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What do the input CAT ratings mean?

Last Updated: May 01, 2014 12:43PM PDT

Q:  What do the input CAT ratings mean as labeled on various test instruments?

A:  Many test instruments such as oscilloscopes, multimeters, and component testers often have labels near or next to their inputs that indicate CAT following a roman numeric number.  This label generally indicates the safety level of the inputs, based on the amount of voltage it can withstand that meets safety requirements.  For example, is is commong to see a multimeter with the label CAT I (1000V), CAT II (300V).  This means, it is rated for safety up to 1000 V in a category I environment, while only 300 V in a category II environment.  Below is a table that describes each category.  They represent different power safety environments from inside to outside a building.

Category Description
CAT I At the signal level, parts of an electronic equipment
CAT II Portable equipment, appliances, at local level environment
CAT III At a fixed installation, distribution level of AC main power
CAT IV Outside of a building, main power line at service level
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