Electrical Measurements and Instrumentation
What this domain tests
This domain covers analogue and digital instruments, bridges, instrument transformers, transducers, calibration, and measurement error.
Core rules
- PMMC movement responds to average current and has a uniform scale; moving-iron instruments respond to RMS and suit AC and DC but have non-uniform scales.
- Ammeter shunts and voltmeter multipliers extend range. A shunt carries most of the current; a series multiplier drops most of the voltage.
- Wheatstone bridge measures medium resistance; Maxwell bridge measures inductance; Schering bridge measures capacitance and loss angle.
- Instrument transformers scale high voltage and current: CT secondary must never be open-circuited while energised because dangerous voltage develops.
- Accuracy class defines limits of error; burden is the apparent power the instrument transformer must supply.
- Loading effect occurs when instrument impedance changes the circuit being measured.
Worked example
A 100 A, 50 mV shunt is used with a meter movement of 1 mA and 50 ohm full scale. Find the shunt resistance. Full-scale meter voltage = 1 mA x 50 ohm = 50 mV, matching the shunt rating. Shunt resistance = 50 mV/100 A = 0.5 milliohm. The meter current is negligible compared with 100 A, so the shunt carries almost the full current.
Traps
- Assuming a PMMC meter reads RMS on a distorted waveform.
- Open-circuiting a live CT secondary.
- Ignoring instrument burden when selecting a CT.
- Forgetting that wattmeter deflection depends on both voltage and current coil polarity.