5
Measurement categories (Overvoltage catego-
ries)
To ensure safe operation of measurement instru-
ments, IEC 61010 establishes safety standards for
various electrical environments, categorized as
CAT I to CAT IV, and called measurement catego-
ries. These are defined as follows.
Higher-numbered categories correspond to electrical
environments with greater momentary energy. So a
measurement device designed for CAT III environ-
ments can endure greater momentary energy than a
device designed for CAT II.
Using a measurement instrument in an environment
designated with a higher-numbered category than
that for which the instrument is rated could result in a
severe accident, and must be carefully avoided.
Never use a CAT I measuring instrument in CAT II,
III, or IV environments.
CAT I Secondary electrical circuits connected to an
AC electrical outlet through a transformer or
similar device.
CAT II Primary electrical circuits in equipment con-
nected to an AC electrical outlet by a power cord
(portable tools, household appliances, etc.)
CAT III
Primary electrical circuits of heavy equipment
(fixed installations) connected directly to the
distribution panel, and feeders from the distribu-
tion panel to outlets.
CAT IV The circuit from the service drop to the service
entrance, and to the power meter and primary
overcurrent protection device (distribution panel).
