Hand-held test meters should never be connected to any electrical equipment or system operating at a voltage that exceeds the meter’s rating. While this is an important safety precaution when using any meter, it is even more important with DMMs.
Digital meters are more sensitive than older analog models to transient over voltages caused by utility switching, motor starting, and capacitor switching. High-voltage transients can damage the electronic circuitry inside DMMs, and in severe cases cause meters to explode.
DMMs have internal fuses that function to protect the test instrument (and the person using it) from harm when taking readings on systems of higher voltage or current rating than the DMM.
However, it is still extremely important never to try to take a reading on a system whose voltage or current is higher than the rating of the DMM itself.
Underwriters Laboratories Inc. has established safety ratings for DMMs. UL standard 3111-1 defines four energy-rating categories for test and measurement equipment, with CAT IV offering the highest level of protection.
CAT IV covers utility connections and all outdoor conductors (because of lightning hazards). Examples include service entrance equipment, watt-hour meters, and switchboards/switchgears.
CAT III covers power distribution equipment within buildings and similar structures. This includes panelboards, feeders, busways, motors, and lighting.
CAT II covers single-phase, receptacle-connected loads located more than 10 m from a CAT III power source or more than 20 m from a CAT IV source.
CAT I covers electronic and low-energy equipment.
DMMs are certified to these four categories by UL and other independent testing laboratories. The certification level is marked directly on the DMMs, and often included in advertising for them. Higher-rated meters can safely be used for lower-level measurement functions.
The category number of a DMM is more important than its voltage rating when determining the degree of protection that it provides. In other words, a CAT III, 600 V meter offers better protection against high-energy transients than a CAT II, 1000 V meter.