The myths of instruments and safety
Well worth a look.

              http://www.pacifictest.com.au/documents/April_May.pdf

Goes well beyond the usual  ... CAT 3, CAT 4 meter discussion and
covers points easily missed.

______________

Well known (or should be).
Worth repeating:

BEWARE OF GLASS FUSES
Never be tempted to use glass fuses in any instruments used on live
mains, even as a temporary measure. They have virtually no fault
clearing capacity and in the situation above, the instrument is likely
to explode in your hand!

_____________________

Myth 1 of 5

MYTH NO 1
A CAT 4, 600V INSTRUMENT IS REQUIRED FOR LIVE MAINS
The perception is, an instrument used on an unprotected 240/415V
network, a Cat 4, 600V (600V being the closest rating to 415V)
instrument is the appropriate choice =96 WRONG!
The voltages which IEC61010 uses for the Cat and Voltage are
PHASE to NEUTRAL (or earth) voltages, not phase to phase as often
thought! Therefore to work on the unprotected 240/415V (or
230/400V) system requires a Cat 4, 300V instrument!
This all becomes clear in the table from IEC61010 (Figure 2).
There are actually two voltage columns, one phase to neutral, the
other phase to phase. IEC has elected to quote the phase to neutral
voltages to identify the ratings.
The table also shows that to achieve the various ratings,
instruments are required to withstand a nominated transient overvoltage.
We can also see that several ratings share a common
transient over-voltage. e.g. Cat 3, 1000V and Cat 4, 600V both
have to withstand a transient over-voltage of 8kV!
Similarly a Cat 3, 600V and a Cat 4, 300V instrument both have
to withstand 6kV. Some instruments are marked with dual ratings
but if not, an instrument marked Cat 3, 600V can be used in a
Cat 4, 300V situation with
complete safety. Both must
withstand the same 6kV
transient over-voltage.
Persons misinterpreting
the ratings will drastically
reduce their choice of
instruments and will find that
some types of instruments
are not manufactured with
unrealistic ratings!

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