Earthing and Bonding Definitions
It is essential to understand the difference between earthing and bonding.
Earthing is defined in BS7671 as: “Connection of the exposed-conductive-parts of an installation to the main earthing terminal of an installation”.
Equipotential bonding is defined as: “Electrical connection maintaining various exposedconductive-parts and extraneous-conductiveparts at substantially the same potential”. (N.B. referred to as bonding throughout this document).
Both of these functions are carried out using protective conductors (as defined in the BS7671 Regulations). Protective conductors for Earthing are known as Circuit Protective Conductors (CPCs), protective conductors for Bonding are known as Bonding Conductors and may be either Main Protective or Supplementary.
The Earthing Conductor is defined as “A protective conductor connecting the Main Earthing Terminal of an installation to an earth electrode or to other means of earthing”.
For a TN systems the means of earthing is either a Protective Multiple Earthing (PME) terminal or a separate conductor which may be the cable sheath. In a TT system the means of earthing is the earth electrode which must comply with the requirements of sub-section 542.2.
The definition of an extraneous-conductive-part is “a conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation”. Isolated metal door handles, shelf brackets, metal windows etc, are unlikely to be extraneous-conductive-parts.