Clamp-on testers provide a convenient, effective way to test current without the need to place a test instrument in the circuit. The meter’s jaws simply clamp around a live conductor and sense the electromagnetic field produced by the current.

The trend with clamp-ons continues to be toward digital models with high-end products able to perform multiple functions. Even so, analog testers still are sold and used.

“Today’s clamps focus on versatility and safety,” said Jim Gregorec, group manager of Ideal Industries test and measurement division. “Traditionally, it is to measure AC/DC current in amps by a convenient, indirect means versus having to place the instrument in series in the circuit. Now, clamp meters can also be full-fledged multimeters with many functions, ranges and good accuracies, allowing them to accurately measure voltage, resistance, frequency, capacitance, and indicate continuity. The most advanced clamps allow electricians to consolidate test tools in their bag.”

Bob Greenberg, product planner at the Fluke Corp., separates clamp meters into three basic categories:

  • A basic clamp meter shows whether all three phases on feeders are pulling the same current.
  • An advanced clamp meter with some logging capability helps troubleshoot intermittent breaker trips.
  • A specialized clamp meter that can accurately measure motor inrush current is useful for motor maintenance on production conveyor motors, HVAC motors and plant air compressors.

Representatives of seven leading suppliers offer these comments:

Amprobe, Jarek Bras, senior product manager: “Digital signal processing and support circuits permit more features to be added to clamp-ons while becoming smaller, lighter and less expensive. Clamp-ons today compete with multimeters and specialized power quality meters. New jaw technology uses lighter copper coil sensors to replace heavy, expensive steel jaws. The result is a significantly lighter meter, which is easier to carry in a heavy electrician’s bag and also more resistant to damage from drops.

“Analog clamp-on testers are still available. They are liked by veteran electricians and by those that don’t want to rely on a battery-operated product. Also, they are indispensable in cold weather conditions, where LCD screens of digital meters can’t work,” Bras said.

Extech Instruments, Scott Black, product manager: “The popularity of the clamp-on meter has not changed over the last two years. The basic functions of clamps are similar from one unit to another. Companies have added additional functions to clamp-on meters in an attempt to differentiate their products from one another. Changes are made to increase the capabilities of products, not necessarily improve them. Changes are driven more by marketing than engineering.”

Fluke Corp., Bob Greenberg, product planner: “Clamp meters vary from each other in measurement abilities. Some clamp meters allow you to see what is going on in the power supply to a particular motor during start up, using special algorithms and high-speed digital signal processing to filter out noise and capture the starting current exactly as the circuit protector sees it.

“Functions found on the most popular multi- function products include reading hold, min/max, frequency, inrush current measurements, backlighting, and true-RMS. [These] are all important features,” Greenberg said. “Variables to compare among models include resolution, digits, counts, accuracy and crest factor. Clamp meter safety ratings are an essential point of comparison. A meter should meet the IEC category and voltage rating approved for the environments where measurement are made.

“Specialized clamp meters were developed because existing meters did not show end-users what motor circuit protectors experienced, even with peak hold, max hold, and min/max hold. Nobody had studied the current draw profile of a motor in startup to see how that profile affects breakers and overload units. The industry needed a way of synchronizing the measurements with the motor startup so the measurements would be accurate and predictable. The result is a series of clamp meters that will detect an inrush condition and immediately start recording a large number of samples during a 100 RMS period,” Greenberg said.

Greenlee Textron, Chera M. Ellis, senior product manager: “The majority of field electricians utilize clamp-on meters more frequently than multimeters. There have been no drastic recent changes as the trend continues to shift toward digital clamp-on models.

“There has been a trend to add more functions, such as infrared, to clamp-on meters, however the response is mixed. Many electrical workers prefer separate tools for separate applications; however, another segment of electrical workers prefer as much functionality as possible into one product.

“The biggest change is the addition of auto functions. For example, to reduce the potential for error, there are clamp-on meters available that can automatically detect if the test is measuring amperage, voltage or resistance,” Ellis continued. “Changes in clamp-on meters are most often due to the evolution to make the product better. Auto-functionality has been well received and leads to the development of other auto-function testers.

“Cost and comfort are the two factors that influence the decision between analog and digital. Typically, an analog meter is very economical; however, digital clamp-on meters continue to become closer in terms of price.

“It is important to categorize the products according to the category safety rating, that is, Category I, II, III, or IV. A product may have all the capabilities and cost parameters desired by an electrical worker however not carry the appropriate category safety rating,” Ellis concluded.

Ideal Industries, Jim Gregorec, group manager—T&M division: “From focus groups, we learned the two things that frustrate electricians most when measuring current are getting the clamp meter onto a conductor and viewing the measurement in the display once the clamp was on.

“Tapered jaws ease the fit of the clamp head between conductors in a tight panel while a hook tip on the end of the clamp head can also separate one wire from a bundle to make getting the clamp meter onto a conductor in a tight area easier. A second display at the bottom of a clamp meter allows for easy viewing of the measurement at any angle.

“Super-bright backlights with large, bold numbers and symbols on the displays also make it easier to see—regardless of the quality of your vision. The underlying, fundamental need for commercial/industrial environments is safety, safety, safety. Therefore, clamps with safety in mind can have features like a second display in the bottom of the meter, voltage presence warning on all functions, and UL Listed to Cat IV/600V.

“A second display allows viewing at arm’s length, the user does not have to get close to hazardous voltage sources to view the current readings. And a high-voltage warning ensures that users are alerted of voltage through the test leads even if the meter is incorrectly set on the wrong function,” Gregorec said.

Megger, Jeffrey R. Jowett, senior applications engineer: “There has been no revolution in clamp-on technology, but rather incremental improvements. Improvements come about to address new market needs and as farsighted engineering uses new technologies. AC units have been gradually improved by the evolution of better core materials, alloys making up the core around which the jaw winding is wrapped. Hall Effect performance has been improved by advancements in the sensitivity of supporting electronics. Improvements in both affordability and field convenience have steadily increased the popularity and demand for these meters.

“Multipurpose models with various combinations of features and functions designed to meet specific applications or challenges have made this area of instrumentation one of the most competitive and dynamic,” Jowett said. “An example of improved field application is the emergence of dual-display units, where readings are displayed from both the front and bottom in order to facilitate working in limited-access areas.

“Meters today have widely differing requirements depending on how they are to be used. The basic rule is you get what you pay for. A residential electrician proofing a job doesn’t necessarily need high accuracy or more than two or three functions. So, why pay for capabilities that are not going to be effectively utilized? In troubleshooting and repair applications, however, multiple functions can be critical, as the operator may need to have as many capabilities as possible in order to maximize efficiency and effectiveness,” Jowett said.

UEi, David Wheaton, product manager: “The popularity of clamp-on meters continues to increase in part because technology now allows more functionality to be added. Today, most clamp-on meters include the features of basic multimeters, and some include more diagnostic capability or specialty test functions. Some of the functions that are on our most popular clamp-on meters are frequency, duty cycle, capacitance and peak capture.

“Changes have come through utilizing improving technology to make the meter a better fit for contractor needs. In the past, some combinations of features would be impossible due to size constraints, cost limits, or not possible due to limits in technology. Today many of the features can be designed into the microprocessor keeping the size and cost within limits.

“One new feature is an optional ‘hook’ current probe that permits the user to access tight spaces and hook the probe around the current-carrying conductor without having to open the jaw,” Wheaton said. “This was directly in response to repeated comments about the difficulty of measuring in tight spaces with a standard type of current clamp and would not have been possible in the past. The smaller tool is easier and safer to use because the contractor isn’t reaching his hand into the space where possible live contact points may exist.”    EC

GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or