Using your meter
What you’ll learn
- The three measurements you will make every day: continuity, voltage and current.
- Where the leads go for each, and why.
- How a clamp meter reads current without touching anything.
- The live–dead–live test, and why a “dead” reading proves nothing on its own.
Your meter is how you see electricity, and on the job there are three questions you will ask it constantly: is there a path (continuity), is there a push (voltage), and is anything flowing (current)? Each has its own lead position and its own rules. The deeper page on using a multimeter walks the dial; this lesson is the reasoning.
First: test before you touch
Before any other measurement, the one that keeps you alive: proving a circuit dead after it has been isolated and locked out. Use a two-pole tester or a meter set to volts, rated (CAT) for where you are working, and do it in three steps:
- Live — test on a known live source. The meter works.
- Dead — test the circuit: every conductor to every other, and each to ground.
- Live — test the known live source again. The meter still works, so the dead reading was real.
A meter with a blown fuse, a dead battery, a broken lead or the dial on the wrong function reads “0 V” on everything. Live–dead–live is the only way to know the zero came from the circuit and not the meter.
Continuity and resistance
Continuity asks: is there an unbroken path between these two points? The meter pushes a small current of its own and beeps if the resistance is low. It is how you ring out a cable, check a fuse, find which conductor is which, or confirm a switch opens and closes. Because the meter supplies its own current, the circuit must be de-energized and isolated — and anything in parallel with what you are testing will give you a false path.
Tension
Voltage is a difference between two points, so the leads go across — in parallel with the thing you are measuring. On a 120/240 V panel you should see about 120 V hot to neutral on each leg, about 240 V hot to hot, 120 V hot to ground, and near 0 V neutral to ground.
Worked: reading a switch
- A 120 V light circuit, lamp installed, switch closed (light on). Across the switch terminals you read about 0 V — both terminals are at the same point, the hot.
- Switch open (light off). One terminal is still hot; the other connects through the cold lamp filament to the neutral. You read about 120 V across the gap.
- So a voltage across a closed device means that device is the problem: a closed switch or a good fuse should read nearly nothing across it. This is the basis of the troubleshooting method.
Courant
Current is flow, so to measure it the meter has to see the flow. A multimeter does it in series: open the circuit and let the current run through the meter, with the red lead in the amps socket. On the job you will nearly always use a clamp meter instead: the jaws go around one conductor and read the magnetic field its current makes. Clamp one conductor only — around a whole cable, the current out and back cancel and you read almost nothing.
Worked: clamping a heater circuit
- A 1,440 W heater on 120 V should draw 1,440 ÷ 120 = 12 A.
- Clamp the hot alone: about 12 A. Clamp the neutral alone: about 12 A.
- Clamp the whole cable: 12 A out, 12 A back — about 0 A. Not a fault; you clamped both.
Try one: 0 V across a closed fuse, or 240 V?
A good, closed fuse in a live, loaded circuit reads nearly 0 V across it. A blown fuse reads the full voltage across it, because it is now the open in the circuit.
Sur le chantier
Tips from the job
- Check the meter before the job: leads undamaged, the CAT rating right for where you are working, the battery good.
- Put the black lead in COM and leave it there. Move only the red one — and put it back in the V socket when you finish an amps reading.
- Take voltage readings hot–neutral, hot–ground and neutral–ground. The three together tell you more than any one.
- Clamp one conductor. If you are reading near zero on a load you know is running, check you have not got two conductors in the jaws.
Common mistakes
- Leaving the red lead in the amps socket and then touching it across a voltage — a dead short through the meter.
- Trusting a dead reading without checking the meter on a known live source first and after.
- Measuring ohms or continuity on a live circuit.
- Using a meter with the wrong CAT rating (or none) at a panel or service.
Check yourself
7 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
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To measure voltage, the meter’s leads go…
Answer and why
B. Voltage is a difference between two points, so the leads touch those two points — across the load or the source.
-
Why does a clamp meter around a whole two-conductor cable read almost zero?
Answer and why
C. The hot carries current out and the neutral brings the same current back. Their magnetic fields cancel inside the jaws. Clamp one conductor at a time.
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Before you trust a “dead” reading, you should…
Answer and why
A. Live–dead–live proves the meter was working before and after. A meter with a blown fuse or a broken lead reads “dead” on everything.
-
Continuity and resistance tests are done…
Answer and why
B. The ohms and continuity functions push a small current of their own. On a live circuit you get a wrong reading at best and a damaged meter at worst.
-
On a 120/240 V panel, what should you read from one hot bus to the other?
Answer and why
D. Each hot is 120 V to neutral and they are opposite, so hot-to-hot is 240 V.
-
A switch is open and the light it controls is off. Across the open switch you read about…
Answer and why
B. With the switch open, one terminal is at the hot and the other is connected back through the lamp to the neutral. The whole supply voltage appears across the gap. Close it and the reading drops to near 0 V.
-
An in-line (series) amps reading with a multimeter means…
Answer and why
B. The meter has to carry the current, so the circuit is opened and the meter completes it. That is why most field current readings are done with a clamp instead.
Questions fréquentes
Why does my meter show a voltage on a wire that is disconnected?
A digital meter has a very high input resistance, so it can pick up a “ghost” voltage coupled from a live conductor running alongside the dead one. It has almost no current behind it. Some meters have a low-impedance (LoZ) setting that loads the circuit enough to make a ghost voltage collapse. Treat any unexpected reading as live until you have proved otherwise.
Is a non-contact tester enough to prove a circuit dead?
No. A non-contact tester is a quick screen that tells you something is probably live. It can miss a live conductor (shielded cable, a dead battery, how you hold it) and it can beep on a dead one. Proving dead is done with a two-pole tester or meter, touching the conductors, with the live–dead–live check.
Voir aussi
- Using a multimeter — the full walk-through, dial position by dial position.
- Meter safety and CAT ratings — which meter where.
- Lockout / tagout walk-through — where the test-before-touch fits.
- Trade math refresher — the lesson before this one.
- First-year review quiz — test the whole year.
- All first year lessons
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