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Worksheet: the multimeter

Name:Class:Date:

Material: the free multimeter simulator at examrobotic.com (examrobotic.com/en/tools/multimeter-simulator). Time: about 45 minutes.

Aim: choose the right function and socket, interpret readings on a three-phase supply and on a 24 V control circuit, and find faults by measuring section by section.

1. On the three-phase supply

  1. Set the selector to AC volts and measure. Write down the readings:
    L1-L2: ______ V   L1-N: ______ V   L1-PE: ______ V   N-PE: ______ V
  2. Why is the phase-to-phase voltage √3 times the phase-to-neutral voltage?
  3. What would several volts between N and PE indicate?
  4. With the red lead in the A (amps) socket, measure between L1 and N. What happens? Why is it so dangerous with a real meter?

2. On the 24 V control circuit

  1. With the circuit working, measure the voltage across the K1 coil (A1-A2): ______ V. And across S0 when it is not pressed? ______ V. Why does a closed contact read almost 0 V?
  2. Switch the supply off and measure the coil in ohms: ______ Ω. Try continuity: does it beep? Why?
  3. Why should you never measure ohms or continuity on a live circuit?

3. Find the fault

Press «Surprise fault» three times. Measure each point to 0 V to find where the voltage is lost, and write down your method and the fault.

Answers (for the teacher)

  1. About 400 V phase to phase, about 230 V phase to neutral and phase to earth, and almost 0 V neutral to earth.
  2. The three phase voltages are 120° apart; the difference between two of them is √3 times each one (230 × √3 ≈ 400 V).
  3. A heavily loaded or unbalanced neutral, or a poor neutral connection: it needs checking.
  4. It is a short circuit through the meter: its fuse blows. With a real meter, if the fuse is not the right type, it can explode and cause an arc flash.
  5. About 24 V across the coil; almost 0 V across the closed contact, because it has no resistance and there is no voltage difference between its two sides. An open contact would show the full voltage.
  6. A few hundred ohms. It normally does not beep: the meter only beeps below a few tens of ohms.
  7. Because the meter injects its own current to measure: with the circuit live, the reading is meaningless and the meter can be damaged.