Clamp meter ABCs

What is a clamp meter, and what can it do? What measurements can be made with a clamp meter? How do you get the most out of a clamp meter? Which clamp meter is best suited to the environment the meter will be used in?

With technological advances in electrical equipment and circuits come more challenges for electricians and technicians. These advances not only require more capability in today’s test equipment, but more skills on the part of the people who use them.

An electrician who has a good grounding in the fundamentals of test-equipment use will be better prepared for today’s testing and troubleshooting challenges. The clamp meter is an important and common tool found in the toolboxes of electricians and technicians alike.

A clamp meter is an electrical tester that combines a voltmeter with a clamp-type current meter. Like the multimeter, the clamp meter has passed through the analog period and into the digital world. Originally created primarily as a single-purpose test tool for electricians, today’s models have incorporated more measurement functions, more accuracy, and, in some instruments, some very special measurement features. Today’s clamp meters have most of the basic functions of a digital multimeter (DMM), but with the added feature of a current transformer built into the product.

The transformer action

The ability of clamp meters to measure large ac currents is based on simple transformer action. When you clamp the instrument’s jaws or flexible current probe around a conductor carrying ac current, that current is coupled through the jaws, similar to the iron core of a power transformer, and into a secondary winding that is connected across the shunt of the meter’s input. A much smaller current is delivered to the meter’s input due to the ratio of the number of secondary windings versus the number of primary windings wrapped around the core. Usually, the primary is represented by the one conductor around which the jaws or flexible current probe is clamped. If the secondary has 1,000 windings, then the secondary current is 1/1000 the current flowing in the primary, or in this case the conductor being measured. Thus, 1 A of current in the conductor being measured would produce 0.001 Amps or 1 milliAmp of current at the input of the meter. With this technique, much larger currents can be measured by increasing the number of turns in the secondary.

Clamp meters measure any combination of alternating and direct current. This includes static dc and charging dc, as well as ac. Clamp meters measure dc current using Hall effect sensors. A Hall effect sensor, basically a kind of magnetometer, can sense the strength of an applied magnetic flux. Unlike a simple inductive sensor, the Hall effect sensor will work when the applied magnetic flux is static, not changing. It will work for alternating magnetic fields, as well. A clamp meter contains a toroidal iron core that clamps together with a Hall effect chip in the gap between the two halves, so that the induced magnetic flux from the current-carrying wire is channeled through it.

Resolution, digits and counts

Resolution refers to how fine a measurement a meter can make. By knowing the resolution of a meter, you can determine if it’s possible to see a small change in the measured signal. For example, if the clamp meter has a resolution of 0.1 A on a 600 A range, it’s possible to see a change of 0.1 A while reading 100 A.

You wouldn’t buy a ruler marked in 1-in. segments if you had to measure down to a quarter inch. Similarly, you must choose a meter that can display the resolution you need to see in your measurements.

Accuracy

Accuracy is the largest allowable error that will occur under specific operating conditions. In other words, it is an indication of how close the meter’s displayed measurement is to the actual value of the signal being measured.

Accuracy for a clamp meter is usually expressed as a percent of reading. An accuracy of 3% of reading means that for a displayed reading of 100 A, the actual value of the current could be anywhere between 97 and 103 A.

Specifications may also include a range of digits added to the basic accuracy specification. This indicates how many counts the digit to the extreme right of the display might vary. So the preceding accuracy example might be stated as ±(2% + 2). Therefore, for a displayed reading of 100 A, the actual current could then be estimated to be between 97.8 and 102.2 A.

Crest factor

With the growth of electronic power supplies, the current drawn from today’s electrical distribution system are no longer pure 60- or 50-cycle sine waves. These currents have become fairly distorted, due to the harmonic content these power supplies generate.

However, 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. If we want to check an electrical circuit for overloading, we need to measure the rms current and compare the measured value to the rated value for the component in question. Therefore, today’s test equipment must be able to accurately measure the true rms value of a signal, regardless of how distorted the signal might be.

Crest factor is a simple ratio of a signal’s peak value to its rms value. For a pure ac sine wave, the crest factor would be 1.414. However, a signal that has a very sharp pulse would cause the ratio, or crest factor, to be high. Depending on the width of the pulse and its frequency, you can see crest factors of 10:1 or higher. In real power distribution systems, crest factors of greater than 3:1 are rarely seen. So as you can see, crest factor is an indication of a signal’s distortion.

A crest factor specification will be found only in specifications for meters that can make true rms measurements. It indicates how much distortion a signal can have and still be measured within the meter’s accuracy specification. Most true rms reading clamp meters have crest factor specifications of 2:1 or 3:1. That rating will handle most electrical applications.

Current measurement

One of the most basic measurements of a clamp meter is current. Clamp meters are capable of measuring both ac and dc current. Typical current measurements are taken on various branch circuits of an electrical distribution system. Determining how much current is flowing in various branch circuits is a fairly common task for the electrician.

How to make current measurements

1. Select Aac or Adc.
2. Open the jaws of the clamp meter and close the jaws around a single conductor.
3. View the reading in the display.

By taking current measurements along the run of a branch circuit, you can tell how much each load along the branch circuit is drawing from the distribution system. When a circuit breaker or transformer appears to be overheating, it’s best to take a current measurement on the branch circuit to determine the load current. However, make sure you are using a true rms responding meter so you can get an accurate measurement of the signal heating up these components. Some meters will not give a true reading if the current and voltage are non-sinusoidal due to non-linear loads.

Voltage measurement

Another common function for a clamp meter is measuring voltage. Today’s clamp meters are capable of measuring both ac and dc voltage. AC voltage is usually created by a generator and then distributed through an electrical distribution system. An electrician’s job is to be able to take measurements throughout the system to isolate and fix electrical problems.

Another common voltage measurement would be testing battery voltage. In this case, you would be measuring dc voltage.

Testing for proper supply voltage is usually the first thing measured when troubleshooting a circuit. If there is no voltage present, or if it is too high or too low, the voltage problem should be corrected before investigating further.

A clamp meter’s ability to measure ac voltage can be affected by the frequency of the signal. Most clamp meters can accurately measure ac voltages with frequencies from 50 Hz to 500 Hz, but a digital multimeter’s ac measurement bandwidth might be 100 kHz or higher.

This is why the reading of the same voltage by a clamp meter and DMM can have very different results. The DMM allows more of the high-frequency voltage through to the measurement circuitry, while the clamp meter filters out some of the voltage contained in the signal above the bandwidth of the meter.

When troubleshooting a variable frequency drive (VFD), the input bandwidth of a meter can become very important in getting a meaningful reading. Due to the high harmonic content in the signal coming out of a VFD to the motor, a DMM would measure most of the voltage content, depending on its input bandwidth. Measuring the voltage output of a VFD is now a common measurement. A motor connected to a VFD only responds to the average value of the signal, and, to measure that power, the input bandwidth of the clamp meter must be narrower than its DMM counterpart.

How to make voltage measurements

1. Select Vac or Vdc, as desired.
2. Plug the black test probe into the COM input jack. Plug the red test probe into the V input jack.
3. Touch the probe tips to the circuit across a load or power source, in parallel to the circuit. 
4. View the reading, being sure to note the unit of measurement. 
5. One option is to press the hold button to freeze the reading in the display and then remove the meter from the live circuit to read the display when you are safely clear of the electrical hazard.

By taking a voltage measurement at the circuit breaker and then at the input of the load on that breaker, you can determine the voltage drop that occurs across the wires connecting them. A significant drop in voltage at the load might affect how well the load functions.

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

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.

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 [email protected].