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CAT I: Secondary electrical circuits connected to
an AC electrical outlet through a
transformer or similar device.
CAT II: Primary electrical circuits in equipment
connected 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 distribution 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).
Higher-numbered categories correspond
to electrical environments with greater
momentary energy, so a measurement
device designed for CAT III environments
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.