Overvoltage categories CAT I-IV (IEC 60664) — not to be confused with the CAT classification of measuring instruments

The measuring-instruments and CAT-category guide covers the CAT classification (CAT I through CAT IV) printed on a multimeter or clamp meter, under the measuring-instrument standard IEC 61010. This article covers a different standard that happens to use the same CAT I-IV terminology, but for a completely different purpose: IEC 60664-1, which describes the overvoltage category of the installation or equipment itself — not of the measuring instrument used to measure on it.

The difference in one sentence

IEC 61010 CAT classification: how well is the measuring instrument protected against transient overvoltages at the location where it is used?
IEC 60664-1 overvoltage category: at what point in the installation is the equipment or the fixed wiring itself located, and what transient overvoltage must the insulation of that equipment be able to withstand?

Both standards use the same CAT I-IV scale and the same logic ("the closer to the grid connection point, the higher the expected transient overvoltage"), but they classify fundamentally different things. A clamp meter with a CAT III rating says nothing about the overvoltage category of the equipment it is being connected to — these are two independent classifications that happen to share the same naming.

The four overvoltage categories of IEC 60664-1

Category	Typical application
CAT I	Equipment connected to a circuit in which measures have been taken to limit transient overvoltages to a low level — for example protected electronics behind their own surge protection.
CAT II	Domestic and similar plug-connected equipment, with individual protection (fuse/circuit breaker) — for example a washing machine or a hand tool.
CAT III	Equipment that is part of the fixed installation itself, inside a distribution board or connected directly to the fixed wiring — for example a fixed distribution cabinet, a fixed industrial motor, or a permanently connected panel.
CAT IV	Equipment and measuring points at or near the grid connection point — for example the main supply itself, the meter-cabinet side of the main switch, or an overhead line/cable directly from the grid.

The closer to the origin of the installation (the grid), the higher the category and the higher the transient overvoltage the equipment's insulation must withstand — a lightning strike or switching operation elsewhere in the grid causes a higher transient voltage peak closer to the source than further into the installation, where cable and equipment impedance already partially damp the peak.

Why the confusion is so common

Because both standards use the same CAT I-IV scale and comparable reasoning, it is easy in practice to assume that a CAT III clamp meter is "therefore" suitable for any CAT III equipment, or that the CAT label on a piece of equipment says something about which measuring instrument may be used on it. That is not the case: the CAT classification of the measuring instrument (IEC 61010) determines the technician's safety while measuring; the overvoltage category of the equipment (IEC 60664-1) determines the required insulation coordination of that equipment itself in its installation position. Both must be correct for the situation at hand, but they are independent assessments.

Practical relevance

When selecting equipment for a given position in the installation (for example a surge protective device, a switch, or electronics placed close to the main supply), the overvoltage category under IEC 60664-1 must be assessed relative to that equipment's position in the installation. This is a separate assessment from — and must not be confused with — choosing a suitably CAT-rated measuring instrument (see the measuring-instruments CAT guide) for taking measurements at that same position.

Common mistakes

Confusing the CAT classification of a measuring instrument (IEC 61010) with the overvoltage category of equipment (IEC 60664-1) — these are two different standards that happen to use the same scale names.
Assuming that CAT III-marked equipment is automatically safe to measure with a CAT III clamp meter — the equipment's classification says nothing about which measuring instrument belongs with it; that follows from the position where the instrument is used to measure, not from the equipment's own CAT label.
Ignoring the overvoltage category of equipment when selecting a surge protective device — a surge protective device designed for a CAT II position provides insufficient protection at a CAT III or CAT IV position closer to the origin.
Assuming the category is only about voltage level — the category concerns the position in the installation relative to the source of transient overvoltages, not the equipment's nominal operating voltage.