In electrical testing, the safety of the user is of paramount importance. In order to ensure safety, we need to understand the capabilities of the system we are dealing with and knowledge of the electrical system, and we need to have the corresponding technical knowledge when using electrical test equipment.

Purpose of the 1. category rating system

The category rating system is designed to protect users from the unpredictable effects of high-energy pulses (transients) caused by grid or load failures. The risk to life and safety of these transients, such as lightning strikes, transients and surges, depends on where the test is performed within the power path. The category is related to the energy in the pulse, not just the peak voltage it reaches.

2. IEC 61010-031

IEC 61010-031 (or equivalent national product safety standard) defines 3 measurement categories, from CAT II to CAT IV. The higher the rating, the more stringent the safety requirements for measuring accessories. In other words, the higher the CAT rating, the more energy the device must withstand and protect the user from transients. It should be noted that IEC 61010-031 is only applicable to low-voltage test equipment with a rated voltage up to 1000V.

3. measurement categories and examples

●CAT II

Purpose: Measurement of circuits directly connected to low-voltage device sockets.

Safety requirements: There should be at least three levels of overcurrent protection between these circuits and the power transformer.

Example: Measurement of power supply nodes in household appliances, portable tools, and similar devices.

●CAT III

Purpose: Measurement of the distribution part of the building's low-voltage power supply device.

Safety requirements: There shall be at least two levels of overcurrent protection between these points and the power transformer.

Examples: measurements on distribution boards, circuit breakers, wiring (including cables, bus bars, junction boxes, switches, sockets in fixtures), as well as industrial equipment and some other equipment (for example, fixed motors permanently connected to fixtures).

●CAT IV

Purpose: Measurements at the source of a low-pressure installation in a building.

Safety requirements: There shall be at least one level of overcurrent protection between these points and the power transformer.

Example: Measurement of electric meters and primary overcurrent protection devices and ripple control units.

No Rated Measurement Category (CAT I)

Purpose: Used for measurements on circuits that are not directly connected to the main power supply.

Example: Special protection (internal) circuits from power supply, telephone, vehicle electrical devices.

The following table shows that there are predefined operating voltages of 50V, 100V, 150V, 300V, 600 V and 1000 V in each category. The higher the voltage in a given class, the higher the level of protection required. For measurements below CAT II, no transient protection is recommended.

Safe use of 4. measuring accessories

● CAT rating marking:

All measurement accessories suitable for category rated test circuits are marked with CAT rating.

CAT ratings are used to indicate the safe range of use of a probe or measurement accessory.

● Class rating of active differential probes:

Active differential probes clearly indicate the class rating and its associated voltage rating in the probe and specifications.

Taking the CAT II 1000V rated voltage probe as an example, although its transient protection voltage is the same as the CAT III 600V rated voltage probe, the two are not equivalent.

Each probe is designed for a specific CAT grade and does not have a higher grade.

● Relationship between the rated test product and the marked rating:

Some probes may be rated for test products higher than the rating marked on the probe, but this does not mean that they can be used at a higher CAT rating.

● Features of CAT III products:

CAT III products are designed with higher insulation, creepage distance and clearance distance to provide higher safety.