Electrical Measuring Instruments and Testing
What this domain tests
This domain covers instrument operating principles, correct connection, range extension and fault testing. Numerical work includes shunts, multipliers and bridge balance.
Core rules
- PMMC instruments respond to average DC and deflect proportionally to current; moving-iron instruments work on both AC and DC; dynamometer types suit power measurement.
- An ammeter is connected in series and should have very low resistance; a voltmeter is in parallel and should have very high resistance.
- Shunt resistance extends ammeter range: Rsh = IgRg/(I − Ig). Multiplier resistance extends voltmeter range: Rm = V/Ig − Rg.
- An energy meter measures kWh, not kW; a wattmeter measures power directly. A megger tests insulation, and a multimeter combines voltage, current and resistance ranges.
- The Wheatstone bridge balances when P/Q = R/S, giving zero galvanometer deflection.
Worked example
A moving-coil meter reads full scale at 1 mA and has 50 Ω resistance. To read 10 mA, the shunt carries 9 mA at the same voltage: Rsh = (1 × 10⁻³ × 50)/(9 × 10⁻³) = 5.56 Ω. The low shunt value diverts most current, so the meter still sees only 1 mA at full scale.
Traps that cost marks
Connecting an ammeter in parallel (it short-circuits the supply); using a voltmeter in series; expecting a PMMC meter to read AC directly; and computing a shunt with the total current instead of the shunt current.