What exactly is a clamp meter? As its name implies, it resembles a pair of pliers and functions similarly to a multimeter. This tool is widely used by electricians. A clamp meter is essentially short for a clamp ammeter. It’s a portable device capable of measuring the AC current of a circuit without needing to disconnect it. Its unique clamp-like opening allows it to be called a clamp ammeter. Most clamp meters don’t measure AC/DC voltages above 1kV. It's advisable not to attempt measuring voltages higher than this, as it could result in an electric shock.
When using a clamp meter, the first thing to remember is that the two power cables need to be separated, and you cannot test stranded wires. Typically, there are three jacks on a clamp multimeter. 1. The VΩ jack is for voltage and resistance measurements; usually, the red test lead is inserted here. 2. COM is a common test hole for all settings, generally housing the black test lead. 3. Some clamp meters also feature a hole marked with a lightning symbol, which is intended for testing live wires—insert the red test lead here, while the black test lead remains unconnected to the multimeter. When the dial is set to the lightning bolt symbol position, you can measure charged objects using the red test lead.
The image below illustrates the proper use of a clamp meter. Let me walk you through it in detail.
[Image description: Diagram showing the correct way to use a clamp meter.]
Let’s take the FLUKE 337 as an example to explain the steps for testing AC current using a clamp ammeter.
1) First, correctly select the meter's gear position. As shown in the image, choose the AC current setting.
[Image description: Picture of the FLUKE 337 with the AC current setting highlighted.]
2) Open the clamp meter’s jaws and attach it to the circuit being tested.
The method shown below is incorrect because it won't measure the circuit's current consumption but instead measures the circuit's leakage current, which is typically zero.
[Image description: Incorrect method of using the clamp meter.]
3) Open the clamp meter’s jaws and properly connect it to the circuit under test. At this point, you can measure the actual current of the household appliance.
The method shown below is correct. Only the single power cable of the circuit under test should pass through the jaws. Testing two wires separately should yield equal readings in theory; otherwise, the wiring might be abnormal, or the leakage current could be significant. The measured current value for the test circuit shown is 4.5A.
[Image description: Correct method of using the clamp meter.]
4) To test the starting current of a household appliance, press the “INRUSH†button on the meter and then activate the appliance.
[Image description: Button labeled “INRUSH†on the FLUKE 337.]
5) If the “INRUSH†button is pressed and the appliance isn’t yet activated, the meter will display a reading like the one shown below.
[Image description: Meter displaying no current.]
6) After starting the appliance, the measured starting current is as shown below.
[Image description: Meter displaying a high current spike.]
Now let’s analyze the key points of using a clamp multimeter.
When in use, turn the range switch to the appropriate current position. Hold the meter body and press the switch with your thumb to open the jaws, introducing the wire under test into the center of the core. Release the switch, and the iron core automatically closes, generating an alternating magnetic field around the wire being tested. The current value is then reflected on the meter, allowing you to read it directly.
Before using a digital multimeter, always read the instructions carefully to familiarize yourself with the power switch, range switch, input jacks, sockets, and various function keys, knobs, and accessories. Additionally, understand the meter's overload limits, polarity display, low voltage indicators, and other alarms, and learn how the decimal point changes. Always inspect the test leads for cracks and ensure the leads' insulation is intact before testing.
Before each measurement, double-check that the correct measurement item and range switch are set, and that the right input jack is selected. When the meter is initially powered on, the numbers may flicker. Wait until the reading stabilizes before taking note.
Even though digital multimeters have comprehensive internal protection circuits, avoid operational errors such as using the current block to measure voltage, using the voltage block to measure current, or using the capacitance block to measure charged capacitors. These actions could damage the meter.
If the highest digit shows only "1" and all other digits are blank, this indicates that the meter is overloaded, and a higher range should be selected.
It’s prohibited to toggle the range switch when measuring voltages above 100V or currents above 0.5A to prevent arcing and burning of the transfer switch contacts.
The hazard symbol next to the input jack indicates the maximum voltage or current that can safely be input. Exceeding this limit could damage the instrument and potentially harm the user.
Clamp meters must never be used to measure the current of high-voltage lines. The voltage of the tested line should not exceed the meter's specified voltage level (typically over 500 volts) to prevent insulation breakdown and electric shock.
When measuring, estimate the current size and select an appropriate range. Do not use a small range to measure large currents.
Before measuring, make sure the range switch is set to the corresponding AC current setting. You cannot use the voltage or resistance settings to measure current. Remember, never use the resistance setting and current setting to measure voltage. Otherwise, if you accidentally do so, the meter could be destroyed.
Only one wire can be clamped at a time. Place the wire in the center of the jaws to improve measurement accuracy. It’s best to hold the meter steady with your hand to prevent the wires from leaning against the jaws.
After completing the measurement, turn the range switch to the maximum voltage range position and then turn off the power switch to ensure safe use for the next session.
In conclusion, mastering the proper use of a clamp meter is essential for electricians and anyone working with electrical systems. By following these guidelines, you can ensure accurate and safe measurements.
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