Electrical Measuring Instruments and Tariffs
Scope
Electrical Measuring Instruments and Tariffs tests moving-coil and moving-iron instruments, wattmeters, energy meters, CTs and PTs, meter constants, and electricity bill calculation.
Core rules
- PMMC instruments respond to average values and are used for DC; moving-iron instruments work on AC and DC.
- A dynamometer wattmeter measures true power; an energy meter integrates power over time.
- In a CT, the secondary current is a scaled version of the primary current; in a PT, secondary voltage is scaled.
- Energy in kWh = power in kW × time in hours.
- Tariff may include fixed charge, energy charge, slab rates, power-factor penalty or rebate, and meter rent.
Worked example
A 240 V, 10 A load at 0.8 power factor runs 5 h/day for 30 days. P = 240 × 10 × 0.8 = 1920 W = 1.92 kW. Energy = 1.92 × 5 × 30 = 288 kWh. At ₹5.50/kWh plus ₹100 fixed charge, the bill is 288 × 5.50 + 100 = ₹1584 + ₹100 = ₹1684. Using 240 × 10 = 2400 W without the power factor would overstate the energy by a third.
Traps
- Treating VA as W and ignoring power factor.
- Confusing kW demand with kWh consumption.
- Reversing the CT ratio when converting secondary current to primary current.
- Forgetting fixed charges or slab changes in the bill.