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Protective conductors and conductive partsExposed or extraneous. What's the difference?

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All the green and yellow conductors get called "earth", but they have different names, connect to different places and do different jobs. They all share one end goal: automatic disconnection of supply (ADS).

Earthing conductor
Joins the MET (main earthing terminal) to the supply earth (the means of earthing).
CPC
Circuit protective conductor. Joins exposed conductive parts back to the MET.
Main protective bonding conductor (MPBC)
Joins extraneous conductive parts (e.g. incoming gas and water pipes) to the MET.
Supplementary bonding conductor
Links exposed and extraneous conductive parts locally, e.g. in a bathroom.
TN-Stransformerstar pointsupply earth (sheath)C/UMETloadexposedconductive partCPCgas meterstopcockgaswaterbranch goesback into groundearth fault loop1 Ω → 230 A → tripsextraneous conductiveparts (metal from the ground)
Our TN-S installation. The red dashed line is the earth fault loop: from the star point, along the line conductor, through the fault, back along the CPC to the MET, through the earthing conductor and the supply earth to the star point. The metal pipes aren't bonded yet.
Exposed conductive part
A conductive (metal) part of the electrical installation that is not normally live but can become live under a fault.
Examples: an oven, a metal switch, socket or light fitting, metal conduit, trunking and cable tray, the steel wire armour of an SWA cable, a metal consumer unit, a metal kettle.
Extraneous conductive part
A conductive part that is not part of the electrical installation and can introduce a potential, usually earth potential, into the premises.
Examples: metal water and gas pipes, an oil supply pipe, exposed structural steel that goes into the ground.

BS 7671 · Part 2 Definitions · "Exposed-conductive-part" and "Extraneous-conductive-part" · check your copy for the page

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