The Switzerland-based International Electrotechnical Commission (IEC) 61010 establishes category and voltage ratings for electrical environments. For test tool users, these category (or CAT) ratings help you determine whether electrical test equipment inputs have been designed to withstand voltage transients.
The Measurement section of IEC 61010 divides power distribution into categories. These categories (or CATs) are based on how a high-energy transient travels through the resistance of the system in question. For example, a higher CAT number refers to an electrical environment with higher power available and higher energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.
The IEC 61010 standard defines Categories in the chart below:
|CAT IV||Three-phase at utility connection, any outdoor mains conductors|
|CAT III||Three-phase distribution, including single-phase commercial lighting|
|CAT II||Single-phase receptacle connected loads|
-Outlets at more than 10 meters (30 feet) from CAT III source
-Outlets at more than 20 meters (60 feet) from CATD IV source
Caption: This table illustrates measurement categories, IEC 1010 applies to low-voltage (<1000 V) test equipment
Luckily, the concept of categories is not new and exotic. It is simply an extension of the same common-sense concept that people who work with electricity professionally apply every day. Here are some shortcuts to understanding categories:
- The general rule of thumb is that the closer you are to the power source, the higher the category number and the greater the potential danger from transients.
- It also follows that the greater the short-circuit current available, the higher the CAT number.
- The greater the source impedance (e.g. a 2 Ohm source allows for 6x the amount of current or power compared to a 12 Ohm source) the lower the CAT number.
The fact is that a well-built multimeter will include a CAT rating and will be certified by an independent test lab. If the meter isn’t rated or certified, there is no way to tell if the meter offers protection against transients and arc flashes. When it comes to testing equipment, it’s critical to err on the side of safety. Select a multimeter rated to the highest category in which it could possibly be used. In some cases, investing in a Cat IV-rated meter eliminates the need to question the CAT level of each measurement.
For more information on Electrical Safety, see our online course available at the Fluke eLearning Center.