Electrical Measuring Instruments and Testing
Electrical Measuring Instruments and Testing
This domain tests the construction, principle, connection and limitations of common instruments, plus practical tests such as insulation resistance and energy-meter checks.
Core rules
- PMMC: moving coil in a permanent-magnet field, torque proportional to current; used for DC, uniform scale, needs a shunt as ammeter and a multiplier as voltmeter.
- Moving iron: works on AC and DC, reads RMS, scale cramped at the start; common in panel meters.
- Electrodynamometer: used in wattmeters and power-factor meters; current coil in series, voltage coil in parallel.
- Induction energy meter: registers kWh; the braking magnet controls speed; creep is prevented by a two-hole brake or stalling device.
- CT and PT step current and voltage down for measurement; ratio error and phase error matter for metering.
- Megger: 500 V for low-voltage insulation testing; a sound LV installation typically reads at least 1 MΩ.
- Clamp meter measures current without breaking the circuit by sensing the magnetic field.
Worked example
An energy meter marked 1200 revolutions/kWh makes 10 revolutions in 4 minutes while a 100 W lamp is connected. Is it accurate? Energy registered = 10/1200 = 0.00833 kWh. Time = 4/60 = 0.0667 h. Indicated power = 0.00833/0.0667 = 0.125 kW = 125 W. The true load is 100 W, so the meter reads about 25% fast and needs calibration or brake-magnet adjustment.
Traps that cost marks
- Connecting an ammeter across the supply; its near-zero resistance short-circuits the circuit.
- Connecting a wattmeter with the voltage coil in series or with reversed polarity.
- Assuming a moving-iron meter is DC-only; it responds to both AC and DC.
- Operating a megger before isolating and discharging the circuit.
- Forgetting to multiply a CT reading by the ratio.