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Mega-ohm meter is commonly known as the shaking table. It is a portable instrument designed to detect the insulation resistance of electrical equipment and power supply lines. The quality of electrical equipment insulation is related to the normal operation of electrical equipment and the personal safety of operators.
When a megohm meter is working, it generates a high voltage, and the measuring object is an electrical device. Therefore, it must be used correctly, otherwise it will cause an accident to the person or equipment. This article summarizes some of the correct use and maintenance methods for your reference.
I. The structure and selection principle of megohmmeter It consists of a hand-cranked generator, header and three terminals (namely L: line terminal, E: ground terminal, G: shield terminal), "G" (ie, shielding The end) is also called a protection ring.
The choice of rated voltage level: high voltage electrical equipment insulation resistance requirements are high, must use high voltage megohmmeter to test; low voltage electrical equipment internal insulation materials can withstand high voltage, in order to ensure equipment safety, should choose low voltage European table. Under normal circumstances, equipment with a rated voltage below 500V should use 500V or 1000V megohmmeters: equipment with a rated voltage above 500V and 1000V to 2500V megohmmeters.
Selection of resistance range: The principle of selecting the megohmmeter is not to make the measurement range excessively exceed the value of the measured insulation resistance, so as to avoid a large reading error due to the thick scale. Also note that some megohmmeters start with a scale other than zero, but 1MΩ or 2MΩ. This megohmmeter should not measure the insulation resistance of low-voltage electrical equipment in a humid environment, because the insulation resistance of the equipment in this environment is relatively small, there may be less than 1MΩ, readings can not be read on the meter. It is easy to mistake that the insulation resistance is 1MΩ or zero. There are two small black points on the dial of the megohmmeter. The area between the black points is the accurate measurement area. Therefore, when the table is selected, the insulation resistance of the device under test should be within the accurate measurement area.
Second, the correct use of megohmmeter method 1 must be measured before cutting off the device power, and short-circuit discharge to the ground. Never allow the device to be charged for measurement to ensure personal and equipment safety. For devices that may induce high voltages, this possibility must be eliminated before measurements can be made.
2 The surface of the tested object should be cleaned. Reduce contact resistance to ensure correct measurement results.
3 The megohmmeter should be open and short circuited before the measurement to check if the megohmmeter is good. That is, before the megohmmeter is connected to the measured object. Shake the handle to make the generator reach the rated speed (120r/min), observe whether the pointer is in the “∞†position of the scale. Short the terminal "L (L) and Ground (E)", shake the handle slowly, and observe whether the pointer is in the "0" position of the scale. If the pointer cannot point to the position of the finger, it indicates that the megohmmeter is faulty. Should be overhauled before use.
The 4 Megohmmeter should be placed in a stable, firm place and away from large external current conductors and external magnetic fields.
5 must be properly wired. There are generally three terminals on a megohmmeter, where L is connected to the conductor of the measured object and the earth, and E is connected to the shell or ground of the object to be measured. G is connected to the shield of the measured object or the part that does not require measurement. When measuring insulation resistance, only the "L" and "E" ends are generally used. However, when measuring the insulation resistance of the cable to the ground or the leakage current of the device under test is severe, the "G" end is used, and the "G" is terminated to the shield or the shell. After the line is connected, turn the crank clockwise. The shaking speed should be slow and fast. When the rotation speed reaches about 120 rotations per minute (ZC-25 type), it keeps rotating at a constant speed and reads after 1 minute. And to shake while reading. Can't stop reading.
Note: When using a megohmmeter to measure the insulation resistance of electrical equipment, it must be noted that the “L†and “E†ends cannot be reversed. The correct connection method is: "L" wire end button connected to the device under test conductor, "E" end button grounded device housing. The "G" shield terminates the insulation of the device under test. If "L" and "E" are reversed, the leakage current flowing through the insulator and the surface is collected by the shell. From the ground through the "L" into the measuring coil. The "G" loses the shielding effect and brings a big error to the measurement. In addition, the degree of insulation between the inner lead of the "E" end and the housing is lower than that of the "L" end. When the megohmmeter is used on the ground and the correct connection mode is adopted, the insulation resistance of the “E†end to the instrument housing and the housing to the ground is equivalent to a short circuit, which will not cause errors, and when “L†and “E†are reversed Time. The insulation resistance of "E" to ground is in parallel with the measured insulation resistance. To make the measurement result small, it brings a big error to the measurement.
6 The megohmmeter is placed in the horizontal position during the jog test. When the handle is turned, no short-circuiting is allowed between the knobs. Swing the handle should be gradually slower, if you find the pointer pointing to the measured insulation may have a short circuit, then you can not continue to shake the handle. In order to prevent the table coil heat damage.
7 reading is complete. Discharge the device under test. The discharge method is to use the rhythm ground wire from the megohmmeter to take a short connection with the device under test (not a megohmmeter discharge). III. Precautions in measurement 1. Do not measure the insulation resistance during lightning or near the high voltage equipment, only when the equipment is not powered. It is also measured without induction.
2. During the measurement process. No one can work on the device under test.
3. Megohmmeters cannot be twisted together. To separate.
4. Before the megohmmeter has stopped rotating or the device under test has not discharged. Do not touch it with your hands. Do not touch the metal part of the lead when removing the thread.
5. At the end of the measurement. For large capacitive devices to discharge.
6. Measured from the megohmmeter terminals, the measuring cord insulation should be good, and the distance between the two wires and between the wire and the ground should be kept properly so as not to affect the measurement accuracy.
7. In order to prevent leakage resistance of the device under test, when using a megohmmeter. The middle layer of the device under test (such as the inner insulation between the cable cores) should be connected to the protection ring.
8. To verify its accuracy regularly.
An introduction to the correct use and maintenance of megohmmeters
Abstract : The megohmmeter is commonly known as the shaking table. It is a portable instrument designed to detect the insulation resistance of electrical equipment and power supply lines. The quality of electrical equipment insulation is related to the normal operation of electrical equipment and the personal safety of operators. When a megohm meter is working, it generates a high voltage, and the measuring object is an electrical device. Therefore, it must be used correctly, otherwise it will cause an accident to the person or equipment. This article summarizes some of the correct use and maintenance methods for your reference.