ABC’s of Clamp Meters

Like the multimeter, the clamp meter has passed through the analogue period and into the digital world of today. Created primarily as a single-purpose test tool for electricians, today’s models have incorporated more measurement functions, more accuracy, and in some instruments, some very special measurement features. Today’s clamp meters have many of the basic functions of a digital multimeter (DMM), but with the added feature of a current transformer built into the product.

A clamp meter is an electrical tester that combines a voltmeter with a clamp type current meter. Choosing a clamp meter not only requires looking at specifications, but also looking at features, functions, and the overall value represented by a meter’s design and the care taken in its production.

Resolution, digits and counts: Resolution refers to how fine a measurement a meter can make. By knowing the resolution of a meter, you can determine if it’s possible to see a small change in the measured signal.

Accuracy: Accuracy is the largest allowable error that will occur under specific operating conditions. In other words, it is an indication of how close the meter’s displayed measurement is to the actual value of the signal being measured.

The accuracy for a clamp meter is usually expressed as a percent of reading. An accuracy of 1% of reading means that for a displayed reading of 100 amps, the actual value of the current could be anywhere between 99.0 and 101.0 amps.

Measuring ac current: One of the most basic measurements of a clamp meter is ac current. Typical ac current measurements are taken on various branch circuits of an electrical distribution system. Determining how much current is flowing in various branch circuits is a fairly common task for the electrician. By taking current measurements along the run of a branch circuit, you can easily tell how much each load along the branch circuit is drawing from the distribution system.

True RMS: With the growth of electronic loads, the currents drawn from today’s electrical distribution system are no longer pure 60- or 50-cycle sine waves. These currents have become fairly distorted; due to the harmonic content these non-linear loads generate. However, electrical power system components such as fuses, bus bars, conductors and thermal elements of circuit breakers are rated in RMS current because their main limitation has to do with heat dissipation.

If we want to check an electrical circuit for overloading, we need to measure the RMS current and compare the measured value to the rated value for the component in question. Therefore, today’s test equipment must be able to accurately measure the True-RMS value of a signal regardless of how distorted the signal may be.

Measuring voltage: Another common function for a clamp meter is measuring voltage. Today’s clamp meters are capable of measuring both ac and dc voltage. An electrician’s job is to be able to take measurements throughout the system to isolate and fix electrical problems.

Another common voltage measurement would be testing battery voltage. In this case, you would be measuring dc voltage. Testing for proper supply voltage is usually the first thing measured when troubleshooting a circuit. If there is no voltage present, or if it is too high or too low, the voltage problem should be corrected before investigating further.

Resistance: Resistance is measured in ohms (V). Resistance values can vary greatly, from a few miliohms (mV) for contact resistance to billions of ohms for insulators. Most clamp meters measure down to 0.1 V. When the measured resistance is higher than the upper limit of the meter, or the circuit is open, ‘OL’ appears in the meter’s display.

A very common electrical resistance measurement is reading the resistance of a contactor coil. Meaningful resistance measurements need to be made with the circuit power off. Better-designed clamp meters provide protection in the ohms mode in case of accidental contact with voltages, but many will be damaged or worse, cause injury. The level of protection varies greatly among different clamp meter models.

Continuity: Continuity is a quick go/no-go resistance test that distinguishes between an open and a closed circuit. A clamp meter with a continuity beeper allows you to complete many continuity tests easily and quickly. The meter beeps when it detects a closed circuit, so you don’t have to look at the meter as you test.

The level of resistance required to trigger the beeper varies from meter to meter. Typical resistance values to turn on the beeper is a reading of less than somewhere between 20 and 40 ohms.

Special functions: A fairly common measurement function is reading the frequency of an ac current waveform. With the clamp meter’s jaws wrapped around a conductor carrying ac current, switch on the Frequency function and the meter’s display will indicate the frequency of the signal flowing in the conductor. This is a very helpful measurement when tracking down harmonic problems in an electrical distribution system or, with some instruments, measuring the output frequency of VFD’s or generators and UPS’s.

Another feature that can be found in some clamp meter models is MIN MAX storage. When this feature is activated, each reading the clamp meter takes is compared against any previously stored readings and the highest and lowest values are stored for later review.

Clamp meter safety: Making measurements safely starts with choosing the proper meter for the environment in which the meter will be used. CSA C22.2 No. 6010 defines the electrical environment according to categories and then voltages within those categories. Most industrial users will need an instrument with a minimum 600V Cat III rating. Many instruments on the market have lower levels of protection, depending how they are being used.

Only use instruments where the stated protection level is applicable to all functions, ranges and inputs. It’s also essential that the instrument has been independently verified as meeting the standard and has CSA or equivalent approval to show it. Once the proper meter has been chosen, you should use it by following good measurement procedures as shown in CSA Z462 or NFPA70E.

With technological advances in electrical equipment and circuits come more challenges for electricians and technicians. These advances not only require more capability in today’s test equipment, but more skills on the part of the people who use them.

An electrician who has a good grounding in the fundamentals of test equipment use will be better prepared for toda
y’s testing and troubleshooting challenges. The clamp meter is one such important and common tool found in the toolboxes of electricians and technicians alike.

Colin Plastow has been with Fluke Electronics Canada since 1987 in various support and product management positions. Today, as industrial product manager for Fluke, he brings his expertise in electronic test and measurement to customers in high-tech and industrial markets. He may be contacted at colin.plastow@fluke.com.