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Safety Notes
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Measurement categories
To ensure safe operation of measurement instruments, IEC 61010
establishes safety standards for various electrical environments,
categorized as CAT II to CAT IV, and called measurement categories.
CAT II : Primary electrical circuits in equipment connected to an AC
electrical outlet by a power cord (portable tools, household
applianceas, etc.)
CAT II covers directly measuring electrical outlet receptacles.
CAT III : Primary electrical circuits of heavy equipment (fixed
installations) connected directly to the distribution panel, and
feeders from the distribution panel to outlets.
CAT IV : The circuit from the service drop to the service entrance, and to
the power meter and primary overcurrent protection device
(distribution panel).
Using a measurement instrument in an environment designated with a
higher-numbered category than that for which the instrument is rated
could result in a severe accident, and must be carefully avoided.
We define measurement tolerances in terms of f.s. (full scale), rdg.
(reading) and dgt. (digit) values, with the following meanings:
f.s. (maximum display value or scale length)
The maximum displayable value or the full length of the scale.
This is usually the maximum value of the currently selected range.
rdg. (reading or displayed value)
The value currently being measured and indicated on the measuring
instrument.
dgt. (resolution)
The smallest displayable unit on a digital measuring instrument, i.e., the
input value that causes the digital display to show a "1".
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