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Locations that can and cannot be measured (measurement categories and CAT labeling)


Just because an instrument has a measurement range of 1000 V doesn’t mean it can be used to make measurements anywhere. Instrument specifications define not only a measurement range, but also specific types of locations where the device can be used. Such specifications are put in place because the magnitude of the sudden voltage (transient overvoltage) that could occur in any given application depends on the location being measured.

 

Approach to measurable locations

Consider the difference between measuring 200 V at a distribution panel and 200 V at a power outlet.
You might assume that only the location being measured is different, and that any instrument could be used to make the measurement. However, distribution panels, which are located closer to the supply of power, are more likely to experience a sudden voltage (transient overvoltage). Examples of possible events include a motor starting to operate, a switch operating, and lightning strike. Transient overvoltages also occur at power outlets, of course, but they involve lower voltages than overvoltages at distribution panels.
Measurement locations are classified using a system of measurement categories based on the magnitude of transient overvoltage that could occur.

Where is the measurement category noted?

Ordinarily, the measurement category (“CAT”) is noted together with a line-to-neutral (line-to-ground) voltage value in specifications, on instrument terminals, on clamp sensors, and on probe tips.

This current clamp meter has the following specifications:

In this way, measurement categories indicating locations where the instrument can be used are noted alongside line-to-neutral voltages.
Instruments are designed to withstand the overvoltages that are possible in the indicated locations. By contrast, using an instrument with the wrong measurement category poses the risk of an accident.

Examples of measurement categories

Let’s review some typical examples before proceeding to an explanation of measurement categories and line-to-neutral voltages.

Measurement location Measurement category Line-to-neutral voltage Required specifications
100 V power outlet Outlets are a CAT II location. 100V CAT II 300 V or higher
Distribution panel (single-phase) circuit breaker Distribution panels are a CAT III location. 100V CAT III 300 V or higher
Three-phase power supply in a electrical room Electrical rooms are a CAT IV location. 240V CAT IV 300 V or higher
Digital circuit (5 V) in an electronic product Products’ internal circuits are CAT III locations. 5V CAT II 300 V or higher

Measurement category

Measurement locations are classified as one of three categories depending on the anticipated transient overvoltage: CAT II, CAT III, and CAT IV.
CAT II locations are on the consumption side, while CAT IV locations are on the supply side, and CAT IV instruments provide a higher level of safety.

CAT II (measurement category II) From the power plugs of devices that are connected to power outlets to the devices’ power supply circuits
CAT III (measurement category III) Power wiring and power supply circuits of devices that are connected directly to a distribution panel (for example, permanently installed equipment) and wiring from distribution panels to wiring terminals on the back side of power outlets
CAT IV (measurement category IV) Buildings’ electric service drops and circuits connecting service drops to electric meters and distribution panels

Never use an instrument that does not satisfy CAT requirements as doing so could lead to a serious accident.

Line-to-neutral voltage

Instruments are classified based on the line-to-neutral voltages that occur at measurement locations. Ordinarily there are three types–300 V, 600 V, and 1000 V–although other figures are occasionally used.
The key factor to note here is that voltages are given relative to neutral. A 3-phase/4-wire 400 V circuit has a line-to-line voltage of 415 V, but a line-to-neutral voltage of 240 V.


3-phase/4-wire


Safety standards set forth the transient overvoltage that an instrument must be able to withstand based on a measurement category and line-to-neutral voltage, as summarized in the table below.

Line-to-neutral voltage [V] Transient overvoltage value [V]
CAT II CAT III CAT IV
300 2500 4000 6000
600 4000 6000 8000
1000 6000 8000 12000

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