KYORITSU 3127 - Manual (Page 7)

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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 O 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 CAT.II.
O : Circuits which are not directly connected to the mains power supply.
CAT.II : 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).