If you’ve ever bought electrical equipment, you may have seen that it is often given a safety rating known as ‘CAT’. This is usually defined as one of four categories, and each tells you that the measurement device is suitable in certain situations, but may not be suitable for others.

It is important to have an understanding of each of the different CAT ratings and how exactly each will influence you.

The first thing you need to know is using equipment outside of its designated safety rating can be extremely dangerous. It can also really throw off the accuracy of tests, as it isn’t built for use within that area.

Here’s a breakdown of the ratings category by category.

Measurement Category I (CAT I)

This category refers to anything that is used for measuring voltages from specially protected secondary circuits. Examples of this include signal levels, special equipment, limited-energy parts of equipment, circuits powered by regulated low-voltage sources and other electronics.

These devices are not intended to be connected to the mains supply. It is uncommon to see CAT I referenced on equipment; as the available energy is so low, many manufacturers decided instead to put a rated voltage and current readings.

Measurement Category II (CAT II)

CAT II is used to define circuits which are intended to be connected into mains sockets, or any other similar points. Any energy output must be limited to below 100 A continuously, although it can be changed to 500 A for voltages not exceeding 150V.

Generally these kind of measurements are performed on stuff such as household appliances, portable tools and similar modules.

Measurement Category III (CAT III)

If you need to make measurements of any fixed or hard-wired equipment in an area, you'll need a device suitable for CAT III testing. This includes general wiring, cables, bus bars, junction boxes, switches, socket outlets and any other fixed equipment you may come across.

Measurement Category IV (CAT IV)

The highest level of CAT available - this refers to testing anything that is connected directly to the source of power in a given building. Since there's a chance of arc flash and high levels of energy present, you MUST have a CAT IV device to test this form of equipment.

Examples would include measurements on a cable connecting a power transformer to a building.