National Instruments NI 781xR

Chapter 1 Introduction

Operate the NI781 xR at or below the measurement category1 listed in the

Environmental section of AppendixA, Specifications. Measurement

circuits are subjected to working voltages2 and transient stresses

(overvoltage) from the circuit towhich they are connected during

measurement or test. Measurement categories establish standard impulse

withstand voltage levels that commonly occur in electrical distribution

systems. The following list describes installation categories:

•Measurement Category I—Measurements performed on circuits not

directly connected to the electrical distribution system referred to as

MAINS3 voltage. 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.

•Measurement Category II—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).

Examples of Measurement Category II are measurements performed

on household appliances, portable tools, and similar products.

•Measurement Category III—Measurements performed in the

building installation at the distribution level. This category refers to

measurements on hard-wired equipment such as equipment in fixed

installations, distribution boards, and circuit breakers. Other examples

are wiring, including cables, bus-bars, junction boxes, switches,

socket-outlets in the fixed installation, and stationary motors with

permanent connections to fixed installations.

•Measurement Category IV—Measurements performed at the

primary electrical supply installation (<1,000 V). Examples include

electricity meters and measurements on primary overcurrent

protection devices and on ripple control units.

1 Measurement categories, also referred to as installation categories, are defined in electrical safety standard IEC 61010-1.

2 Working voltage is the highest rms value of an AC or DC voltage that can occur across any particular insulation.

3 MAINS is defined as a hazardous live electrical supply system that powers equipment. Suitably rated measuring circuits may

be connected to the MAINS for measuring purposes.