Overvoltage Installation Categories
The most important single concept to understand about the new standards is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (See Figure 1). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (ac resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards.
Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter.
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