National Instruments NI 7350

Chapter 2 Configuration and Installation

Remove power from signal lines before connecting them to or

disconnecting them from the device.

Operate the device at or below the measurement category1 marked on the

hardware label. 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 is a description of

measurement categories:

• Measurement Category I is for 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 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

bya standard wall outlet (for example, 115 AC voltage for U.S. or

230 ACvoltage for Europe). Examples of Installation Category II are

measurements performed on household appliances, portable tools,

and similar devices/modules.

• Measurement Category III is for 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 is for 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.