v
Safety
We define measurement tolerances in terms of rdg.
(reading) and dgt. (digit) values, with the following
meanings:
rdg. (reading or displayed value)
The value currently being measured and indicated
on the measuring product.
dgt. (resolution)
The smallest displayable unit on a digital measuring
product, i.e., the input value that causes the digital
display to show a "1".
Overvoltage Categories
To ensure safe use of measurement, IEC 60664
establishes safety level standards for different
locations, classified as CAT I through CAT IV, and
called overvoltage categories. These are defined as
follows.
CAT I: Secondary electrical circuits that are
connected to a wall outlet through a
transformer or similar device.
CAT II: Primary electrical circuits in equipment
connected to a wall outlet via a power
cord (portable tools, household appliances,
etc.)
CAT III: Primary electrical circuits of heavy
equipment (fixed installations) connected
directly to the distribution panel, and
feeders between the distribution panel and
outlets.
CAT IV: The circuit from the service drop to the
service entrance, then to the power meter
and to the primary overcurrent protection
device.
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