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This article introduces several commonly used instruments by electricians and their usage methods and precautions, mainly including multimeters, ohmmeters, voltmeters, ammeters, and resistance measuring instruments.
1. Usage of Multimeter
The multimeter can measure DC current, DC voltage, AC voltage, and resistance, and some can also measure power, inductance, and capacitance, making it one of the most commonly used instruments by electricians.
1. Basic Structure and Appearance of Multimeter
The multimeter mainly consists of the display part, measuring circuit, and switching device. The display part is usually a magnetic electric microammeter, commonly known as the meter head; the measuring part converts the measured electrical quantity into a small DC current suitable for the meter head, usually including shunt circuits, voltage divider circuits, and rectifier circuits; the measurement of different types of electrical quantities and the selection of ranges are achieved through the switching device.
500 series multimeter↓
2. How to Use a Multimeter
(1) Correctly select the terminal (or socket)
The red probe connection line should be connected to the red terminal (or socket marked with a “+” sign), and the black probe connection line should be connected to the black terminal (or to the socket marked with a “-” sign). Some multimeters are equipped with measurement terminals for AC and DC 2500 volts, when using, the black test rod should still be connected to the black terminal (or “-” socket), and the red test rod should be connected to the 2500 volts terminal (or socket).
(2) Correctly select the position of the switch
Turn the switch to the required position according to the measurement object. For example, to measure current, turn the switch to the corresponding current gear, and to measure voltage, turn it to the corresponding voltage gear. Some multimeters have two switches on the panel, one for selecting the type of measurement and the other for selecting the measurement range. When using, you should first select the type of measurement, then select the measurement range.
(3) Choose an appropriate range
Based on the approximate range of the measured value, turn the switch to the appropriate range for that 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) Correctly take readings
There are many scales on the scale dial of the multimeter, each applicable to different measured objects. Therefore, when measuring, you should pay attention to reading on the corresponding scale while also considering the match between scale readings and range gears to avoid errors.
(5) Correct use of the ohm range
① Choose the appropriate multiplier range:
When measuring resistance, the choice of multiplier range should allow the pointer to stop in the less dense part of the scale, the closer the pointer is to the middle of the scale, the more accurate the reading; the further left, the denser the scale lines, the less accurate the reading.
② Zero adjustment:
Before measuring resistance, the two test rods should touch each other, and the “zero adjustment knob” should be turned until the pointer just points to the zero position on the ohm scale. This step is called ohm range zero adjustment. This step should be repeated each time you change the ohm range to ensure measurement accuracy. If the pointer cannot be adjusted to the zero position, it indicates that the battery voltage is insufficient and needs to be replaced.
③ Do not measure resistance while powered:
When measuring resistance, the multimeter is powered by a dry battery, and the measured resistance must not be powered to avoid damaging the meter head. During the use of the ohm range, do not short-circuit the two test rods to avoid wasting battery power.
(6) Pay attention to operational safety
① When using the multimeter, be careful not to touch the metal parts of the test rods to ensure safety and measurement accuracy.
② When measuring high voltages or large currents, do not switch the switch while powered, otherwise, it may burn out the switch.
③ After using the multimeter, it is best to turn the switch to the highest range for AC voltage, this range is the safest for the multimeter to prevent damage due to negligence during the next measurement.
④ Before the test rod contacts the measured circuit, a comprehensive check should be performed again to see if there are any errors in the positions of each part.
2. Usage of Megohmmeter
The megohmmeter, commonly known as the shake meter, is used to measure large resistances and insulation resistances, with its unit being megaohms (MΩ), hence the name megohmmeter. There are many types of megohmmeters, but their functions are generally similar.
Hand-cranked megohmmeter↓
Electronic megohmmeter↓
1. Selecting a Megohmmeter
The specified 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 circuits rated below 500V, a 500V or 1000V megohmmeter can be used;
When measuring insulation resistance of equipment or circuits rated above 500V, a 1000V to 2500V megohmmeter should be used; when measuring insulators, a 2500V to 5000V megohmmeter should be used.
In general, when measuring insulation resistance of low-voltage electrical equipment, a megohmmeter with a range of 0 to 200MΩ can be used.
2. Measuring Insulation Resistance
The megohmmeter has three terminals, with the upper two larger terminals marked as “Ground” (E) and “Line” (L), and a smaller terminal below marked as “Protective Ring” (or “Shield”) (G).
(1) Insulation Resistance of the Line to Ground
Connect the megohmmeter’s “Ground” terminal (i.e., E terminal) securely to the ground (generally to a certain grounding body), and connect the “Line” terminal (i.e., L terminal) to the measured line, as shown in the figure below.
After connecting, turn the megohmmeter clockwise, gradually increasing the speed, and maintain a speed of about 120 revolutions per minute, and when the speed stabilizes, the pointer will also stabilize, and the value indicated by the pointer is the insulation resistance value of the measured object.
In actual use, the E and L terminals can also be connected arbitrarily, i.e., E can be connected to the measured object, and L can be connected to the grounding body (i.e., to ground), but the G terminal must not be connected incorrectly.
(a) Measure the insulation resistance of the line
(b) Measure the insulation resistance of the motor
(c) Measure the insulation resistance of the cable
(2) Measuring the Insulation Resistance of Motors
Connect the megohmmeter E terminal to the machine shell (i.e., ground) and 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 ground insulation resistance value of that phase.
(3) Measuring the Insulation Resistance of Cables
When measuring the insulation resistance between the conductive core of the cable and the cable outer shell, connect the terminal E to the cable outer shell, the terminal L to the core, and the terminal G to the insulation layer between the cable shell and core, as shown in figure c.
3. Usage Precautions
(1) Perform open circuit and short circuit tests before use. Ensure that the L and E terminals are in a disconnected state, shake the megohmmeter, and the pointer should point to “∞”; short-circuit the L and E terminals, slowly rotate, and the pointer should point to “0”. If both conditions are met, it indicates that the megohmmeter is functioning well.
(2) When measuring the insulation resistance of electrical equipment, the power must be cut off first, and the equipment should be discharged to ensure personal safety and measurement accuracy.
(3) The megohmmeter should be placed horizontally, and it should be pressed down firmly to prevent shaking during operation, with a shaking speed of 120 revolutions per minute.
(4) The connecting wires should be made of multi-stranded soft wire with good insulation properties, and the two connecting wires should not be twisted together to avoid inaccurate measurement data.
(5) After measuring, the measured object should be discharged immediately, and do not touch the measured part or remove the wires with bare hands before the shaking handle of the shake meter has stopped moving and the measured object has been discharged to prevent electric shock.
3. Ammeters
Ammeters are connected in series with the circuit being measured to measure its current value. Depending on the nature of the measured current, they can be divided into DC ammeters, AC ammeters, and dual-purpose ammeters. According to their measurement range, there are microammeters, milliamp meters, and amp meters. Based on their working principle, they can be divided into magnetic electric type, electromagnetic type, and electric type.
Portable clamp ammeter↓
1. Selecting an Ammeter
When measuring DC current, the most common choice is a magnetic electric type instrument, but electromagnetic or electric type instruments can also be used. When measuring AC current, the electromagnetic type instrument is more commonly used, but electric type instruments can also be used. For situations requiring high measurement accuracy and sensitivity, magnetic electric type instruments should be used; for situations where measurement precision is not strict and the measured value is larger, low-cost electromagnetic type instruments are often chosen for their strong overload capacity.
The range of the ammeter should be selected based on the size of the measured current, ensuring that the measured current value falls within the range of the ammeter. If the size of the measured current is unclear, it is advisable to first use an ammeter with a larger range to avoid damage due to overload.
2. Usage Methods and Precautions
(1) The ammeter must be connected in series with the circuit being measured.
(2) When measuring DC current, the polarity of the ammeter connection terminals “+” and “-” must not be reversed, otherwise, it may damage the instrument. The magnetic electric ammeter is generally only used for measuring DC current.
(3) Select an appropriate range based on the size of the measured current. For ammeters with two ranges, it has three connection terminals, and when using, check the range markings on the connection terminals carefully, connecting the common terminal and one range terminal in series with the measured circuit.
(4) Choose an appropriate accuracy to meet the measurement needs. Ammeters have internal resistance, and the smaller the internal resistance, the closer the measurement result is to the actual value. To improve measurement accuracy, an ammeter with a smaller internal resistance should be used as much as possible.
(5) When measuring larger AC currents, a current transformer is 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 AC ammeter used in conjunction should also have a range of 5 amperes. The indicated value of the ammeter multiplied by the current transformer’s transformation ratio gives the actual current value being measured. When using a current transformer, ensure that the secondary coil and core are reliably grounded, and a fuse should not be installed on one end of the secondary coil, and it is strictly prohibited to operate with an open circuit.
4. Voltmeters
Voltmeters are connected in parallel with the circuit being measured to measure the voltage value of the circuit. Depending on the nature of the measured voltage, they can be divided into DC voltmeters, AC voltmeters, and dual-purpose voltmeters. According to their measurement range, there are millivolt meters and volt meters. Based on their working principle, they can be divided into magnetic electric type, electromagnetic type, and electric type.
1. Selecting a Voltmeter
The principles and methods for selecting voltmeters are basically the same as those for ammeters, mainly considering factors such as the measurement object, measurement range, required accuracy, and instrument price. For measurements with less stringent accuracy requirements, electromagnetic voltmeters are generally used. For measurements requiring high precision and sensitivity, magnetic electric multi-range voltmeters are commonly used, among which the voltage range of the multimeter is most widely used.
2. Usage Methods and Precautions
(1) The voltmeter must be connected across the two ends of the circuit being measured.
(2) The range of the voltmeter should be greater than the voltage of the measured circuit to avoid damaging the voltmeter.
(3) When using a magnetic electric voltmeter to measure DC voltage, pay attention to the polarity markings on the voltmeter connection terminals “+” and “-”.
(4) Voltmeters have internal resistance; the larger the internal resistance, the closer the measurement result is to the actual value. To improve measurement accuracy, a voltmeter with larger internal resistance should be used as much as possible.
(5) When measuring high voltages, a voltage transformer should be used. The primary coil of the voltage transformer is connected in parallel with the circuit being measured, and the secondary coil rated voltage is 100 volts, connected to a voltmeter with a range of 100 volts. The indicated value of the voltmeter multiplied by the voltage transformer’s transformation ratio gives the actual voltage value being measured. During operation, care must be taken to prevent the secondary coil from short-circuiting, and a fuse is usually set in the secondary coil as protection.
5. Ground Resistance Measuring Instrument
Ground resistance refers to the resistance of the grounding body buried underground and the soil dispersal resistance.
Usage Methods:
1. Disconnect the grounding line from the grounding body, or disconnect all grounding branch connections on the grounding line.
2. Insert two grounding rods into the ground to a depth of 400mm, one 40m away from the grounding body and the other 20m away from the grounding body.
3. Place the shake meter on a flat area near the grounding body, and then proceed with wiring.
(1) Connect one wire from the meter’s terminal E to the grounding body E′.
(2) Connect one wire from the meter’s terminal C to the grounding rod C′, which is 40m away from the grounding body.
(3) Connect one wire from the meter’s terminal P to the grounding rod P′, which is 20m away from the grounding body.
4. Adjust the coarse adjustment knob according to the required ground resistance of the measured grounding body (there are three adjustable ranges).
5. Shake the shake meter evenly at a speed of about 120 revolutions per minute. When the needle deflects, immediately adjust the fine-tuning dial until the needle is centered. The reading after fine-tuning multiplied by the coarse adjustment resistance positioning multiple gives the ground resistance of the measured grounding body. For example, if the fine-tuning reading is 0.6 and the coarse adjustment resistance positioning multiple is 10, the measured ground resistance is 6Ω.
6. To ensure the reliability of the measured ground resistance value, change the position and re-measure. Take the average of several measured values as the ground resistance of the grounding body.
Source: Network, infringement deletion
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