Energy Meter Types
Domain 3: Energy Meter Types
This domain tests recognition and selection of meter technologies: induction, static electronic, single-phase, three-phase, direct-connected, CT-operated, prepaid and smart meters.
Core rules
- Induction meter: voltage coil, current coil, aluminium disc and brake magnet; robust but has friction and creep error. IS 722 covers AC induction meters.
- Static meter: shunt or CT, ADC, microcontroller; records kWh, kvarh, kVAh, maximum demand and supports communication.
- Direct-connected meters suit small loads; CT-operated meters suit higher currents, and the CT secondary must never be opened under load.
- Prepaid meters disconnect on zero balance; smart meters support remote reading, tamper alerts and time-of-day registers.
- Select by supply phase, voltage, current, accuracy class and tariff requirement.
Worked example
A 50 kW three-phase load at 415 V and PF 0.8 draws I = 50,000/(√3 × 415 × 0.8) ≈ 87 A. A 100 A direct meter is marginal; a CT-operated meter with 100/5 CTs is safer. The CT ratio is 20, so the register difference must be multiplied by 20. Reasoning: compute line current first, then compare with meter and CT rating.
Traps that cost marks
- Using a single-phase meter on a three-phase service.
- Choosing a CT ratio from kW without power factor.
- Opening a live CT secondary.
- Assuming every electronic meter is a smart meter.
- Ignoring accuracy class when replacing a meter.