Cable Heating Effects due to Harmonic Distortion in Electrical Installations

The increasing use of non-linear loads in electrical installations has exacerbated the problems of harmonic distortion in industrial and commercial electrical systems. In the UK the current practice to determine the cable size for an electric circuit is to use BS7671. However, previously the 16th edition IEE Wiring Regulations only dealt with situations where cables attain the conductor temperature generated by sinusoidal currents at the fundamental power frequency. This paper outlines the methods available to determine the minimum size of line conductors for protection against overload currents, taking into account the harmonic content of the load current, and explains the harmonic rating factor Cf introduced in 2008 for cables that are under significant harmonic influences. Since the effect of harmonic currents is to increase the joule losses in a cable, the ampacity of the cable will need to be corrected to ensure the maximum conductor operating temperature is not exceeded. An experiment on how cable temperature can be measured under harmonic influence is described, and several sets of measurements taken on a typical cable are analysed. The paper concludes that direct usage of the BS7671 rating factor for harmonics appears to be rather conservative and could lead to over-sizing of the line conductors for three-phase circuits, but is deemed beneficial in the long run.