What is an “Adequately Rated” Meter According to NFPA 70E?

NFPA 70E is a vital standard for establishing practical and safe workplaces in order to avoid electrical hazards. Within NFPA 70E, the phrase “adequately rated meter” pops up frequently. The term “adequately rated” is often misunderstood throughout industries that follow NFPA 70E and use electrical testing meters. Read on to learn what “adequately rated” means and how to properly select the right meter for your work.

Note: This blog offers an in-depth exploration of Step 7 within NFPA 70E, Section 120.6: Process for Establishing and Verifying an Electrically Safe Work Condition. Our comprehensive coverage of Section 120.6 can be found here, or view the entire training series here.

To begin, let’s review Step 7 of Section 120.6:

Use an adequately rated portable test instrument to test each phase conductor or circuit part at each point of work to test for the absence of voltage. Test each phase conductor or circuit part both phase-to-phase and phase-to-ground.”

Now, let’s dive into what the term “adequately rated” means when selecting electrical testing meters.

Meter Categories

Electrical testing meters are classified into categories ranging from CAT 1 to CAT 4. A basic description of each meter is available below. For more detailed descriptions, refer to the chart in the video.

  • CAT 1 meter: Suitable for small electrical jobs, such as TV and electronics repair. These meters are readily available to the public through common big-box retailers. They are not appropriate for commercial or industrial use.
  • CAT 2 meter: Suitable for work related to single-phase receptacle connected loads. These meters are also available from common big-box retailers. They are also not appropriate for commercial or industrial use.
  • CAT 3 meter: Suitable for three-phase distribution, including single-phase commercial lighting and appliances. This meter covers most tasks needed for commercial or industrial use. However, this meter is not suited for close-range work with a power source or main service due to the available fault currents.
  • CAT 4 meter: Suitable for tasks related to three-phase utility connections, or any kind of high incident energy, such as outdoor conductors or large transformers.

Choosing a Meter

Based on the categories above, using an “adequately rated” meter means using a meter with a rating that’s suitable for the voltage requirements of the task you’re performing.

To choose the correct meter, determine the CAT level at which you’re operating. Then, select a meter that has a higher voltage rating than the maximum voltage you’re testing. A higher CAT number indicates  an electrical environment with a higher power threshold and higher voltage transients. For example, a CAT 3 meter is resistant to higher voltage transients than a CAT 2 meter.

Note: When selecting a meter, use caution – not all meters are created equal. Meters often have dual ratings, such as CAT 1/2. As shown in the video, the example CAT 1/2 meter is rated CAT 1 at 1000V and CAT 2 at 600V. In comparison, a CAT 3 meter is rated higher at 600V, indicating greater resistance to voltage transients.

What Happens if I Use a Meter That’s Not Rated Properly for My Work?

Electrical equipment maintenance often requires juggling multiple electrical measurement types, such as AC volts, DC volts, ampacity, and continuity ohms. For example, troubleshooting equipment can involve checking fuses, continuity on a motor starter, or contacts on a speed drive – tasks that  require switching your meter to the correct measurement.

Imagine you are testing equipment with a CAT 1 meter that lacks built-in protection. When switching to ohms and getting back on the voltage, an arc flash is extremely likely, as shown in the video. The meter can explode, causing an arc flash and significantly increasing your chance of burns, injury, or even death.

Additionally, meter explosions can release carbon dust, copper vapor, or ionized air – all of which are conductive. So, when a single explosion occurs near any contacts with large potential, the resulting arc flash can expand and create a much larger explosion.

For an example, consider an elevated power line along a roadside. If an arc flash happens on one phase of the power line, the resulting explosion and debris can hit one of the other phases above. This chain reaction creates a phase-to-phase explosion, intensifying and expanding the arc flash.

Bottom Line

Make sure you’re using a meter that matches the voltage requirements of the work being performed. Pay close attention to the CAT rating on your meter and make sure it has a higher voltage rating than the maximum voltage you’re testing. Also, test your meter frequently to ensure correct operation.

If you’re interested in exploring NFPA 70E further, Tyndale’s “NFPA 70E Made Easy” video series is our educational resource hub dedicated to this essential standard, offering comprehensive insights and guidance. The series delves into the intricacies of NFPA 70E, a vital tool since 1979 for establishing practical and safe workplaces.

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