Measurement category - a method of classification of live electric circuits used in measurement and testing of installations and equipment, usually in the relation within a building (residential or industrial).
The categories take into account the total continuous energy available at the given point of circuit, and the occurrence of impulse voltages. The energy can be limited by circuit breakers or fuses, and the impulse voltages by the nominal level of voltage.
The definitions for each measurement category are as follows.
There are four categories, which are always stated with the designated voltage, for instance "CAT III, 150 V" or "CAT IV, 1000 V". This has important safety implications for impulse voltages and insulation clearances.
CAT I is applicable to instruments and equipment, which are not intended to be connected to the mains supply.
Because the available energy is very limited, this category is normally not marked on the equipment - instead simply rated voltages and currents are stated (e.g. in multimeters).
Examples: low voltage electronic circuits, load circuits of bench power supplies, etc.
CAT II defines circuits which are intended for direct connection into mains sockets or similar points. The energy in such installations should be limited to below 100 A continuously (or below 500 A for voltages not exceeding 150 V).
The maximum available continuous power must be limited (for instance by a circuit breaker) to not more than 22 000 VA.
Example: a device connected to a 240 V mains socket with 13 A fuse (energy limited to 3100 VA)
CAT III is for circuits which can be connected to the mains installation of a building. Energy is limited by circuit breakers to less than 110 000 VA with the current not exceeding 11 000 A.
CAT IV includes circuits which are connected directly to the source of power for a given building. There are very high levels of available energy (e.g. limited only by the power transformer) and arc flash can occur.
Example: measurements on a cable connecting the power transformer and a building (i.e. before the circuit breakers in the building).
The value of clearance relates to the electrical insulation, and the possibility of arc flash between two electrically energised parts (or between live and grounded parts). Higher voltages require higher clearances. For double insulation the clearances must be doubled.
The required values can vary from 0.04 mm for single insulation CAT II, 50 V, to 28 mm for double insulation CAT IV, 1000 V. The exact values are defined in the international standards. Such standards should be followed rigorously during the design process of the appropriate equipment.
Impulse withstand voltages
Similarly to the clearances the value of required impulse withstand voltage varies from 500 V (CAT II, 50 V), to 12 000 V (CAT IV, 1000 V).
- IEC 61010-20-030, Safety requirements for electrical equipment for measurement, control, and laboratory use - Part 30 Special requirements for testing and measuring circuits