Published In February 2001
When it comes to clamp-on meters, status quo is not always the way to go. While you may have enough test meters on hand, replacement or supplementation—with new digital models that offer a high degree of accuracy, greater safety than some earlier models, and additional efficiency-boosting features—could be a good investment. (And with the high cost of labor these days, the purchase price of many of the units is likely to be less of a concern, vis-à-vis return on investment, than it used to be.) High-quality digital clamp-on meters with a host of capabilities can expand the range of everyday field diagnostics and troubleshooting while cutting down on the effort. Because there are so many choices, you should be able to pick and choose among feature sets, without overbuying. Another consideration in selecting a meter is the skill of the user. Some basic test tools with simple light-emitting diode (LED) readouts can now do rather sophisticated measurements, so an electrician with less skills can take measurements and, if a problem is found, call in someone with more skills and a more capable instrument to diagnose and correct the problem. While many of today’s digital meters are slimmer and smaller-jawed than yesteryear’s—the better to fit into tight panel boxes and provide easy toting—you will likely still need a large-jawed meter to encircle large-diameter wires that carry higher currents. Features worth considering include: * auto shut-off (so that an instrument left on inadvertently will automatically shut off after a set time, to conserve batteries); * autoranging (with the unit automatically selecting the range with the best resolution and accuracy for the application rather than requiring manual setting of a dial); * ability to perform AC/DC current and voltage measurements, ability to display peak as well as minimum/maximum readings; and * the ability to perform true Root Mean Square (RMS) readings (the accuracy of which is often needed by contractors who work in environments heavy with electronic lighting ballasts, variable-speed motor drivers, computers, and other digital equipment). Data capture (of the lowest and highest readings encountered in a measurement sequence) and data hold (maintaining display of the last reading) allow users to capture a particular spike and analyze it against collected readings. Other possibilities on digital clamp-ons, including graphical displays, analog bargraphs, averaging, and the ability to connect to recorders or data loggers, which could be handy when troubleshooting power quality problems or machinery over time. Some new digital meters feature non-contact voltage testing, whereby the user places the unit near, but not on, the wire and the tester picks up the magnetic field around the wire to determine if it is live with voltage. More and more manufacturers are complying with the safety ratings and the overvoltage categories as well as maximum voltage ratings that are listed as part of the International Electrotechnical Commission (IEC) 1010 standards. Buyers should also look for UL ratings (UL 3111, which incorporates the IEC standards), with category ratings that reflect the degree of risk at any given point in the electrical distribution system. Different categories are assigned depending on the risk at any given point in the electrical distribution system. The higher the number, the greater the risk, and therefore, the higher the safety standard for that class of meter, said Jim Wetzel, technical application specialist at Greenlee/Textron. The casings of IEC-approved units, for example, are double-insulated so if one level fails there is another level of safety, he said. Clamp-on meters that can measure frequency of the wave form in both voltage and current modes could be especially handy when working with a generator set because they can advise the electrician if the frequency is right or if the worker is dealing with lines that have a lot of harmonics. True RMS meters are considered highly accurate when measuring distorted AC waveforms. “The combination of complete peak and true RMS sensing gives you an idea of how good your wave form is,” said Mel Henrickson, application engineer at Amprobe. The combination enables you to determine the crest factor (the peak divided by true RMS). “If it comes out to 1.4, you are dealing with a sine wave, which is fine, but if it is higher, you are dealing with a distorted wave form, which could help cause possible nuisance tripping of circuit breakers or overheating of transformers.” The Gardner Bender GCM 620 Digital Clamp-On meter, an AC/DC unit featuring true RMS, peak hold, autoranging, and jaws that can clamp around one 750 MCM or two 350 MCM cables, can measure up to 1,000 amps. In addition, the unit works with an optional line splitter that, once the tester is plugged into the wall and the appliance is plugged into it, splits the hot and the neutral wires so the operator can take an accurate reading. This eliminates the need to deal with the readings from the hot and neutral wires canceling each other out, the company said. IDEAL Industries has a clamp-on, 61-702, that features non-contact voltage testing and a vibration mode that uses a digital motor (a shaker) to indicate the presence of voltage. The meter is also able to test whether there is voltage present on a machine that is hooked up but not “on.” The unit incorporates a fairly basic harmonics indicator that could signal trouble ahead-an LED display stays on when “clean power” is present and goes out to alert the worker when 5 percent or more harmonics is present in the circuit. If the display shows harmonics (which can indicate voltage being off by 10 to 40 percent), more extensive testing with a power analyzer or circuit analyzer could follow by that worker or a more experienced worker, said Jim Gregorec, group manager at Ideal. Some digital meters offer “record mode,” a new feature that allows the user to either hook up the voltage leads or clamp it on a line and, after time, view maximum/minimum and average readings to get a clear idea of what the loading on the line has been. This data could be helpful if the worker is considering putting additional loads on a transformer, for example, to help ensure the overall load will not exceed the rating of the transformer. (The new units, which are typically double-insulated, should conform to the Category III 600-volt safety ratings from IEC or the 1,000-volt standard.) Some manufacturers, including Hioki and AVO International, are offering clamp-on meters that, used with optional proprietary hardware and software, have the capability of recording data for analysis and archiving. With this type of unit hooked up to a recorder, you not only get a reading directly on the clamp but can also generate a graphical hard copy, as well. Hioki 3284/3285 Clamp On AC/DC HiTESTER, with 200 amps and 2,000 amps capability respectively, for example, works with the Hioki Memory HiCORDER, so a user concerned with power quality and basic motor evaluations can see and print the parameters being analyzed as waveforms. The HiCORDER, which can function essentially as a data logger, stores information at intervals and can be left on up 600-plus days for viewing of min/max and average amps. “You can set the points when you want it to take a recording or you can let it record continuously until the memory is full,” according to John Luchka, vice president of marketing at Hioki. “You can also measure a peak with a motor run or start or surge and, if a new peak current or voltage occurs, the instrument will capture that automatically.” AVO International Megger DCM2000P Power Clampmeter can transmit both stored and live measurements to a PC running AVO PowerLog software for further analysis and archiving. Measuring up to 2,000 amps and 600V, the unit displays waveforms, harmonics, chart trends, and up to five parameters at a time on a backlit dot matrix LCD. 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.