What Is Clamp Meter?

by Matthew Mills | Last Updated: February 10, 2021
What Is Clamp Meter?

For individuals working with electrical systems, it is vital to measure the current flow in a circuit or a contact. It tells them that the circuit is live and also allows them to calculate power consumption.

While such measurements were earlier hazardous and quite difficult, these days we have sophisticated instruments that allow for safe measurement of current. Here, we shall discuss one such instrument, known as the clamp meter. So What Is Clamp Meter?

A clamp meter or a tong tester is a reliable and safe testing instrument used for measuring a live conductor without damaging the circuit in any way or powering it down. Many technicians prefer this instrument because it lets them measure a strong current without having to power off the circuit being tested.

Clamp meters are mainly of three types, namely current transformer clamp meters, flexible clamp meters, and Hall Effect clamp meters.

When current flows through a circuit or a conductor, a magnetic field is created. This can be detected in order to find the value of the said current. It is safe and convenient to use for technicians since the current flow is not disrupted because of this.

Earlier, measuring in-line current was a bit of a hassle as you had to cut into the wire and place the measuring instrument’s test probes inside the circuit to do so. This device has a pair of transformer clamps, so you don’t need to bring it in contact with a conductor while taking measurements.

The different parts of this instrument are as follows:

  • Jaws: These are also known as the clamps and they are used to detect the magnetic field formed due to the current passing through the circuit.
  • Opening Trigger: This is a trigger used for opening or closing the clamps.
  • Power Button: Used for powering the clamp meter up or down.
  • Back Light Button: When you press this button, the LCD display lights up, thus letting you read the display in low light conditions.
  • Negative Input Terminal: This is the terminal for connecting the meter cable’s negative jack.
  • Hold Button: By pressing this button you can display the previous value.
  • Positive Input Terminal: At this terminal, you can connect the meter cable’s positive jack.
  • Functional Rotary Switch: You can use this switch to select the range and type of the measured current.
  • LCD display: This is where the values are displayed.
What Is Clamp Meter?

How does a clamp meter work?

Let us look at the working principles for both current transformer and Hall Effect clamp meters.

Current Transformer Clamp Meter

The first type of clamp meter has a couple of clamps constructed with Ferrite Iron. Copper coils are wrapped around these clamps, resulting in the formation of a magnetic core. The instrument works on the electromagnetic principle that talks about magnetic flux being generated due to the flow of current through a conductor or a circuit.

Let us consider the conductor, through which the current is flowing to be the transformer’s primary winding. The flow of current creates a magnetic field in the conductor and when you place the clamp meter arm in order to measure the current, it forms the secondary arm of the transformer.

Current Transformer Clamp Meter

The clamp tester arm has an iron core that concentrates the conductor’s magnetic field. Due to this, a current is produced via electromagnetic induction and this is proportional to the primary current. The clamp tester’s arm is brought in contact with the circuit, after which we get the final value of the current.

The magnetic field that is formed in the clamps of the meter is proportional to the turn ratio. The clamp meter input receives a small current because the number of secondary windings is much higher than the number of primary windings.

For instance, take the number of secondary windings to be 1000. In this case, the secondary current is 0.001% of the primary current. Thus, it is possible to measure larger values of current by adding a few extra turns to the secondary winding. You have the option of converting the measured current into voltage since many instruments are now provided with an mV output.

Hall Effect Clamp Meters

You can use these types of clamp meters for measuring both AC and DC current up to the range of 1 kHz. Similar to the current transformer type clamp meters, these instruments concentrate the magnetic field encircling the conductor using solid iron clamps.

How does a clamp meter work?

In this clamp meter, the clamps are not encircled by copper wires. Rather, the magnetic field is concentrated around the gaps in the iron core after you clamp the arms of the meter around the conducting material.

The gap is formed when the clamp tips of the instrument meet, thus creating an air pocket. This gap prevents the saturation of the core by limiting the magnetic flux.

Conversely, the clamps of a current transformer type clamp meter are level when they are closed. The clamp tips show the metal core when you open them. Inside the air gap, there is a semiconductor called the Hall Effect sensor, which is enclosed in a thin plastic covering. This is a sensor that reacts to magnetic fields by changing its output voltage.

Here the magnetic field is that of the conducting material or the circuit through which the current is flowing. The main objective of the Hall Effect sensor is to provide a direct measurement of the magnetic flux. Once you scale and amplify the output voltage produced by this sensor, you can get an accurate reading of the current that flows through the conductor.

Hall Effect Clamp Meters

In such devices, the direct current magnetic fields are focused around the core, similar to a piece of iron holding a permanent magnet.

Taking into consideration the earth’s magnetic fields and others lying in the vicinity, the reading must be zeroed before taking the final measurements. This allows you to get rid of the offset errors.


How to use a clamp meter?

You can use a clamp meter for measuring AC and DC current, voltage, and even resistance. However, before taking a measurement, you must make sure the test probes are not connected to the meter and that your fingers are placed behind the tactile barrier. This is a safety measure that will ensure you don’t suffer from an injury or electrical shock.

Measuring AC current through a flexible current probe

Before carrying out this measurement, you need to take some steps to prevent injury and electrical shock.

  • You should not put the flexible probe around a live conductor or try to remove it from the conductor.
  • Great care needs to be taken while fitting and removing the probe.
  • The installation to be tested must be de-energized and protective gear should be worn if needed.
Measuring AC current through a flexible current probe
  1. First, attach the current lead to the clamp meter.
  2. The flexible tube of the probe should be connected around the conductor. Make sure to latch and close the flexible probe’s end after opening it for the connection. When the probe locks itself in place, you should hear a sound.
  3. When using the device to measure current, you need to center the conducting material and make sure no live conductors are lying nearby. The conductor should be kept at a minimum distance of an inch away from the probe coupling.
  4. Adjust the dial accordingly until the display indicates that the flexible probe is being used for taking the measurements. The same rule about the display flashing based on the current being less than or more than 0.5 A also applies here.
  5. Note the value of the current from the display.

In case the flexible probe is malfunctioning, you should check the coupling system for damage and ensure that it’s closed and connected in the proper way. The system will not get closed if there is an unknown material present. You should also check whether the dial is in the right position and whether the cable is damaged.

Measuring voltage using a clamp meter

These days, clamp meters can be used for measuring both AC and DC voltage. An electrician often has to take voltage measurements to fix electrical issues. This is a bit more complicated compared to measuring current since you need to involve the test probes instead of using just the clamps.

Measuring voltage using a clamp meter
In order to measure the voltage, you need to follow these steps:

  • Select the type of voltage to measure, i.e. DC or AC.
  • Connect the test probes, connecting the black probe to the COM slot and the red probe to the V/O slot.
  • Similar to current measurement, you need to select the voltage range to be measured.
  • To get an accurate reading, you need to bring the probe tips in contact with the conducting material.
  • If you wish to stay away from the source of electricity and recall the voltage reading at the same time, you can use the hold function. This freezes the reading, so you can put some distance between you and the electrical source without forgetting it.

Measuring resistance using a clamp meter

There is a wide range of resistance values and the lowest value measurable through a clamp meter is 0.1 ohm. When you are measuring an open circuit or the resistance is too high and thus beyond the scope of the meter, you will see “OL” displayed on the LCD screen.

You must cut off power to the circuit before measuring the resistance because otherwise, you can damage it. In case the meter comes into contact with the voltage accidentally, some instruments have protective measures in place.
Measuring resistance using a clamp meter

Follow these steps to measure resistance:

  1. Cut off power to the circuit.
  2. Adjust the dial to select resistance.
  3. Connect the black test lead to the COM input slot.
  4. Connect the red test lead to the VΩ input slot.
  5. Determine the part of the circuit whose resistance value you need to measure and connect the probe leads across it.
  6. Note the reading from the screen.
  7. Before you start taking measurements for resistance, you must ensure there is no power flowing into the circuit.

How to measure DC amps with a clamp meter?