Electrical Measuring Instruments Notes for SSC JE Electrical Engineering
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Three Torques in an Indicating Instrument
- 2.Types of Instruments
- 3.Ammeter and Voltmeter
- 4.Wattmeter and Energy Meter
- 5.Two Wattmeter Method (Three Phase Power)
- 6.Bridges for Measurement
- 7.Other Instruments
- 8.Terms of Measurement
- 9.Quick Tips
- 10.Quick Revision Points
- 11.Practice Questions With Answers
- 12.How to Practise This Chapter
- 13.Frequently Asked Questions
PDF Notes: Electrical Measuring Instruments Notes for SSC JE Electrical Engineering
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Electrical measuring instruments are used to measure the current, voltage, power and energy in a circuit. This chapter is a favourite in the SSC JE Electrical paper, because the questions are direct: the type of instrument for AC or DC, the range extension and the bridges. These electrical measuring instruments notes for SSC JE explain every instrument in simple tables with solved examples and practice questions.
Three Torques in an Indicating Instrument
| Torque | Purpose |
|---|---|
| Deflecting torque | Moves the pointer by an amount that depends on the quantity measured |
| Controlling torque | Brings the pointer back to zero and gives a definite deflection; usually by a spring or gravity |
| Damping torque | Stops the oscillation of the pointer; usually by air friction or eddy currents |
Types of Instruments
| Instrument | Works on | Used for | Scale |
|---|---|---|---|
| PMMC (permanent magnet moving coil) | Force on a current-carrying coil in a magnetic field | DC only | Uniform (linear); high accuracy and sensitivity |
| Moving iron (MI) | Attraction or repulsion of iron pieces | AC and DC | Non-uniform; cheap and rugged |
| Dynamometer | Force between a fixed and a moving coil | AC and DC; mainly wattmeters | Non-uniform for voltmeter and ammeter; uniform for wattmeter |
| Induction type | Eddy currents in a disc | AC only; energy meters | Used in energy meters |
| Rectifier type | A rectifier with a PMMC | AC measurement with a PMMC | Calibrated for the RMS value |
| Hot wire | Heating effect of current | AC and DC | Non-uniform; slow |
Ammeter and Voltmeter
- An ammeter is connected in series with the circuit and has a very low resistance.
- A voltmeter is connected in parallel with the circuit and has a very high resistance.
- A shunt (a low resistance in parallel with the meter) extends the range of an ammeter: Rsh = Rm Im / (I − Im).
- A multiplier (a high resistance in series with the meter) extends the range of a voltmeter: Rs = V / Im − Rm.
- A current transformer and a potential transformer are used to extend the range of AC meters.
Example 1. A meter has a resistance of 100 ohm and a full scale current of 1 mA. Find the shunt to read up to 1 A.
Rsh = 100 × 0.001 / (1 − 0.001) = about 0.1 ohm.
Example 2. The same meter is to be used as a voltmeter of 10 V range. Find the series resistance.
Rs = 10 / 0.001 − 100 = 9900 ohm.
Wattmeter and Energy Meter
| Instrument | Measures | Facts |
|---|---|---|
| Wattmeter | Power in a circuit | Dynamometer type; has a current coil (in series) and a pressure (voltage) coil (in parallel) |
| Energy meter | Electrical energy in kWh | Induction type; 1 unit = 1 kilowatt hour; the disc rotates in proportion to the energy |
| Power factor meter | Power factor of the load | |
| Frequency meter | Frequency of the supply |
Two Wattmeter Method (Three Phase Power)
- Total power P = W1 + W2.
- Power factor angle: tan φ = √3 (W1 − W2) / (W1 + W2).
- At unity power factor, W1 = W2.
- At 0.5 power factor, one wattmeter reads zero.
- At zero power factor, the two readings are equal in value and opposite in sign.
Bridges for Measurement
| Bridge | Measures |
|---|---|
| Wheatstone bridge | Medium resistance; balanced when R1 / R2 = R3 / R4 |
| Kelvin double bridge | Low resistance |
| Maxwell bridge | Inductance |
| Hay bridge | Inductance of a coil with a high Q |
| Schering bridge | Capacitance and dielectric loss |
| Anderson bridge | Self-inductance |
Other Instruments
- Megger: measures the insulation resistance of cables and machines.
- Multimeter: measures voltage, current and resistance (AVO meter).
- CRO (cathode ray oscilloscope): displays the waveform of voltage and measures frequency and phase.
- Earth tester: measures the earth resistance.
Terms of Measurement
| Term | Meaning |
|---|---|
| Accuracy | How close the reading is to the true value |
| Precision | How close the repeated readings are to each other |
| Resolution | The smallest change that can be measured |
| Sensitivity | The change in output for a change in input |
| Calibration | Comparing an instrument with a standard |
Quick Tips
- Remember that PMMC is for DC only and moving iron is for both AC and DC.
- Remember that the ammeter is in series with a low resistance and the voltmeter in parallel with a high resistance.
- Learn the bridge and its measured quantity in pairs.
- Remember the readings of the two wattmeters at unity, 0.5 and zero power factor.
Quick Revision Points
- PMMC has a uniform scale and works only on DC.
- Moving iron instruments work on both AC and DC.
- Energy meter is an induction type instrument.
- Shunt is in parallel with an ammeter and multiplier is in series with a voltmeter.
- The two wattmeter method gives P = W1 + W2.
Practice Questions With Answers
- Which instrument can be used only for DC measurement?
Answer: PMMC instrument - Which instrument is used for both AC and DC?
Answer: Moving iron instrument - How is an ammeter connected in a circuit?
Answer: In series - How is a voltmeter connected in a circuit?
Answer: In parallel - Which instrument measures electrical energy?
Answer: Energy meter (induction type) - Which bridge is used to measure capacitance?
Answer: Schering bridge - Which bridge is used to measure inductance?
Answer: Maxwell bridge - Which instrument measures the insulation resistance?
Answer: Megger - What is the total power in the two wattmeter method?
Answer: W1 + W2 - What does a multiplier extend?
Answer: The range of a voltmeter
How to Practise This Chapter
Solve one shunt and one multiplier numerical daily, and revise the table of instruments and bridges. Then solve questions from our practice papers and the previous year papers page. Read the previous chapter, Three Phase Induction Motor, and see all topics on the SSC JE Electrical Engineering notes page. For basic circuits, read the Network Theorems note.
Frequently Asked Questions
What are the three torques in a measuring instrument?
They are the deflecting torque, the controlling torque and the damping torque.
What is a PMMC instrument?
A PMMC instrument has a coil that moves in the field of a permanent magnet. It is accurate but works only on DC.
Which instrument is used for AC and DC?
The moving iron instrument and the dynamometer instrument can be used for both AC and DC.
What is the use of a shunt?
A shunt is a low resistance connected in parallel with an ammeter to extend its range.
What is the use of a multiplier?
A multiplier is a high resistance connected in series with a voltmeter to extend its range.
Which instrument is used to measure power?
A wattmeter, which is a dynamometer type instrument with a current coil and a pressure coil, measures power.
Which type of meter is used as an energy meter?
An induction type meter is used as an energy meter for AC supply.
What is the two wattmeter method?
It is a method of measuring three phase power with two wattmeters. The total power is the sum of their readings.
What is a Wheatstone bridge?
A Wheatstone bridge is a circuit used to measure an unknown resistance by balancing two arms of the bridge.
What is a megger used for?
A megger measures the insulation resistance of cables, motors and transformers.
Are measuring instruments important for SSC JE Electrical?
Yes. They give direct questions every year on the type, range extension and bridges.
How many questions come from measuring instruments in SSC JE?
Usually three to five questions come from measurements and instruments, but the number changes from year to year.
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Three Phase Induction Motor Notes for SSC JE Electrical Engineering
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Basic Electrical Circuits and AC Fundamentals for SSC JE Electrical: RMS, Impedance, Power Factor, Resonance and Three-Phase
Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.