Electrical Safety Requirements for Using DMMs

Electrical Safety Requirements for Using DMMs

Learn about how to audit digital multimeter safety practices to help satisfy NFPA 70E requirements and comply with OSHA’s definition of qualified persons.

From Fluke Corp.

When performing the routine tasks of maintenance and troubleshooting, a technician might place a test probe on a live, energized terminal, leaving fingertips perhaps only an inch or so from this terminal. Should that test lead or the test tool it is connected to fail, or should a fault occur while the technician is observing a reading, or even if the technician makes a human error, the results can be deadly.

Engineers, electricians, and technicians must follow electrical safe work practices when it comes to using multimeters, including inspection before use. The most effective method to help achieve safety, and also the method required by the Occupational Safety and Health Administration (OSHA), is for employees to demonstrate their ability to select, inspect, use and maintain their test equipment.

Procedural Guidance

OSHA regulations and the NFPA 70E: Standard for Electrical Safety in the Workplace® provide procedural guidance for test equipment inspection. For example, NFPA 70E® states that only “qualified persons” are allowed to perform tasks that include the use of test equipment on systems 50V and greater. The digital multimeter (DMM) is the most commonly used instrument.

Since 2007, OSHA regulations require technicians to “demonstrate” their skills to their employer to be considered a qualified person. Thus, employers must verify an individual’s ability to safely use DMMs.

Properly Rated for the Circuit

A DMM must be properly rated for the circuit on which it is to be applied, and technicians must be able to explain these ratings. This includes ratings of any test probes, flexible clamps or other accessories to be used.

The first step is to identify the nominal system voltage of the circuit to be tested. This is the voltage class assigned to systems and equipment and can be found on nameplates and drawings. Typical nominal voltages found in plants are 120/240, 208Y/120 and 480Y/277.

Technicians need to be aware of the extreme danger of using an inadequately rated DMM. Using a 1,000V-rated DMM on medium-voltage circuits has, unfortunately, happened more than once with tragic results.

Nationally Recognized Testing Laboratory (NRTL)

In addition to proper voltage and current ratings, test tools must be listed by a Nationally Recognized Testing Laboratory (NRTL) and properly labeled with the NRTL’s mark. OSHA lists which NRTLs have been approved to test and verify that DMMs meet consensus-based standards.

This testing reasonably assures that products are safe for use. Once the equipment meets the testing laboratory’s criteria, the tool can be labeled with the NRTL’s recognized mark. Any test equipment without such a label should not be used.

Among the most common testing laboratory marks found on DMMs are Underwriters Laboratories Inc. (UL), Canadian Standards Association (CSA) and TUV. Technicians must demonstrate their ability to recognize and identify these labels and explain their importance.

CAT Ratings

Electrical workers must also be familiar with the Category Rating, or CAT, found on DMMs. The CAT rating indicates the multimeter’s ability to withstand transient overvoltage conditions that could destroy the meter and injure personnel.

DMMs used in distribution systems should be at least CAT III rated. CAT IV offers a greater degree of protection. Most industrial DMMs are rated CAT III for use on 1,000V and below systems and CAT IV for use on 600V and below systems. Electrical workers should be able to identify the CAT rating needed for their job.

Designed for Environment and Use

Technicians must verify that the test instruments and their accessories are designed for both the environment and the manner in which they’ll be used. For example, when examining a DMM for proper design, ask, “Will this DMM be used in a hazardous location?” When taking voltage readings, it’s possible for a very small electrical arc to be drawn when placing a test probe on, or removing a test probe from, an energized point.

Using a procedural checklist to demonstrate proper digital multimeter selection, inspection and usage is a best practice to help verify only qualified persons are performing electrical measurement activities. Click the image to enlarge and download a sample checklist. [CLICK IMAGE TO ENLARGE OR DOWNLOAD]

The National Electrical Code® (NEC®) identifies environments as Hazardous (Classified) locations if explosive atmospheres are present. Intrinsically safe DMMs are designed for use in such locations and technicians must look for such identification if applicable.

This also is a good reason to place and remove test probes at a ninety-degree angle to the terminal, and not let the probe “slide” from one terminal to another.

Visual Inspection

The visual inspection must include not only the test tool itself, but all associated test leads, cables, power cords, probes, and connectors. Look for any obvious external defects. It is not uncommon to find damaged test leads or probes, which must be replaced before use.

One good method is to slowly pull test leads between your fingers as you perform a visual inspection of the lead. The fingers often can feel damaged insulation even if you miss seeing them. All test leads should have a shroud around the end that inserts into the DMM. This prevents accidental shock should the test lead become unplugged from the test tool while the probe is still on an energized component.

Test probes — both voltage and current probes — will have a voltage and category rating. The IEC symbol for “double-insulated” (one square box inside of another) indicates that one single insulation failure will not result in personnel being exposed to dangerous shock levels.

Don’t discount the use of clamps, flex clamps and test probes for current measurements for visual inspections. Such devices should be marked with a maximum current rating. They also should have the NRTL label. Many test probes are double insulated and marked with the double insulated symbol.

Check the Condition

Never hesitate to remove a tool from service if there’s any question about its condition. Make sure some method, such as tagging, is used so that someone else doesn’t inadvertently use the defective test equipment before repairs are completed. A simple item — a frayed copper strand protruding from a test lead — can put you in the hospital!

Consider the need for a protective case to protect the equipment from mechanical shock and general rough handling during a day’s work. A storage case may also be needed for more complete protection when transporting or storing the test tool.

It would be foolish to use a DMM that didn’t operate correctly. Daily, before first use, DMMs should be verified to operate properly on voltage by testing on a known voltage source, such as a receptacle. Before measuring, short together resistance test leads to verify proper operation in the resistance function. This quick test also verifies continuity of test leads. Make sure a low battery warning doesn’t appear on the display during this check.


Using a procedural checklist to demonstrate proper DMM selection, inspection and usage is a best practice to help verify only qualified persons are performing electrical measurement activities. Auditing DMM safe work practices helps satisfy NFPA 70E requirements for test equipment and complies with OSHA’s definition of qualified persons. A technician who can successfully demonstrate his or her ability to select, inspect and properly use a DMM has taken a significant step in making a safe return home at the end of the day.

Fluke Corp. is a participating EncompassProduct Partner in the Rockwell Automation PartnerNetwork™ program. Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.

The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.


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