A megger, also called a megger, is mainly used to measure the insulation resistance of electrical equipment. It is composed of an alternator voltage doubler rectifier circuit, a meter and other components. When the megger shakes, it generates a DC voltage.
There are grounding megometers and insulation megometers. The grounding megometer measures the earth resistance. The insulation megometer measures the insulation resistance of wires and cables. Grounding megometer can also be called earth resistance tester. can also be called insulation resistance tester.
How to use the electrician's megger (illustration)
How to use the electrician's megger: Reliably ground the terminal E of the megohmmeter L. is connected to the line tested. Shake the oscillating watch generator handle inclockwise from slow to fast for about 1 minute. Wait for the oscillating watch hand to stabilize and take a reading.
The megohmmeter scale is in megaohms (MΩ). Meggers are composed of medium and large scale integrated circuits. This meter has large output power, high short circuit current value and multiple output voltage levels (each model has four voltage levels).
Digital meggers are composed of medium- and large-scale integrated circuits. This meter has large output power, high short circuit current value and multiple output voltage levels (there are four voltage levels). The working principle is that the high DC voltage generated by the internal battery as a power source through DC/DC conversion flows from the E pole through the test product to the L pole, thereby generating a currentfrom the E pole to the L pole, which is completed by the divider after the I/V conversion. The calculation directly displays the measured insulation resistance value on the LCD screen.
Precautions for using electrician megger
1. When measuring the insulation resistance of the equipment, the power supply to the equipment should first be turned off. . Equipment containing inductors and capacitors (such as capacitors, transformers, motors and cable lines) should be discharged first. Off-duty personnel must request corresponding work tickets before they can work.
2. The oscillator must be placed horizontally. Before wiring, you need to shake the oscillator and observe whether the pointer is at "∞". Then short-circuit terminals L and E and slowly shake the watch. The needle should be at zero. After open circuit and short circuit tests, it is cConfirmed that the megger is intact before the measurement can be made.
3. The lead wire of the megger should be a multi-stranded soft wire and the two wires should not be twisted together to avoid inaccurate measurement data.
4. After the megger measurement is completed, the measured object should be unloaded immediately. Do not touch the measuring part of the measured object or disassemble it with your hands until the megger stops rotating and the measurement is completed. the object is not discharged to avoid electric shock.
5. The surface of the object to be measured should be wiped clean and there should be no dirt (such as paint, etc.) to avoid inaccurate measurement data.
An instrument used to measure high value resistance and insulation resistance is called a megger. Nowadays, new products are often called insulation resistance testers or metersinsulation resistance meters, sometimes also called megohmmeters. Among them, the appearance of the old flip-flop meter is shown on the right below:
The main structure of the flip-flop meter (insulation resistance meter) is composed of a manual generator and an electromagnetic meter head without mechanical reaction. There are connection terminals outside (L: line section, E: ground terminal, G: shield terminal). Newer insulation resistance testers generally resemble digital multimeters.
The working principle of megometer (insulation resistance meter) is as shown in the figure below. Its magnetoelectric meter has two moving coils located at a certain angle to each other. of a notched cylindrical iron core is fixed on a rotating shaft with the needle, forming the moving part of the watch head. It is placed in a permanent magnet. The gapr between the magnetic poles of the magnet and the iron core is unequal. . Since the pointer does not have a shock absorber spring, the pointer can remain in any position when the instrument is not in use.
Shake the handle and the DC generator can produce current. Among them, a current I1 flows in the loop between coil 1 and the measured resistance Rx, and the other current. I2 flows through coil 2 and through the resistance loop Rf, assuming that the resistance of coil 1 is R1 and the resistance of coil 2 is R2, according to Ohm's law:
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I1=U÷(Rc+R1+Rx), R2=U÷(Rf+R2)
The coil excited in the magnetic field is acted upon by the force of the magnetic field, causing coil 1 to generate a rotational torque M1 and coil 2 to generate a rotational torque M2, since the two coils are wound in opposite directions, the directions of M1 and M2 are opposite and the moment resulting from the two couples causes. the pointer to deflect. When M1 = M2, the pointer is stationary and what it points to at this time is the insulation resistance value of the device under test.
When Rx is disconnected (i.e. X=∞ (infinity)), I1=0, M1=0, the pointer deviates to the left under l action of M2, and finally points towards the mark. At the scale Rx=∞, if Rx is shorted (i.e. Rx=0), I1 will be the largest and M1 will be the largest at that time, causing the pointer to deviate in the clockwise, and the pointer points to the point Rx=0 on the scale. According to this principle, the quality of the megger can be tested.
First, let's talk about megger selection. For equipment with a voltage rating below 500 volts, choose a 500 volt or 1000 volt megger (megger) with. a nominal voltage of 500 volts. For voltages above 1,000 volts, use a megger with a range of 1000 volts to 2,500 volts.
How to use the megohmmeter
The megohmmeter (megohmmeter) has three measuring terminals and one line terminal (L) . , another is the ground terminal button (E) and another is the shield terminal button (G). Generally, when measuring the insulation resistance of photos or power lines to ground, only terminals L and E are used, and the wiring is as shown in the left figure below. Terminal “L” is connected to the “fire” or “phase” terminal of the device under test, and terminal “E” is connected to the “earth” terminal of the device under test. When measuring the insulation resistance of cables to ground or when the leakage current of the equipment under test is large, Use the terminal button “G”. The right figure below shows the wiring diagram for measuring cable insulation resistance.
Once the lines are connected, you can turn the handleturn the clock generator clockwise so that the voltage emitted by the generator rotor can be used for measurement. The crank rotation speed should be slow to fast. When the rotation speed reaches a certain value, the rotation speed should be kept uniform and stable. (Typically, the rotation speed of a regular megger is around 120 rpm). When the speed of the megger generator stabilizes, the pointer on the dial also stabilizes. At this time, the value indicated by the needle is the measured insulation resistance value.
Notes
Before measurement, the megohmmeter (megohmmeter) should be tested for open circuits and shorts circuits to check if the meter is in good condition? If both connecting wires are open circuited and the handle is gently shaken, the pointer should point to "∞". If the two sons of a bitchxion are shorted again, the pointer should be at "0", indicating that the meter is open circuited. in good condition. There is an error in the table.
The equipment under test should be disconnected from the power supply and the capacitive equipment should be completely discharged to ensure personal safety and accurate measurement.
During the telemetry process, no one can work on the device under test.
When measuring motors, transformers, cables and capacitors with large capacitances, there must be a certain charging time. The larger the capacity, the longer the charging time generally. , a megger is used to turn the meter. The reading after one minute serves as a reference. Unload immediately after the measurement is completed to ensure safety. The discharge method involves removing the ground wire used during measurement from the megger andto short-circuit it to the object to be measured.
It is prohibited to measure insulation during lightning or on equipment with high voltage conductors nearby. The measurement can only be carried out when the equipment is not loaded and cannot be charged due to induction from other power sources.
Do not touch the measuring part of the equipment or the terminals of the megohmometer with your hands until the megohmometer stops rotating. When removing wires, do not directly touch exposed parts of the wires.
The megometer should be calibrated regularly: the calibration method is to directly measure the standard resistance with a certain value and measure whether it has a measurement error and whether it is within the authorized limits. range.
The working principle of the megometer (insulation resistance meter)
There are twocoils connected to the megometer needle, a Connect the additional resistor R in the same meter in series; connect the other in series with the measured resistance R, then connect them together to the hand generator. When the generator is started by hand, current flows through both coils at the same time and torques in opposite directions are generated on the two coils. The watch hands flex at a certain angle depending on the combined torque of the two couples. This deviation angle is determined by the ratio of the two currents. The additional resistance is constant, so the current value only depends on the size of the resistance to be measured.
Principles of selection of megometers (insulation resistance meters)
(1) Selection of rated voltage levels . Generally speaking, for equipment with a voltage rating below 500 V, a 500 V or 1000 V meggermust be used; For equipment with a rated voltage above 500V, a 1000V~2500V megometer should be used.
(2) Resistance range selection. There are two small black dots on the scale of the megger dial, and the area between the small black dots is the precise measurement area. Therefore, when selecting a table, the insulation resistance value of the equipment under test should be within the precise measurement area.
Use of a megometer (insulation resistance meter)
(1) Calibration of the meter. Before measurement, the megometer should be subjected to open circuit and short circuit test to check whether the megometer is in good condition. Open the two connecting wires, shake the handle, the pointer should point to "∞", then short circuit the two connecting wires, the pointer should point to "0", if the above conditions are met, it is good, otherwise it doesn't mattert not be used.
(2) the equipment under test is disconnected from the line and the large capacity equipment must be unloaded.
(3) Select a megger with a constant voltage level.
(4) When measuring insulation resistance, generally only the "L" and "E" terminals are used, but when measuring the resistance cable insulation to ground or current leakage of the equipment under test. In more serious cases, use the "G" terminal and connect the "G" terminal to the shielding layer or shell. After the circuit is connected, you can turn the shake handle clockwise. Shaking speed should be slow to fast. When the rotating speed reaches about 120 revolutions per minute (ZC-25 type), continue rotating at a constant speed and take. reading after 1 minute. Take the measurements by shaking and don't stop to read.
(5) Repull the wires and unload. When the reading is complete, shake slowly while removing the wires, then discharge the device under test. The discharge method involves removing the ground wire used during measurement from the megometer and short-circuiting it with the device under test (without discharging the megometer).
Measuring steps of megometer (insulation resistance meter)
The use of megometer is divided into following steps:
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(1) Power failure, power test and discharge of the equipment under test
(2) Select the appropriate equipment; depending on the equipment tested The megometer;
(3) Check the megger, which is divided into open circuit inspection and short circuit inspection. An open circuit means both tests. the pens do not touch each other and the grip and pointer rotate at a constant speed. Checking for short circuits consists of rotating the lowcule at a constant speed, the two test leads are in contact and the pointer points to zero;
(4) First, connect the E segment to the ground and rotate the seesaw at a constant speed to reach 120 revolutions per minute, then connect the L segment as a line ;
(5) Once the measurement is completed, unload.
Precautions for megometer (insulation resistance meter)
(1) It is prohibited to measure during lightning or near high temperatures. Voltage Equipment Insulation resistance can only be measured when the device is not loaded and no electricity is induced.
(2) During the shaking test, no one may work on the equipment under test.
(3) The meter wires cannot be twisted together, but must be separated.
(4) It is strictly forbidden to touch the megger with bare hands untilit does not stop rotating or before the device under test is discharged. When removing the wires, do not touch the metal parts of the cables.
(5) At the end of the measurement, large capacity equipment must be unloaded.
(6) Its accuracy must be checked regularly.