National Instruments NI 9229 - Manual (Page 26)

NI 9229/9239 26 ni.com
Measurement Category I is for measurements performed on
circuits not directly connected to the electrical distribution system
referred to as MAINS voltage. MAINS is a hazardous live
electrical supply system that powers equipment. This category is
for measurements of voltages from specially protected secondary
circuits. Such voltage measurements include signal levels, special
equipment, limited-energy parts of equipment, circuits powered by
regulated low-voltage sources, and electronics.
Caution Do not connect to signals or use for
measurements within Measurement Categories II,
III, or IV.
Measurement Category II is for measurements performed on
circuits directly connected to the electrical distribution system.
This category refers to local-level electrical distribution, such as
that provided by a standard wall outlet, for example, 115 V for U.S.
or 230 V for Europe.
Caution Do not connect to signals or use for
measurements within Measurement Categories III or IV.