How to use a multimeter
Multimeters can measure DC current, DC voltage, AC voltage and resistance. Some can also measure power, inductance and capacitance. They are one of the most commonly used instruments among electricians.
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1. Basic structure and appearance of multimeter
The multimeter mainly consists of three parts: indicating part, measuring circuit and conversion device. The indicating part is usually a magnetoelectric microammeter, commonly known as the meter head; the measuring part converts the measured electricity into a tiny DC current suitable for the meter head, and usually includes a shunt circuit, a voltage divider circuit and a rectifier circuit; the measurement of different types of electricity The selection of measurement range is achieved through a conversion device.
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500 type multimeter
2. How to use a multimeter
1. The terminal button (or jack) must be selected correctly
The red test lead connecting wire should be connected to the red terminal button (or into the jack marked with "+"), and the black test lead connecting wire should be connected to the black terminal button (or into the jack marked with "-") , some multimeters are equipped with AC and DC 2500 volt measurement terminals. When in use, the black test rod is still connected to the black terminal (or the "-" jack), and the red test rod is connected to the 2500 volt terminal (or inside the jack).
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2. The selection of the transfer switch position must be correct.
Turn the switch to the required position according to the measurement object. If measuring current, turn the switch to the corresponding current range and measure voltage to the corresponding voltage range. Some multimeters have two switches on the panel, one to select the measurement type and the other to select the measurement range. When using, you should first select the measurement type and then select the measurement range.
3. The measurement range should be selected appropriately.
According to the approximate range of the measurement, turn the switch to the appropriate range for that type. When measuring voltage or current, it is best to keep the pointer within one-half to two-thirds of the range for more accurate readings.
4. Take correct readings
There are many scales on the dial of a multimeter, and they are suitable for different objects being measured. Therefore, when measuring, while reading on the corresponding scale, you should also pay attention to the coordination between the scale reading and the measuring range to avoid errors.
5. Correct use of ohm gear
1. Select the appropriate magnification file
When measuring resistance, the magnification gear should be selected so that the pointer stays in the thinner part of the scale. The closer the pointer is to the middle of the scale, the more accurate the reading will be. The further to the left, the more crowded the scale line will be, and the more accurate the reading will be. Difference.
2. Zero adjustment
Before measuring resistance, you should touch the two test rods together and turn the "zero knob" at the same time so that the pointer just points at the zero position of the ohm scale. This step is called ohm zero adjustment. Each time you change the ohm level, repeat this step before measuring resistance to ensure measurement accuracy. If the pointer cannot be adjusted to zero, the battery voltage is insufficient and needs to be replaced.
3. Do not measure resistance while charging
When measuring resistance, the multimeter is powered by dry batteries. The resistance being measured must not be charged to avoid damage to the meter head. When using the ohm mode, do not short-circuit the two test rods to avoid wasting the battery.
6. Pay attention to operational safety
① When using the multimeter, be careful not to touch the metal part of the test rod with your hands to ensure safety and measurement accuracy.
② When measuring higher voltage or larger current, do not turn the transfer switch while it is powered, otherwise the switch may burn out.
③ After using up the multimeter, it is best to turn the switch to the highest range of AC voltage. This is the safest range for the multimeter to prevent damage to the multimeter due to negligence during the next measurement.
④ Before the test rod comes into contact with the circuit under test, a comprehensive inspection should be made to see if the position of each part is wrong.
How to use a megger
Ohmmeter, commonly known as megohmmeter, is used to measure large resistance and insulation resistance. Its measurement unit is megaohm (MΩ), so it is called megohmmeter. There are many types of megohmmeters, but they all serve roughly the same purpose.
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Hand cranked megger
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Electronic megger
1. Selection of megger
It is stipulated that the voltage level of the megohmmeter should be higher than the insulation voltage level of the object under test. Therefore, when measuring the insulation resistance of equipment or lines with a rated voltage below 500V, a 500V or 1000V megger can be used;
When measuring the insulation resistance of equipment or lines with a rated voltage above 500V, a 1000~2500V megohmmeter should be used; when measuring insulators, a 2500~5000V megohmmeter should be used.
Under normal circumstances, when measuring the insulation resistance of low-voltage electrical equipment, a megohmmeter with a range of 0~200MΩ can be used.
2. Measuring method of insulation resistance
The megohmmeter has three terminals. The two larger terminals at the upper end are marked "ground" (E) and "line" (L) respectively. The smaller terminal at the bottom is marked "protection ring" ( or "shielding") (G).
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1. Insulation resistance of line to ground
Connect the "ground" terminal (i.e. E terminal) of the megohmmeter to the ground reliably (usually to a grounding body), and connect the "line" terminal (i.e. L terminal) to the line under test, as follows As shown in the figure.
After the connection is completed, shake the megohmmeter clockwise. The speed will gradually increase. Keep it at about 120 rpm and then shake it at a constant speed. When the speed is stable and the pointer of the meter is also stable, the value indicated by the pointer is the insulation resistance of the object being measured. value.
In actual use, the two terminals E and L can be connected arbitrarily, that is, E can be connected to the object under test, and L can be connected to the ground body (that is, grounded), but the G terminal must not be connected incorrectly.
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(a) Measure the insulation resistance of the line
(b) Measure the motor insulation resistance
(c) Measure cable insulation resistance
2. Measure the insulation resistance of the motor
Connect the E terminal of the megohmmeter to the chassis (that is, grounding), and connect the L terminal to the winding of a certain phase of the motor. As shown in the figure above, the measured insulation resistance value is the insulation resistance value of a certain phase to ground.
3. Measure the insulation resistance of the cable
When measuring the insulation resistance of the conductive core of the cable and the cable shell, connect terminal E to the cable shell, connect terminal L to the wire core, and connect terminal G to the insulation layer between the cable shell and the core. As shown in Figure c above.
3. Note on use
(1) Open circuit and short circuit tests should be conducted before use. Make the L and E terminals disconnected, shake the megohmmeter, the pointer should point to "∞"; short-circuit the L and E terminals, turn slowly, the pointer should point at "0". Both of these items meet the requirements, indicating that the megohmmeter is good.
(2) When measuring the insulation resistance of electrical equipment, you must first cut off the power supply and then discharge the equipment to ensure personal safety and accurate measurement.
(3) The megohmmeter should be placed in a horizontal position when measuring, and the megohmmeter should be pressed firmly to prevent it from shaking during shaking. The shaking speed is 120 rpm.
(4) The lead wire should be a multi-stranded soft wire with good insulation properties. The two leads should not be twisted together to avoid inaccurate measurement data.
(5) After the measurement, the object under test should be discharged immediately. Do not touch the measurement part of the object under test or remove the wires with your hands before the handle of the megger stops rotating and the object under test is not discharged to prevent electric shock.
Ammeter
An ammeter is connected in series to the circuit being measured to measure its current value. According to the properties of the measured current, it can be divided into DC ammeter, AC ammeter and AC-DC ammeter. In terms of their measurement range, there are microammeters, milliammeters and ammeters. According to the action principle, it is divided into magnetoelectric type, electromagnetic type and electric type.
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Portable clamp ammeter
1. Selection of ammeter
When measuring DC current, it is more common to use magnetoelectric instruments, but electromagnetic or electric instruments can also be used. When measuring AC current, electromagnetic instruments are mostly used, but electric instruments can also be used. Magnetoelectric instruments should be used where high measurement accuracy and sensitivity are required. Electromagnetic instruments with low price and strong overload capability are often chosen for occasions where measurement accuracy is not strict and the measured size is large.
The range selection of the ammeter should be determined according to the size of the measured current, so that the measured current value should be within the range of the ammeter. When the magnitude of the current being measured is unclear, an ammeter with a larger range should be used for testing first to avoid damage to the meter due to overload.
2. Usage methods and precautions
1. Be sure to connect the ammeter in series to the circuit under test.
2. When measuring DC current, the "+" and "-" polarities of the ammeter terminals must not be connected incorrectly, otherwise the meter may be damaged. Magnetoelectric ammeters are generally only used to measure DC current.
3. The appropriate range should be selected according to the size of the current being measured. For an ammeter with two ranges, it has three terminals. When using it, you must see the terminal range marks clearly and connect the common terminal and one range terminal in series in the circuit under test.
4. Choose the appropriate accuracy to meet the needs of the measurement. The ammeter has internal resistance. The smaller the internal resistance, the closer the measurement result is to the actual value. In order to improve the accuracy of measurement, an ammeter with smaller internal resistance should be used as much as possible.
5. When measuring AC currents with large values, current transformers are often used to expand the range of the AC ammeter. The rated current of the secondary coil of the current transformer is generally designed to be 5 amps, and the range of the AC ammeter used with it should also be 5 amps. The ammeter indication value multiplied by the current transformer ratio is the measured actual current value. When using a current transformer, the secondary coil and core of the transformer should be reliably grounded. A fuse must not be installed at one end of the secondary coil, and it is strictly prohibited to open the circuit during use.
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Voltmeter
The voltmeter is connected in parallel in the circuit under test to measure the voltage value of the circuit under test. According to the nature of the voltage measured, it is divided into DC voltmeter, AC voltmeter and AC-DC voltmeter. In terms of their measurement range, they are divided into millivoltmeters and voltmeters. According to the action principle, it is divided into magnetoelectric type, electromagnetic type and electric type.
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1. Selection of voltmeter
The selection principles and methods of voltmeters are basically the same as those of ammeters, mainly considering the measurement object, measurement range, required accuracy and instrument price. The measurement accuracy requirement is not high, and electromagnetic voltmeters are generally used. For those with high requirements on measurement accuracy and sensitivity, magnetoelectric multi-range voltmeters are often used, among which the voltage range of a multimeter is commonly used.
2. Usage methods and precautions
1. Be sure to connect the voltmeter in parallel with both ends of the circuit under test.
2. The range of the voltmeter should be greater than the voltage of the circuit under test to avoid damaging the voltmeter.
3. When using a magnetoelectric voltmeter to measure DC voltage, pay attention to the "+" and "-" polarity marks on the terminals of the voltmeter.
4. The voltmeter has internal resistance. The greater the internal resistance, the closer the measurement result is to the actual value. In order to improve the accuracy of measurement, a voltmeter with a larger internal resistance should be used as much as possible.
5. Use a voltage transformer when measuring high voltage. The primary coil of the voltage transformer is connected in parallel to the circuit under test. The rated voltage of the secondary coil is 100 volts and connected to a voltmeter with a range of 100 volts. The indicated value of the voltmeter multiplied by the transformation ratio of the voltage transformer is the value of the actual measured voltage. During operation of the voltage transformer, it is necessary to prevent short circuit in the secondary coil. A fuse is usually installed in the secondary coil for protection.
Ground resistance measuring instrument
Ground resistance refers to the resistance of the ground body buried underground and the soil diffusion resistance.
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Instructions
1. Disconnect the connection point between the ground trunk line and the grounding body, or disconnect the connection points of all ground branch lines on the ground trunk line.
2. Insert two grounding rods into the ground 400mm deep respectively. One is 40m away from the grounding body, and the other is 20m away from the grounding body.
3. Place the oscillator on a flat place near the grounding body, and then perform wiring.
(1) Use a connecting wire to connect the wiring post E on the meter and the grounding body E′ of the grounding device.
(2) Use a connecting wire to connect the terminal post C on the meter and the grounding rod C′ 40m away from the grounding body.
(3) Use a connecting wire to connect the wiring pile P on the meter and the grounding rod P′ 20m away from the grounding body.
4. According to the ground resistance requirements of the ground body under test, adjust the coarse adjustment knob (there are three adjustable ranges).
5. Shake the watch evenly at a speed of about 120 rpm. When the hands deflect, adjust the fine-tuning dial until the hands are centered. Multiply the reading after the fine adjustment dial is adjusted by the coarse adjustment positioning multiple, which is the ground resistance of the ground body being measured. For example, the fine-tuning reading is 0.6, and the coarse-tuning resistance positioning multiple is 10, then the measured ground resistance is 6Ω.
6. In order to ensure the reliability of the measured ground resistance value, the orientation should be changed and retested. Take the average value of several measured values as the grounding resistance of the grounding body.
