Training Trends: Know the Rating

May 19, 2003
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Last month, Training Trends discussed the someArial deadly effects of failed electrical measurement equipment. This month, we'll take a look at the measurement tool category rating and how to determine which equipment should be used where.

Before starting each job, take a minute to check the category rating and certification label of your digital multimeter or other test tool. It's not just a good work practice--it could save your life. Each tool should conform to IEC (International Electrotechnical Commission) 1010, an international standard developed to help design test tools that will stand up to today's electrical test environment.

The front of the meter should carry a designation of its CAT (category) overvoltage rating. Some also carry the marks of independent laboratories guaranteeing they meet IEC standards. Those standards are largely voluntary, and many meters claim to meet the standards but don't. That's why the seal of an independent lab is worth the extra cost.

Ranging from Category I, dealing with low energy circuits to Category IV, covering installations with lines exposed to outdoor environments, the rigorous IEC standards are designed for safety. Meters designed to the standards can withstand hazards caused by transients and other dangers. Be sure your measurement tool category rating matches how you are using the tool, even if that means switching from meter to meter throughout the day. Here's a look at the category ratings:

Category I--typically covers electronic equipment

  • Protected electronic equipment
  • Equipment connected to source circuits in which measures are taken to limit transient overvoltages to an appropriately low level
  • Any high-voltage-low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a copier.

    Category II--single-phase receptacle connected loads

    • Appliances, portable tools and other household and similar loads
    • Outlet and long branch circuits
    • Outlets at more than 10 meters from CAT III source
    • Outlets at more than 20 meters from CAT IV source.

    Category III--three-phase distribution, including single-phase commercial lighting

    • Equipment in fixed installations, such as switchgear and polyphase motors
    • Bus and feeders in industrial plants
    • Feeders and short branch circuits, distribution panel devices
    • Lighting systems in larger buildings
    • Appliance outlets with short connections to service entrance.

    Category IV--three-phase at utility connection, any outdoor conductors

    • Origin of installations, such as where low-voltage connection is made to utility power
    • Electricity meters, primary overcurrent protection equipment
    • Outside and service entrance, service drop from pole to building, run between meter and panel
    • Overhead line to detached building, underground line to well pump.

    Category II conditions are most prevalent, but that shouldn't lull those testing electricity lines and sources into complacency. Moving from inside a house or garage to outside or into an industrial setting, probably means dealing with Category III or Category IV. Large industrial motors are in Category IV territory.

    The bottom line is matching the meter to the application. One meter doesn't fit all applications.

    Rosemary Baisch is a product marketing manager with Fluke Corp. (Everett, WA). For more information, she can be contacted at rosemary.baisch@fluke.com.

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