The Categories for Electrical Measurements

Sometimes you find in specifications terms like CAT III or CAT IV. These terms are given in IEC 61010-1. The original title is:
Safety requirements for electrical equipment for measurement, control, and laboratory use

The standard specifies general safety requirements for the following types of electrical equipment and their accessories, wherever they are intended to be used:
a) Electrical test and measurement equipment
b) Electrical industrial process-control equipment
c) Electrical laboratory equipment
So you won't find this classification on energy meters, but on portable or stationary test equipment.

The primary categories

CAT I
This category is for measurements of voltages from specially protected secondary circuits. These voltage measurements include signal levels, circuits powered by regulated low-voltage sources and electronics. There is a very little chance for danger or overloading.

CAT II
This category is sufficient for portable measurement tools on household level.

CAT III
This category is qualified for distribution wiring including mains bus bars, feeders and branch circuits.

CAT IV
This category refers to measurements on primary over-current protection devices.

Rated Voltage	CAT IV	CAT III	CAT II	CAT I
150V	4 kV	2.5 kV	1.5 kV	800 V
300 V	6 kV	4 kV	2.5 kV	1.5 kV
600 V	8 kV	6 kV	4 kV	2.5 kV
1000 V	12 kV	8 kV	6 kV	4 kV

The tests for the different categories are done with impulse voltage test equipment.

What do we learn?

First, you will never use test equipment with voltages higher than the rated one for the specific equipment.
For testing of energy meters, we consider a rated voltage of 600 V. As we are operating on distribution level, the equipment needs to have at least CAT III.

Thank you for reading and stay safe.

Editor's note: This article was originally published in January 2021 and has been updated for comprehensiveness.