Part V — Measurements and Instrumentation

Chapter 5: Electrical Measurement Principles

5.1 Measuring Instruments

An ammeter measures current and is always connected in series in a circuit (ideally with near-zero internal resistance, to minimise its effect on the circuit being measured). A voltmeter measures voltage and is always connected in parallel (ideally with very high internal resistance, to draw negligible current from the circuit). A wattmeter measures electrical power, combining both a current coil (connected in series, like an ammeter) and a voltage/pressure coil (connected in parallel, like a voltmeter) to compute power directly.

5.2 Types of Instruments

Moving Coil (PMMC — Permanent Magnet Moving Coil) instruments work only on DC and are known for high accuracy and sensitivity (used in DC ammeters/voltmeters, and with a rectifier for AC measurement). Moving Iron (MI) instruments can work on both AC and DC and are commonly used for general-purpose AC ammeters/voltmeters, though generally less accurate than moving coil instruments. Dynamometer-type instruments (used in wattmeters) can also work on both AC and DC.

5.3 The Multimeter and Megger

A multimeter is a versatile instrument combining ammeter, voltmeter, and ohmmeter functions in one device, widely used for general electrical troubleshooting. A Megger (insulation resistance tester) measures very high resistance values, specifically used to test the insulation resistance of cables, windings, and equipment (typically applying a high test DC voltage, e.g., 500V or 1000V, and measuring the resulting very small leakage current) — a critical maintenance test to detect insulation degradation before it causes a fault.

5.4 Energy Meters

The common household electricity energy meter measures energy consumption in kilowatt-hours (kWh, colloquially "units"); traditional electromechanical meters use an induction-type mechanism with a rotating aluminium disc, while modern installations increasingly use digital/smart energy meters, which additionally allow remote reading and time-of-use billing.

5.5 Practice Set — Measurements (14 MCQs)

1. An ammeter is always connected in a circuit:
   (a) In parallel (b) In series (c) Neither in series nor parallel (d) Only across the source terminals
2. An ideal ammeter should have:
   (a) Very high internal resistance (b) Near-zero internal resistance (c) Infinite resistance (d) Exactly equal resistance to the load
3. A voltmeter is always connected in a circuit:
   (a) In series (b) In parallel (c) Only in a bridge configuration (d) Directly across the ammeter
4. An ideal voltmeter should have:
   (a) Near-zero internal resistance (b) Very high internal resistance (c) Resistance equal to the load (d) Negative resistance
5. A wattmeter combines which two elements to measure power?
   (a) Two current coils (b) Two voltage coils (c) A current coil (series) and a voltage/pressure coil (parallel) (d) A single resistor only
6. Moving Coil (PMMC) instruments work only on:
   (a) AC (b) DC (c) Both AC and DC equally well without any modification (d) Neither AC nor DC
7. Moving Iron (MI) instruments can work on:
   (a) DC only (b) AC only (c) Both AC and DC (d) Neither AC nor DC
8. A multimeter typically combines the functions of:
   (a) Ammeter, voltmeter, and ohmmeter (b) Only an ammeter (c) Only a wattmeter (d) Only a frequency meter
9. A Megger is specifically used to measure:
   (a) Power factor (b) Insulation resistance of cables/windings (c) Frequency (d) Energy consumption in kWh
10. A Megger typically applies a high test voltage of approximately:
    (a) 5V (b) 12V (c) 500V or 1000V (d) 50,000V
11. Household electricity energy meters measure energy consumption in:
    (a) Watts (b) Kilowatt-hours (kWh) (c) Amperes (d) Volts
12. Traditional electromechanical energy meters use an induction-type mechanism with:
    (a) A rotating aluminium disc (b) A digital display only (c) A mercury column (d) A bimetallic strip
13. Modern smart energy meters, compared to traditional electromechanical meters, additionally allow:
    (a) Remote reading and time-of-use billing (b) Measurement of insulation resistance (c) Measurement of magnetic flux directly (d) Elimination of the need for any billing
14. Dynamometer-type instruments, used in wattmeters, can operate on:
    (a) DC only (b) AC only (c) Both AC and DC (d) Neither

Answer Key

1.(b)

2.(b)

3.(b)

4.(b)

5.(c)

6.(b)

7.(c)

8.(a)

9.(b)

10.(c)

11.(b)

12.(a)

13.(a) 14.(c)