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Megohm Insulation Resistance Test Method

time:2020/1/22   source:Huatian Electric Power  reading:1177 time

The insulation resistance tester can perform three different tests, and a deep understanding of these common test methods is an important tool for gaining the ability to determine electrical insulation conditions and quality. Usually, a dc voltage is applied to the conductor under test and the current flowing through the insulation layer (called "leakage current") is measured.And into the equipment's non-current-carrying gold components for testing.

1. Short or field reading tests

Short-time or dot-read testing is suitable for electrical equipment with very small capacitance, such as short circuits in house wiring or electrical panels.

In this approach, you simply connect the megohm meter to the insulating layer to be tested and apply the appropriate test voltage for a short specified time period (usually recommended 60 seconds). By recording these measurements over time, the actual condition of the insulation can be better judged, and even though readings may be higher than the recommended minimum, any sustained downtrend is usually a reasonable warning of future trouble. A reading below the recommended value may be acceptable as long as they are consistent. One megohm rule. Normally, the insulation resistance should be about 1 megohm per 1000 volts of work, with a minimum of 1 megohm, which is known as the "one megohm rule". For example, a motor rated at 5000V should have a minimum insulation resistance of 5 megohm. 

2. Time resistance method

Unlike dot-read testing, the time-resistant approach is completely temperature independent and can provide you with definitive information without recording previous tests. This test method is sometimes referred to as the "absorption test" because it is based on the absorption of a good insulator, not a wet or contaminated insulator, and even a dot reading indicating an insulation phenomenon can give you a clearer idea of the quality of the insulation.Acceptable terms.

In this way, you can connect megometers as you would in a short or fixed-point reading test, while taking continuous readings at a given time and paying attention to the differences in readings. A typical curve shows the dielectric absorption effect of a capacitive device, such as a large motor winding, in a "time resistance" test. Good insulation shows a continuous increase in internal resistance over a period of time (about 5 to 10 minutes) - this is caused by the charge formed on the capacitor plate attracting the opposite polarity of charge in the insulation, moving and thus absorbing the current.The charging effect of good insulation is much longer over a period of time than the charging time of the insulating capacitor. Perform time resistance tests on large switchgear, transformers, bushing, motors and cables, especially at higher voltages, requiring high insulation resistance ranges and clean, consistent test voltages. These types of equipment should be tested using linear dynamometers.

3. Dielectric absorption and polarization index

The ratio of two time resistance readings (for example, a 60-second reading divided by a 30-second reading) is called dielectric absorption, and if the ratio is a 10-minute reading divided by a 1-minute reading, the value is called the polarization index. These values are very useful for determining the quality of the insulation, which can be tested in 60 seconds with a manual test instrument, much easier than a 30-second first read.

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