Tenmars TM-86 User Manual page 8

 Measurement category III is for 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, for example, stationary motors
with permanent connection to fixed
installation.
 Measurement category II is for measurements
performed on circuits directly connected to the low
voltage installation.
Examples are measurements on household
appliances, portable tools and similar
equipment.
 Measurement category I is for measurements
performed on circuits not directly connected to
MAINS.
Examples are measurements on circuits not
derived from MAINS, and specially protected
(internal) MAINS-derived circuits. In the latter
case, transient stresses are variable; for that
reason, the norm requires that the transient
withstand capability of the equipment is made
known to the user.
EN - 7
TM-86