New clamp meters offer more capability, accuracy, safety

Clamp meters are familiar tools for electrical engineers, and technicians — they're the wrench of the trade for electricians. But as electrical system loads become more complex, you should make sure your clamp meters are keeping pace.

The two types of clamp meters in use today are average responding and true-RMS sensing. The difference between the two is measurement accuracy of nonlinear loads. While average-responding clamp meters are widely used and are usually lower in cost, they give correct readings for linear electrical loads only. Linear loads include standard induction motors, resistance heaters, and incandescent lights. This type of load typically is associated with residential electrical systems.

Examples of nonlinear loads include equipment that contains semiconductors, such as industrial automation controls, personal computers, photocopiers, energy-efficient lighting, commercial air conditioning units, and adjustable speed drives. When loads are nonlinear, typical average-responding meters read low — with as much as 40% error. This much error can result in a measurement that appears to indicate a properly operating system, while in reality there could be dangerous overheating taking place.

For this reason, the best possible clamp meter for all plant applications is one that measures true RMS (see sidebar titled "But what does true RMS really mean?"). The newest generation of clamp meters has other valuable new functions as well (Fig. 1). These include the ability to make min/max, frequency, and true inrush current measurements.

Before attempting to make electrical measurements or service electrical equipment, ensure you are working safely. Refer to applicable manuals and heed equipment warnings (see sidebar titled "Working safely").

Min/max is a function typically offered in a digital multimeter (DMM). But it's also useful in a clamp meter for measuring load variations. Min/max allows you to measure voltage and current extremes over a period of time without having to monitor the meter closely to record the readings yourself. This function is particularly useful in troubleshooting programmable logic controllers (PLCs) or nuisance trips on breakers that reset mysteriously.

To make a min/max measurement, place the jaws of the clamp meter around the cable, press the min/max button on the clamp meter, and leave it for a period long enough to measure the event that you suspect is causing the problem. The display will give you the minimum and maximum readings taken during that period.

High-end clamp meters also have the ability to measure frequency on variable speed drives and many other types of equipment. An advanced clamp meter provides a better frequency reading than a digital multimeter because its filters allow it to reject the high-frequency components of the pulse-width modulation (PWM) waveform inherent to drives.

To make frequency measurements on a 3-phase motor connected to a variable speed drive, first check the output frequency indicated on the drive's display. Then set the clamp meter to frequency mode by pressing the button or turning the selector switch to a position labeled "Hz" on most clamp meters. Place the clamp around one of the phase conductors between drive and motor; the meter accurately displays the frequency.

It is estimated that more than 78% of the electricity consumed by U.S. industry is used to power electric motors. With the cost of energy on the rise, there is increased urgency to install high-efficiency motors in plants.

While high-efficiency motors consume less electricity than their older, less efficient counterparts, they are more likely to trip the circuit protector when started. The initial startup current, or inrush current, causes the trips. Inrush current can be several times greater than the operating, or steady-state current. For example, in a 3-phase motor, inrush current generally lasts between 75 and 150 msec with a current spike between 500% and 1200% of normal levels. Although short lived, this surge can create problems.

Among the most annoying problems is a nuisance trip of the circuit protector. If the protector is not sized to handle the amount of inrush current that could occur, the device can trip upon energizing the circuit or during circuit operation.

Excessive inrush current may also shorten the life of switches and circuit protectors. Switches are most susceptible since the current spike occurs as the contacts are closed, causing the contacts to become pitted. In severe cases, excess current can weld switch contacts together.

Because of this, precise measurement of inrush current is more than just a convenience — it's a critical element of motor installation and maintenance. Some of the latest clamp meters have the ability to accurately measure inrush current with the touch of a button. They use high-speed, digital signal processing to filter out electronic noise and capture the starting, or inrush, current as the circuit protector sees it.

To make an inrush measurement, first "arm" the inrush function of the clamp meter by pressing the inrush button with the clamp hooked around the conductor. Activate the load and the measurement will be triggered by the inrush current. Once triggered, the meter takes a large number of samples during a 100-msec period. The clamp meter digitally filters and processes these samples to calculate the actual starting current. The result is a highly accurate, synchronous indication of the start current.

As distribution systems and loads become more complex, the possibility of transient overvoltage increases. Motors, capacitors, and power conversion equipment, such as variable speed drives, can be significant generators of spikes. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients.

When taking measurements on electrical systems, these transients are "invisible" and largely unavoidable hazards. They occur regularly on low-voltage power circuits, and can reach peak values in the many thousands of volts. In these cases, the user's protection depends on the safety margin built into the meter. The voltage rating alone will not indicate how well that meter was designed to survive these high-voltage transients.

The International Electrotechnical Commission (IEC) has developed a safety standard for electrical measurement equipment titled IEC 61010-1. This safety standard emphasizes protection against the increasing danger of high-voltage transients. In a plant environment, it's critical to use a meter that is designed to meet this standard and is marked with a relevant category and voltage rating (see sidebar titled "Explanation of IEC category ratings").

Historically, clamp meters have been an indispensable tool in managing electrical loads in plants. But the recent improvements in functionality and form factor have changed these old workhorses into an even more valuable and versatile tool for electrical professionals.

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If you have questions on clamp meters, contact the author. Steve Clark can be reached at 800-443-5853 or fluke-info@fluke.com. Article edited by Jack Smith, Senior Editor, 630-288-8783, jsmith@reedbusiness.com.