6 Keysight U1401B User’s and Service GuideMeasurement CategoryThe Keysight U1401B has a safety rating of CAT II 150 V.Measurement CAT I Measurements performed on circuits not directlyconnected to the AC mains. Examples are measurements on circuits not derivedfrom the AC mains and specially protected (internal) mains-derived circuits.Measurement CAT II Measurements performed on circuits directly connectedto a low-voltage installation. Examples are measurements on householdappliances, 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 fixedinstallation, and equipment for industrial use, and some other equipmentincluding stationary motors with permanent connection to the fixed installation.Measurement CAT IV Measurements performed at the source of thelow-voltage installation. Examples are electricity meters and measurements onprimary overcurrent protection devices and ripple control units.