What is a clamp meter?

This article will detail the basics of a clamp meter and its measurement principles.

When it comes to instruments that measure current, many people may think of analog or digital multimeters. However, if you want to make measurements without disconnecting the circuit, a clamp meter is recommended.

Direct wiring method Figure 1 Clamp method

In this article, we will introduce in detail the basic knowledge of the clamp meter, the principle of measurement, the precautions to be taken when selecting a clamp meter, and the basic methods of using a clamp meter.

■ What you can measure with a clamp meter

Shaped like a clothespin, a clamp meter is a measuring instrument that can measure current in an energized state by simply clamping a wire. When measuring with an ordinary multimeter, you need to disconnect the circuit of the object to be measured and then connect the multimeter in series to measure it, while a clamp meter can read the magnetic field and measure the current by simply clamping the wire between the clamps.

This means that it is possible to measure machines and electronic devices in operation with current flowing.

Figure 2

There are various types of clamp meters, but the main types that can measure are

DC current measurement

AC Current Measurement

Leakage Current Measurement

Measurement of start-up current

Voltage Measurement

・Conductivity check, resistance measurement, diode measurement

Capacitance Measurement, Temperature Measurement

Of course, not all clamp-on ammeters are capable of measuring all parameters. You need to choose the clamp meter that best suits your needs.

■ Measuring principle of clamp meter

As mentioned above, a clamp meter can be used to make safe measurements by simply clamping the wire without disconnecting the circuit. It works by sensing the magnetic field emitted by the current flowing in the wire and measuring the value of the current from that field.

When a current flows, a concentric magnetic field is generated. The magnitude of the magnetic field is proportional to the magnitude of the current, and the direction of the magnetic field is the same as the direction of the screw's advance when the screw is tightened. This law is known as Ampere's law (also known as the right hand screw rule). Clamp meters are manufactured using Ampere's Law.

Figure 3

In addition, a change in the magnetic field produces an electric current in the conductor penetrating the coil. The magnitude of the current is determined by the increase or decrease in the magnetic field. A clamp meter determines the magnitude of the current in the magnetic field and the conductor by measuring the induced current.

■ Points for selecting a clamp meter

There are many types of clamp meters, so you need to select the right one for your purpose. Selection points are as follows.

Measured current

Clamp meters can be categorized according to the type of current being measured. There are leak current measurement type and load current measurement type. Load current measurement type clamp meter is used for normal current measurement. Leakage current measurement type clamp meters are required for measurements under short-circuit conditions.

Rectification method

When using a load current measurement clamp meter, you need to be aware of the difference in rectification methods. There are two types, the average value type and the true RMS type. One point to note is that the average value type clamp meter cannot correctly measure the RMS value of the distorted waveform.

Is it DC or AC?

Whether the current in the DUT is DC or AC is also an important influence on the choice of clamp meter. In addition, if AC current is being measured, the frequency band of the clamp meter should be checked as the frequency may vary.

Diameter of wire that can be clamped

The diameter of the wire that can be clamped varies from clamp meter to clamp meter. It is important to check what the diameter of the wire to be measured is before selecting a clamp meter.

Various models with different jaw diameters

Figure 4

・Measurement of electrical flow

Make sure that the maximum current value of the load of the wire to be tested is within the current range of the clamp meter.

Other clamp meters have many other functions, so we recommend that you choose the right clamp meter for your needs.

■ Basic use of a clamp meter

Clamp meters are simple to use: basically, you measure current by clamping a wire with a clamp meter. To measure the load current, you only need to clamp a wire.

Figure 5

Load current measurements cannot be made correctly if 2 single-phase wires are clamped. According to Ampere's law, the same amount of reversed magnetic field is formed in the 2 wires going back and forth. If the magnetic field is measured in this case, the two fields cancel each other out and the test current value becomes zero.

Leakage current measurements may clamp not only one but also two wires (or one wire if the leakage current is measured with a ground wire). Leakage current is the current that flows through the insulation resistance of a load and normally does not exceed a few µA.

Leakage current measurement (single-phase 2-wire)   Figure 8		Leakage current measurement (grounding wire) Figure 9

■ Select the appropriate clamp meter for your application.

Clamp meters measure current by simply clamping a wire, so they can be used for a wide range of measurements, including machines in operation. Therefore, before selecting a clamp meter, please check which measurements you need to make.