Detecting intermittents with a DMM

The elusive electrical intermittent is one of the most difficult problems for anyone to troubleshoot. The difficulty lies primarily in the timing. Being able to look at the symptoms while the problem is present simplifies the troubleshooting process considerably. However, the old adage “A watched pot never boils” seems to be applicable to electrical intermittents as well. Being present, as well as being able to take an electrical measurement while the problem exists, are the two biggest challenges in tracking down an intermittent. There are a number of test tools that can help make the intermittent troubleshooting process a little easier. These tools range from complex signal analyzers and storage oscilloscopes to handheld digital multimeters (DMM). Of course, you may not have these tools available or the location of the problem makes it difficult to bring a large analyzer to the problem site. A DMM may be able to tell you a lot about an intermittent without having to go back to the shop and haul that storage scope to the job site.

DMM features for hunting intermittents

Couple the basic measurement features of a DMM (ac volts, dc volts and resistance) with some form of measurement recording ability, and you have a tool for detecting the symptoms of intermittents. Not too long ago, you could buy a voltage or current measuring tool that was built around a mechanical strip chart recorder. Just place the input on a voltage, or clamp a current transformer around a conductor and the recorder would make progressive marks on a strip of paper fed under the marking pen. The maximum length of the recording was determined by the amount of paper that could be placed on a roll of paper. This same strip chart recorder technique has been incorporated into some of today’s DMMs. Fluke’s 170 and 180 Series DMMs have a feature called MIN MAX AVG Recording Mode. Just like the strip chart recorder, the DMM takes a reading of the input at regular intervals. But instead of saving each reading, it compares the reading to two previously stored values to determine if it is higher than previous highest reading or lower than the previous lowest reading. If it is, the new reading replaces the old value stored in high or low reading register. After allowing the recording process to continue for a period of time, you can recall these registers to the display and see the highest and lowest reading taken during the recording time. As an added bonus, these DMMs will also compute and store the average value of all readings taken during a given time period.

TIP: Do not remove the test leads from the circuit being measured until you have either pressed the HOLD button to stop the recording, or looked at and documented all the stored values. Removing the leads while recording will result in the DMM processing the values present on the disconnected leads and affect the AVERAGE value and possibly the lowest or highest stored values taken during the time the leads were connected to the circuit.

Multimeter safety by the numbers

A common misconception in choosing a multimeter is that as long as you choose one with a high enough voltage rating, you’re covered. However, the voltage rating is only part of the story. Engineers who analyze multimeter safety often discover that failed units were subjected to a much higher voltage than the user thought was being measured. This can occur when a meter, rated for low voltage (1000 V or less), is inadvertently used to measure medium voltage. Or, it can result from a momentary high voltage spike or transient that hits the multimeter input without warning. These transients are “invisible” and largely unavoidable but they occur regularly on low-voltage power circuits, and can reach peak values in the many thousands of volts. Your safety depends on the safety margin built into your meter. And that safety margin is based on several specifications beyond just the voltage rating alone.

The key to safety compliance

How can you tell if you’re getting a genuine CAT IV, CAT III, or CAT II meter? It’s not always easy. A manufacturer can self-certify that its meter is at a certain CAT level without any independent verification. The IEC develops and proposes standards, but it is not responsible for enforcing those standards. So the best way to determine that a meter’s CAT certification is genuine is to look for the symbol and listing number of an independent testing lab such as UL, CSA, TÜV or other recognized approval agency. That symbol can only be used if the product successfully completed testing to the agency’s standard, which is based on national/international standards. UL 61010-1, for example, is based on IEC 61010-1. Beware of wording such as “Designed to meet specification ...” Designer’s plans are never a substitute for an actual independent test.

• The general rule-of-thumb is that the closer you are to the power source, the higher the CAT number, and the greater the potential danger from transients.
• The greater the short-circuit current available at a particular point, the higher the CAT number.
• The greater the source impedance, the lower the CAT number. Source impedance (or total impedance) includes the impedance of the wiring between the point where you are measuring and the power source. Source impedance is what dampens transients.
• A transient voltage surge suppression (TVSS) device installed at a panel must have higher energy-handling capacity than one installed right at the computer. In CAT terminology, the panel board TVSS is a CAT III application, and the computer is a receptacle-connected load and therefore, a CAT II installation.
• A single piece of equipment may have more than one category. For example, in office equipment, from the 120 V/240 V side of the power supply back to the receptacle is CAT II. The electronic circuitry, is CAT 0. In building control systems, such as lighting control panels, or industrial control equipment such as programmable controllers, it is common to find electronic circuits (CAT 0) and power circuits (CAT III) existing in close proximity. Always select a multimeter rated to the highest category in which it could possibly be used.

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