoring and interpreting any changes in these periodic measurements. After noting the absolute value, the variation over time should be analyzed. Thus, a measurement showing a relatively low insulation value which is nevertheless stable over time is, in theory, less of a concern than a significant decrease in the insulation value over time, even if the insulation is higher than the recommended minimum. In general, any sudden fall in the insulation resistance is evidence of a problem requiring investigation.
2) Time resistance test
Unlike the short time/spot reading test, the time resistance method test can provide fairly conclusive results without the luxury of past test measurements. This test method is based on taking successive readings at fixed time intervals, and then plotting the readings. This is an especially effective method when moisture and other contaminants might be present. As mentioned earlier, absorption current starts out high and gradually decreases over time as voltage is applied. In a machine with healthy insulation, this trend will continue for several minutes and show an increasing level of resistance. On the other hand, if the insulation is poor, the level of resistance will flatten out after an initial burst The best way to quantify the results of a time resistance test is through a dielectric absorption ratio. The dielectric absorption ratio consists of two time resistance readings. A commonly used set of intervals is a 60-second reading divided by a 30-second reading. Another frequently used set is a 10-minute reading divided by a 1-minute reading. This resulting value is referred to as the polarization index.
3) Step voltage test
A step voltage test involves testing the insulation at two or more voltages and comparing the results. Good insulation will show a relatively consistent resistance reading regardless of the voltage applied. On the other hand, when the resistance level drops as the voltage level increases, it’s usually an indication that the insulation is aging, contaminated, or brittle. This occurs because small imperfections like pinholes and cracks reveal themselves under increased electrical stress. When performing a step voltage test, it’s important that you start with the lowest test voltage and then move to a higher voltage level. Test duration is typically 60 seconds.
Digital Or Analog Model Insulation Tester
It’s up to you! There’s no fundamental reason why one or the other should be used in a given situation; each has its strengths and weaknesses. Digital has the advantage of high accuracy, resolution, and an obvious, clearly-stated reading devoid of interpretation. Analog has the advantage of exhibiting the time dynamics of a test, and revealing subtle changes in the behavior of the test item under voltage, which could indicate specific conditions. In general,
digital models are better for less experienced operators and simpler applications, while analogs are preferred by experts. Digital models require no interpretation, and are less prone to “human factor”. The reading is right there to observe, proclaimed to the eye in bold digits. This makes it easy on inexperienced workers, and bolsters the validity of test reports which may have to be submitted to skeptical third parties. But the “dancing digits” that scroll across the display before the reading begins to settle down are hard to interpret for the time dynamics of the test. The sweep of the analog pointer is easy on the eye, and facilitates the ready observation and interpretation of the whole test, from initiation to termination. Experienced personnel come to watch pointer travel as much as, or more than, the final reading. Is the pointer steady, or jittery? Does it travel smoothly, or oscillate? Does it rhythmically drop back
and rise again? Does it “freeze” on the reading, or “stutter”? All these observations are difficult to determine on a digital display, but can be invaluable in recognizing and diagnosing a problem. The actual reading, however, is not as clear-cut as on a digital, and open to human interpretation and possible error. Actual values must be extrapolated between the markings on the scale, and parallax can introduce a further uncertainty.
Some key questions to think before choosing an Insulation tester:
• What is the maximum test voltage necessary?
• Which measurement methods will be used (spot measurements, PI, DAR, DD, step voltage)?
• What is the maximum insulation resistance reading required?
• How will the megohmmeter be powered?
• What are the measurement storage capabilities?
Insulation Resistance Testers Available From World Leading Manufacturers:
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He is a Electronics & Telecommunication Engineering graduate and an application expert with wide experience in Test & Measurement Instrument industry. Worked for products of World leader companies in TMI industries like Agilent, Tektronix, Fluke, Lecroy, Keithley, Rohde & Schwarz, Dranetz-Bmi. He is also a Level 1 (ASNT) Certified Thermographer (Snell Group).
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