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Digital clamp meters are popular testers for troubleshooting and diagnostics. Combining a voltmeter with a clamp type current meter, they enable safe and easy precise measurements under a variety of conditions. Many models offer the primary functions of a digital multimeter — the ability to measure AC and DC voltage, current, resistance, and continuity — and then add a lot more functionality.
Today's crop of clamp meters are a small, light fleet compared to earlier generations. Typically, they are all a lot easier and safer to use — as well as more accurate and more rugged — than the analog clamp meters of yesteryear.
Clamp meters for use by electricians or plant maintenance technicians should be independently tested for safety compliance and carry the symbol of an independent testing lab, such as UL, CSA or TUV. They should be safety-rated CAT III 600V, at minimum.
When selecting a clamp meter, look at the features and specifications. While some meters are plenty sophisticated, if you don't need to perform the tests they enable, a less full-featured model may suffice.
Several user-friendly capabilities that maintain position or readings enhance productivity. These include auto range, auto hold, data hold, and min/max storage.
Auto range automatically sets the best resolution and accuracy for the specific application. Auto hold enables the operator to press the activating button before positioning the meter. Once the unit stabilizes, the meter beeps and locks the reading into the display. Data hold (also called display hold) retains the displayed value for later reading (regardless of how the reading is taken), which is handy when taking measurements in tight or poorly lit locations where you cannot clearly read the display. Min/max storage enables comparison of each reading against any previously stored readings, automatically replacing the previous highest or lowest reading with any new high or new low reading.
Some units are true root mean square (TRMS) meters, having the capability for measuring waveforms and distorted waveforms in environments with non-linear loads that cause harmonics, such as variable speed motors, electrical lighting ballasts, computers and copiers.
Convenience features include a backlit digital display for easy viewing in low-light situations; auto shut-off, which saves the battery if you forget to turn off the tester; and integration of a temperature probe that can detect an elevated reading which might indicate an incipient problem before it evidences other symptoms.
Clamp meters are available that also measure frequency and duty cycles. Some units have dual displays, which can show voltage/frequency, current/frequency, current/duty, and voltage/duty, and/or capability to perform audible continuity and diode tests.
Other features to look for: overvoltage protection; extra-large jaws to accommodate heavy gauge cables; and overbooting on the tester for on-the-job protection against drops and hard knocks.
Some meters work with optional recorder and data-logging software, enabling downloading single or multiple measurements to a computer for analysis and storage. Software applications are available for recording and preserving attended measurements and, if the tester is left behind in place, for recording and preserving events throughout a delineated time period, regardless of whether or not a technician is present.
Specifications that may differ among models include resolution, which correlates to how fine a measurement the meter can make, expressed as a decimal e.g. 0.1 amp; accuracy, reflecting the largest allowable error with respect to the actual value of the signal being measured, generally expressed as a percentage of the reading and also, in some units, by how many counts the digit to the extreme right on the display may vary; count capability (between 4,000 and 20,000 count is typical); number of digits in the LCD display; and whether or not there is also a digital bargraph display, and if there is, the number of segments.
author: By William & Patti Feldman