There is no specific level set for power measurement in Green Mark, but the overall measurement uncertainty has to be within ±5%. It is recommended to have at least a minimum accuracy class of 1 for the power meters and an accuracy class of 0.5 for the current transformers. These are common specifications for power meter and current transformer in the market.
A minimum of four (4) power meters are required for each of the following groups of equipment:
- Chilled water pump(s),
- Cooling Tower(s), and
- Condenser water pump(s).
The Current Transformer is the main component affecting power measurement(s). The accuracy class of the CT has to be better than that of the power meter in order to achieve the required measurement accuracy. Power Meter suppliers will usually specify the minimum CT accuracy class required to achieve the meter measurement accuracy.
Presently, type test reports from manufacturers are acceptable. Nevertheless, we strongly encourage the use of verified power meters, with individual test certificate prior to the installation of any power meter(s) and current transformer(s) in order to achieve the overall chilled water plant measurement accuracy.
The simple averaging of meter readings only works for pure sine-waves. However, such pure sine-waves do not exist in the real world of an electrical installation. A true RMS meter works by taking the square of the instantaneous value of the input current, averaging over the time and then displaying the square root of this average. This is accurate and applicable for all waveforms. True RMS power measurement is essential in any installation where there are non-linear loads (e.g. computers, electronic ballasts, compact fluorescent lamps, etc).
The power meter should capable of measuring the following:
a) Active energy (kWh) for 1 min per interval
b) Reactive energy (kVarh) for 1 min per interval
c) Active power (kW) for 1 min per interval
d) Reactive power (kVar) for 1 min per interval
e) Power factor for each phase