Testing and Calibration
Domain 8: Testing and Calibration
This domain tests accuracy tests, starting and creep checks, meter constant and error calculation.
Core rules
- Accuracy error (%) = ((measured energy − standard energy) / standard energy) × 100.
- Class 1 meters should stay within ±1%; class 2 within ±2% under specified test conditions. IS 13779 applies to static watt-hour meters.
- Starting test: the meter must start and register at the specified low current.
- Creep test: with voltage applied and no current, the meter must not register continuously.
- Use a test bench, standard meter, stopwatch and pulse counter; record load, power factor and temperature.
Worked example
A meter marked 100 imp/kWh gives 50 pulses, so measured energy = 50/100 = 0.50 kWh. The standard meter records 0.49 kWh. Error = ((0.50 − 0.49)/0.49) × 100 = +2.04%. For a class 2 meter this is just outside the ±2% limit at that test point, so investigate before sealing. Reasoning: convert pulses to energy, then compare with the standard.
Traps that cost marks
- Using measured energy as the denominator.
- Mixing pulses and disc revolutions.
- Testing at only one load point.
- Ignoring power factor during the test.
- Adjusting a sealed meter without authorization.