Published: August 2003
There’s more to this tester than just the basics
The clamp-on meter is one of the most basic testing devices, providing a fast, non-intrusive method for measuring electrical current.
Clamp-on meters measure basic power quality and energy parameters for diagnosing and troubleshooting problems, to analyze disturbances on a facility’s electrical distribution network, and to ensure adequate quality and quantity of electric supply at various points of utilization throughout the facility, noted Richard Bingham, director of engineering, Dranetz-BMI.
The key feature of all clamp-on meters is the ability to make tests without interrupting the flow of current.
To take readings, a meter’s jaws simply clamp around a live conductor and sense the electromagnetic field produced by the current—the higher the current, the higher the magnetic field intensity, said George Allen, marketing manager Wavetek Meterman Test Tools. The user does not need to break apart the circuitry under test in order to measure the current. Range of meters typically is from 0 to 2,000A or higher, depending on the model.
The first clamp-on meters were analog models, and electricians have used them for years. Digital clamp-ons came on the scene later, but they, too, have been on the market for some time. As the digital revolution grew and expanded to more and more products, digital clamp-ons became preferred by more and more users. While many analog clamp-on meters remain in use, the trend definitely has been toward digital, and most clamp-ons sold today are digital models.
Manufacturers point out that digital models with easy-to-see LCD readout windows are more accurate, have safety features unavailable on analog units, and have capabilities extending beyond basic testing.
Cost and comfort level are the two factors that influence the decision between analog and digital, according to Chera M. Ellis, Greenlee Textron senior product manager.
“Typically,” she explained, “an analog meter is very economical, although digital clamp-on meters are closing the price gap. But some users still are more comfortable with an analog meter, especially if they have never used a digital model.”
Analogs are still used to view quick changes in current and for fast “go/no-go” inspections, observed Jim Gregorec, T&M division group manager, Ideal Industries.
“Because analog models don’t require batteries, they are capable of working when cold temperatures drop below freezing,” added Mel Hendrickson, applications engineer for Amprobe Advanced Test Products. “However, along with other limitations, analog clamps cannot measure DC current or TRMS [true root mean squared] values.”
Durability and safety factors also favor digital meters. “Analog clamp-on meters usually are not as rugged mechanically or electrically as digital models,” said Bob Greenberg, product planner at Fluke Corporation, “and analog meters lack input protection, whereas clamp-on meters can carry a category safety rating.”
While some analog meters offer multiple functions, they cannot match digital meters in this respect.
“Typically, digital clamp-on meters today are designed to be multimeters, not just current meters, and for convenience’s sake, they include voltage and resistance measurements,” said Jeffrey Jowett, Megger applications engineer. “Probably the most important distinction is between true RMS and average-responding measurement and, also, accuracy. Other features include reading AC, DC, or AC plus DC; peak and data holds; continuity with buzzer; lead zero; and output ports for connection to loggers or other recording devices. Not to be overlooked are conformances to safety standards, as these units are often used in potentially dangerous environments.”
Fortunately, prices have not increased at the same pace as new features and improved capabilities.
“In the last five years the prices of digital clamps with TRMS response have come down to a point that they are available for any user,” said John O’Brien, ExTech applications engineer. “The trend toward digital units with better accuracy and reliability and at lower prices will continue.”
“There are two basic types of clamp-on meters today,” explained Greenberg. “The first type is for AC current only. These utilize a magnetic core with a coil of wire wrapped around it, utilizing transformer principles to take measurements. The second type measures both AC and DC and utilizes a Hall Effect sensor inserted in an air gap in the magnetic core.”
“The most basic meters measure AC Amps,” Bingham said. “As meters progress in capability and cost, parameters incorporated include RMS AC/DC volts and Amps, resistance, and diode testing. The next level is the addition of power, frequency, power factor and basic power quality events such as voltage sags and swells.
“Advanced models expand capabilities with additional power quality parameters, including total harmonic distortion, measurement of ripple on DC, and individual harmonics. These models also provide expanded memory for longer-term data logging [over 10,000 readings], ability to measure and display multiple parameters simultaneously, and more sophisticated software for in-depth analysis on a personal computer.”
Demand is increasing for meters for measuring non-linear current loads used by motors, computers and other electronic equipment and only models with TRMS capabilities can provide accurate readings for such tests.
With the broad selection of clamp-on meter models available today, buyers can select the meters that best suit their specific requirements, paying only for the features they need.
Changing with the industry
Capabilities of clamp-ons have steadily evolved as the needs of the electrical industry changed.
Among significant improvements, Fluke’s Greenberg cited improved filtering for quieter readings and frequency measurement on adjustable speed drives, one-button DC, zero-on AC/DC (Hall Effect) clamp-on models to eliminate DC measurement offset, and improved protection conforming with UL, CSA, IEC and CE worldwide safety standards.
“An additional option for electricians, technicians, or engineers is to use a digital multimeter—which they already own—with a clamp-around adapter,” said Greenberg. “For measuring very small current levels, one needs a much smaller jaw which can be found in clamp-around adapters. There are also flexible coil—Rogowski-type—clamp-around adapters for measuring around very large conductors or entire bundles with very high current levels up to 10,000A.”
Said Megger’s Jowett: “Improvements to clamp-on meters are largely the same as those with test instrumentation in general—improved microcircuitry, which permits multiple functions to be added and improved accuracy and performance. Also of significance is the imposition of international safety standards in design.
“Nearly all clamp-on meters have migrated to digital circuitry, have a backlight, and offer higher safety ratings with an amalgamation of DMM capability of the functions of full-featured DMMs. True RMS capability also is becoming more common. In the next 18 months we likely will see some new features, such as temperature and specialized functions will come on specific models in the near future. More task-specific clamp-on meters will be available to meet needs of dedicated industries. Full-featured clamp-on meters are coming down in price so that affordable, accurate and easy-to-use units are within the budgets of many more customers.”
Bingham identified three basic changes in clamp-ons in the past few years:
“The electronics within these instruments have become more sophisticated at a lower cost, resulting in the ability to collect more data—both power quantity and quality—at a lower cost. The software and analytical support for these instruments has advanced, enabling the field technician to receive immediate results and additional information for troubleshooting. And the instruments have become more rugged and safe for field use.”
Clamp-on meters, he said, will continue to advance in calculation sophistication and internal memory, enabling them to be more reliable for the identification of intermittent problems, as well as operate for longer periods on battery.
Ellis believes improvements in safety are coming.
“Clamp-on meters will provide more safety for the end user,” she said. “Measurement category ratings of Cat IV 600V will become common. Many manufacturers are going to have to invest tooling dollars in order to meet the spacing requirements outlined in the IEC Standard 61010-1.”
“Today’s equipment,” concluded Gregorec, “measures capacitance for checking capacitors on motors, peak hold for capturing in-rush current during motor startups, ‘min/max’ for voltage fluctuations over time, true RMS for measuring in harmonic environments, and backlight for viewing in dimly lit locations. As clamp-on meters become more versatile, they are being accepted as a one-tool-does-all type of meter.” EC
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at 405.748.5256 or firstname.lastname@example.org.