Clamp Meter (Tong Tester) is one of the most useful test equipment. This post will discuss Tong Tester types, Operating Principle and how to operate it.

Introduction to Clamp Meter (Tong Tester)

Clamp meter is often known as Tong Tester also. It is an easy to use test equipment and is useful in the measurement of a live-conductor without damaging / powering down the circuit. One of the biggest advantage of this equipment is that one may take the measurement of a high value current even without powering off/shutting down the circuitry under test.

The only disadvantage of this test equipment (Clamp meter or Tong tester) is that the accuracy of the tong-tester is considerably low.

Fig. 1 – Clamp Meters or Tong Testers

Parts of Clamp Meter (Tong Tester)

Generally, a Clamp Meter or Tong Tester has the following parts: –

1.  Transformer Clamps (Jaws)

It senses the magnetic field of the current flowing through the conductor.

2.  Clamp Opening Trigger

It is used to open/ close the clamps.

3.  Power Switch

As the name suggests, it is used to power on/ off the meter.

4.  Back Light Button

It is used to light up the LCD display for easy reading of displayed value in dark places or at night.

5.  Hold Button

It is used to hold the last value on the display.

6.  Negative/ Ground Input Terminal

It is used to connect the negative/ ground jack of the meter cable.

7.  Positive Input Terminal

It is used to connect the positive jack of the meter cable.

8.  LCD Display

It displays the measured value.

9.  Functional Rotary Switch

It is used to select the the type and range of current being measured.

Fig. 2 – Parts of Tong Tester (Clamp Meter)

Types of Clamp Meter (Tong Tester)

There are two basic types of tong testers available in the market depending on the nature of current to be measured. They are: –

• Current Transformer Clamp Meter (Tong Tester) – Used to measure AC current.
• Hall Effect Clamp Meter (Tong Tester) – Used to measure AC as well as DC current.

Current Transformer Clamp Meter (Tong Tester) – Operating Principle and How it works

A current transformer based Tong Tester (Clamp meter) consists of two clamps made of Ferrite Iron. These clamps are independently wrapped up with Copper coils. Together both of them forms a magnetic core which actually does the measurements. The Electro-magnetic principle says that “whenever a current flows through a conducting material, it causes the generation of magnetic-flux”.

Fig. 3 – Operating Principle of Current Transformer Tong Tester (Clamp Meter)

Now, let us assume that the conducting material from which the current is passing through is the primary arm of the transformer. Due to the current’s movement, a magnetic field is generated in it. When the arm of tong tester is placed for measurement, it acts as a secondary winding of the transformer. The iron core of the arm of tong tester concentrates the magnetic field of the primary winding (i.e. conductor) and hence a current in proportion to the primary-current is generated by electromagnetic-induction. This arm of the tong tester is connected to a measurement circuitry which in turn provides the current-reading finally.

The measuring unit of magnetic field is called as Magnetic Flux. It is denoted by the Greek letter ‘Phi’ (Φ).

Current transformer based clamp meters respond only to ac waveforms.

Fig. 4 – Open View of Tong Tester (Clamp Meter)

How large currents can be measured using clamp meter?

The magnetic field induced in the arms of clamp meter is directly proportional to the number of turns in the secondary winding. Since the number of turns in the secondary winding is much more compared to the number of turns in the primary winding wrapped around the core, a very small magnitude of current goes to the measuring unit’s input.

Let’s understand it with an example. If the number of turns in the secondary winding is 100, then the current induced in the secondary winding will be 1/100th of the current flowing through the primary winding. This means that for measuring 100 amp of current, only 1 amp will reach to the input of meter. Using this methodology, we can easily measure even large currents with clamp meter just by increasing the number of turns in the secondary winding.

Hall Effect Clamp Meters – Operating Principle and How it works

Hall Effect clamp meters are capable of measuring both AC and DC current.

Fig. 5 – Operating Principle of Hall Effect Tong Tester (Clamp Meter)

Similar to current transformer Clamp Meters, Hall Effect clamp meters also consists of two clamps made of Ferrite Iron. However, these clamps are not wrapped up with copper wires like in case of Current transformer clamp meters. In Hall Effect clamp meter, the magnetic field induced by the current flowing through the conductor is concentrated in the gap(s) (one or more) of the core whenever the clamps close around the conductor.

Fig. 5 – Gaps on the clamp tip of Hall Effect Clamp Meter

When you carefully see the clamp tips in the figure above, you can notice the existence of Gap. When both the clamps of Hall Effect clamp meter meet/ close, the Gap in the Clamp core creates an air pocket where the magnetic field is concentrated. The gap also ensures that the core should not be saturated by limiting the magnetic flux.

There is a small semiconductor component inside the gap which is known as Hall Effect Sensor. This Hall Effect Sensor is not visible as it is covered with a plastic molding. The Hall Effect Sensor is a special transducer which can vary its output voltage depending on the increase or decrease of magnetic field due to current flowing in the conductor. This property of Hall Effect sensor is used to measure current. The voltage generated by the Hall Effect Sensor is further amplified and scaled for representing the current flowing through the conductor.

The Hall Effect Clamp meter can also measure DC current as the core is capable of concentrating DC magnetic fields also. However, When measuring the current, these clamp meters must be zeroed to avoid presence of errors due to magnetic field of Earth or any other magnetic field source in near vicinity.

Fig. 6 – How to Operate Tong Tester (Clamp Meter)

How to Operate Clamp Meter (Tong Tester)

• Switch ON Clamp meter (Tong tester).
• Only a higher scale-value should be selected in order to avoid any harm to the circuit. Slowly decrease the scale reading selector if current is less.
• Clip ON ammeter around each cable in turn and measure current.
• Keep it in mind that current of only one cable to be measured at one time.