1. Correct Use of the Multimeter
To use the multimeter correctly, familiarize yourself with the dial before use. Adjust the two zero position regulators gently. Choose the correct terminals, ensuring the red and black probes are inserted into the right holes.
Turn the switch to the correct range. Choose the appropriate measurement range for accurate readings. Ensure you are looking at the scale straight on for accurate readings.
After measuring, switch the probes back to the high voltage range. Regularly check the internal battery; deterioration can lead to electrolyte leakage. Store the instrument in a good environment, free from vibration, humidity, and weak magnetic fields.
2. Correct Use of the Ohm Range on the Multimeter
To use the ohm range correctly, you should know and be able to perform eight tasks. Ensure the battery voltage is sufficient and that the circuit being tested is not powered. Select the appropriate range, aiming for the middle of the scale.
Each time you change the range, you must recalibrate the zero resistance. Ensure good contact at the probe tips, and do not touch the probe ends with your hands. For measuring continuity in wires, use a range above one thousand ohms.
To test diodes, different ranges will yield different resistances. 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 take. Be aware of the internal resistance of the meter, and ensure someone is monitoring the process. The multimeter should be connected in parallel with the circuit being tested, and do not change the range while powered.
When measuring DC voltage, clarify the positive and negative terminals of the circuit. For measuring reactive circuits, do not disconnect the power supply during the measurement. For testing high voltages in kilovolts, use specialized probe leads.
Induced voltage to ground can vary significantly across different ranges.
4. Method for Measuring DC Current with a Multimeter
To measure current with a multimeter, set the switch to the milliamp range, determine 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. DC Method to Identify the Ends of a Three-Phase Motor Stator Winding
To identify the ends of a three-phase motor winding, use the DC method. Set the multimeter to the milliamp range and use a DC power source like a battery. Connect one phase winding to the meter and the other to the battery.
When powered, if the needle moves in either direction, the positive terminal is the start. If it does not reverse, switch the connections and test the remaining phase 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 running, use the residual magnetism method to identify the ends. Mark the output terminals 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 move significantly, the three ends are connected together. If the needle swings left and right, it indicates two starts and one end connected together.
Swap the ends of one phase winding and test again until the needle stabilizes, indicating the ends are connected separately.
7. Loop Method to Identify the Ends of a Three-Phase Motor Stator Winding
For a motor that has been running, use the loop method to identify the ends. Connect the three-phase windings in a triangle configuration. 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 move significantly, the ends of the windings are connected. If the needle swings widely, one phase winding is reversed.
The two connection points and wire ends are the start and end terminals.
8. Measuring the Speed of a Three-Phase Motor with a Multimeter
To measure the speed of a three-phase motor, use a multimeter. Open the motor junction box and disconnect the terminal connections. Set the multimeter to the milliamp range and connect it to any phase winding.
Rotate the rotor for one complete turn and observe how many times the needle swings. A two-pole motor will swing once, indicating a synchronous speed of three thousand RPM. A four-pole motor will swing twice, indicating a synchronous speed of one thousand five hundred RPM.
Continue this method to determine the speed, which will be slightly lower than the synchronous speed.
9. Testing the Ground Resistance of Household Grounding Protection Lines
To test the ground resistance of household grounding lines, set the multimeter to the voltage range and connect a kilowatt electric stove to the phase and neutral. Measure the voltage at the stove terminals to calculate the working current value.
Reconnect the stove to the phase ground line and measure the voltage again. The difference in voltage readings divided by the working current value gives the ground resistance value, with an approximate error of five percent.
10. Identifying the Phase Line and Neutral Line of Low Voltage AC Power Supply
In a low voltage three-phase four-wire system, identify the phase line and neutral line. 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 line.
If the needle deflects significantly, the probe is touching the phase line. If the needle does not move or only slightly deflects, the probe is touching the neutral line.
11. Testing the Polarity and Condition of a Diode
To test the polarity of a diode, set the multimeter to the kilo-ohm range. A reading below one thousand ohms indicates forward resistance of the diode. Connect the black probe to the anode and the red probe to the cathode.
A reading of several thousand ohms indicates reverse resistance. Connect the red probe to the anode and the black probe to the cathode.
To determine if the diode is good, set the multimeter to the kilo-ohm range. A significant difference between forward and reverse resistance indicates a good diode. If the values are close, the diode is likely faulty.
If both readings are zero, the diode is shorted. If both readings are infinite, the diode is open.
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 apply 220V AC. Set the range to 250V DC and connect the silicon stack in the forward direction.
A reading above thirty volts indicates it is functioning properly; if the needle does not move, there is a fault. If the reading is 220V AC and the needle does not move, the silicon stack is open.
13. Testing the Condition of a Capacitor
To test the condition of a microfarad capacitor, set the multimeter to the kilo-ohm range and connect the red and black probes to the terminals. The needle should swing left and right; the greater 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 has shorted.
14. Using a Digital Multimeter’s Buzzer Function to Test Electrolytic Capacitor Quality
To test the quality of an electrolytic capacitor, use a digital multimeter. Set the switch to the buzzer function and connect the probes to the positive and negative terminals. A short beep indicates the capacitor is good, and the symbol will appear on the display.
If the buzzer sounds for a long time, the capacitor has a large capacity. If the buzzer continuously sounds, the capacitor is shorted. If there is no sound, the capacitor is open.
15. Safety Regulations When Using a Clamp Meter
When using a clamp meter, remember the safety regulations. Testing on high voltage circuits must be done by two people. The potential of the wire being tested must not exceed the voltage rating of the clamp meter.
Always wear gloves and stand on an insulated platform. Maintain a safe distance from any live parts. When measuring low voltage bus currents, ensure insulation barriers are in place.
Do not use the clamp meter on poorly insulated or bare wires.
16. Correct Use of a Clamp Meter
When using a clamp meter, select the appropriate model and specifications. Start with the maximum range for rough measurements, then select the appropriate range. Position the wire in the center of the clamp jaws, ensuring good contact between the moving and stationary cores.
Once the clamp is around the wire, do not change the range while powered. Clamp meters measure current and voltage separately. Do not insert both wires of a lighting circuit into the clamp at the same time.
After each test, set the range back to the maximum.
17. Techniques for Measuring Three-Phase Three-Wire Current with a Clamp Meter
To measure three-phase three-wire current with a clamp meter, apply Kirchhoff’s law. Insert the clamp around one wire to read the current of that phase.
Insert the clamp around two wires to read the current of the third phase. If the clamp surrounds all three wires, the balanced load will read zero.
18. Techniques for Measuring Small AC Current with a Clamp Meter
To measure small AC currents with a clamp meter, wrap the insulated wire around the clamp core. Divide the reading by the number of turns plus one to obtain the actual current value.
19. Detecting Phase Loss Faults in a Three-Phase Resistance Furnace
To detect phase loss in a three-phase resistance furnace, use a clamp meter. 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 Faults in Low Voltage Distribution Lines
In long low voltage distribution lines, locating short circuit ground faults can be challenging. If the faulty phase wire is connected to an electric stove, use a single-pole switch to connect to the power supply. Use a clamp meter to measure the current in segments along the line.
The point where the current changes indicates the location of the short circuit.
21. Testing Thyristor Rectifier Devices
To test thyristor rectifier devices, use a clamp meter. Connect the clamp to the anode connection wire and observe the current reading on the meter. If the needle indicates zero current, the component is not functioning.
For three-phase components, the current values should be balanced, indicating normal operation. Significant current imbalance suggests phase shifting issues. If there are faults in the AC section, the rectifier transformer may be missing a phase.
22. Detecting User Phase Theft
If a user’s single-phase energy meter shows low or no readings, use a clamp meter to check before and after the energy meter. If the clamp is around the phase wire and neutral wire, and the meter shows a non-zero reading, it indicates phase theft.
Significant differences in current readings between the phase and neutral wires indicate phase theft, typically in a one-phase to ground configuration.
23. Safety Regulations When Using an Insulation Resistance Tester
When using an insulation resistance tester, adhere to safety regulations. When measuring high voltage equipment, two people must be present. Ensure the equipment is completely powered down and adequately discharged.
When measuring insulation on lines, obtain permission from the other party. Both circuits must be powered down, and measurements should not be taken during thunderstorms. When measuring near live equipment, ensure the operator is positioned safely.
Maintain a safe distance and ensure monitoring to prevent electric shock.
24. Correct Use of an Insulation Resistance Tester
When using an insulation resistance tester, select the appropriate voltage level. Ensure the equipment is powered down 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. Keep it level and perform both open and short circuit tests. Use two insulated single-core leads that do not tangle.
Ensure the terminal connections are clear and correct. Rotate the handle clockwise, gradually reaching a constant speed. There is no fixed time for testing; record the reading when the needle stabilizes.
25. Important Considerations When Using an Insulation Resistance Tester
When using an insulation resistance tester, remember these eight important considerations. Do not touch the connection terminals during testing. Do not wipe the glass of the meter during the testing process.
When testing the insulation of equipment to ground, connect the ground terminal to the outer casing. For large capacitive equipment, disconnect at rated speed. When testing electrolytic capacitors, connect the ground terminal to the positive terminal.
For multiple tests on the same equipment, it is best to use the same meter. Record the temperature during insulation testing. Do not test resistances below one hundred kilo-ohms, and do not use the meter for continuity testing.
26. Connecting a Diode in Series to Prevent Discharge from the Device Being Tested to the Insulation Resistance Tester
Connect a diode in series with the insulation resistance tester. This prevents discharge currents from large capacitive devices during testing. This will stabilize the needle and ensure accurate readings.
After testing, stop rotating the handle to avoid damaging the meter.
27. Methods to Increase the Voltage of the Insulation Resistance Tester
For low voltage insulation resistance testers, connect them in series to measure insulation. The combined voltage levels will add up, resulting in a higher insulation resistance reading.
28. Insulation Absorption Ratio of Power Transformers
To assess the quality of transformer insulation, use an insulation resistance tester. At a normal temperature of around twenty degrees Celsius, start timing from the moment of measurement: check the reading at fifteen seconds and the stable value after a few seconds.
The ratio of the two insulation resistance values is called the insulation absorption ratio. A value greater than 1.3 is good, while a value 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 an insulation resistance tester to measure the minimum insulation resistance value, using thirty-five degrees Celsius as a reference, reducing by half for every ten degrees increase.
For every ten degrees decrease, double the reading to determine if it is acceptable. 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, repeat the test three times. If the results are consistent, the motor is good; otherwise, it is faulty.
30. Testing the Condition of High Voltage Silicon Stacks with an Insulation Resistance Tester
To test the condition of high voltage silicon stacks, use an insulation resistance tester. Connect the two leads to the silicon stack terminals. Measure the forward and reverse resistance; a significant difference indicates a good stack.
If the readings are very close, the silicon stack is likely faulty. If both readings are infinite, the stack is open. If both readings are close to zero, the stack has shorted.
31. Testing the Condition of Self-Ballasted High Voltage Mercury Lamps with an Insulation Resistance Tester
To test the condition of high voltage mercury lamps, use a kilovolt insulation resistance tester. Connect the leads to the lamp terminals. Test in a dim environment, gradually increasing the speed of the tester.
If the reading is below half a megaohm, the lamp is functioning well. If the lamp does not light, the reading is zero, indicating a short circuit inside. If the needle indicates infinity, there is an open circuit fault.
32. Testing the Quality of Fluorescent Tubes with an Insulation Resistance Tester
To test the quality of fluorescent tubes, use a kilovolt insulation resistance tester. Set the multimeter to the voltage range, with a setting of 500V DC. Connect the multimeter in parallel, ensuring the polarity is correct.
Connect the leads to the ends of the tube. If the lamp lights at rated speed, it is normal if the reading is above three hundred volts. If the tube glows slightly, it indicates aging above three hundred volts.
If the tube does not light at all, it indicates that the tube is damaged.
33. Testing the Condition of Starters for Fluorescent Lamps with an Insulation Resistance Tester
To test the condition of starters for fluorescent lamps, use an insulation resistance tester. Connect the leads to the starter terminals. Slowly and gently rotate the handle; the neon bulb should flash red.
If the starter is functioning well, it will light up; otherwise, it is faulty.
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