1. Correct Use of Multimeter
To correctly use a multimeter, familiarize yourself with the dial before use. Adjust the two zero position regulators gently. Choose the correct terminal, and insert the red and black probes into the correct holes.
Rotate the switch to the correct gear. Choose the appropriate range for accurate measurements. Look at the range scale carefully, and read the values directly from the surface.
After measurement, switch the probes to the high voltage range. Regularly check the battery inside the meter; deterioration can cause electrolyte leakage. Store the meter in a good environment, free from vibration, humidity, and weak magnetic fields.
2. Correct Use of the Ohm Range on a Multimeter
To correctly use the ohm range, you should know eight points. The battery voltage must be sufficient, and the circuit being tested must be de-energized. Select the appropriate multiplier range, and ensure the needle points to the middle of the scale.
Each time you change the multiplier range, you must recalibrate the resistance to zero. Ensure good contact at the probe tips, and do not touch the probe ends with your hands. Measure the continuity of the circuit with ranges above a thousand ohms.
To test a diode component, different multipliers provide different resistances. When measuring transformer windings, be cautious as touching may result in electric shock.
3. Precautions When Measuring Voltage with a Multimeter
When measuring voltage with a multimeter, there are eight precautions to follow. Understand the internal resistance of the meter; there must be someone supervising. The meter is connected in parallel to the circuit being tested, and do not change the range when energized.
When measuring DC voltage, clarify the polarity of the circuit. For measuring reactive circuits, do not interrupt the power supply during the measurement. For testing kilovolt high voltages, use specialized probe wires.
Induced voltage to ground varies significantly with different ranges.
4. Methods for Measuring DC Current with a Multimeter
To measure current with a multimeter, set the switch to the milliamp range, confirm the polarity of the circuit, and connect the meter in series. Select a higher range to minimize the impact on the circuit.
5. DC Method to Identify the Ends of a Three-Phase Motor Stator Winding
Three-phase motor windings can be identified using the DC method. Set the multimeter to the milliamp range, using a DC power source like a battery. Connect one winding to the meter and touch the other winding to the battery.
When powered, if the needle moves, both positive and negative ends are the start. If it does not reverse, swap the connections and test the remaining winding in the same manner.
6. Residual Magnetism Method to Identify the Ends of a Three-Phase Motor Stator Winding
For a motor that has been in operation, use the residual magnetism method to identify the ends. Mark the output ends of the three-phase windings and connect them in parallel. Set the multimeter to the milliamp range and connect to the common point.
Slowly rotate the motor shaft while observing the meter needle. If the needle does not swing significantly, the three ends are connected. If the needle swings left and right, it indicates two starts and one end.
Swap the ends of one winding and measure again until the needle stabilizes, confirming the start and end connections.
7. Current Loop Method to Identify the Ends of a Three-Phase Motor Stator Winding
For a motor that has been in operation, use the current loop method to identify the ends. Connect the three-phase winding outputs in a triangular configuration. Set the multimeter to the milliamp range and connect in series with the three-phase windings.
Rotate the motor shaft uniformly while observing the meter needle. If the needle does not swing significantly, the ends are connected. If the swing is large, one winding’s ends are reversed.
The two connection points are the start and end of the windings.
8. Use a Multimeter to Measure the Speed of a Three-Phase Motor
To measure the speed of a three-phase motor, use a multimeter. Open the motor junction box and remove the connecting links. Set the multimeter to the milliamp range and connect to any phase winding.
Rotate the rotor one full 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 process to determine the speed, which will be slightly lower than the synchronous speed.
9. Testing the Ground Resistance of Household Ground Protection Wires
To test the ground resistance of household wiring, set the multimeter to the voltage range, connecting a kilowatt electric stove to the phase and neutral. Measure the voltage at the stove’s terminal to calculate the working current value.
Reconnect the stove to the phase ground wire and measure the voltage again. The difference in terminal voltage divided by the working current gives the ground resistance value, with an approximate 5% margin of error.
10. Identifying Phase and Neutral Wires in Low Voltage AC Power Sources
In a low voltage three-phase four-wire system, identify the phase and neutral wires. Set the multimeter to the voltage range, with an AC range of 250 volts. Connect one probe to the ground and touch the other probe to the power line.
If the needle deflects significantly, you are touching a phase wire. If the needle does not move or only slightly deflects, you are touching the neutral wire.
11. Testing the Polarity and Integrity of Diodes
To test the polarity of a diode, set the multimeter to the kilo-ohm range. If the measured resistance is less than a thousand ohms, the diode is forward-biased. Connect the black probe to the positive terminal and the red probe to the negative terminal.
If the measured resistance is several thousand ohms, the diode is reverse-biased. Connect the red probe to the positive terminal and the black probe to the negative terminal.
To determine if the diode is functioning, set the multimeter to the kilo-ohm range. A large difference between forward and reverse resistances indicates a good diode. If the values are close, the diode is likely faulty.
If both forward and reverse resistances are zero, the diode is shorted. If both are infinite, the diode is open.
12. Testing the Integrity of High Voltage Silicon Rectifiers
To check the integrity of silicon rectifiers, set the multimeter to the voltage range. Connect the rectifier in series with the multimeter, using a 220 volt AC source. Set the DC range to 250 volts and connect the rectifier in the forward direction.
If the reading is greater than 30 volts, it is acceptable; if the needle does not move, there is a fault. If the reading on the AC range is 250 volts, it indicates a short circuit. If the needle does not move and the reading is zero, the rectifier is open.
13. Testing the Integrity of Capacitors
To test the integrity of microfarad capacitors, set the multimeter to the kilo-ohm range. Connect the red and black probes to the two terminals. If the needle swings left and right once, the larger the swing, the better the capacitor.
If the needle does not move, the capacitor is open. If the needle drops to zero and does not return, the capacitor is shorted.
14. Testing Electrolytic Capacitor Quality with a Digital Multimeter in Buzzer Mode
To test the quality of electrolytic capacitors, set the digital multimeter to buzzer mode. Connect the red and black probes to the positive and negative terminals, respectively. A short beep indicates a good capacitor; if the sound stops, it shows the capacitance is too high.
If the buzzer sounds continuously, the capacitor is shorted. If there is no sound, the capacitor is internally open.
15. Safety Regulations When Using Clamp Meters
When using clamp meters, remember the safety regulations. Testing high voltage circuits must be done by two persons. The potential of the wire being tested must not exceed the clamp meter’s voltage rating.
Always wear gloves and stand on insulated platforms. Maintain a safe distance from energized parts of the body. When measuring low voltage bus currents, ensure insulated barriers are in place.
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. For rough measurements, set it to the maximum range and choose the appropriate measurement range. Place the wire in the center of the clamp, ensuring good alignment of the moving and stationary cores.
After inserting the wire into the clamp, do not change the range while energized. Use separate clamp meters for current and voltage measurements. Do not insert both wires of a lighting circuit into the clamp simultaneously.
After each test, set the range to the maximum.
17. Techniques for Measuring Three-Phase Three-Wire Currents with Clamp Meters
When using clamp meters to measure three-phase three-wire currents, apply Kirchhoff’s law to derive measurement techniques. Insert one wire into the clamp to read the current value of that phase.
Insert two wires into the clamp to read the current of the third phase. If all three wires are inserted, the load balance will read zero.
18. Techniques for Measuring Small AC Currents with Clamp Meters
To measure small AC currents with clamp meters, wrap the insulated wire around the core of the clamp. Divide the reading by the number of turns plus one to obtain the actual current value.
19. Detecting Phase Loss 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 reads zero, that phase’s resistance wire is burnt out.
20. Finding Short Circuit Ground Faults in Low Voltage Distribution Lines
In long low voltage distribution lines, finding short circuit ground faults can be challenging. If the faulty phase wire is connected to an electric furnace, use a single-pole switch to connect to the power supply. Use clamp meters to measure the current in segments of the line.
Identifying the presence of current at a junction indicates the short circuit ground fault.
21. Testing Thyristor Rectifier Units
To test thyristor rectifier units, use clamp meters. Connect the positive lead of the clamp to the anode and observe the current reading. If the meter reads zero, the component is not operational.
If the current values of three-phase components are balanced, it indicates normal operation. If there is significant imbalance in current, the components may have inconsistent phase shifting. If there is a fault in the AC portion, the transformer may have a phase loss.
22. Detecting User Phase Theft
If a user’s single-phase energy meter measures less than expected or does not register at all, use clamp meters to check before and after the energy meter. If the clamp meter reads a non-zero value when clamped around the phase and neutral wires, it indicates phase theft.
If the readings from the phase and neutral wires differ significantly, it confirms phase theft through a ground wire.
23. Safety Regulations for Measuring Insulation with an Insulation Resistance Tester
When using an insulation resistance tester, adhere to safety regulations. When measuring high voltage equipment, two persons must be present. Ensure the equipment is fully de-energized and adequately discharged.
When measuring line insulation, obtain permission from the other party. Ensure both circuits are de-energized, and do not measure during thunderstorms. When measuring near energized equipment, ensure proper positioning.
Maintain a safe distance and ensure supervision to prevent electric shock.
24. Correct Use of Insulation Resistance Testers
When using insulation resistance testers, select the appropriate voltage level. Ensure the equipment is fully de-energized and adequately discharged before testing. Clean the surface of the equipment to remove any dirt.
Position the tester appropriately, away from electric and magnetic fields. Ensure it is placed horizontally without tilting, and conduct open and short circuit tests. Use two colored single-core soft leads that are insulated and do not intertwine.
Clearly identify the connection terminals and ensure correct connections. Turn the handle clockwise to gradually reach a constant speed. There is no fixed time for the test; record the reading when the needle stabilizes.
25. Important Notes for Insulation Resistance Testing
When conducting insulation resistance testing with an insulation resistance tester, remember these eight important notes. Do not touch the connection terminals during testing. Avoid cleaning the glass of the meter during the test.
When measuring insulation to ground, connect the grounding terminal to the outer casing. For large capacitive devices, ensure to detach at rated speed. When testing electrolytic capacitors, connect the grounding terminal to the positive terminal.
For devices on the same platform, it is best to use the same tester for all measurements. When measuring insulation, record the temperature at the time of measurement. Do not measure resistances of hundreds of kilo-ohms, and do not use the tester for general purposes.
26. Connecting Diodes in Series to Prevent Equipment from Discharging the Insulation Resistance Tester
Connect diodes in series with the insulation resistance tester terminals to prevent large devices from discharging during testing. This will eliminate needle oscillations and ensure accurate readings.
Stop rotating the handle after measurement; the meter will not be damaged.
27. Increasing the Voltage Level of Insulation Resistance Testers
For low voltage insulation resistance testers, connect them in series to increase the voltage level for testing. The total insulation resistance reading will be the sum of the individual readings.
28. Insulation Absorption Ratio of Power Transformers
The quality of transformer insulation can be assessed using insulation resistance testers. At a temperature of about twenty degrees, start timing from the moment of measurement: observe the reading at fifteen seconds, and then the stabilized value after a few seconds.
The ratio of the two insulation resistances is called the insulation absorption ratio. A value greater than 1.3 indicates good insulation, while a value less than 1.3 suggests moisture.
29. Quick Assessment of Low Voltage Motor Integrity
To assess the integrity of low voltage motors, open the junction box for testing. Use an insulation resistance tester to measure the minimum insulation value in mega-ohms, referencing thirty-five degrees as the standard; decrease by half for every increase of ten degrees.
For every decrease of ten degrees, multiply by two. The readings exceeding the threshold indicate good motors. Set the multimeter to the milliamp range and use the star connection method for the motor. Connect the probes to any two phase heads and rotate the shaft slowly.
If the meter needle swings significantly, repeat the test three times. If the results are consistent, the motor is good; otherwise, it is unusable.
30. Testing the Integrity of High Voltage Silicon Rectifiers with Insulation Resistance Testers
To assess the integrity of high voltage silicon rectifiers, use insulation resistance testers. Connect the grounding terminal to the two leads of the rectifier. Measure the resistance in both forward and reverse directions; a significant difference indicates a good rectifier.
If the two readings are very close, the rectifier is likely faulty. If both readings are infinite, the rectifier is open. If both readings are close to zero, the rectifier is shorted.
31. Testing the Integrity of Self-Ballasted High Pressure Mercury Lamps with Insulation Resistance Testers
To test the integrity of high pressure mercury lamps, use a kilovolt insulation resistance tester. Connect the two leads to the lamp terminals. Gradually increase the speed of the tester while in a dim environment.
If the reading is less than half a mega-ohm, the lamp is functional. If the lamp does not light up, the reading is zero, indicating a short circuit inside the lamp. If the needle points to infinity, there is an open circuit fault inside the lamp.
32. Testing the Quality of Fluorescent Tubes with Insulation Resistance Testers
To test the quality of fluorescent tubes, use a kilovolt insulation resistance tester. Set the multimeter to the voltage range, with a DC range of 500 volts. Connect the multimeter in parallel, ensuring polarity is consistent.
Connect the two leads to the ends of the tube. If the lamp glows at the rated speed, it is normal if the voltage is below three hundred volts. If the tube glows slightly, it indicates aging above three hundred volts.
If the tube does not light up, it indicates that it is damaged.
33. Testing the Integrity of Starters for Fluorescent Lamps with Insulation Resistance Testers
To test the starters of fluorescent lamps, use insulation resistance testers. Connect the grounding terminal to the two leads of the starter. Gently rock the handle of the tester; if the neon bulb flashes red, the starter is functional.
If it does not flash, the starter is faulty.