(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 “∞â€. 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 “Lâ€, 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 outer casing 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 small. , bringing a large error to 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.
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