KYORITSU 2009R - Manual (Page 6)

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○Measurement categories (Over-voltage categories)
To ensure safe operation of measuring instruments, IEC61010
establishes safety standards for various electrical environments,
categorized as CATⅠ to CATⅣ, and called measurement
categories. Higher-numbered categories correspond to electrical
environments with greater momentary energy, so a measuring
instrument designed for CATⅢ 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 over-current protection
device (distribution panel).