Measurement category


Measurement category is a method of classification by the International Electrotechnical Commission of live electric circuits used in measurement and testing of installations and equipment, usually in the relation within a building.
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.

Measurement categories

Measuring circuits are subjected to working voltages and transient stresses from the circuit to which they are connected during measurement or test. When the measuring circuit is used to measure mains, the transient stresses can be estimated by the location within the installation at which the measurement is performed. When the measuring circuit is used to measure any other electrical signal, the transient stresses must be considered by the user to assure that they do not exceed the capabilities of the measuring equipment. In this standard, circuits are divided into the following measurement categories:

CAT I

Category I is for measurements performed on circuits not directly connected to mains.
Examples are measurements on circuits not derived from mains, and specially protected mains-derived circuits. In the latter case, transient stresses are variable; for that reason IEC 61010-1-5.4.1 requires that the transient withstand capability of the equipment is made known to the user.

CAT II

Measurement category II is for measurements performed on circuits directly connected to the low-voltage installation.
Examples are measurements on household appliances, portable tools and similar equipment.

CAT III

Measurement category III is for measurements performed in the building installation.
Examples are measurements on distribution boards, circuit-breakers, wiring, including cables, bus-bars, junction boxes, switches, socket-outlets in the fixed installation, and equipment for industrial.

CAT IV

Measurement category IV is for measurements performed at the source of the low-voltage installation.
Examples are electricity meters and measurements on primary overcurrent protection devices and ripple control units.

Clearances

The value of clearance relates to the electrical insulation, and the possibility of arc flash between two electrically energised 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, to 12,000 V :
Working VoltageTransient OvervoltageTransient OvervoltageTransient OvervoltageTransient Overvoltage
CAT IVCAT IIICAT IICAT I
150 V4,000 V2,500 V1,500 V800 V
300 V6,000 V4,000 V2,500 V1,500 V
600 V8,000 V6,000 V4,000 V2,500 V
1000 V12,000 V8,000 V6,000 V4,000 V
Source Impedance2 Ohms2 Ohms12 Ohms30 Ohms