7
Safety Notes
Accuracy
We define measurement tolerances in terms of rdg. (reading) and dgt. (digit) values,
with the following meanings:
f.s.
(Maximum display value or scale length/range)
The maximum displayable value or scale length. This is usually the name 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” as the least-significant digit.
Measurement categories
To ensure safe operation of measuring instruments, IEC 61010 establishes safety
standards for various electrical environments, categorized as CAT II to CAT IV, and
called measurement categories.
DANGER
•
Using a measuring 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.
•
Using a measuring instrument without categories in an environment
designated with the CAT II to CAT IV category could result in a
severe accident, and must be carefully avoided.
CAT II:
When directly measuring the electrical outlet receptacles of the primary
electrical circuits in equipment connected to an AC electrical outlet by a
power cord (portable tools, household appliances, etc.)
CAT III: When measuring the primary electrical circuits of heavy equipment (fixed
installations) connected directly to the distribution panel, and feeders from
the distribution panel to outlets
CAT IV: When measuring the circuit from the service drop to the service entrance,
and to the power meter and primary overcurrent protection device
(distribution panel)
T
CAT I V
CAT I I I
CAT I I
Service entrance
Distribution panel
Service drop
CAT IV
CAT III
CAT II
Outlet
Power meter
Fixed installation
Internal wiring
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