1. What is Clamp Meter

Clamp meter is a widely used instrument in high current measurement scenarios such as power maintenance, industrial testing, etc. This article discusses and analyze its typical calibration methods.

2. The Principle and Classification of Clamp Meter

2.1 Working Principle of Clamp Meter

Clamp meter is a kind of meter which measures the current value by putting the open and close magnetic circuit on the current-carrying conductor, and the current value of the measured circuit can be obtained without disconnecting the circuit during the measurement. Its working principle is to determine the relationship between the magnetic field strength around the current-carrying conductor and the current through Biosaval's Law. Knowing that the current in an infinite conductor is I, the magnetic flux density generated at point A, which is r from the current-carrying conductor, is calculated according to equation (1):

                                                                   B: Magnetic flux density at point A in Tesla(T);

                                                        μo: Constant, magnetic permeability in vacuum, (4πX10-7H/m)

                                                        H: Magnetic field strength at point A in amperes (A/m)

                                                                     I: Current flowing through the conductor (A)

                                                                    r: Vertical distance from point A to the center of the conductor in meters (m).

From equation (1), it can be seen that the magnetic flux density at a specific location around a current-carrying conductor is directly proportional to the magnitude of the current value and inversely proportional to the distance between that location and the conductor.

Therefore, the current value can be measured indirectly through the measurement of magnetic flux density, measurement of the conductor is often perpendicular to the clamp ammeter jaws through the geometric center (see Fig 2).

2.2 Classification of Clamp Meters

According to the principle of measurement, clamp meter can be divided into mutual inductance, Hall effect type and magnetic modulation type. Here are the principles of measurement objects and characteristics:

Table 1 The description of principles of the measurement of clamp meter

2.3 Typical Parameter

3. Clamp Meter Calibration Methods and Comparative Analysis

3.1 Measurement and Error Analysis

The working principle of the clamp ammeter is to indirectly measure the current by detecting the magnetic field. The ideal calibration method is to use a standard current source to apply a test current to an infinitely long wire and make the wire pass vertically through the geometry of the clamp meter being calibrated. center. However, in actual work, the wire cannot be infinitely long, which will bring certain errors.

As shown in Fig 3, the magnetic flux density B0 generated by the conductor W0 at point P is calculated according to equation (2); and the magnetic flux density BX  generated by the conductor W1  at point P  is calculated according to equation (3)

B0: Magnetic flux density at point P in the theoretical model, in Tesla (T)

μo: Constant, magnetic permeability in vacuum, (4πX10-7H/m)

I: Current flowing through the current-carrying wire in amperes (A).

r: Vertical distance from point P to the center of the current-carrying wire (m)

BX : The flux density at point P in Tesla (T).

Θ: The angle between the line between point P and the end point of the current-carrying conductor and the current-carrying conductor, in degrees.(o)

The value of current measured by a clamp meter is proportional to the magnetic flux density it detects, and this gives the system relative error γ (%) due to finite-length current-carrying conductors according to the equation (4).

In practice, the clamp meter used to measure the current of a finite length of wire will produce an error, Fig 5 shows a list of different values of D and L under the error γ (%):

Fig. 5 Errors for different values of D and L

As can be seen from the figure:

• The larger the diameter D of the clamp meter detection coil, the larger the systematic error generated by the measurement;

• When the clamp meter detection coil diameter D is fixed, the clamp current output ring of the current-carrying wire length L is larger, the smaller the systematic error.

3.2 Equivalent Ampere-Turns Method of Clamp Meter Calibration

Clamp meter calibration requires a high-precision standard high-current source as the primary current, but the design and manufacture of high-precision AC and DC high-current sources of 1kA level is difficult and costly, so the equivalent ampere-turns method is usually used in the early days for calibration.

The equivalent ampere-turns method is to pass a standard current source of tens of amperes through a multi-turn coil and through the jaws of the clamp meter to be calibrated. At this time, the current detected by the clamp ammeter is:

       Equivalent Ampere-Turns Current = Standard Current Source Current x Number Of Coil Turns.

Fig. 6 Typical Structure of Equivalent Ampere-Turn Method Clamped Current Output Ring

w/2 represents the centerline distance between the C0 column and the C1 and C2 columns, and h represents the height of the centerline of the current-carrying conductor of the clamped current output ring.

During the winding process, good insulation should be maintained between turns and turns, and the entire clamped current output ring fabrication should be kept as centrosymmetric as possible.

Equivalent ampere-turns method is affected by leakage current, coil shape, external magnetic field and other factors. and the accuracy is low. When the number of turns increases, rapid square-fold increase in inductance, which raises high requirements for the calibration source's ability to carry inductive loads. This type of calibration is only suitable for calibration of low accuracy clamp meters.

3.3 Single-Turn Method of Clamp Meter Calibration

The single-turn method is a method of calibrating a clamp meter by passing a large kA level current through the jaws of the clamp meter via a single-turn standard current ring. A typical single-turn method clamp-on current ring output structure is shown in Fig 7.

A copper ring should maintain A-A 'into the axis of symmetry, and the diameter is not less than 1m, and the current source terminals should maintain a reliable and firm connection and good contact. The temperature rise should be no more than 20°C during continuous high current operation.

The copper ring used in the single-turn method of is convenient to disassemble, which makes it easy to calibrate the device for AC and DC current measurements using a high-grade core-piercing current transformer or DC comparator. During calibration, the uncertainty of the high-level traceability device should not be greater than 1/4 of the absolute value of the allowable error of the calibration device, and the geometric center of the core should be placed where the clamp meter jaws are placed. The single-turn method is the main method for calibrating clamp ammeters, which is in line with the development direction of high accuracy levels of clamp ammeters.

Table 2 Comparison of single-turn and equivalent ampere-turn methods

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