6 Keysight U1281A/U1282A Service Guide
Measurement Category
The U1281A/U1282A has a safety rating of CAT III, 1000 V and CAT IV, 600 V.
Measurement CAT I   Measurements performed on circuits not directly 
connected to the AC mains. Examples are measurements on circuits not derived 
from the AC mains and specially protected (internal) mains-derived circuits.
Measurement CAT II   Measurements performed on circuits directly connected 
to a low-voltage installation. Examples are measurements on household 
appliances, portable tools, and similar equipment.
Measurement CAT III   Measurements performed in the building installation. 
Examples are measurements on distribution boards, circuit- breakers, wiring, 
including cables, bus-bars, junction boxes, switches, socket outlets in the fixed 
installation, and equipment for industrial use, and some other equipment 
including stationary motors with permanent connection to the fixed installation.
Measurement CAT IV   Measurements performed at the source of the 
low-voltage installation. Examples are electricity meters and measurements on 
primary overcurrent protection devices and ripple control units.