HT instruments M71 - Manual (Page 5)

M71
EN – 4
1.4. DEFINITION OF MEASUREMENT CATEGORY (OVERVOLTAGE)
Standards IEC/EN61010-1 (Safety requirements for electrical equipment for measurement,
control and laboratory use, Part 1: General requirements) define what a measurement
category (usually called “overvoltage category”) is. At § 6.7.4: Measuring circuits it quotes:
Circuits are divided into the following measurement categories:
• Measurement category IV is for measurements performed at the source of a low-
voltage installation.
Examples are electricity meters and measurements on primary excess current protection
devices and ripple control units.
• Measurement category III is for measurements performed in the building installations.
Examples are measurements on distribution boards, circuit breakers, wiring, including cables,
bus-bars, junction boxes, switches, socket-outlets in the fixed installations, and equipment for
industrial use as well as some other equipment, for example, stationary motors with permanent
connection to fixed installations.
• Measurement category II is for measurements performed on circuits directly
connected to the low voltage installations.
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
(internally) MAINS-derived circuits. In the latter case, transient stresses are variable; for this
reason, the norm requires that the transient withstand capability of the equipment is made
known to the user.