Megohmmeters are indispensable instruments for power, post and telecommunications, communications, electromechanical installation and maintenance, and industrial enterprises that use electricity as an industrial power or energy source. It is suitable for measuring the resistance value of various insulating materials and the insulation resistance of transformers, motors, cables and electrical equipment. However, it is one of the necessary test instruments for electricians, so the relevant knowledge must be mastered. Let's explore the relevant knowledge of the megohmmeter with the small series. Maybe there will be unexpected gains!
How can a megohmmeter judge good or bad?
A good watch can be zero when the two wires are shorted, and when the two wires are disconnected, the pointer points to infinity when the shaking speed reaches 120 rpm. And when you stop shaking, the needle can be pointed to any place.
How to discharge after the megohmmeter testAfter the insulation resistance test is completed, first stop the megohmmeter power supply output, discharge the test product through the resistor grounding, and finally directly discharge the ground. For large-capacity samples, such as long cables, capacitors, and large motors, discharge for more than 5 minutes to allow the charge on the sample to be exhausted. In addition, when there is a possibility of inductive electrostatic voltage to nearby power equipment, it should also be discharged or short-circuited in advance. After sufficient discharge, the sample can be accessed. For the voltage doubler rectification device assembled on site, it is necessary to discharge the capacitors of each stage step by step before changing the wiring or ending the experiment and removing the wiring.
For power cables, capacitors, generators, transformers, etc., the test sample must be discharged with appropriate discharge resistance. If it is discharged directly to the ground, an extremely high frequency oscillating overvoltage may occur, which is harmful to the insulation of the sample. The discharge resistance depends on the test voltage and the capacitance of the sample, and must have sufficient resistance and heat capacity. Water resistance is usually used, and the resistance value can be approximately 200 to 500 Ω per kilovolt. The effective length between the two stages of the discharge resistor can be selected with reference to the length L of the high voltage protection resistor. The length L of the insulating portion of the discharge rod shall comply with the safety regulations and not less than the effective length of the discharge resistor.
NC680 insulation resistance tester (digital megohmmeter) voltage level is 500V, 1000V second gear fast switching function.
NC681 insulation resistance tester (digital megohmmeter) voltage level is 500V, 1000V, 2500V three-speed fast switching function.
NC682 insulation resistance tester (digital megohmmeter) voltage level is 500V, 1000V, 2500V, 5000V four-speed fast switching function.
Note: Be sure to pay attention to safety when conducting insulation test and pressure test!
Meteor test method and steps for testing power cable insulation resistance(1) Preparation work.
• Measure the insulation resistance of a 10kV power cable with a 2500V megger (with test leads). Before the measurement, the megohmmeter should be inspected to see if the pointer is normal, the clamp lead and the clip terminal are connected well, and the measuring lead is insulated with a shield.
• Prepare the grounding rod and prepare for discharge. After the power cable is powered off, it is first discharged and then measured.
• Check the marking on the power cable. For example, if the rated insulation voltage in the model matches the voltage level of the shaker, the range of insulation values ​​of the device should be known when measuring.
• Wipe the dirt near the cable core with a dry, clean, soft cloth.
· Before starting the measurement, use the rope to lift the cable head to about 1.5m, and the other end of the cable should be up, preferably erect, to avoid the cable head hitting the wall and ground, and there is a person responsible for the care.
(2) Testing. The test items are mainly the insulation resistance values ​​of phase and ground, namely UV, W, ground; VU, W, ground; WU, V, ground, a total of three times.
· Wiring as required should be correct. For example, if the relative insulation resistance is shaken, the measured phase is shielded and connected to the G terminal of the megger; the two cores of the non-measured phase are connected to the metal sheath of the cable and grounded together, and the grounding is common. The wire is connected to the M-ohm terminal of the Megohmmeter; a test wire is connected to the L terminal of the megohmmeter, and the other end of the test wire (L-line) is not wired at this time, as shown in the figure.
Diagram of the wiring diagram of the megohmmeter test power cable insulation resistance
· One person wears insulated gloves and holds the insulated part of the L test line with the hand. The other person rotates the megohmmeter handle to make the generator speed reach 120r/min, and the L line is in contact with the core. After 1min (after the pointer is stable), record Its insulation resistance value.
• Remove the L-line from the core, stop turning the handle, and then discharge. After the discharge is completed, the measurement of the one-phase cable core has ended.
Repeat the above steps twice to measure the insulation resistance of the other two-phase cable cores. If the measured value is not within the acceptable range, it indicates that the insulation resistance of the tested power cable is unqualified; if the three measurements are within the standard range, the insulation resistance of the power cable is qualified.
The standard for judging whether the insulation resistance of a power cable is qualified is as follows:
(1) 10kV power cables with a length of 500m or less are measured with a 2500V megohmmeter. When the cable temperature is 20°C, the insulation resistance value should not be lower than 400MΩ.
(2) Between three phases, the insulation resistance value should be almost the same; if it is inconsistent, the imbalance factor should not be greater than 2.5.
(3) The value of this measurement and the value measured last time shall not decrease by more than 30% when converted to the same temperature.
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