In our last issue, we identified the category ratings of electrical test instruments in ascending order of transient voltage withstand capability as CAT I, CAT II, CAT III, and CAT IV. You can find CAT definitions in IEC 61010-1. In an industrial environment, you should be using either CAT III or CAT IV.
Although you can safely use CAT II instruments on most 120V and 240V branch circuits in a building, the reality is that if you have a DMM hanging from your belt that's the one you're going to use. Carry a CAT III instrument so you can work on anything you're likely to encounter below the CAT IV level (you'll need CAT IV test equipment to work on utility service equipment).
In addition to the CAT rating, check the instrument's voltage and current rating before using.