Which category rating would a meter need to have to take voltage readings at the service switchgear of an industrial facility?

• A. CAT II
• B. CAT III
• C. CAT I
• D. CAT IV

Answer: (B)

Category ratings describe how much transient voltage a meter can safely withstand in different parts of an electrical system. The service switchgear sits in the distribution portion of a facility’s electrical system—the point where power enters the building and is distributed to feeders and equipment. In this area, switching events, motor starts, and other load changes can generate noticeable transients. A meter used here needs a rating that can tolerate those transients, which is why CAT III is appropriate.

CAT II covers end-user outlets and portable loads, CAT I is for low-energy signal circuits, and CAT IV applies to measurement at the utility service entrance or other supply-origin points. Measuring inside the distribution network of the facility calls for CAT III, balancing safety with relevance to the actual installation environment.