Multimeter Basic Function Quiz

Ten questions on using a multimeter safely and reading it correctly. Pick an answer to see whether you are right and why. Related reading: The Code: controls.

1. To measure voltage, the meter leads are connected:

   1. In series with the load
   2. In parallel, across the two points being measured
   3. To ground only
   4. With the circuit de-energized

   Voltage is a difference in potential between two points, so the meter goes across them, with the circuit energized.

2. The safest common way to measure current on an HVAC circuit is to:

   1. Put the meter leads in series with the motor
   2. Clamp a clamp meter around a single conductor
   3. Clamp around both conductors of a circuit at once
   4. Measure resistance and divide

   A clamp meter senses the magnetic field around one conductor without breaking the circuit. Clamping both conductors cancels the fields and reads near zero.

3. Before measuring resistance, you must:

   1. Energize the circuit
   2. Turn off the power and isolate the component
   3. Set the meter to AC volts
   4. Ground both leads

   An ohmmeter supplies its own small current. Measuring a live circuit can damage the meter and gives a false reading, and parallel paths in the circuit skew it too.

4. A meter reading OL on the ohms setting across a switch usually means:

   1. The switch is closed
   2. The path is open, with resistance beyond the meter's range
   3. The meter battery is dead
   4. The switch is shorted to ground

   OL means over limit: the resistance is too high to measure, which on a switch or a winding means an open.

5. Before testing a run capacitor's microfarads, you should:

   1. Leave it connected and energized
   2. Safely discharge it and disconnect at least one lead
   3. Warm it up
   4. Measure it on AC volts

   Capacitors can hold a dangerous charge after the power is off. Discharge it safely, then measure capacitance with the leads disconnected.

6. A run capacitor rated 45 µF ±6% measures 40 µF. It is:

   1. Within tolerance
   2. Below tolerance and should be replaced
   3. Above tolerance
   4. Fine, as long as it is not bulging

   Six percent of 45 is 2.7, so the acceptable range is about 42.3 to 47.7 µF. A reading of 40 µF is below it.

7. A continuity test checks whether:

   1. Voltage is present
   2. There is a complete, low-resistance path between two points
   3. A motor is drawing the right current
   4. A capacitor is charged

   Continuity mode beeps when resistance is very low, confirming an unbroken path. It is an ohms test, so the circuit must be de-energized.

8. What does the CAT rating on a meter, such as CAT III, describe?

   1. The meter's accuracy
   2. The level of voltage spikes the meter is built to withstand where it is used
   3. The battery type
   4. The maximum current it can read

   Measurement categories rate a meter and its leads for the transient overvoltages found at different points in an electrical system. Use a meter rated for where you are working.

9. You read almost 0 volts across the contacts of a closed contactor that is carrying load current. This means:

   1. The contacts are open
   2. The contacts are making good contact
   3. There is no power to the unit
   4. The coil is shorted

   Good closed contacts have very little voltage drop across them. A noticeable voltage across closed contacts under load points to pitted or burned contacts.

10. Which meter setting is used to check a residential 24-volt control circuit?

    1. DC millivolts
    2. AC volts
    3. Ohms
    4. Microamps

    The control transformer supplies about 24 volts AC, and it often reads a few volts higher when lightly loaded.