Learning Library · Multimeters, explained · 4

CAT ratings: measurement categories

A meter rated 1000 V is not automatically safe on a 480 V service entrance. Measurement categories describe the voltage spikes, or transients, that a meter has to survive, and those get bigger the closer you work to the utility supply. Pick where you are measuring and what is there.

Three categories, one idea

Lightning strikes and utility switching send short, very high voltage spikes down the supply. Every transformer, length of wire, and breaker they pass through absorbs some of that energy, so a spike is largest at the service entrance and smallest at the end of a branch circuit. IEC 61010, the safety standard for test equipment, divides an installation into measurement categories by how much energy a spike there can carry. Part 2-030 of the standard defines them.

Category	Where	Examples
CAT IV	The source of the installation	Service entrance, utility meter, primary overcurrent protection, outdoor and underground feeders
CAT III	The building installation	Distribution panels and breakers, bus bars and feeders, the wiring to outlets, hard-wired motors, industrial equipment
CAT II	Utilization points of the installation	Wall outlets and similar points, and the appliances and portable tools plugged into them

For each category and voltage, the standard sets a test transient the meter must survive without harming the user:

Voltage class	CAT II	CAT III	CAT IV
150 V	1,500 V	2,500 V	4,000 V
300 V	2,500 V	4,000 V	6,000 V
600 V	4,000 V	6,000 V	8,000 V
1000 V	6,000 V	8,000 V	12,000 V
Test source impedance	12 Ω	2 Ω	2 Ω

Wall outlets: some guides list outlets under CAT III. IEC 61010-2-030 defines CAT II as circuits connected directly to utilization points, such as socket outlets, so a measurement at the outlet is CAT II. The wiring that feeds the outlet, back to the panel, is CAT III.

The source impedance matters as much as the voltage. A CAT III or CAT IV transient comes from a 2 Ω source, so it can drive six times the current of a CAT II transient at the same voltage. That current is what turns a meter's input into an arc.

Try this

• At a main panel with 480 V, a CAT III 600 V meter is rated for the job: the location can produce 6,000 V and the meter is tested to 6,000 V.
• Move to the service entrance. The same 480 V now needs CAT IV, an 8,000 V transient, and the CAT III 600 V meter is no longer rated for it.
• Select CAT III 1000 V / CAT IV 600 V. Both halves of that rating carry the same 8,000 V transient, which is why they are so often listed together. It covers the 480 V service entrance.
• Take a CAT II 300 V meter to a 120 V plug-in appliance: rated, because the appliance can produce 1,500 V and the meter is tested to 2,500 V. Take it to the 120 V panel that feeds the appliance and it is not, because the panel needs CAT III, 2,500 V.
• Set 1000 V at the service entrance. Only CAT IV 1000 V, a 12,000 V transient, will do.

Reading a meter's rating

A rating is always a pair: a category and a voltage. Treat it as a ceiling in both directions. A higher category at the same voltage is fine, and so is a higher voltage in the same category. UNI-T's UT171B, UT181A, and UT196 handheld multimeters are rated CAT III 1000 V and CAT IV 600 V. The UT197PV is rated CAT IV 1000 V and CAT III 1500 V.

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