CAT II, CAT III and CAT IV ratings

How to read it

A high-energy spike such as a lightning strike is dampened as it travels through the impedance of the wiring, Fluke’s application note ABCs of multimeter safety explains, so the categories step down from the utility connection inward. A higher CAT number means more power available and higher-energy spikes.

In Fluke’s table, CAT IV is the origin of the installation, where the low-voltage connection is made to utility power: electricity meters, primary overcurrent protection, the service drop from pole to building and the run between meter and panel. CAT III is distribution: switchgear, polyphase motors, feeders and distribution panel devices. CAT II is loads plugged into receptacles: appliances, portable tools, and outlets more than 10 m (30 ft) from a CAT III source. Below them is protected electronic equipment, which this note calls CAT 0 and Fluke’s 114 to 117 manual counts as CAT I. Extech’s EX330 manual gives the same examples for CAT IV, electricity meters and primary over-current protection, and household, office and laboratory appliances for CAT II.

Also searched as: CAT II, CAT III 600V, CAT IV, CAT rating, IEC 61010, measurement category, overvoltage category.

The category and the voltage together

The voltage beside the category is the working voltage, and the two together set the spike the meter is tested to survive. In Fluke’s table a CAT III 600 V meter is tested with 6000 V spikes from a 2 Ω source, and a CAT II 1000 V meter with 6000 V spikes from a 12 Ω source, which delivers a sixth of the current; the note says the CAT III 600 V meter offers better protection, although its voltage rating looks lower. Within one category the higher voltage is the higher rating: CAT III 1000 V meters are tested with 8000 V spikes, as are CAT IV 600 V meters. A meter can carry two ratings, as Extech’s EX330 does: CAT II 1000 V and CAT III 600 V.

Where the rating is printed

Fluke’s safety guide says CAT ratings are usually near the input jacks. Test leads carry ratings too: Fluke’s note says IEC 61010-1 applies to test leads, which should be certified to a category and voltage as high as the meter’s or higher.

Different names for the same rating

Fluke’s manuals call them measurement categories, and Extech’s EX330 manual calls them overvoltage installation categories, under IEC 1010. Klein’s MM400 is rated CAT III 600 V, which its manual defines as test and measuring circuits on the distribution part of a building’s low-voltage mains installation.