While a
person is conducting tests with an instrument on live line supply, there is a possibility
of a sharp rise in voltage for a short duration.
This may
result in an arc or flash between measurement terminals of the testing device.
In addition, if a heavy flash occurs, it may critically injure a person
handling the instrument.
To safeguard
the person using the measuring instrument and to classify the various instruments
as per the application they are used in, the IEC has classified instruments in the
following categories:
• Category
IV: Distribution systems, service connections, and primary overcurrent protection
for larger installations
• Category
III: Three-phase and single-phase distribution within a premises
• Category
II: Appliances, lighting points, socket-outlets
• Category
I: Transient-protected electronic equipment.
The IEC
Standard 61010 provides guidelines for manufacturers to follow safety norms for
testing devices. It is to be noted that irrespective of their maximum voltage rating,
a Category IV device provides a greater degree of transient protection than Category
III, etc.
The Category
III device is suitable for most of the testing undertaken by electricians.
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