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METERING The importance of – commercial implications of inaccurate meters With the increasing use of energy and its rising cost, the sustained accuracy of energy meters is more important now than ever before. In many countries, there are indirect commercial implications to the use of energy. For example, if the estimation of tax levied on a manufacturing organisation is based on its electricity consumption. Consumption recorded by meters is also used to estimate carbon emissions. Why does a meter become inaccurate with time? How can the inaccuracy be minimised? The durability of voltage and current sensors and power supply are important factors in achieving sustained accuracy in electricity meters. Most popular voltage sensors are resistive potential dividers because of their low cost, linearity and they are not affected by a magnetic field. This resistive divider is directly connected to the mains voltages that are to be measured. Hence a mains facing resistor should be a high voltage resistance; typically high ohmic/high voltage By Bal Mukund Vyas metal glaze resistors. Any change in value of the this resistor will directly affect the accuracy of the meter. The value of resistance can change due to loading, pulsed loading, ESD and environmental cycling. The typical ageing specification of such resistors is shown in Table 1. These specifications indicate that a new resistor might change its value by a significant amount after the meter is installed in field. When a new meter is calibrated during manufacture, the resistor is new. The change in value of resistance after the calibration will increase the errors in energy recorded by the meter. One of the methods to minimise this is to pre-burn the resistors before manufacturing the meters. Current sensors are usually current shunts, current transformers, Hall Effect sensors etc. The resistance of a current shunt might change and this will directly affect the error in energy recorded by the meters. A typical circuit of current transformers used in meters is shown in Figure 2. Figure 2: Typical architecture of a modern electronic meter from a metrological view point The resistance connected to the secondary circuit of this current transformer has to be of high stability because any change in its value will directly affect the error in the meter. Also it is important to ensure the correct power rating when selecting a resistor. High power dissipation in a resistor will increase the temperature of the film in the resistor and might seriously affect Change in its value will directly affect the error” Max. Resistance Change for Resistance Range, ∆R max., after: Load (1000 h) Figure 1: Typical architecture of a modern electronic meter from a metrological view point METERING INTERNATIONAL ISSUE – 2 | 2015 (1.5 % R + 0.1 ohm) Long Term Damp Heat Test (100 Days) , ∆R max (1.5 % R + 0.1 ohm ) Table 1. Typical ageing specification of resistors 17