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You must insulate signal connections for the maximum voltage for which 
the device is rated. Do not exceed the maximum ratings for the device. 
Remove power from signal lines before connection to or disconnection 
from the device. 

National Instruments measurement products may be classified as either 

Measurement Category I or II. Operate products at or below the Measurement Category 
level specified in the hardware specifications.

1

: Measurement circuits are subjected to working 

voltages

2

 and transient stresses (overvoltage) from the circuit to which they 

are connected during measurement or test. Measurement Category 
establishes standardized impulse withstand voltage levels that commonly 
occur in electrical distribution systems. The following is a description of 
Measurement (Installation

3

) Categories:

•

Measurement Category I is for measurements performed on circuits 
not directly connected to the electrical distribution system referred to 
as MAINS

4

 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 by a 
standard wall outlet (e.g., 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 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.

1   

Measurement Categories as 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   

Measurement Category is also referred to as Installation Category.

4   

MAINS is defined as the (hazardous live) electrical supply system to which equipment is designed to be connected for the 
purpose of powering the equipment. Suitably rated measuring circuits may be connected to the MAINS for measuring 
purposes.