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Correct measurement of insulation resistance

time:2021/1/5   source:华天电力  reading:529 time

When using a megohmmeter, it will generate a very high voltage. Since the measurement object is usually electrical equipment, it must be used correctly, otherwise it will cause a safety accident or equipment accident.

1. Preparation

Make the following preparations before use:

1. The power supply of the device under test must be cut off and short-circuited to ground. Measurement is not allowed when the device is live.

2. For those devices that may induce high voltage, this possibility must be eliminated before measurement can be performed.

3. Note that the surface of the measured object must be kept clean, reduce the surface resistance, and ensure the accuracy of the measurement results.

4. Check whether the megohmmeter is in a normal state, mainly check its "0" and "∞", that is, shake the handle to make the motor reach the rated speed. When the megohmmeter is short-circuited, the pointer should point to the "0" position. The pointer should point to the "∞" position when it is open.

5. Note that the megohmmeter should be placed steadily and firmly, and away from large current conductors and strong magnetic fields.

Insulation Resistance Tester.png

2. Correct measurement

When measuring, pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors. There are three terminals of the megohmmeter: one is "L", which is the line end; the other is "E", which is the ground end; The other is "G", that is, the shielding end (also called the protection ring). Generally, the insulating object under test is connected between "L" and "E". However, when the surface of the insulator under test has serious leakage, the test object must be The shielding end or the part that does not need to be measured is connected to the "G" end.

In this way, the leakage current flows directly back to the negative terminal of the generator through the shielded terminal "G" to form a loop, instead of flowing through the measuring mechanism (ratiometer) of the megohmmeter, which fundamentally eliminates the effect of surface leakage current, especially It should be noted that when measuring the insulation resistance between the cable core and the outer surface, the shielding end "G" must be connected, because when the air humidity is high or the cable insulation surface is dirty, the leakage current will be very large. In order to prevent For the impact of the measured object on its internal insulation measurement due to leakage, generally a metal shielding ring is added to the outer surface of the cable and connected to the "G" end of the megohmmeter.

When measuring the insulation resistance of electrical equipment with a megohmmeter, you must pay attention to the "L" and "E" terminals cannot be reversed. The correct connection method is: "L" terminal is connected to the conductor of the device under test, and "E" terminal is grounded. The equipment shell is connected, and the "G" terminal is connected to the insulated part of the device under test. If the "L" and "E" terminals are reversed, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell, from the ground to the "L" Flowing into the flow ratio meter makes "G" lose its shielding effect and bring greater error to the measurement.

In addition, because the degree of insulation between the internal lead of the "E" end and the case is lower than the insulation degree between the "L" end and the case, place the megger on the ground and use the correct wiring method. The insulation resistance to ground is equivalent to a short circuit, which will not cause measurement errors; and when "L" and "E" are connected reversely, the insulation resistance of "E" to ground will be connected in parallel with the measured insulation resistance, making the measurement result too small , Resulting in larger errors.

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