Modern life is changing with each passing day, and people can't live without electricity for a moment. There are electrical safety issues in the process of electricity use, in electrical equipment, such as motors, cables, household appliances, and so on. One of their normal operations is the degree of insulation of the insulating material, ie the value of the insulation resistance. When heated and damp, the insulation ages. Its insulation resistance is reduced. This causes electrical equipment leakage or short-circuit accidents. In order to avoid accidents, it is required to constantly measure the insulation resistance of various electrical equipment. Determine if the insulation level meets the needs of the equipment. Common resistance measurements are usually measured at low voltage and at high voltage. The insulation resistance is higher due to the general value (generally megaohms). Measurements at low voltages do not reflect true insulation resistance values ​​operating at high voltages. The megohmmeter is also called an insulation resistance meter. It is the most commonly used meter for measuring insulation resistance. It has a high-voltage power supply when measuring the insulation resistance, which is the difference between it and the resistance meter. The megohmmeter is convenient and reliable for measuring insulation resistance. However, if it is used improperly, it will bring unnecessary errors to the measurement. We must use the megohmmeter insulation resistance for measurement.

When the megohmmeter is working, it generates high voltage itself, and the measuring object is electrical equipment, so it must be used correctly, otherwise it will cause personal or equipment accidents. Before using, you must first make the following preparations:

(1) Before the measurement, the power of the device under test must be cut off, and the ground should be short-circuited. The device must not be energized for measurement to ensure the safety of the person and equipment.
(2) For equipment that may induce high voltage, the possibility must be eliminated before measurement can be made.
(3) The surface of the object to be tested should be cleaned to reduce the contact resistance and ensure the correctness of the measurement results.
(4) Before the measurement, check whether the megohmmeter is in normal working condition, mainly check the two points of “0” and “∞”. That is, the handle is shaken to make the motor reach the rated speed. The megger should be in the “0” position when it is short-circuited and “∞” when it is open.
(5) The megohmmeter should be placed in a stable and stable place and away from large external current conductors and external magnetic fields.

After the above preparations are made, the measurement can be carried out. When measuring, pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even errors.

There are three terminals of the megohmmeter: one is “L”, that is, the line end, one “E” is the ground end, and another “G” is the shield end (also called the protection ring). Generally, the insulation resistances to be tested are connected. Between the "L" and "E" ends, but when the surface of the insulator to be tested is seriously leaky, the shield ring of the test object or the portion not to be measured must be connected to the "G" end. Thus, the leakage current flows directly back to the negative end of the generator via the shield end "G" to form a loop, rather than flowing through the megohmmeter measuring mechanism (moving coil). This fundamentally eliminates the influence of surface leakage current. In particular, it should be noted that when measuring the insulation resistance between the cable core and the outer surface, the shield terminal "G" must be connected because the air humidity is high or the cable is insulated. When the surface is not clean, the leakage current on the surface will be very large. In order to prevent the measured object from being affected by leakage, its internal insulation measurement is generally added to the outer surface of the cable with a metal shielding ring, and the megger of the megohmmeter. "End connected.

When using the megohmmeter to measure the insulation resistance of electrical equipment, be sure to note that the "L" and "E" ends cannot be reversed. The correct connection is: "L" line end button is connected to the conductor of the device under test, "E" The grounding device is grounded, and the “G” shield terminates the insulated part of the device under test. If the "L" and "E" are reversed, the leakage current flowing through the insulator and the surface is collected to the ground through the outer casing, and the ground flows through the "L" into the measuring coil, so that the "G" loses the shielding effect and gives the measuring tape. There is a big error. In addition, because the inner lead of the "E" end is insulated from the outer casing by a degree of insulation lower than the "L" end and the outer casing, when the megohmmeter is placed on the ground, when the correct wiring is used, the "E" end is opposite to the outer casing of the instrument. The insulation resistance of the shell to the ground is equivalent to a short circuit and does not cause an error. When the "L" and the "E" are reversed, the insulation resistance of the "E" to the ground is in parallel with the measured insulation resistance, and the measurement result is biased. Small, causing large errors in the measurement.

It can be seen that in order to accurately measure the insulation resistance of electrical equipment, etc., the megohmmeter must be used correctly, otherwise the accuracy and reliability of the measurement will be lost.

How to use megohmmeter and its requirements

1. Before measuring, keep the megohmmeter in a horizontal position, hold the body with your left hand, shake the megger with the right hand, the speed is about 120r/min, the pointer should point to infinity (∞), otherwise the megohmmeter is faulty.

2. Before measuring, the power of the tested electrical equipment and circuit should be cut off, and the relevant components should be temporarily grounded and discharged to ensure the safety and measurement results of the personal and megohmmeter.

3. It must be wired correctly when measuring. The megohmmeter has three terminals (L, E, G). When measuring the resistance of the loop to ground, the L end is connected to the exposed conductor of the loop, and the E end is connected to the grounding wire or the metal casing; when measuring the insulation resistance of the loop, the first end and the tail end of the loop are respectively connected with L and E; measuring the insulation of the cable In order to prevent the leakage current on the cable surface from affecting the measurement accuracy, the shield of the cable should be connected to the G terminal.

4. The measuring cord insulation drawn from the megohmmeter terminal should be good. The proper distance should be maintained between the two wires and between the wire and the ground to avoid affecting the measurement accuracy.

5. When shaking the megohmmeter, do not touch the terminal of the megohmmeter and the circuit under test to prevent electric shock.

6. After shaking the Megger, the terminals should not be shorted to avoid damage.

Second, the use of electronic megohmmeter and requirements

Before the measurement, the instrument is normal A, the power-on check display, normal display OL; B, see if the gear can be converted normally (generally there is gear selection, voltage selection) C, press the test button to check whether there is corresponding voltage output, method : Use a common multimeter to select the highest voltage of the DC voltage, then insert the test lead into the output of the megohmmeter, and press the megohmmeter test button to observe the display of the corresponding voltage value on the multimeter;
After the preparation work is completed, the measurement is performed after the measurement is completed. If the OL is displayed during the measurement, the measured resistance may exceed the measurement range of the instrument and the gear position may be changed (MΩ, GΩ, according to the function configuration of the instrument itself); B, the instrument has no voltage output. Unable to test, can be tested according to the relevant introduction in the first paragraph;
The electronic megohmmeter uses a voltage doubler circuit. When the fifth battery or the nine-volt battery is used for power supply, the power supply current is large. Therefore, it is necessary to turn off the power when not in use (even if the automatic shutdown function is used up, manually shut down)

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