Understanding Input CAT Ratings for Test Equipment

What are test equipment CAT ratings?

There are several widely recognized CAT (categories) ratings, such as CAT I, CAT II, CAT III, and CAT IV. Each category defines the necessary level of protection and the specific electrical environments where equipment can safely operate. The rating is applicable to test equipment as well as the probes used to connect the equipment to a power source. For example, a multimeter will have a CAT rating. The multimeter probe will also have a CAT rating.

Failure to observe test equipment CAT rating could result in death to the operator and catastrophic equipment damage due to arc and blast.

Tech Tip: The lowest CAT rating defines the level of protection. If a multimeter is CAT IV rated and the probes are CAT III rated, the combination is considered CAT III safe.

Why are CAT ratings necessary?

These categories provide crucial guidelines for both manufacturers and users, ensuring that electrical equipment is rated to handle a range of conditions such as voltage spikes, transient voltages, and other electrical disturbances that could endanger equipment and operators. By specifying the safety level of equipment inputs in terms of voltage endurance that meets established safety standards, these categories help maintain safety and reliability in diverse electrical environments.

What are the test equipment CAT designations?

CAT I is suitable for low-risk circuits such as electronic and battery-operated devices, offering basic insulation and protection against contact with conductive parts.

CAT II encompasses local electrical installations, such as appliances plugged into wall outlets, providing additional protection against overvoltages from the mains supply.

CAT III is designed for distribution systems or fixed equipment directly connected to the distribution panel or service entrance, built to withstand higher-energy transients and surges from the electrical supply.

CAT IV provides the highest level of protection, typically at the source of the installation where transient overvoltages are most severe.

Many Oscilloscopes, Multimeters, and other Test Equipment typically have their CAT ratings indicated near their inputs, or listed in the owner’s manual.

Tech Tip: A multimeter’s internal fuse is a critical component associated with its CAT rating. You must replace the fuse with the manufacturer’s recommended type. Failure to follow this warning could inadvertently lower the meter’s CAT rating. Stated another way, a compromised meter could explode injuring the operator. Under extreme condition it could also cause an arc blast event with catastrophic results.

Example CAT rating for a multimeter

A multimeter with the rating CAT I (1000V), CAT II (600V), CAT III (300V) means it is rated for safety up to 1000 V in a category I environment, up to 600 V in a category II environment, while only up to 300 V in a category III environment. Provided matching CAT rated probes are used.
Image of a meter showing its CAT ratings