The measuring category specifies the permissible areas of application of measuring and testing devices for electrical equipment and installations (e.g. voltage testers, multimeters, VDE test devices) for use in the area of low-voltage power distribution. The classifica- tion of the measuring category is defined by IEC 61010-2-030 (Safety requirements for electrical equipment for measurement, control, and laboratory use - Part 2-030: Particular requirements for test and measurement circuits).
The measurement categories are similar or identical to the overvoltage categories in terms of values, but are based on other standards and describe different circumstances.
The measurement category CAT II to IV is of particular importance for safety in measurements, since low-resistance circuits have higher short-circuit currents and overvoltage’s and transients due to load switching, lightning strikes or phase errors. They must be withstood by the measuring device without endangering the user through electric shock, burns, mechanical hazards, fire, sparking, arcing or explosion. Due to the low impedance of the public power supply network, the short-circuit currents are highest at the house infeed. Within the house installation, the maximum short-circuit currents are reduced by the series of resistances of the installation.
Technically, compliance with the category is achieved, among other things, through contact safety of plugs and sockets, insulation and stable flame-retardant housing, sufficient air and creepage distances, adequate conductor cross-sections and, in particular, a fuse with a high breaking capacity (typically 10 kA at nominal voltage).
The measurement category is marked on the measuring instruments with Roman numerals. If the indication is missing, the device may only be used for measurements “without category” (before category 1 (CAT I).
The following categories and intended uses are defined in IEC 61010-2-030:
|Without measurement category
||Measurements on circuits that have no direct connection to the mains (battery operation), e.g. devices of protection class 3 (operation with safety extra-low voltage), batteryoperated devices, 12...24 V car electrics.
||Measurements on circuits that have a direct connection to the low-voltage mains by means of a plug, e.g. household appliances, portable electrical appliances.
||Measurements within the building installation (stationary consumers with non-pluggable connection, distribution connection, permanently installed devices in the distribution board), e.g. sub-distribution.
||Measurements at the source of the low-voltage installation (meter, main connection, primary overcurrent protection), e.g. meter, low-voltage overhead line, house connection box.
Source: https://de.wikipedia.org/wiki/Messkategorie, translated into English 15.12.2021