Calibration of Energy Meter by Direct Loading Method:
There are three methods for testing/calibration:
1. Long period dial test
2. Using rotary sub- standard meter
3. Using precision grade instruments
All above methods uses connection of energy meter under test in parallel with rotary substandard meter or precision grade meter. As in figure the current coils are connected in series hence both the instruments carry same currents and pressure coils are connected in parallel so that same voltage is applied across them. The meters are started and stopped at the same time. The energy readings at the end are compared and error can be calculated and meter is corrected.
Let D = registration of meter under test in kWh &
Ds = registration of substandard meter in kWh
Then % error = [(D – DS)/DS] x 100