In an ideal world, electrical current sent through a conductive wire would reach its intended destination. But the world is far from ideal, so some of the sent current is lost along the way for numerous reasons. All wires are insulated with a resistant sheathing in order to contain the conductivity of the aluminium or copper core, but even then, some of the current is still lost. Just like a leak in a water pipe, imperfections in a wire's insulation will have it steadily lose the flow of electricity, which can be detrimental to both electrical circuits and machinery. That being said, performing insulation resistance tests on the regular is an important part of maintaining an electrical system and to ensure the insulation is performing at a safe and effective level. Doing this will identify potential problems before they result in equipment failure or injury.
But insulation is subjected to many elements that can cause it to underperform. Excessive cold or heat, vibration, oil, dirt, moisture and corrosive vapors can all negatively impact insulation. Testing the integrity of insulation resistance requires measuring the amount of total current across the wire. A low level of resistance means that a significant amount of current is leaking through the insulation, whereas a high level of resistance indicates that very little current is leaking through the insulation. By pressurising a conductor with a provided voltage, you can use Ohm's law to apply a numerical representation to your resistance measurements. If you divide the voltage by the current escaping through the insulation and returning to the meter, you'll get the capacitive current, leakage current and absorption current.
The capacitive current represents the initial burst of current that happens when voltage is initially applied to a conductor. In other words, it's similar to the rush of water flowing through a hose - it will generally start out high and then drop quickly as soon as the conductor is fully charged. Absorption current, just like capacitive current, starts out high and then falls, but at a much slower rate. As voltage builds up, the absorption level in the insulation drops. This gradual change represents the storage of potential energy along and inside the insulation. Additionally, absorption current is a crucial part of the time resistance method of insulation testing. Lastly, leakage current is the small and steady current present over and through the insulation. Any increase in this current is an indication of deteriorating insulation, which is noted on the insulation testers as a decrease in resistance.
Now that you have some basic understanding of the definition of insulation testing and why it's important, you can think about when and how to perform the tests.
Obviously, you'll want to perform insulation testing when installing new electrical equipment or machinery, for two important reasons. First and foremost, it ensures that the insulation is adequate to start operation. This test is referred to as proof test. And secondly, it provides baseline readings to use as reference points in future testing. Because of fluctuating factors such as temperature and moisture, insulation testing is based on relative measurements. That being said, a reading of 1.5 megaohms will be ruled insignificant without having a previous set of measurements to compare it against. So performing routine measurements can give you valuable information about the insulation's quality, as conditions can vary.
Proof testing is an important step in protecting against defective and miswired equipment. It's often referred to as a go or no go test because it tests the equipment or machinery for errors or improper installation. This test is performed by applying a direct current voltage through a de-energised circuit using an insulation resistance tester. If no failures are registered during the measurement, the test will be a success. Keep in mind that proof testing voltage is much higher than the voltage used in routine maintenance methods.
Short time testing is a testing method where the tester is connected across the insulation of the motor windings of the equipment. A voltage is applied for a minute, and you'll be testing the consistency of the current throughout the duration. Once the test is done, the resistance measurement will be recorded. As briefly aforementioned, a single test can only act as a reference point for the quality of insulation. With that said, short time testing should be performed more frequently so that long-term trends can be examined. Again, it's important to understand that factors such as moisture and temperature can impact the readings. Usually, insulation deteriorates an a gradual but consistent pace, so a significant downward decrease is a sign of insulation breakdown.
The time resistance test, unlike the short time resistance test, can provide conclusive results without the need for past measurements. This method is based on taking readings at fixed time intervals, then plotting them. This is an effective method when contaminants like moisture, dirt, oil and other contaminants are present. The best way to quantify time resistance test results is though the dielectric absorption ratio, which consists of two time resistance readings. The most commonly used set of intervals is a one minute reading divided by a half minute reading, or a ten minute reading divided by a one minute reading. The resulting value is known as polarisation index.
And the step voltage test tests for the insulation at multiple voltages and compares the results. Good insulation will provide you with consistent readings regardless of the amount of voltage applied. When the resistance level drops as the voltage level is increased, that's an indicator of the insulation being contaminated, aging or becoming brittle. This can occur from small imperfections like cracks and pinholes that reveal themselves under high electrical stress. That being said, when performing the step voltage test, start with the lowest test voltage and then move to higher voltage levels.
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