1. Correct Use of the Multimeter
Correct use of the multimeter requires familiarity with the dial. Adjust the two zero position adjusters gently. Choose the right terminal, and plug the red and black probes into the correct holes.
Rotate the switch to the correct range. Choose the appropriate range for accurate measurements. Look closely at the scale line and read the measurement correctly from above.
After measurement, switch the probes back, and turn the switch to the high voltage range. Regularly check the battery inside the meter, as deterioration can cause leakage of electrolyte. Store the instrument in a good environment, free from vibration, humidity, and weak magnetic fields.
2. Correct Use of the Ohm Range of the Multimeter
When using the ohm range correctly, you should know and be able to perform eight tasks. The battery voltage must be sufficient, and the circuit under test must be free of voltage. Choose the appropriate multiplier range, and aim for the middle of the scale.
Each time you change the multiplier range, you must reset the resistance to zero. Ensure good contact at the probe tips without touching the probe ends with your hands. Measure circuit continuity using a range above one thousand ohms.
Test diodes with different resistance values according to the multiplier. When measuring transformer windings, avoid touching them as it may cause electric shock.
3. Precautions when Measuring Voltage with a Multimeter
When measuring voltage with a multimeter, there are eight precautions to consider. Be aware of the internal resistance of the meter, and ensure someone is monitoring. The meter should be connected in parallel with the circuit under test, and do not change the range while powered.
When measuring DC voltage, clarify the positive and negative terminals of the circuit. When measuring voltage across a reactive circuit, do not disconnect the power supply during the test. For testing kilovolt high voltage, use dedicated probe leads.
Induced voltage to ground can have a large difference in values depending on the range.
4. Methods for Measuring DC Current with a Multimeter
To measure current with a multimeter, switch to the milliamp range, identify the positive and negative terminals of the circuit, and connect the meter in series with the circuit. Choose a higher range to minimize the impact on the circuit.
5. Using DC Method to Identify the Start and End of a Three-Phase Motor Stator Winding
For three-phase motor windings, use the DC method to determine the start and end. Set the multimeter to the milliamp range, and use a DC power supply like a battery. Connect one phase winding to the meter and the other to the battery.
At the moment of powering on, if the needle moves, the terminal connected to the positive is the start. If it does not reverse and you swap the connections, use the same method for the other phase winding.
6. Using Residual Magnetism Method to Identify the Start and End of a Three-Phase Motor Stator Winding
For motors that have been operated, use the residual magnetism method for identification. Mark the output terminals of the three-phase windings and connect them in parallel. Set the multimeter to the milliamp range and bridge the common point.
Slowly rotate the motor shaft while observing the needle of the meter. If the needle does not swing significantly, the three starts and ends are connected. If the needle swings left and right, one phase is reversed.
Swap the ends of one phase winding and use the same method to test. Continue until the needle does not swing, indicating the start and end are connected.
7. Using Loop Method to Identify the Start and End of a Three-Phase Motor Stator Winding
For motors that have been operated, use the loop method for identification. Connect the output terminals of the three-phase windings in a triangle. Set the multimeter to the milliamp range and connect it in series with the three-phase windings.
Rotate the motor shaft evenly while observing the meter needle. If the needle does not swing significantly, the windings are connected. If the needle swings widely, one phase winding is reversed.
8. Measuring the Speed of a Three-Phase Motor Using a Multimeter
To measure the speed of a three-phase motor, use the multimeter. Open the motor junction box and disconnect the connection terminals. Connect the multimeter to the milliamp range, and bridge any phase winding.
Rotate the rotor for one complete turn and observe how many times the needle swings. A two-pole motor swings once, indicating a synchronous speed of 3000 RPM. A four-pole motor swings twice, indicating a synchronous speed of 1500 RPM.
Continue this method to determine speed, which is slightly lower than the synchronous speed.
9. Testing the Grounding Resistance of Household Grounding Protection Wires
For household grounding wires, test the grounding resistance values. Set the multimeter to the voltage range and connect the electric stove to the phase and neutral. Measure the voltage at the stove end to calculate the working current value.
Reconnect the stove to the phase ground wire and measure the voltage again. The difference between the two voltage readings divided by the working current value gives the grounding resistance value, with about 5% error.
10. Identifying Phase and Neutral Wires of Low Voltage AC Power Supply
In a low voltage three-phase four-wire system, identify the phase and neutral wires. Set the multimeter to the voltage range, with a range of 250V AC, connect one probe to the ground, and the other to the power wire.
If the needle deflects significantly, the probe touches the phase wire. If the needle does not move or only slightly deflects, the probe touches the neutral wire.
11. Measuring the Polarity and Condition of a Diode
To measure the polarity of a diode, set the multimeter to the kilo-ohm range. A small resistance reading indicates the diode’s forward resistance. Connect the black probe to the anode and the red probe to the cathode.
If you obtain a resistance reading in the tens of thousands of ohms, this indicates the diode’s reverse resistance. Connect the red probe to the anode and the black probe to the cathode.
To determine the diode’s condition, set the multimeter to the kilo-ohm range. A large difference in forward and reverse resistance values indicates a healthy diode. If the values are close, the diode is likely faulty.
If both values are zero, the diode is shorted. If both values are infinite, the diode is open internally.
12. Testing the Condition of a High Voltage Silicon Stack
To check the condition of a silicon stack, set the multimeter to the voltage range. Connect the silicon stack in series with the multimeter and bridge it to 220V AC. Set the range to 250V DC and connect the silicon stack in the forward direction.
A reading greater than 30V indicates good condition; if the needle does not move, there is a fault. If the reading is 220V AC, it indicates a short circuit. If the needle does not move and the reading is zero, the silicon stack is open internally.
13. Measuring the Condition of a Capacitor
To check the condition of a microfarad capacitor, set the multimeter to the kilo-ohm range and connect the probes to the terminals. The needle should swing left and right; the larger the swing, the better the capacitor.
If the needle does not move at all, the capacitor is open. If the needle drops to zero and does not return, the capacitor is shorted.
14. Using a Digital Multimeter’s Buzzer Function to Test Electrolytic Capacitor Quality
The quality of an electrolytic capacitor can be tested using a digital multimeter. Set the switch to the buzzer function and connect the probes to the positive and negative terminals. A short beep indicates a good capacitor, and the symbol will appear when the sound stops.
If the buzzer sounds for a long time, the capacitor has a large capacitance. If it sounds continuously, the capacitor is shorted. If there is no sound, the capacitor is open internally.
15. Safety Regulations for Using Clamp Meters
When using clamp meters, remember the safety regulations. Testing on high voltage circuits must be conducted by two people. The potential of the wire being tested must not exceed the clamp meter’s voltage level.
Always wear gloves and stand on an insulated platform. Maintain a safe distance from any live parts of the body. For low voltage busbar measurements, use insulated barriers.
Do not use clamp meters on poorly insulated or bare wires.
16. Correct Use of Clamp Meters
When using clamp meters, choose the appropriate model and specifications. Start with the maximum range for rough measurements, then choose the right range. Center the wire in the jaws of the clamp meter.
Once the wire is clamped, do not change the range while powered. For clamp voltmeters, measure current and voltage separately. Do not clamp both wires of the lighting circuit at the same time.
After each test, set the range to the maximum.
17. Techniques for Measuring Three-Phase Three-Wire Current with Clamp Meters
To measure three-phase three-wire current using clamp meters, apply Kirchhoff’s law. Clamp around one wire to read that phase’s current. Clamp around two wires to read the third phase’s current. If all three wires are clamped, the reading should be zero, indicating balanced load.
18. Techniques for Measuring Small AC Current with Clamp Meters
To measure small AC currents using clamp meters, wrap the insulated wire around the core of the clamp meter. Divide the reading by the number of turns plus one to obtain the actual current value.
19. Detecting Phase Loss Faults in Three-Phase Resistance Furnaces
To detect phase loss in three-phase resistance furnaces, use clamp meters. If the current values of two phase wires are both less than the rated current, and one phase wire shows zero current, that phase’s resistance wire is burnt out.
20. Locating Short Circuit Ground Fault Points in Low Voltage Distribution Lines
In long low voltage distribution lines, it is difficult to locate short circuit ground fault points. If the fault phase wire is connected to an electric stove, and the single-pole switch is connected to the power supply, use clamp meters to measure the current segment by segment.
If there is no current at the boundary point, that is the short circuit ground fault point.
21. Testing Thyristor Rectifier Devices
To test thyristor rectifier devices, use clamp meters. Connect the clamp around the anode connection wire and observe the current reading on the meter. If the reading is zero, the device is not operational.
If the current values of the three-phase devices are balanced, this indicates normal operation. Significant imbalance indicates phase shifting issues with the devices. If there is a fault in the AC part, the rectifier transformer may have a phase loss.
22. Measuring User Phase Theft
If a user’s single-phase energy meter records less energy or does not run, use clamp meters to detect the current. Clamp around the phase wire and neutral wire; if the reading is not zero, there is a phase theft.
23. Safety Regulations for Measuring Insulation with Insulation Resistance Meters
When using insulation resistance meters, adhere to safety regulations. Measurement of high voltage equipment must be conducted by two people. Ensure the equipment is fully powered down and adequately discharged.
When measuring insulation of lines, obtain permission from the other party. Both circuits must be powered down, and measurements during thunderstorms are prohibited. When measuring near live equipment, choose the appropriate position.
Maintain a safe distance and ensure monitoring to prevent electric shock.
24. Correct Use of Insulation Resistance Meters
When using insulation resistance meters, choose the appropriate voltage level. Ensure the equipment is fully powered down and adequately discharged before measurement. Clean the equipment’s surface to remove dirt.
Position the meter appropriately, away from electric and magnetic fields. Place it horizontally without tilting, and test both open and short circuits. Use two insulated single-core leads that do not tangle.
Identify the terminals clearly, and connect the test leads correctly. Rotate the handle clockwise, gradually achieving a constant speed. The measurement time is not fixed; note the reading when the needle stabilizes.
25. Important Notes When Using Insulation Resistance Meters
When using insulation resistance meters, remember eight important notes. Do not touch the terminal buttons during the test. Keep the glass of the meter head free of dust during the measurement process.
When measuring the insulation of equipment to ground, connect the ground terminal to the shell. When testing large capacitive devices, detach at rated speed. For testing electrolytic capacitors, connect the ground terminal to the positive terminal.
It is best to use the same meter for multiple tests on the same equipment. When measuring the insulation of devices, note the temperature at the time of measurement. Do not measure resistances of hundreds of kilo-ohms, and do not use it as a general meter.
26. Connecting Diodes in Series to Prevent Discharge to Insulation Resistance Meters
Connect a diode in series with the insulation resistance meter terminals. When testing large capacitive devices, this prevents the device from discharging current. This eliminates needle fluctuations and ensures accurate readings.
After the measurement, stop rotating, and the meter will not be damaged.
27. Methods to Increase the Terminal Voltage of Insulation Resistance Meters
For low voltage insulation resistance meters, connect them in series to measure insulation. The combined voltage levels add up, and the insulation resistance readings sum as well.
28. Insulation Absorption Ratio of Power Transformers
The quality of transformer insulation can be measured using insulation resistance meters. At room temperature around twenty degrees Celsius, start measuring: observe the reading at fifteen seconds, and stabilize the value after a lapse of time.
The ratio of two insulation resistances is called the insulation absorption ratio. A value greater than 1.3 is good; less than 1.3 indicates moisture.
29. Quickly Assessing the Condition of Low Voltage Motors
To assess the condition of low voltage motors, open the junction box for testing. Use insulation resistance meters to measure the minimum insulation resistance value at thirty-five degrees Celsius, reducing by half for every ten degrees increase.
For every ten degrees decrease, multiply by two; only values exceeding the threshold are considered good. Set the multimeter to the milliamp range and use the star connection method. Connect the probes to any two phase terminals and slowly rotate the shaft.
If the needle swings significantly left and right, the results of three tests must be consistent. If the motor under test is good, otherwise, it may not be usable.
30. Measuring the Condition of High Voltage Silicon Stacks with Insulation Resistance Meters
To check the condition of high voltage silicon stacks, use insulation resistance meters. Connect the grounding wires to the two leads and contact the two terminals of the silicon stack. Shake the meter to measure the forward and reverse resistance; a significant difference indicates good condition.
If the readings are very close, the silicon stack may have failed. If both readings are infinite, the stack is open internally. If both readings are near zero, the stack is shorted internally.
31. Testing the Condition of Self-Ballasted High-Pressure Mercury Lamps with Insulation Resistance Meters
To check the condition of high-pressure mercury lamps, use insulation resistance meters at kilovolt levels. Connect the grounding wires to the two terminals of the lamp. Shake the meter slowly and then quickly.
If the reading is less than half a mega-ohm, the lamp emits a halo indicating good condition. If the lamp does not light, the reading is zero, indicating a short circuit inside the lamp. If the needle indicates infinity, there is an open circuit fault inside the lamp.
32. Testing the Quality of Fluorescent Tubes with Insulation Resistance Meters
To evaluate the quality of fluorescent tubes, use insulation resistance meters at kilovolt levels. Set the multimeter to the voltage range, with a direct current of five hundred volts. Connect the multimeter in parallel, ensuring the polarity is consistent to measure voltage.
Connect the grounding wires to the two ends of the lamp. If the lamp lights up at rated speed, it is normal if the voltage is less than three hundred volts. If the lamp glows slightly, it is aging if above three hundred volts.
If the lamp does not illuminate, it indicates that the lamp is damaged.
33. Testing the Quality of the Starter of Fluorescent Lamps with Insulation Resistance Meters
To test the starter of fluorescent lamps, use insulation resistance meters. Connect the grounding wires to the two terminals of the starter. Shake the handle slowly; if the neon bulb discharges with a red glow, the starter is good, otherwise, it is faulty.
(Source: Internet, copyright belongs to the original author)
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