What are earthed and unearthed systems?
What are the requirements of BS 7671?
What are the advantages and disadvantages of the various types of earthing systems?
This article, which is based on IEE Guidance notes, is intended to provide information that it is hoped will prove helpful.
BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT. T = Earth (from the French word Terre) N = Neutral S = Separate C = Combined I = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth. All exposed-conductive-parts of an installation are connected to an earth electrode.)