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Essential tools in an electrician’s toolbox
Include multimeters, ohmmeters,
voltage meters, and ammeters
Resistance measuring instruments, etc.
These tools are indispensable in actual
electrical work.
Come check out the detailed operation of each tool!
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1. How to Use a Multimeter
A multimeter can measure DC current, DC voltage, AC voltage, and resistance, and some can also measure power, inductance, and capacitance. It is one of the most commonly used instruments by electricians.
1. Basic Structure and Appearance of a Multimeter
A multimeter mainly consists of an indicator part, a measuring circuit, and a switching device. The indicator part is usually a moving coil 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, typically including shunt circuits, voltage divider circuits, and rectifier circuits; the measurement and range selection of different types of quantities are achieved through the switching device.
Model 500 multimeter↑
2. How to Use a Multimeter
(1) Select the correct terminal (or socket)
The red probe connection should connect to the red terminal (or to the socket marked with a “+” sign), and the black probe connection should connect to the black terminal (or to the socket marked with a “-” sign). Some multimeters have measurement terminals for AC and DC up to 2500 volts; when using, the black test probe should still connect to the black terminal (or the “-” socket), while the red test probe connects to the 2500-volt terminal (or socket).
(2) Select the correct position for the switch
Turn the switch to the required position based on the measurement object. For measuring current, turn the switch to the corresponding current range; for measuring voltage, turn it to the corresponding voltage range. Some multimeters have two switches on the panel, one for selecting the measurement type and the other for selecting the measurement range. When using, first select the measurement type, 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 of that type. When measuring voltage or current, it’s best to keep the needle within one-half to two-thirds of the range for more accurate readings.
(4) Correctly read the values
There are many scales on the multimeter’s dial, each suitable for different measured objects. Therefore, when measuring, while reading the value on the corresponding scale, also pay attention to the match between the scale reading and the range to avoid errors.
(5) Correct use of the ohm range
① Select an appropriate multiplier range:
When measuring resistance, the multiplier range should be chosen to keep the needle in the less dense part of the scale; the closer the needle is to the middle of the scale, the more accurate the reading will be, while the further left it goes, the denser the scale lines become, making the reading less accurate.
② Zero adjustment:
Before measuring resistance, the two test probes should be touched together, and at the same time, the “zero adjustment knob” should be turned to make the needle point exactly at the zero position on the ohm scale. This step is called zero adjustment of the ohm range. Each time the ohm range is changed, this step must be repeated before measuring resistance to ensure measurement accuracy. If the needle cannot be adjusted to zero, 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 resistance being measured must not be live to avoid damaging the meter head. During the use of the ohm range, do not short-circuit the two test probes to avoid wasting battery.
(6) Pay attention to operational safety
① When using a multimeter, be careful not to touch the metal parts of the test probes to ensure safety and measurement accuracy.
② When measuring higher voltages or larger currents, do not turn the switch while powered; otherwise, it may burn out the switch.
③ After using the multimeter, it’s best to turn the switch to the highest AC voltage range; this range is the safest for the multimeter to prevent damage during the next measurement due to negligence.
④ Before the test probes touch the measured circuit, a comprehensive inspection should be performed again to ensure that all parts are correctly positioned.
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2. How to Use a Megohmmeter
Hand-cranked megohmmeter↑
Electronic megohmmeter↑
A megohmmeter, commonly known as a shake meter, is used to measure high resistance and insulation resistance, with the unit of measurement being megohms (MΩ), hence the name megohmmeter. There are many types of megohmmeters, but their functions are generally similar.
1. Selecting a Megohmmeter
The voltage rating of the megohmmeter should be higher than the insulation voltage rating of the measured object. Therefore, when measuring insulation resistance of equipment or circuits rated below 500V, a 500V or 1000V megohmmeter can be selected;
When measuring insulation resistance of equipment or circuits rated above 500V, a 1000-2500V megohmmeter should be selected; when measuring insulators, a 2500-5000V megohmmeter should be used.
Generally, when measuring low-voltage electrical equipment insulation resistance, a megohmmeter with a range of 0-200MΩ can be selected.
2. Measuring Insulation Resistance
A megohmmeter has three terminals; the two larger upper terminals are marked with “Ground” (E) and “Line” (L), and the smaller lower terminal is marked with “Protective Ring” (or “Shield”) (G).
(1) Insulation Resistance from Line to Ground
Connect the megohmmeter’s “Ground” terminal (i.e., terminal E) reliably to the ground (generally to a grounding body), and connect the “Line” terminal (i.e., terminal L) to the measured line, as shown in the figure below.
After the connection is made, turn the megohmmeter’s crank clockwise, gradually increasing the speed, maintaining a speed of about 120 revolutions per minute, and when the speed is stable, the pointer of the meter will also stabilize, indicating the insulation resistance value of the measured object.
In actual use, the E and L terminals can also be connected arbitrarily; that is, E can be connected to the measured object, and L can be connected to the grounding body (i.e., grounded), but terminal G must not be connected incorrectly.
(a) Measuring the insulation resistance of the line
(b) Measuring the insulation resistance of a motor
(c) Measuring the insulation resistance of a cable
(2) Measuring the Insulation Resistance of a Motor
Connect the megohmmeter’s E terminal to the motor casing (i.e., ground), and connect the L terminal to one phase of the motor winding, as shown in the figure above. The insulation resistance value measured will be the insulation resistance value of that phase to ground.
(3) Measuring the Insulation Resistance of a Cable
When measuring the insulation resistance between the conductive core of the cable and the cable shell, connect terminal E to the cable shell, terminal L to the core, and connect terminal G to the insulation layer between the cable shell and core, as shown in figure c above.
3. Usage Precautions
(1) Before use, perform open circuit and short circuit tests. Ensure that terminals L and E are in a disconnected state, turn the megohmmeter, and the pointer should point to “∞”; short-circuit terminals L and E, slowly turn, and the pointer should point to “0”. If both requirements are met, it indicates that the megohmmeter is functioning well.
(2) When measuring the insulation resistance of electrical equipment, power must be cut off first, and the equipment must be discharged to ensure personal safety and measurement accuracy.
(3) The megohmmeter should be placed horizontally during measurement, and it should be held firmly to prevent shaking while cranking; the cranking speed should be 120 revolutions per minute.
(4) The connecting wires should be made of multi-strand flexible wire with good insulation properties; two connecting wires should not be twisted together to avoid inaccuracies in measurement data.
(5) After measuring, immediately discharge the measured object. Do not touch the measured part or remove the wires before the crank of the megohmmeter stops turning and the measured object is discharged to prevent electric shock.
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3. Ammeters
Portable clamp ammeter↑
An ammeter is connected in series with the circuit being measured to measure its current. Depending on the nature of the measured current, it can be classified as a DC ammeter, AC ammeter, or dual-use ammeter. According to the measuring range, it can be divided into microammeter, milliammeter, and ammeter. Based on the principle of operation, it can be magnetic, electromagnetic, or electrodynamic.
1. Choosing an Ammeter
When measuring DC current, the most common choice is a moving coil instrument, but electromagnetic or electrodynamic instruments can also be used. For measuring AC current, electromagnetic instruments are more commonly used, but electrodynamic instruments can also be used. In situations where high accuracy and sensitivity are required, magnetic instruments should be used; for less strict accuracy requirements and larger measured quantities, lower-priced electromagnetic instruments with strong overload capability are often chosen.
Selection of the ammeter’s range should be determined based on the size of the current being measured, ensuring that the measured current value falls within the ammeter’s range. If the size of the measured current is unclear, a larger range ammeter should be used for preliminary testing 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’s terminals “+” and “-” must not be reversed; otherwise, it may damage the instrument. Moving coil ammeters are generally only used for measuring DC current.
(3) Choose an appropriate range based on the size of the current being measured. For ammeters with two ranges, they typically have three connection terminals, so pay attention to the range markings while connecting terminals to the measured circuit.
(4) Select an appropriate accuracy to meet the measurement needs. Ammeters have internal resistance; the smaller the internal resistance, the closer the measurement result will be to the actual value. To improve measurement accuracy, it is advisable to use ammeters with lower internal resistance.
(5) When measuring large AC currents, a current transformer is often used to extend 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 with it should also have a range of 5 amperes. The ammeter’s reading multiplied by the transformation ratio of the current transformer 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; it is strictly prohibited to operate with an open circuit.
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4. Voltmeters
Voltmeters are connected in parallel with the circuit being measured to measure the voltage of the circuit. Depending on the nature of the voltage being measured, they can be classified as DC voltmeters, AC voltmeters, or dual-use voltmeters. According to the measuring range, they can be divided into millivolt meters and volt meters. Based on the principle of operation, they can be magnetic, electromagnetic, or electrodynamic.
1. Choosing a Voltmeter
The principles and methods for selecting a voltmeter are basically the same as those for selecting an ammeter, considering factors such as the measurement object, measuring range, required accuracy, and instrument price. For lower accuracy requirements, electromagnetic voltmeters are commonly used, while for high accuracy and sensitivity requirements, moving coil multi-range voltmeters are often used, with the voltage range of multimeters being widely utilized.
2. Usage Methods and Precautions
(1) The voltmeter must be connected across the two ends of the circuit being measured.
(2) The voltmeter’s range should be greater than the voltage of the circuit being measured to avoid damaging the voltmeter.
(3) When using a moving coil voltmeter to measure DC voltage, pay attention to the polarity markings on the voltmeter’s terminals “+” and “-”.
(4) A voltmeter has internal resistance; the larger the internal resistance, the closer the measurement result will be to the actual value. To improve measurement accuracy, it is advisable to use voltmeters with higher internal resistance.
(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’s rated voltage is 100 volts, which is connected to a voltmeter with a range of 100 volts. The voltmeter’s reading multiplied by the voltage transformation ratio gives the actual voltage being measured. During operation, care must be taken to prevent short circuits in the secondary coil; a fuse is usually set in the secondary coil as protection.
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5. Ground Resistance Measuring Instrument
Ground resistance refers to the resistance of the grounding body buried underground and the soil dispersion resistance.
How to use:
1. Disconnect the grounding line from the grounding body or disconnect the connection points of all grounding branches 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.
3. Place the shaking meter on a flat area near the grounding body, then connect the wires.
(1) Connect one wire to the terminal E of the meter and the grounding device’s grounding body E′.
(2) Connect one wire to the terminal C of the meter and the grounding rod C′ that is 40m away from the grounding body.
(3) Connect one wire to the terminal P of the meter and the grounding rod P′ that is 20m away from the grounding body.
4. Adjust the coarse adjustment knob according to the grounding resistance requirements of the measured grounding body (there are three adjustable ranges available).
5. Crank the shaking meter at a uniform speed of about 120 revolutions per minute. When the needle deflects, adjust the fine-tuning dial until the needle is centered. The reading after fine-tuning, multiplied by the coarse adjustment positioning multiplier, is the grounding resistance of the measured grounding body. For example, if the fine-tuning reading is 0.6 and the coarse adjustment resistance positioning multiplier is 10, then the grounding resistance measured is 6Ω.
6. To ensure reliable grounding resistance values, change the orientation and re-measure. Take the average of several measured values as the grounding resistance of the grounding body.
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