Keysight U1251B Guia Del Usuario página 7

Measurement Category
The Keysight U1251B and U1252B Handheld Digital Multimeter has a safety
rating of CAT III 1000 V/ CAT IV 600 V.
Measurement CAT I are measurements performed on circuits which are not
directly connected to the AC mains. For example, measurements on circuits not
derived from the AC mains or specially protected (internal) mains-derived circuits.
Measurement CAT II are measurements performed on circuits that are directly
connected to a low voltage installation. For example, measurements on
household appliances, portable tools and other similar equipment.
Measurement CAT III are measurements performed on building installations.
For example, measurements on distribution boards, circuit-breakers, wiring
(including cables), bus-bars, junction boxes, switches, socket outlets within the
fixed installation, equipment for industrial use and equipment that is permanently
connected to the fixed installation such as stationary motors.
Measurement CAT IV are measurements performed at the source of the
low-voltage installation. For example, measurements on electricity meters,
primary over-current protection devices and ripple control units.
Keysight U1251B and U1252B User's and Service Guide 7