How to use and precautions for instruments commonly used by electricians

　　This article introduces the usage and precautions of several instruments commonly used by electricians, mainly including a brief introduction to multimeter, ohmmeter, voltmeter, ammeter, resistance meter, etc.

　　1. How to use a multimeter

　　A multimeter can measure DC current, DC voltage, AC voltage, resistance, etc. Some can also measure power, inductance, capacitance, etc. It is one of the most commonly used instruments by electricians.

　　1. Basic structure and appearance of multimeter

　　The multimeter is mainly composed of three parts: the indicating part, the measuring circuit and the conversion device. The indicating part is usually a magnetoelectric microammeter, commonly known as the meter head; the measuring part converts the measured electrical quantity into a tiny DC current suitable for the meter head, usually including a shunt circuit, a voltage divider circuit and a rectifier circuit; the measurement of different types of electrical quantities and the selection of ranges are achieved through the conversion device.

　　Model 500 Multimeter↓

　　2. How to use a multimeter

　　(1) Select the correct terminal (or jack)

　　The red test lead wire should be connected to the red terminal (or the socket marked with a "+" sign), and the black test lead wire should be connected to the black terminal (or the socket marked with a "-" sign). Some multimeters are equipped with 2500 volt AC and DC measurement terminals. When in use, the black test stick is still connected to the black terminal (or the socket marked with a "-"), and the red test stick is connected to the 2500 volt terminal (or socket).

　　(2) The switch position should be selected correctly

　　Turn the conversion switch to the required position according to the measurement object. For example, when measuring current, turn the conversion switch to the corresponding current range, and when measuring voltage, turn the conversion switch to the corresponding voltage range. Some multimeters have two conversion switches on the panel, one for selecting the measurement type and the other for selecting the measurement range. When using, you should first select the measurement type and then select the measurement range.

　　(3) Choose the appropriate range

　　According to the approximate range of the measurement, turn the switch to the appropriate range of the type. When measuring voltage or current, it is best to keep the pointer within the range of one-half to two-thirds of the range for more accurate readings.

　　(4) Read the numbers correctly

　　There are many scales on the dial of the multimeter, which are suitable for different objects to be measured. Therefore, when measuring, while reading the corresponding scale, you should also pay attention to the coordination of the scale reading and the range to avoid mistakes.

　　(5) Correct use of the ohmmeter

　　①Select the appropriate magnification:

　　When measuring resistance, the multiplier should be chosen so that the pointer stays on the part with sparse scale lines. The closer the pointer is to the middle of the scale, the more accurate the reading. The further to the left, the more crowded the scale lines are and the less accurate the reading.

　　②Zero adjustment:

　　Before measuring resistance, you should touch the two test rods together and turn the "zero adjustment knob" at the same time so that the pointer just points to the zero position of the ohm scale. This step is called ohm range zero adjustment. Each time you change the ohm range, you should repeat this step before measuring resistance to ensure measurement accuracy. If the pointer cannot be adjusted to zero, it means that the battery voltage is insufficient and needs to be replaced.

　　③ Do not measure resistance when powered on:

　　When measuring resistance, the multimeter is powered by dry batteries. The measured resistance must not be charged to avoid damaging the meter head. When using the ohm range, do not short-circuit the two test rods to avoid wasting batteries.

　　(6) Pay attention to operational safety

　　① When using a 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 conversion switch while energized, otherwise the switch may burn out.

　　③After using the multimeter, it is best to turn the conversion switch to the highest AC voltage range. This range is the safest for the multimeter to prevent the multimeter from being damaged due to negligence during the next measurement.

　　④ Before the test rod touches the circuit under test, a comprehensive inspection should be carried out to see if there is any error in the position of each part.

　　2. How to use the megohmmeter

　　The megohmmeter is commonly known as the shaking meter. It is used to measure the maximum resistance and insulation resistance. Its unit of measurement is megohm (MΩ), so it is called a megohmmeter. There are many types of megohmmeters, but their functions are roughly the same.

　　Manual megohmmeter↓

　　Electronic megohmmeter↓

　　1. Selection of megohmmeter

　　It is stipulated that the voltage level of the megohmmeter should be higher than the insulation voltage level of the object being measured. Therefore, when measuring the insulation resistance of equipment or lines with a rated voltage below 500V, a 500V or 1000V megohmmeter can be used;

　　When measuring the insulation resistance of equipment or lines with a rated voltage of more than 500V, a 1000~2500V megohmmeter should be used; when measuring insulators, a 2500~5000V megohmmeter should be used.

　　Generally, a megohmmeter with a range of 0~200MΩ can be used to measure the insulation resistance of low-voltage electrical equipment.

　　2. Insulation resistance measurement method

　　The megohmmeter has three terminals. The two larger terminals at the top are marked "Ground" (E) and "Line" (L), and the smaller terminal at the bottom is marked "Guard Ring" (or "Shield") (G).

　　(1) Insulation resistance of line to ground

　　Reliably ground the "ground" terminal (i.e., the E terminal) of the megohmmeter (generally connected to a grounding body), and connect the "line" terminal (i.e., the L terminal) to the circuit under test, as shown in the figure below.

　　After the connection is completed, shake the megohmmeter clockwise, gradually increase the speed, maintain 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 value of the object being measured.

　　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 grounding body (that is, grounded), but the G terminal must not be connected incorrectly.

　　(a) Measuring the insulation resistance of the circuit

　　(b) Measuring the insulation resistance of the motor

　　(c) Measuring cable insulation resistance

　　(2) Measuring the insulation resistance of the motor

　　Connect the E terminal of the megohmmeter to the casing (i.e. ground), 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 between the conductive core and the cable casing, connect terminal E to the cable casing, terminal L to the core, and terminal G to the insulation layer between the cable casing and the core, as shown in Figure c above.

　　3. Use precautions

　　(1) Open circuit and short circuit tests should be performed before use. Leave the L and E terminals disconnected, shake the megohmmeter, and the pointer should point to "∞"; short-circuit the L and E terminals, and slowly turn them, and the pointer should point to "0". If both of these meet the requirements, the megohmmeter is good.

　　(2) When measuring the insulation resistance of electrical equipment, the power supply must be cut off first, and then the equipment must be discharged to ensure personal safety and accurate measurement.

　　(3) When measuring, the megohmmeter should be placed in a horizontal position and pressed down firmly to prevent it from shaking. The shaking speed is 120 rpm.

　　(4) The lead wire should be made of multi-strand soft wire with good insulation performance. The two lead wires should not be twisted together to avoid inaccurate measurement data.

　　(5) After the measurement is completed, the object under test should be discharged immediately. Before the megohmmeter's crank stops turning and the object under test is discharged, do not touch the measuring part of the object under test or remove the wires to prevent electric shock.

　　3. Ammeter

　　The ammeter is connected in series in the circuit being measured to measure its current value. According to the nature of the measured current, it can be divided into DC ammeter, AC ammeter and AC/DC dual-purpose ammeter. According to its measuring range, it can be divided into microammeter, milliammeter and amperemeter. According to the operating principle, it can be divided into magnetoelectric type, electromagnetic type and electric type.

　　Portable clamp type ammeter↓

　　1. Selection of ammeter

　　When measuring DC current, magnetoelectric meters are more commonly used, but electromagnetic or electric meters can also be used. When measuring AC current, electromagnetic meters are more commonly used, but electric meters can also be used. Magnetoelectric meters should be used in situations where high measurement accuracy and high sensitivity are required; electromagnetic meters with low prices and strong overload capacity are often selected for situations where measurement accuracy is not strictly required and the measured object is large.

　　The range of the ammeter should be determined according to the current being measured, and the current value being measured should be within the range of the ammeter. When the current value being measured is not clear, an ammeter with a larger range should be used for test first to avoid damage to the instrument due to overload.

　　2. Usage and precautions

　　(1) The ammeter must be connected in series with the circuit being measured.

　　(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 current being measured. For an ammeter with two ranges, it has three terminals. When using it, you must clearly see the terminal range markings and connect the common terminal and one range terminal in series in the circuit being measured.

　　(4) Select 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 the measurement, the ammeter with a smaller internal resistance should be used as much as possible.

　　(5) When measuring large AC currents, 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 amperes, and the range of the AC ammeter used with it should also be 5 amperes. The ammeter indication multiplied by the current transformer's transformation ratio is the actual current value measured. When using a current transformer, the secondary coil and the core of the transformer should be reliably grounded. A fuse should not be installed at one end of the secondary coil, and it is strictly forbidden to open the circuit during use.

　　4. Voltmeter

　　The voltmeter is connected in parallel to the circuit being measured to measure the voltage value of the circuit being measured. According to the nature of the voltage being measured, it is divided into DC voltmeter, AC voltmeter and AC/DC dual-purpose voltmeter. According to its measuring range, it is divided into millivoltmeter and voltmeter. According to the operating principle, it is divided into magnetoelectric type, electromagnetic type and electric type.

　　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. If the measurement accuracy requirement is not high, electromagnetic voltmeters are generally used. If the measurement accuracy and sensitivity requirement are high, magnetoelectric multi-range voltmeters are often used, among which the voltage range of the multimeter is commonly used.

　　2. Usage and precautions

　　(1) Be sure to connect the voltmeter in parallel with both ends of the circuit being measured.

　　(2) The voltmeter range should be greater than the voltage of the circuit being measured to avoid damaging the voltmeter.

　　(3) When using a magnetoelectric voltmeter to measure DC voltage, pay attention to the "+" and "-" polarity markings on the voltmeter terminals.

　　(4) A voltmeter has internal resistance. The larger the internal resistance, the closer the measurement result is to the actual value. In order to improve the accuracy of the measurement, a voltmeter with a larger internal resistance should be used as much as possible.

　　(5) When measuring high voltage, a voltage transformer must be used. The primary coil of the voltage transformer is connected in parallel to the circuit being measured, and the rated voltage of the secondary coil is 100 volts, which is connected to a voltmeter with a range of 100 volts. The voltmeter indication multiplied by the voltage transformer's transformation ratio is the value of the actual voltage measured. During operation, the voltage transformer must be strictly prevented from short-circuiting the secondary coil. Usually, a fuse is set in the secondary coil as protection.

　　5. Ground resistance measuring instrument

　　Grounding resistance refers to the resistance of the grounding body buried underground and the soil diffusion resistance.

　　Instructions:

　　1. Disconnect the connection point between the grounding main line and the grounding body, or disconnect the connection points of all grounding branches on the grounding main line.

　　2. Insert two ground rods into the ground to a depth of 400 mm, one 40 m away from the grounding body and the other 20 m away from the grounding body.

　　3. Place the megohmmeter on a flat surface near the grounding body, and then make the connections.

　　(1) Use a connecting wire to connect the terminal 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 terminal post P on the meter and the grounding rod P′ 20m away from the grounding body.

　　4. Adjust the coarse adjustment knob (there are three adjustable ranges) according to the grounding resistance requirements of the grounding body to be tested.

　　5. Shake the meter evenly at a speed of about 120 rpm. When the needle deflects, adjust the fine-tuning dial until the needle is centered. Multiply the reading after the fine-tuning dial is adjusted by the coarse adjustment multiple to get the ground resistance of the grounding body being measured. For example, if the fine-tuning reading is 0.6 and the coarse adjustment resistance multiple is 10, the ground resistance being measured is 6Ω.

　　6. In order to ensure the reliability of the measured grounding resistance value, the orientation should be changed and re-measured. The average value of several measured values ​​is taken as the grounding resistance of the grounding body.