Clamp meter ABCs

A back-to-basics explanation of how, when, why and what to measure.

By Chris Rayburn, Fluke

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Resistance measurement

Resistance is measured in Ohms (Ω). Resistance values can vary greatly, from a few milliOhms (mΩ) for contact resistance to billions of Ohms for insulators. Most clamp meters measure down to 0.1 Ω. When the measured resistance is higher than the upper limit of the meter, or the circuit is open, OL should appear in the meter’s display.

Resistance measurements must be made with the circuit power off. Otherwise, the meter or circuit could be damaged. Some clamp meters provide protection in the Ohms mode in case of accidental contact with voltages. The level of protection may vary greatly among different clamp meter models.

How to make resistance measurements

  1. Turn off power to the circuit.
  2. Select resistance (Ω). 
  3. Plug the black test probe into the COM input jack. Plug the red test probe into the VΩ input jack. 
  4. Connect the probe tips across the component or portion of the circuit for which you want to determine resistance. 
  5. View the reading in the meter’s display

Make sure the power is off before making resistance measurements.


Continuity is a quick go/no-go resistance test that distinguishes between an open and a closed circuit.

A clamp meter with a continuity beeper allows you to complete many continuity tests. The meter beeps when it detects a closed circuit, so you don’t have to look at the meter as you test. The level of resistance required to trigger the beeper varies from meter to meter. The typical resistance setting to turn on the beeper is a reading less than between 20 Ohms and 40 Ohms.

Special functions

A fairly common measurement function is reading the frequency of an ac current waveform. With the clamp meter’s jaws, or a flexible current probe, wrapped around a conductor carrying ac current, switch on the frequency function and the meter’s display will indicate the frequency of the signal flowing in the conductor. This can be a helpful measurement when tracking down harmonic problems in an electrical distribution system.

Electrical power system components such as fuses, bus bars, conductors, and thermal elements of circuit breakers are rated in rms current because their main limitation has to do with heat dissipation.

Another feature that can be found in some clamp meter models is min, max and average storage. When this feature is activated, each reading the clamp meter takes is compared against any previously stored readings. If the new reading is higher than the reading in the high reading memory, it replaces that reading as the highest reading. The same comparison is made against the low reading memory, and the new reading, if lower, replaces the stored reading. The average reading is updated accordingly. As long as the min, max and average feature is active, all readings are processed in this way. Thus, after a period of time, you can call up each of these memory values to the display and determine the highest, lowest and average reading over a specific period of time.

In the past, not all clamp meters could measure capacitance. The capacitance measurement function is now being incorporated into the feature set of many new clamp meters. This function is useful for checking motor start capacitors or measuring values of electrolytic capacitors contained in controllers, power supplies or motor drives. For electricians who deal with motors in their work, the ability to capture the amount of current drawn by a motor during its startup can tell a lot about a motor’s condition and loading. After clamping the jaws or the flexible current probe around one of the motor’s input leads, activate the inrush mode. Next, turn on the motor. The clamp meter’s display will indicate the maximum current drawn by the motor over the first 100 ms of its start cycle. This proprietary inrush measurement technology filters out noise and captures motor-starting current exactly as the circuit protection sees it.

Clamp meter safety

Making measurements safely starts with choosing the proper meter for the environment in which the meter will be used. Once the proper meter has been chosen, you should use it by following good measurement procedures.

The International Electrotechnical Commission (IEC) established new safety standards for working on electrical systems. Make sure you are using a meter that meets the IEC category and voltage rating approved for the environment where the measurement is to be made. For instance, if a voltage measurement needs to be made in an electrical panel with 480 V, then a meter rated Cat. III — 600 V or higher should be used. This means the input circuitry of the meter has been designed to withstand voltage transients commonly found in this environment without harming the user. Choosing a meter with this rating, which also has a CSA or TÜV certification, means the meter not only has been designed to IEC standards, but has been independently tested and meets those standards.

Many new clamp meters now carry a Cat. IV safety rating, which means they can be used in outdoor or underground settings where lightning strikes or transients can occur more frequently and at higher levels.

Chris Rayburn is marketing manager for clamp meter, Earth ground and insulation test products Fluke. Email him at

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