Mastering Multimeters: 33 Key Points You Should Know

1. Correct Use of Multimeters

To use the multimeter correctly, familiarize yourself with the dial. Adjust the two zero position regulators gently. Choose the correct terminal, ensuring the red and black probes are plugged into the right holes.

Switch the selector to the correct position. Choose the appropriate range to ensure accurate measurements. Look closely at the range scale, and read the display directly above.

After measuring, switch the probes back to the high voltage range. Check the battery inside regularly; deterioration can lead to electrolyte leakage. Store the meter in a good environment, free from vibration, humidity, and weak magnetic fields.

2. Proper Use of the Ohm Setting on a Multimeter

To use the ohm setting correctly, there are eight key points to know. Ensure the battery voltage is sufficient, and the circuit being measured is powered off. Select the appropriate range, aiming for the middle of the scale.

Each time you change the range, recalibrate the zero resistance. Ensure the probe tips have good contact, and do not touch the probe ends with your hands while measuring. Check for continuity in the circuit using the appropriate range above one thousand ohms.

When measuring diodes, note that the resistance will differ with varying ranges. For transformer windings, avoid touching to prevent electric shock.

3. Precautions for Measuring Voltage with a Multimeter

When measuring voltage with a multimeter, keep in mind eight precautions. Know the internal resistance of the meter, and always have someone supervise. Connect the meter in parallel with the live circuit, 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. When testing kilovolt high voltages, use specialized probe leads.

4. Measuring DC Current with a Multimeter

To measure current with a multimeter, set the switch to the milliamp range, confirm the circuit’s polarity, and connect the meter in series with the circuit. Choose a higher range to minimize 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 identify the start and end. Set the multimeter to milliamp range and use a dry battery for power. Connect one phase winding to the meter, and touch the other phase winding to the battery.

When powered, if the needle moves, the positive terminal is the start. If it does not reverse, switch the connections and repeat for the remaining phase winding.

6. Using Residual Magnetism to Identify the Start and End of a Three-Phase Motor Stator Winding

For motors that have been running, use the residual magnetism method to identify the start and end. After marking the three-phase winding terminals, connect them in parallel. Set the multimeter to milliamp range and bridge the common point.

Slowly rotate the motor shaft while watching the meter needle. If the needle does not move significantly, the three ends are connected. If it swings left and right, two ends are connected at one point.

Adjust one phase winding, and repeat the measurement until the needle stabilizes, indicating the start and end are connected.

7. Using Current Loop Method to Identify the Start and End of a Three-Phase Motor Stator Winding

For motors that have been running, use the current loop method to identify the start and end. Connect the three-phase winding terminals in a triangle configuration. Set the multimeter to 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 windings are connected correctly. If the needle swings widely, one phase winding is reversed.

The two connection points 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. Connect the multimeter to the milliamp range across any phase winding.

Rotate the rotor one full turn and observe how many times the needle swings. For a two-pole motor, it will swing once; for a four-pole motor, it will swing twice.

9. Testing the Grounding Resistance of Household Grounding Wires

To test the grounding resistance of household grounding wires, set the multimeter to the voltage range, connecting a kilowatt electric stove to the phase and neutral. Measure the voltage at the stove terminals to calculate the working current.

Reconnect the electric stove to the grounding wire, then measure the voltage again. The difference in voltage readings divided by the working current gives the grounding resistance value, with an error of about 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, the phase line and neutral line are identified. Set the multimeter to the voltage range, with a range of AC 250 volts. Connect one probe to the ground and touch the other probe to the power line.

If the needle deflects significantly, the probe touches the phase line. If the needle barely moves, the probe touches the neutral line.

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 reading below one thousand ohms indicates forward resistance. Connect the black probe to the positive terminal and the red probe to the negative terminal.

A reading of several thousand ohms indicates reverse resistance. Connect the red probe to the positive terminal and the black probe to the negative terminal.

To assess the diode’s condition, check the forward and reverse resistance values. A significant difference indicates a good diode, while similar values suggest the diode is 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 bridge it with AC 220 volts. Set the DC range to 250 volts and connect the silicon stack positively.

If the reading is greater than thirty volts, it is qualified; if the needle does not move, it indicates a fault. For the AC range of 250 volts, a reading of 220 indicates a short circuit. If the needle does not move and the reading is zero, it indicates an open circuit inside the silicon stack.

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 red and black probes to the capacitor 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 without returning, the capacitor has failed.

14. Using a Digital Multimeter’s Buzzer Function to Test the Quality of Electrolytic Capacitors

To test the quality of electrolytic capacitors, 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 a good capacitor.

If the buzzer sounds continuously, the capacitor is shorted. If the buzzer does not sound, the capacitor is open.

15. Safety Regulations for Using Clamp Meters

When using a clamp meter, remember the safety regulations. Testing high-voltage circuits must be conducted by two people. Ensure the potential of the wire being tested does not exceed the clamp meter’s voltage rating.

Always wear gloves and stand on insulated platforms. Maintain a safe distance from any charged objects. When measuring low-voltage bus currents, use insulated barriers.

Avoid using clamp meters on poorly insulated or bare wires.

16. Correct Use of Clamp Meters

When using clamp meters, select the appropriate model and specifications. Start with the maximum range for rough measurements, then select the appropriate range. Center the wire in the clamp jaws for accurate readings.

After inserting the wire into the jaws, 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 simultaneously.

After each test, return the range to the maximum setting.

17. Techniques for Measuring Three-Phase Three-Wire Current with Clamp Meters

When using clamp meters to measure three-phase three-wire current, apply Kirchhoff’s law. Insert one wire into the clamp to read the current of that phase.

Insert two wires to read the current of the third phase. If all three wires are inserted, the balanced load 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 clamp’s core. Divide the reading by the number of turns plus one to get the actual current value.

19. Detecting Phase-Loss Faults in Star-Connected Three-Phase Resistance Furnaces

For phase-loss faults in three-phase resistance furnaces, use clamp meters to measure. If the current values in two phase lines are both below rated current, and one phase line reads zero, 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. Use a clamp meter to measure current in segments of the circuit.

Where current is present indicates the short-circuit ground fault location.

21. Testing Thyristor Rectifier Devices

To test thyristor rectifier devices, use a clamp meter. Connect the anode lead and observe the current reading on the meter. If the reading is zero, the device is not functioning.

If the three-phase current values are balanced, it indicates normal operation. Severe imbalance indicates phase shifting or faults in the AC part or missing phases in the transformer.

22. Identifying User Phase-Cross Theft

If a user’s single-phase energy meter shows low or no reading, use a clamp meter to measure. Clamp around the phase and neutral wires, and if the reading is not zero, it indicates phase-cross theft.

23. Safety Regulations for Measuring Insulation with Insulation Resistance Meters

When using insulation resistance meters, adhere to safety regulations. High-voltage equipment testing must be done by two people. Ensure the equipment is completely powered off and fully discharged.

When measuring line insulation, obtain permission from the other party. Avoid measuring during thunderstorms, and maintain a safe distance from live equipment.

24. Correct Use of Insulation Resistance Meters

When using insulation resistance meters, select the appropriate voltage level. Ensure the equipment is powered off and fully discharged before testing. Clean the surface of the equipment to remove dirt.

Position the meter correctly to avoid electric and magnetic fields. Keep it horizontal and perform both open and short-circuit tests. Use insulated leads that do not tangle.

Ensure terminal markings are clear and connections are correct. Rotate the crank clockwise to achieve a steady speed. There is no set measurement time; stabilize the pointer before recording.

25. Important Notes for Insulation Resistance Meter Testing

When testing with insulation resistance meters, remember eight important notes. Never touch the terminal connections during testing. Do not wipe the glass of the meter’s display during the measuring process.

When testing insulation to ground, connect the ground terminal to the equipment’s shell. For large capacitive equipment, disconnect below rated speed. When testing electrolytic capacitors, connect the ground terminal to the positive terminal.

For multiple tests on the same equipment, use the same meter for consistency. Record the temperature during insulation tests. Avoid testing resistances below one hundred kilo-ohms, and do not use the meter for continuity tests.

26. Connecting Diodes in Series to Prevent Discharge from the Device Being Tested

To prevent discharge from the device being tested, connect a diode in series with the insulation resistance meter. This will stabilize the reading during measurement.

After measuring, stop rotating the crank to avoid damaging the meter.

27. Methods to Increase the Terminal Voltage of Insulation Resistance Meters

For low-voltage insulation resistance meters, connect them in series to increase voltage for testing. The combined voltage will add to the insulation resistance reading.

28. Insulation Absorption Ratio of Power Transformers

To evaluate transformer insulation, use an insulation resistance meter. At around twenty degrees Celsius, measure the resistance at fifteen seconds and again at sixty seconds.

The ratio of the two readings is known as the insulation absorption ratio. A value greater than 1.3 is good, while less than 1.3 indicates moisture.

29. Quickly Assessing the Condition of Low-Voltage Motors

To quickly assess the condition of low-voltage motors, open the junction box and test with an insulation resistance meter. The minimum insulation resistance value should be based on thirty-five degrees Celsius, reducing by half for every ten degrees increase.

If the readings exceed the acceptable range, the motor is good; otherwise, it is faulty.

30. Testing the Condition of High-Voltage Silicon Stacks with Insulation Resistance Meters

To assess high-voltage silicon stacks, connect the leads to the stack’s terminals and measure resistance. A significant difference between the two readings indicates a good stack.

If the readings are close, the stack is faulty. If both readings are infinite, the stack is open. If both readings are near zero, the stack has a short circuit.

31. Testing the Condition of Self-Ballasted High-Pressure Mercury Lamps

To test high-pressure mercury lamps, use a kilovolt insulation resistance meter. Connect the leads to the lamp terminals and measure in a dim environment.

If the reading is below half a megaohm, the lamp is functioning; if it does not light, it indicates a short circuit. If the meter shows infinite resistance, the lamp has an open circuit fault.

32. Testing the Quality of Fluorescent Tubes with Insulation Resistance Meters

To test fluorescent tubes, set the insulation resistance meter to five hundred volts DC. Connect the leads to the tube terminals and measure the voltage.

If the tube lights up at rated speed, it is good; if it barely lights, it shows aging. If the tube does not light at all, it indicates damage.

33. Testing the Quality of Fluorescent Lamp Starters with Insulation Resistance Meters

To test fluorescent lamp starters, connect the leads to the starter terminals and slowly rotate the meter’s handle. If the neon bulb flashes, the starter is good; otherwise, it is faulty.