KYORITSU KEW 4105A Instruction Manual page 6

Measurement Category
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 over-current protection device
(distribution panel).
Incoming wire
CAT.Ⅳ
Socket
Interior wiring
CAT.Ⅲ
CAT.Ⅱ
̶ 4 ̶
O :Device which is
not directly
connected to the
mains power supply