Electrical Measurements and Instruments
Electrical Measurements and Instruments checks instrument selection, connection, reading interpretation and simple calibration checks.
Core rules
- PMMC instruments work on DC only and have a uniform scale; moving-iron instruments read AC (and DC) RMS and are robust but less accurate.
- A wattmeter has a current coil in series and a voltage coil in parallel; reversal of either coil reverses the reading.
- CTs and PTs scale high currents and voltages down. Primary = secondary reading × ratio; for a CT 200/5, multiply by 40.
- An energy meter integrates power over time; its revolutions-per-kWh constant allows a check against a known load.
- Meggers test insulation resistance; earth testers measure electrode resistance; clamp meters measure current without breaking the circuit.
Worked example
An energy meter marked 1200 rev/kWh makes 5 revolutions in 40 s. Energy per revolution = 1/1200 kWh. Energy in 40 s = 5/1200 = 0.004167 kWh. Power = energy/time = 0.004167 × 3600/40 = 0.375 kW = 375 W. If the actual load is a 400 W lamp, the meter is reading low by about 6%, so check for a CT multiplier, incorrect constant or meter fault.
Traps
Do not apply a CT ratio twice; do not connect a wattmeter voltage coil in series; do not use a megger on live equipment or on electronic drives; and do not read a moving-iron meter as if it had a linear PMMC scale.