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.