Rule 39 of 40 · Chapter VI — Tools, Habits, and Judgement
Keep a rated meter you understand
Why this rule exists
Your meter is your primary safety instrument and, if wrong or unrated, a hazard in your hand. It turns I think it is off into it reads zero, but only if it works and is rated for where you use it. The CAT rating, II through IV, reflects how much transient energy the meter can absorb without an internal arc at that point in the system; closer to the service means higher energy and category. A meter you do not understand or maintain can lie to you the moment you trust it with your life.
In practice
Own a quality meter rated for the highest category you will work in, CAT III or IV for panels and service entrances, with correctly rated, undamaged leads. Before trusting a dead reading, prove the meter on a known live source, then re-prove it after (the live-dead-live method). Check the function and range settings; measuring voltage on a current or resistance setting gives a false reading. Inspect leads for cracks and keep the battery fresh. Know what your meter's readings mean, including how it behaves on a floating or induced voltage.
When it doesn't apply
Non-contact detectors and simple testers suit a first look but do not replace a proven meter reading for absence of voltage. Meters can show phantom induced voltages on long dead conductors; a low-impedance setting distinguishes real voltage from ghost. If your meter behaves in a way you cannot explain, stop and resolve it before touching anything.