KYORITSU 3126 - Manual (Page 6)

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○ Measurement categories(Over-voltage categories)
To ensure safe operation of measuring instruments, IEC 61010
establishes safety standards for various electrical environments,
categorized as CAT I to CAT IV, and called measurement
categories.
Higher-numbered categories correspond to electrical environments
with greater momentary energy, so a measuring instrument
designed for CAT III environments can endure greater momentary
energy than one designed for CATII.
CAT I : Secondary electrical circuits connected to an AC
electrical outlet through a transformer or similar device.
CAT II : Primary electrical circuits of equipment connected to an
AC electrical outlet by a power cord.
CAT III : Primary electrical circuits of the equipment 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).