Apprentice path · Stage 2: Tools, meters and materials · Lesson 6

Reading a multimeter without lying to yourself

A meter will happily give you a confident number that means nothing, if the dial, the leads or the units are wrong.

Why this matters on the job

The dangerous reading is not the wrong one you notice. It is the wrong one that looks exactly like a right one -- and the one that most often gets someone hurt is a zero that came from a broken meter.

How to think about it

  1. Set the function first: AC volts, DC volts and resistance are three different settings, not one.
  2. Put the leads in the right jacks -- for voltage, black to COM and red to the V jack. Follow your meter's manual.
  3. Read the units on the display, not just the digits.
  4. Inspect the leads, check the ratings, and prove the tester before and after.

Worked through

The display shows 12.0 -- but 12 mV and 12 V are not the same thing at all, and one letter on the screen is the difference. Milli means one thousandth. 12 mV is twelve thousandths of a volt. If you read the digits and skip the prefix you are off by a factor of a thousand, and you will believe a dead circuit is live or a live one is dead.

Check yourself: Your meter reads 0.00. Does that prove the circuit is safe to touch?

Answer: No. A zero reading alone proves nothing until you have proved the meter itself still works on a known live source.

A meter that has failed reads zero on everything. The reading looks identical whether the circuit is dead or the meter is. Only proving the tester tells you which one you are looking at.

Watch out for: Never measure resistance on an energized circuit. The resistance setting puts the meter's own small voltage onto the circuit, and it expects to find nothing else there.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.