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Junior Lineman / Energy Assistant — ITI Electrical Quick Book · Chapter 6

Meters and Measuring Instruments, Energy Meter Reading

What to remember

  • An ammeter has very low resistance and is connected in series. A voltmeter has very high resistance and is connected in parallel. A shunt (low resistance, in parallel) extends the range of an ammeter; a multiplier (high resistance, in series) extends the range of a voltmeter.
  • Moving-coil (PMMC) instruments work on DC only; moving-iron instruments work on AC and DC. The domestic energy meter is an induction-type instrument with an aluminium disc.
  • Units consumed = present reading − previous reading (multiplied by the multiplying factor for CT-operated meters). One unit is one kWh.

Classification of instruments

  • Absolute instruments give the value of the quantity in terms of the instrument constants (such as a tangent galvanometer). Secondary instruments are calibrated against a standard; these are the instruments in daily use.
  • By function: indicating (show the value at the time, like an ammeter), recording (draw the value against time on paper) and integrating (add up the quantity over time, like an energy meter).
  • By quantity measured: ammeter, voltmeter, wattmeter, energy meter, frequency meter, power factor meter, ohmmeter, megger, tachometer.

Three torques in an indicating instrument

  • 1. Deflecting torque: moves the pointer according to the quantity measured. Produced by magnetic, heating, electrostatic or induction effect.
  • 2. Controlling torque: brings the pointer to rest at the proper reading, and returns it to zero when the quantity is removed. Provided by a spring (spring control) or by gravity.
  • 3. Damping torque: stops the pointer from oscillating and brings it quickly to rest. Provided by air friction (a vane or piston), fluid friction or eddy currents (the most effective).

At the steady reading the deflecting torque equals the controlling torque.

Types of instrument and their working

TypeWorks onSupplyScaleRemarks
PMMC (permanent-magnet moving coil)A coil in a permanent magnet carrying current is deflectedDC onlyUniformAccurate, low power use, sensitive; with a rectifier it can read AC
Moving iron (attraction or repulsion)Iron pieces magnetised by a coilAC and DCNon-uniform (crowded at start)Cheap, robust; used for panel meters
DynamometerFixed and moving coilsAC and DCSquare-law for ammeters, uniform for wattmetersUsed mainly as a wattmeter
Induction typeEddy currents in a disc by alternating fieldsAC onlyUsed for integrating metersEnergy meters
Hot-wire, thermocoupleHeating effectAC and DCSquare lawHigh-frequency work

Range extension

Ammeter shunt: R_sh = I_m R_m / (I − I_m), where I_m is the full-scale meter current, R_m the meter resistance and I the total current to be measured.

Voltmeter multiplier: R_s = V / I_m − R_m.

Worked example 1: A 1 mA, 100 Ω meter is used to read up to 10 mA. R_sh = 0.001 × 100 / 0.009 = 11.1 Ω.

Worked example 2: The same 1 mA, 100 Ω meter is to read up to 10 V. R_s = 10 / 0.001 − 100 = 9900 Ω.

In a moving-coil meter the coil is wound on an aluminium former. Instruments with a high ohm-per-volt rating (sensitivity) disturb the circuit less. The multimeter (analogue or digital) measures AC and DC volts, DC current and resistance. Zero-adjust the ohmmeter before measuring resistance, and never measure resistance in a live circuit.

Other instruments an electrician uses

  • Wattmeter: measures active power; has a current coil (in series) and a pressure (voltage) coil (in parallel); reading = V I cosφ. Example: 230 V, 5 A at p.f. 0.8 reads 920 W.
  • Clamp meter (tong tester): measures AC current without breaking the circuit, using the principle of a current transformer.
  • Megger (insulation tester): measures insulation resistance using a high DC voltage from a hand generator or battery electronics.
  • Earth tester: measures earth electrode resistance.
  • Tachometer: speed of rotation in rpm.
  • Frequency meter and power factor meter: direct reading.
  • Phase sequence indicator: shows the sequence of a three-phase supply.
  • Phase tester (neon tester or screwdriver): checks live wires. Not for accurate voltage reading.
  • Synchroscope: shows when an alternator may be paralleled with the supply.
  • Digital instruments: display the value on an LCD; do not need pointer adjustment; high input impedance.

Errors and good use: parallax error (reading from the wrong angle; reduced by a mirror scale), zero error, and loading effect. Instrument accuracy class numbers (such as 1.0 or 2.0) show the permitted error as a percent of full scale. Read a meter near the upper part of its scale for the best accuracy. Connect the ammeter in series and the voltmeter in parallel. For DC instruments observe polarity.

The single-phase energy meter

An energy meter is an integrating instrument that records the energy in kilowatt-hours (kWh) used by the consumer. The household meter is of the induction type (electromechanical) or a static (electronic) meter.

Parts of an induction-type meter:

  • Current coil (series coil): thick wire, few turns, carries the load current.
  • Pressure coil (potential coil, shunt coil): thin wire, many turns, across the supply.
  • Aluminium disc: rotates because of the torque from eddy currents. The speed is proportional to the power.
  • Braking (permanent) magnet: gives damping torque so the speed is proportional to power.
  • Registering (counting) mechanism: a train of gears and dials giving the units.
  • Creep: the disc rotates slowly even when there is no load. It is stopped by anti-creeping holes or small iron pieces on the disc.

Meter constant is the number of revolutions of the disc per kWh (for example, 1200 rev/kWh) in electromechanical meters, and the impulses per kWh in static meters.

Worked example 3: In a meter with 1200 rev/kWh, 60 revolutions mean energy = 60 / 1200 = 0.05 kWh.

Worked example 4: With the same meter, 20 revolutions in 60 seconds give a load of P = 3600 × 20 / (1200 × 60) = 1 kW.

Worked example 5: A 3 kW load runs for 2 hours, so the meter records 6 units.

Static (electronic) meters have no moving parts, use current and voltage sensors and a microcontroller, and display values on an LCD. They show kWh, and some also show kVA, maximum demand, power factor and time-of-day values. Smart meters also send readings automatically to the utility and allow remote operation.

Larger consumers: high-tension and large low-tension consumers use three-phase meters connected through current transformers (and potential transformers for HT). The meter then reads in proportion, so the final energy is the reading difference times the multiplying factor (MF). MF = CT ratio × PT ratio (where PT is used).

Reading the meter

For a digital (counter-type) meter: read the number shown on the display. The display shows kWh. The consumption of a billing period is the difference between the present and the previous reading.

For a pointer (dial) meter: read the dials from left to right. Adjacent dials turn in opposite directions (one clockwise, the next anticlockwise). If the pointer lies between two digits, take the lower digit. If the pointer is on or very close to a digit, check the next lower dial: if that dial has not yet passed zero, take the lower digit.

Worked example 6: Previous reading 4520, present reading 4630. Consumption = 110 units.

Worked example 7: Same readings with MF = 20 (for example, a CT ratio of 100/5): consumption = 110 × 20 = 2200 units.

Utilities charge for the units consumed with slab or category rates, plus fixed charges. Large consumers are also charged on maximum demand (kVA) and may get a power factor incentive or penalty. Do not state prices from memory; use the latest tariff order.

Good practice: the meter is fixed vertically, at a convenient height, before the main switch. The supply to the meter is brought through a seal. Tampering (bypassing the meter, reversing the connections or interfering with the seal) is illegal and punished as power theft. A meter that is faulty should be reported and tested against a standard by the utility.

Exam traps

  • Ammeter in series, voltmeter in parallel; ammeter resistance low, voltmeter resistance high.
  • Shunt is in parallel with the ammeter; multiplier is in series with the voltmeter.
  • PMMC reads DC only; moving iron reads both AC and DC.
  • Moving-iron scale is non-uniform; PMMC scale is uniform.
  • The wattmeter pressure coil is in parallel (across the supply); the current coil is in series.
  • A clamp meter measures current without opening the circuit; a megger measures insulation resistance, not current.
  • Energy meter reads in kWh, not kW; an integrating instrument, not an indicating one.
  • For a CT-operated meter, multiply the difference of readings by the MF.

One-liners

  • 1. The three torques are deflecting, controlling and damping.
  • 2. Eddy-current damping is the most effective damping.
  • 3. A PMMC instrument has a uniform scale.
  • 4. The induction type is used for the domestic energy meter.
  • 5. Disc of an energy meter is made of aluminium.
  • 6. A braking magnet gives the damping in an energy meter.
  • 7. Creeping is rotation of the disc with no load.
  • 8. A clamp meter is also called a tong tester.
  • 9. A megger measures insulation resistance.
  • 10. Wattmeter reads V I cosφ.
  • 11. Meter constant is in revolutions per kWh.
  • 12. One unit equals one kilowatt-hour.

Practice questions

  1. Which of the statements is/are correct? 1. An ammeter has a very low resistance and is connected in series. 2. A voltmeter has a very low resistance and is connected in series.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A voltmeter has a very high resistance and is connected in parallel.

  2. Which of the statements is/are correct? 1. A moving-iron instrument can be used only on DC. 2. A PMMC instrument can be used directly on DC only.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Moving-iron instruments work on both AC and DC.

  3. Which of the statements is/are correct? 1. The scale of a PMMC instrument is uniform. 2. The scale of a moving-iron instrument is perfectly uniform.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A moving-iron scale is non-uniform (crowded at the lower end).

  4. Which of the statements is/are correct? 1. A multiplier used to extend the range of a voltmeter is connected in parallel with the meter. 2. A shunt used to extend the range of an ammeter is connected in parallel with the meter.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The multiplier is a high resistance in series with the voltmeter.

  5. Which of the statements is/are correct? 1. Eddy-current damping is very effective in indicating instruments. 2. Controlling torque is meant to stop the pointer from oscillating.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Damping torque stops oscillation; controlling torque restores the pointer to zero.

  6. Which of the statements is/are correct? 1. The current coil of a wattmeter is connected across the supply. 2. The pressure coil of a wattmeter is connected across the supply.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The current coil is in series with the load.

  7. Which of the statements is/are correct? 1. A clamp meter can measure AC current without opening the circuit. 2. A clamp meter measures insulation resistance.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Insulation resistance is measured by a megger.

  8. Which of the statements is/are correct? 1. The domestic electromechanical energy meter is of the PMMC type. 2. The domestic electromechanical energy meter uses an aluminium disc.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    It is an induction-type instrument and works on AC only.

  9. Which of the statements is/are correct? 1. An energy meter is an integrating instrument. 2. An energy meter is a recording instrument that draws a graph.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    It adds up the energy; it does not draw a graph against time.

  10. Which of the statements is/are correct? 1. Creeping in an energy meter is prevented by increasing the load. 2. Creeping is the slow rotation of the disc with no load connected.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Anti-creep holes or pieces on the disc stop creeping.

  11. Which of the statements is/are correct? 1. For a CT-operated meter, consumption equals the difference of readings multiplied by the multiplying factor. 2. The multiplying factor is not needed for any meter that is connected through CTs.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The multiplying factor is required to get the actual consumption.

  12. Which of the statements is/are correct? 1. A static meter cannot show kWh readings on a display. 2. A static (electronic) meter has no moving disc.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    It displays kWh on an LCD.

  13. Which of the following is an integrating instrument?

    1. Voltmeter
    2. Wattmeter
    3. Ammeter
    4. Energy meter
    Answer

    D. Energy meter

    An energy meter totals energy over time.

  14. Which torque brings the pointer back to zero when the quantity is removed?

    1. Deflecting torque
    2. Inertia torque only
    3. Damping torque
    4. Controlling torque
    Answer

    D. Controlling torque

    A spring provides the controlling torque.

  15. The damping torque in an energy meter is provided by

    1. a fuse
    2. a spring
    3. a permanent braking magnet
    4. a carbon brush
    Answer

    C. a permanent braking magnet

    Eddy currents in the disc under the magnet give a braking effect.

  16. The material of the rotating disc of an induction energy meter is

    1. steel
    2. copper only
    3. iron
    4. aluminium
    Answer

    D. aluminium

    A light aluminium disc gives eddy-current torque.

  17. A 1 mA, 100 Ω meter is to be used as a 10 mA ammeter. The shunt resistance required is nearly

    1. 900 Ω
    2. 100 Ω
    3. 11.1 Ω
    4. 1.1 Ω
    Answer

    C. 11.1 Ω

    R_sh = I_mR_m/(I − I_m) = 0.1/0.009 = 11.1 Ω.

  18. A 1 mA, 100 Ω meter is to be used as a 10 V voltmeter. The series multiplier required is

    1. 100 Ω
    2. 10 kΩ plus 100 Ω
    3. 9900 Ω
    4. 900 Ω
    Answer

    C. 9900 Ω

    R_s = V/I_m − R_m = 10000 − 100 = 9900 Ω.

  19. A meter marked 1200 rev/kWh has made 60 revolutions. The energy recorded is

    1. 0.5 kWh
    2. 5 kWh
    3. 20 kWh
    4. 0.05 kWh
    Answer

    D. 0.05 kWh

    Energy = 60/1200 = 0.05 kWh.

  20. In a meter with 1200 rev/kWh the disc makes 20 revolutions in 60 seconds. The load is

    1. 2 kW
    2. 0.5 kW
    3. 1 kW
    4. 20 kW
    Answer

    C. 1 kW

    P = 3600 × 20/(1200 × 60) = 1 kW.

  21. A 3 kW load runs for 2 hours. The energy meter records

    1. 60 units
    2. 1.5 units
    3. 6 units
    4. 5 units
    Answer

    C. 6 units

    E = 3 × 2 = 6 kWh.

  22. The previous reading of a meter was 4520 and the present reading is 4630. The units consumed are

    1. 4630
    2. 9150
    3. 1100
    4. 110
    Answer

    D. 110

    4630 − 4520 = 110 units.

  23. A CT-operated meter has a multiplying factor of 20. The difference of its readings is 110. The actual consumption is

    1. 5.5 units
    2. 130 units
    3. 110 units
    4. 2200 units
    Answer

    D. 2200 units

    110 × 20 = 2200 units.

  24. A wattmeter connected to a load at 230 V, 5 A and 0.8 power factor reads

    1. 575 W
    2. 1840 W
    3. 920 W
    4. 1150 W
    Answer

    C. 920 W

    P = V I cosφ = 230 × 5 × 0.8 = 920 W.

  25. The instrument used to measure insulation resistance is the

    1. megger
    2. synchroscope
    3. tachometer
    4. clamp meter
    Answer

    A. megger

    A megger applies a high DC voltage to the insulation.

  26. A tong tester is another name for a

    1. earth tester
    2. clamp meter
    3. megger
    4. wattmeter
    Answer

    B. clamp meter

    It measures AC current by clamping around one conductor.

  27. The instrument that measures the speed of a motor shaft is the

    1. synchroscope
    2. phase sequence indicator
    3. megger
    4. tachometer
    Answer

    D. tachometer

    A tachometer reads rpm.

  28. The instrument used before paralleling an alternator with the supply is the

    1. ohmmeter
    2. hygrometer
    3. synchroscope
    4. wattmeter
    Answer

    C. synchroscope

    A synchroscope shows the phase and frequency difference.

  29. The phase sequence of a three-phase supply is checked with a

    1. frequency meter
    2. megger
    3. phase sequence indicator
    4. clamp meter
    Answer

    C. phase sequence indicator

    It shows whether the sequence is R-Y-B or reversed.

  30. The unit of energy shown on a household meter is

    1. kilovolt
    2. kilowatt-hour
    3. kilowatt
    4. kilovar
    Answer

    B. kilowatt-hour

    One kWh is one unit.

  31. An ohmmeter reading must be taken only when the circuit is

    1. switched off (dead)
    2. connected to a transformer secondary
    3. connected to the earth tester
    4. live at full load
    Answer

    A. switched off (dead)

    The meter has its own battery; a live circuit can damage it.

  32. An instrument with the scale crowded at its lower end is typically the

    1. digital instrument
    2. PMMC instrument
    3. wattmeter with a uniform scale
    4. moving-iron instrument
    Answer

    D. moving-iron instrument

    Deflecting torque is proportional to the square of the current.

  33. The kind of damping most commonly used in moving-coil instruments is

    1. damping by a paper vane
    2. gravity damping
    3. eddy-current damping
    4. electrostatic damping
    Answer

    C. eddy-current damping

    The aluminium former moving in the magnet field gives eddy-current damping.

  34. Parallax error in a meter is reduced by

    1. a larger shunt
    2. a thicker coil
    3. an extra battery
    4. a mirror behind the pointer
    Answer

    D. a mirror behind the pointer

    Pointer and mirror image must coincide at the reading.

  35. For the best accuracy an analogue meter should be read at

    1. only at the zero mark
    2. any point equally
    3. the upper part of its scale
    4. the very beginning of the scale
    Answer

    C. the upper part of its scale

    Percent error is smaller at a larger deflection.

  36. The meter constant of an electronic energy meter is given in

    1. volts per ampere
    2. impulses per kWh
    3. kW per ohm
    4. revolutions per second
    Answer

    B. impulses per kWh

    The LED blinks a fixed number of times per kWh.

  37. The current coil of an energy meter is made of

    1. thick wire with few turns
    2. thin wire with many turns
    3. bare steel wire
    4. superconducting wire
    Answer

    A. thick wire with few turns

    It carries the full load current and so has low resistance.

  38. The pressure coil of an energy meter is made of

    1. thick wire with a few turns
    2. aluminium foil
    3. tin-lead wire
    4. thin wire with many turns
    Answer

    D. thin wire with many turns

    It is across the supply and so has high resistance and high inductance.

  39. The multiplying factor of a CT-operated meter with a 100/5 A current transformer is

    1. 20
    2. 100
    3. 5
    4. 500
    Answer

    A. 20

    MF = 100/5 = 20.

  40. The type of instrument that cannot measure DC directly is the

    1. dynamometer type
    2. induction type
    3. moving iron type
    4. PMMC with a shunt
    Answer

    B. induction type

    The induction principle needs an alternating field.

  41. A voltmeter of high ohm-per-volt rating

    1. draws more current
    2. reads current
    3. is used in series
    4. draws less current from the circuit
    Answer

    D. draws less current from the circuit

    Higher sensitivity means a smaller loading effect.

  42. Which of the statements is/are correct? 1. The wattmeter reading is proportional to V I cosφ. 2. The wattmeter reads apparent power in VA.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A wattmeter reads active power in watts.

  43. Which instrument is best suited to measure the power consumed by a single-phase load directly?

    1. Voltmeter
    2. Wattmeter
    3. Tachometer
    4. Megger
    Answer

    B. Wattmeter

    A wattmeter has both a current coil and a pressure coil.

  44. An energy meter on a 230 V circuit supplies a 2 kW geyser and 500 W of lights for 4 hours. The units recorded are

    1. 8
    2. 12.5
    3. 10
    4. 2.5
    Answer

    C. 10

    Total = 2.5 kW × 4 h = 10 kWh.

  45. Which of the following types of instrument has a uniform scale?

    1. Moving iron
    2. PMMC
    3. Thermocouple ammeter
    4. Hot-wire ammeter
    Answer

    B. PMMC

    Deflection is directly proportional to current in a PMMC.

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