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How to Test Insulation Resistance

written by: Asheesh • edited by: Lamar Stonecypher • updated: 8/28/2010

Worried about how to protect your expensive equipment by testing insulation resistance? Here we will discuss the testing of insulation resistance through three important processes.

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    The testing of insulating resistance is responsible to prevent electric shocks, to increase the safety of workers, and to reduce down time. The testing processes are very helpful to detect deterioration of insulation, and also to check the quality of repairs before using equipment for any operation.

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    Causes of insulation failure

    The causes of insulation failure are due to environmental problems such as dirt, moisture, corrosive vapors, oil, and excessive hot and cold. Some technical causes are also responsible such as vibration, aging, and nicked wiring.

    Many maintenance operations are performed to test insulating resistance, but only three of them are basic operations.

    Equipment required for testing insulating resistance

    • Megohmmeter
    • Temperature indicator
    • Humidity meter
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    How to make connections in the megohmmeter

    Usually, the megohmmeter has three terminals.

    Line terminal- This terminal is indicated as ‘L’ in the equipment and is connected to the conductor whose insulation resistance you are going to measure.

    Earth terminal- This terminal is indicted by the letter “E". It is connected to the other side of the conductor, i.e., the ground terminal.

    Guard terminal- This is denoted as ‘G’. It is used to provide the return circuit that bypasses the meter.


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    Insulation resistance testing

    This is the simplest test among the three. Firstly, make all required connections, and pass the test voltage only for one minute. During this process, the resistance may drop or remain steady. It is seen that larger insulation systems show steady decrease in the resistance, whereas, in smaller systems the resistance remains steady. This is because the capacitive and absorption current drop is faster in smaller systems. Record the readings of resistance after one minute.

    It is very important to correct the readings by the standard readings provided in tables. These tables are available for any connection. Actually, temperature affects the insulation resistance with inverse proportion. You will need to correct the readings with respect to the readings of base temperatures. Usually, these corrections are made by 20 and 40 degree C base temperature. However, you can use a common rule of thumb that shows that the value of insulation resistance decreases by factor 2 for every 10 degree C temperature increase.

    The international Electrical Testing Assoc (NETA) has specified these statistics for the maintenance of equipment. Compare the final reading with previous readings of the equipment, and also with readings taken for similar products. If it is found that the readings differ significantly, then you should go for further investigation of the equipment.

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    Dielectric Absorption Ratio

    This ratio is very helpful to determine whether the equipment is good or not. According to the data provided by NETA, good equipment shows a gradual increase in insulation resistance after the application of a test voltage.

    To find this ratio you will need to record the readings two different times. After this, divide the later reading by the earlier reading. The result will be the dielectric absorption ratio of the equipment. Usually, the readings are taken at 30 and 60 seconds or 60 seconds and 10 minutes.

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    Step Voltage Test

    To perform this test, you will need to apply two or more test voltages through steps. For each step, the test voltage should be applied for the same time. Usually, a time period of 60 seconds is used. The process is based upon the phenomenon that the insulation that is dry, clean, and without any damage will show nearly the same resistance, though different test voltages are applied.

    When the readings have been taken, compare them with each other. If the readings are nearly same then there is no need to worry; however, if the readings decrease continuously with increase in test voltage, then it’s the time to investigate because the insulation may be damaged by dirt, moisture, cracking, and aging.

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    The process of testing insulation resistance is not very difficult. You will need to follow the steps accurately. These tests will help you to investigate whether the equipment is working well or not. You will save the cost of maintenance by doing regular insulation resistance testing.

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    Reference-: megohmmeter-