Level 3 Diploma in Electrical Installations Buildings and

Level 3 Diploma in Electrical Installations (Buildings and Structures) Unit 305: Electrical systems design Electrical systems – earthing Power. Point presentation © 2015 City and Guilds of London Institute. All rights reserved. 1 of 9

Level 3 Diploma in Electrical Installations (Buildings and Structures) Electrical systems – earthing An electrical system consists of: • The source or sources (where the electricity is generated). For example, a generator or – more commonly – a transformer. • The distribution (the means of moving the energy to where it is needed) – such as distribution cables or overhead lines. • The installation – such as the electrical wiring and equipment in a dwelling or shop. © 2015 City and Guilds of London Institute. All rights reserved. 2 of 9

Level 3 Diploma in Electrical Installations (Buildings and Structures) Electrical systems – earthing First letter (relationship between supply and earth) T The supply is connected directly to earth (T coming from the Latin word terra, meaning earth) at one or more points. I The supply is isolated from earth, or one point is connected to earth through a high impedance. Second letter (relationship between earth and the exposed conductive parts of the installation) N Direct electrical connection of the exposed conductive parts to the earthed point of the supply system (for ac systems, the earthed point of the supply system is normally the neutral point). T Direct electrical connection of the exposed‑conductive‑parts to earth, independent of earthing of any point of the supply system. Subsequent letters (arrangement of neutral and protective conductors) C The neutral and protective functions are combined in a single conductor (the PEN conductor). S Protective function provided by a conductor separate from neutral conductor or earthed line (earthed phase in ac systems) conductor. © 2015 City and Guilds of London Institute. All rights reserved. 3 of 9