National Instruments Compact FieldPoint cFP-1808 Quick Start Manual page 19

Operate Compact FieldPoint products at or below the measurement category
marked on the hardware label. Measurement circuits are subjected to working
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voltages and transient stresses (overvoltage) from the circuit to which 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 MAINS
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 (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 is for measurements performed in the building
installation at the distribution level. This category refers to measurements on
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Measurement categories are defined in electrical safety standard IEC 61010-1.
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Working voltage is the highest rms value of an AC or DC voltage that can occur across any particular
insulation.
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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.
cFP-1808 Quick Start Guide 18
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