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Chapter 1 Introduction
Operate the product at or below the installation category
1
marked on the hardware label.
Measurement circuits are subjected to working voltages
2
and transient stresses (overvoltage)
from the circuit to which they are connected during measurement or test. Installation categories
establish standard impulse withstand voltage levels that commonly occur in electrical
distribution systems. The following is a description of installation categories:
• Installation Category I is for measurements performed on circuits not directly connected to
the electrical distribution system referred to as MAINS
3
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.
• Installation 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 Installation Category II are measurements performed on household
appliances, portable tools, and similar products.
• Installation 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.
• Installation 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
Installation categories, also referred to as measurement 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.
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