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During electricity generation, transformation, distribution and consumption, distortions of voltage and current sinusoidal signals occur. Most consumers are non-linear, and a significant content of higher harmonics emerges when they are connected to the supply grid. The algorithms of modern electricity meters take into account most of higher harmonics, whilst with some fallibilities. However, when supply voltage and current have the same amplitude and a various harmonic composition, the readings of electricity meters can vary greatly. This article describes how the harmonic composition of supply voltage and current affects the readings of electricity metering devices. The analysed ratios do not depend on the amplitude of supply voltage.

An electricity meter is a tool that determines the amount of electricity transferred to the network by a producer and consumed by electrically powered loads. Metering devices are composed of sensitive sensors that simultaneously measure currents and voltages and a microprocessor, which calculates required values basing on the received data. This article gives an overview of the most common current sensors, which are used in contemporary electricity metering devices. Their principles, advantages and disadvantages are reviewed, conclusions about the external influences on the sensors, which can lead to the incorrect operation of a meter and wrong definition of consumed electricity, are made.