Nonlinear equipment connected to an electricity supply system causes harmonic distortion of the voltage waveform. This distortion can adversely affect consumers' equipment and so it is necessary to limit the levels of harmonics. Engineering Recommendation G5/4 gives the UK requirements for the connection of nonlinear equipment to the public electricity supply system. Its objective is to ensure that the levels of harmonics are maintained below the compatibility levels so that other equipment connected to the supply system will not maloperate or fail prematurely.