TESTING OF ENERGY METERS
Energy meters are tested at the following conditions:
- At 5% of rated current at unity power factor
- At 100% or 125% of rated current with unity power factor
- At one intermediate load with unity power factor
- At rated current and 0.5 lagging power factor
Creep test With pressure coil supplied with 110% of rated voltage and current coil open circuited, the meter disc should not rotate by more than one revolution, i.e., it should not creep. Starting test At 0.5% of rated current and full rated voltage, the meter disc should start rotating.
Phantom Loading
When the current rating of the meter under test is high, a test with actual loading arrangements would involve considerable wastage of energy and also it is difficult to arrange for such large loads under laboratory test conditions. In such cases, to avoid this, ‘phantom’ or ‘fictitious’ loading arrangements are done for testing of energy meters Phantom loading consists of supplying the shunt magnet pressure coil circuit from a rated voltage source.
The series magnet current coil is supplied from a separate low voltage supply source. It is
possible to circulate rated current through the current coil circuit with the low voltage source since impedance of this circuit is very low. The energy indicated by the meter under phantom loading condition is the same as the energy indication as would have been with a real load. With this arrangement, the total energy consumed for the test is comparatively smaller. The total energy required for the test is that due to the small pressure coil current at rated voltage and small current coil voltage at rated current