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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 VoltageCAT IVCAT IIICAT IICAT I
150V4 kV2.5 kV1.5 kV800 V
300 V6 kV4 kV2.5 kV1.5 kV
600 V8 kV6 kV4 kV2.5 kV
1000 V12 kV8 kV6 kV4 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.
Our portable meter test tool RS350 has an CAT III approval for 600 V. Why does it not have CAT IV? The main reason behind are the 4 mm safety sockets on the equipment. Even the reputed European manufacturers for this component are not giving a CAT IV approval.
The sockets are rated with 1000 V CAT III.
So, to declare equipment with safety sockets as CAT IV is not really honest. We had this discussion during the CE certification of our equipment and this question came recently up again.
So, next time you see a meter test equipment CAT IV with laboratory safety sockets, you know which question to put to the manufacturer.

Thank you for reading and stay safe.

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