Electrical Measuring Instruments and Testing
Scope
This domain tests instrument construction and application: PMMC, moving iron, dynamometer wattmeter, induction energy meter, CTs and PTs, and test instruments such as megger, earth tester and clamp meter.
Core rules
- PMMC: DC only, uniform scale, reads average value; extended with a shunt for current and a multiplier for voltage.
- Moving iron: AC and DC, reads RMS, non-uniform scale, cheaper and robust.
- Dynamometer: used as wattmeter and transfer instrument; deflection proportional to VI cos φ.
- A CT secondary must never be open-circuited while primary current flows; a PT secondary must never be short-circuited.
- Megger: 500 V for LV insulation, 1,000 V for HV; earth tester uses the fall-of-potential method; clamp meter measures current without breaking the circuit.
Worked example
A PMMC movement has full-scale current 1 mA and resistance 100 Ω. To read 10 A, the multiplying factor m = 10/0.001 = 10,000. Shunt R_sh = R_m/(m − 1) = 100/9,999 = 0.0100 Ω. To read 10 V, the multiplier R_mult = V/I_m − R_m = 10/0.001 − 100 = 9,900 Ω. The shunt carries 9.999 A while the movement carries 1 mA, so the shunt must be thick enough to dissipate 9.999² × 0.01 ≈ 1 W.
Traps that cost marks
Trying to use a PMMC meter on AC, opening a CT secondary, using a megger on an electronic circuit, and reading an MI meter as if its scale were linear at low values. In a two-wattmeter method the total power is W₁ + W₂, not their difference, for a balanced load.