Published In February 2002
As electrical systems become increasingly complex—incorporating more computers and other devices that create nonlinear loads (distorted loads)—evaluating the electrical load depends partially on the test equipment’s ability to take accurate readings of several factors. Today’s digital clamp-on meters, with easy-to-read light-emitting diode (LED) displays, safely perform a range of precise measurements under varying conditions. They also offer expanded and less-expensive capabilities that were not available in analog clamp-ons. They will also, generally, do a better job of standing up to job-site hard knocks. Some user-friendly features in new clamp-ons include: • auto ranging, which automatically selects the range with the best resolution and accuracy for an application and • auto hold and data hold, which take readings where it is difficult to view results while the leads and the meter are in place. With auto hold, the operator presses a button prior to positioning the meter. Once the meter is stabilized, it beeps and locks the reading into the display. Data hold retains display of the last reading, for later retrieval. Some digital meters feature “record mode,” which enables the user to hook up the voltage leads or clamp the meter onto a line and, after a period of time has elapsed, view max/min and average readings, for an indication of what the loading on the line has been during that time. The measurements could be useful to ensure that the circuits do not overload. For operator safety, look for meters that comply with the safety ratings, the overvoltage categories, and the maximum voltage ratings that are listed as part of the International Electrotechnical Commission (IEC) 1010 standards. At the least, the meter should have a minimum CATEGORY III, 600-volt rating. Depending upon the type of AC problems you anticipate encountering, you can select between a True Root Mean Squared (TRMS) meter, which yields a continuous true reading, and an average reading meter, which is typically less expensive. Average readings, when measuring AC, give an average value calibrated to the RMS of a sine wave only. If measuring nonlinear loads, you can only get a correct reading with a TRMS meter. (Nonlinear loads come from equipment that doesn’t draw current in linear fashion but draws the current in spurts or pulses, including adjustable speed motor drives, computers, copiers, and other electronic devices.) In commercial applications, nonlinear loads that draw high peak currents are prevalent. Nonlinear loads can lead to overheating of neutral conductors and circuit breaker tripping. Below are representative digital clamp-on meters. Manufacturers typically offer a range of meters with varying combinations of feature sets and capabilities. The Amprobe model AC/DC-3000 is a TRMS clamp-on meter, rated for CATEGORY III-600V. The unit, which sports a backlit fully annunciated LCD display, features AC and DC True RMS current measurement to 1,000 amps and AC and DC TRMS voltage measurement to 600 volts. The meter can also measure resistance and can perform an audible continuity test and diode test. The dual display can show current/frequency, current/duty cycle, voltage/frequency and voltage/duty cycle. A special record feature allows the user to either clamp the meter on the line or attach the voltage leads to a voltage source; walk away; and upon return, view the average, minimum, and maximum readings. The unit also measures frequency and duty cycle and can record those readings, which could be very useful, according to the company, when doing generator work and trying to analyze “flicker,” which is a frequency-related problem. Greenlee Textron compact CM-1000 clamp-on meter is designed to provide a broad range of test capabilities for a variety of in-plant, maintenance, repair, and operations (MRO) and heating, ventilation, and air conditioning (HVAC) service applications. The unit, with voltage and resistance capability, reads to a maximum 1,200 amps AC, and to a maximum 1,000 volts DC and 600 volts AC. It has a data hold function to capture readings for display on a large 4,000-count liquid-crystal display (LCD) high-resolution panel, autoranging, a low-battery indicator, and auto power-off. For enhanced safety, the CM-1000 clamp-on meter has overvoltage protection and is equipped with a large jaw capacity to handle heavy gauge cables. The clamp-on meter has a beeper used when performing continuity tests and is UL certified and complies with Category III 600-volt standards. The new AEMC Instruments Model 514 is a general-purpose professional clamp-on meter that measures AC and DC Amps to 1,000A, AC Volts to 750V, DC Volts to 1,000V, Ohms, Continuity, and Frequency (using either the V or A measurement) and has a Diode Test function and a push-button DC zero. The meter, which provides TRMS measurements, uses Hall sensor technology that provides DC and AC current measurements to 1,000A. The unit, which is safety rated to 600V Category III, has a 4,000-count 3¾-digit LCD display and a 42-segment analog bargraph display. Other features include auto power-off, for conserving battery power after 30 minutes of inactivity, and a low-power indicator. Manufactured with an integrated circuit/firmwave/software system designed for clamp meters, without reliance on multimeter chips, the Fluke 337 Clamp Meter has a compact body with large jaws that can accommodate large conductors up to 1,000 amp rated bus bars and one-hand-operated meter controls that facilitate current measurements with an index finger on clamp opening lever; and thumb-on rotary switch. The TRMS clamp meter measures AC/DC current and voltage, ohms with continuity beeper, motor start-up current (in-rush), and frequency. The unit, which includes MIN/MAX capture and display, has a large, backlit display that enables readings in dim light and a “display hold” that retains measurements in the display window for readings after the meter is removed from the conductors. An auto shut-off maximizes battery life. The meter comes with a three-year warranty. AVO International Megger DCM2000P Power Clampmeter, measuring mixed currents up to 2,000 amps and voltages up to 600 V, displays waveforms, harmonics, chart trends, and up to five parameters simultaneously on a backlit dot matrix liquid crystal display (LCD). The unit features TRMS, peak, crest factor, auto-ranging and auto-zeroing, and has a selector switch position for balanced three-phase measurements. It measures all basic power functions: active power (Watts), apparent power (kVA), re-active power (Vars), plus power factor (in both degrees and cosine). Used with an optional (proprietary) recorder and data-logging software, the unit can record stored and real-time measurement to a PC for analysis and for archiving. With the unit connected to the recorder and the printer-enabled PC, users can get a reading directly on the clamp and generate a graphical hard copy of the result. Gardner Bender GCM-620 True RMS AC/DC clamp-on meter features a rugged design built to withstand abuse and large jaws that fit around one 750 MCM cable or two 350 MCM cables. The unit, which is rated Category III 600V and comes in a nylon carrying case, measures TRMS 45Hz to 1Khz, AC/DC voltage to 600V, AC/DC current to 1,000 amps, and resistance to 400 ohms, and provides auto ranging and data hold. An optional AC current line-splitter separates the positive and neutral conductors of power cords when testing AC amperage draw. Suitable for residential, light commercial, and industrial applications, the Ideal Industries 61-702 200 Amp Clamp Meter functions as a clamp meter, noncontact voltage tester, and solenoid tester. Measuring AC voltage, DC voltage, AC current, resistance, capacitance, and frequency, the Category III-1,000V-rated unit features data hold/max hold, overload protection on all ranges, an audible dangerous voltage warning, and a harmonics indicator. A noncontact voltage sensor on the tester tip indicates the presence of 24-600 VAC without using leads. A vibration mode, using a digital motor, provides a tactile sensation when voltage is present. The meter can also test for the presence of voltage on a machine that is hooked up but not “on.” The meter incorporates a harmonics indicator—an LED display lights up when clean power is present and remains unlit when 5 percent or more harmonics is present. The unit comes with replaceable silicone test leads, carrying case, and a lifetime limited warranty. Wavetek Meterman AC68 professional multifunction clamp digital multimeter, sports eight functions including handling 600 amps AC or DC current and AC and DC voltage (using Hall sensor technology), frequency, resistance, diode, and continuity test beeper, and 11 ranges. The unit, which comes with a one-year instant replacement warranty, features a 4,000-count LCD display, data hold, and AC peak hold, auto power-off, a DC auto zero function, and auto ranging. Clamp-on ground resistance testers are efficient when used with multi-grounded systems because they enable testing without requiring disconnect of the ground under test. Enabling noncontact measurements of ground conductors without the need for auxiliary ground spikes, Extech Instruments Clamp-on Ground Resistance Tester, Model 382356, provides auto-ranging mA and resistance measurements. Suitable for grounding systems in substations, factories, and office buildings, the unit provides ground resistance measurements that range up to 1,500 ohms with 0.01-ohm maximum resolution and leakage current measurement to 15A with 0.01A maximum resolution. Readings are displayed on a high-contrast LCD with data hold. The 0.9-inch (23mm) clamp jaw opening fits large ground conductors and provides high sensitivity. Features include automatic detection of electrical noise on the ground rod, continuity test beeper, and automatic self-calibration when powered on. The tester, which comes with a 9V battery, calibration plate, and carrying case, meets International Electrotechnical Commission (IEC)-1010 (Category III-300V and Category II-600V) requirements. THE FELDMANS provide Web content for companies and write for magazines, trade associations, building product manufacturers, and other companies on a broad range of topics. They can be reached at firstname.lastname@example.org or (914) 238-6272.