Protective bonding conductor: protective conductor provided for protective-equipotential-bonding [this term is defined in the IEC 60050-195-2021].
The following metal parts are not permitted for use as protective bonding conductors:
- metallic water pipes;
- metallic pipes containing potentially flammable materials such as gases, liquids, powder;
- constructional parts subject to mechanical stress in normal service;
- flexible or pliable metal conduits, unless designed for that purpose;
- flexible metal parts;
- support wires; cable trays and cable ladders.
Where protective equipotential bonding conductors are installed, they should be parallel to and in closest contact as possible with d.c. cables and a.c. cables and accessories.
A distinction must be made between an earthed protective bonding conductor and an unearthed protective bonding conductor. I give their definitions according to [2]:
Earthed protective bonding conductor: protective bonding conductor which has a conductive path to local earth.
[2]
Unearthed protective bonding conductor: protective bonding conductor which is isolated from the Earth.
[2]
Protective bonding conductors for connection to the main earthing terminal shall have a crosssectional area not less than half the cross-sectional area of the largest protective earthing conductor within the installation and not less than:
- 6 mm2 copper; or
- 16 mm2 aluminium; or
- 50 mm2 steel.
The cross-sectional area of protective bonding conductors for connection to the main earthing terminal need not exceed 25 mm2 Cu or an equivalent cross-sectional area for other materials.
In Switzerland, if used in conjunction with installations for lightning protection, the minimum crosssectional area of the main protective bonding conductor shall be 10 mm2.
In the UK, particular requirements exist regarding the minimum acceptable cross-sectional areas for protective bonding conductors where Protective Multiple Earthing (PME) conditions apply.
Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation and not less than 6 mm2. The cross-sectional area need not exceed 25 mm2 if the bonding conductor is of copper or a cross-sectional area affording equivalent conductance in other metals.
Except for highway power supplies and street furniture, where PME conditions apply the main protective bonding conductor shall be selected in accordance with the neutral conductor of the supply.
Where an installation has more than one source of supply to which PME conditions apply, a main protective bonding conductor shall be selected according to the largest neutral conductor of the supply.
| Copper equivalent cross-sectional area of the supply neutral conductor | Minimum copper equivalent* cross-sectional area of the main protective bonding conductor |
| 35 mm2 or less | 10 mm2 |
| over 35 mm2 up to 50 mm2 | 16 mm2 |
| over 50 mm2 up to 95 mm2 | 25 mm2 |
| over 95 mm2 up to 150 mm2 | 35 mm2 |
| over 150 mm2 | 50 mm2 |
Table 1 – Minimum cross-sectional area of the main protective bonding conductor in relation to the neutral of the supply [Source: BS7671]
*The minimum copper equivalent cross-sectional area is given by a copper bonding conductor of the tabulated cross-sectional area or a bonding conductor of another metal affording equivalent conductance.
The main protective bonding connection to any gas, water or other service shall be made as near as practicable to the point of entry of that service into the premises. Where there is an insulating section or insert at that point, or there is a meter, the connection shall be made to the consumer’s hard metal pipework and before any branch pipework. Where practicable the connection shall be made within 600 mm of the meter outlet union or at the point of entry to the building if the meter is external.
A protective bonding conductor connecting two exposed-conductive-parts shall have a conductance not less than that of the smaller protective conductor connected to the exposed-conductive-parts.
A protective bonding conductor connecting exposed-conductive-parts to extraneous-conductive-parts shall have a conductance not less than half that of the cross-sectional area of the corresponding protective conductor.
The minimum cross-sectional area of protective bonding conductors for supplementary bonding, and of bonding conductors between two extraneous-conductive-parts, shall be in accordance with 543.1.3 [2].
The cross-sectional area of every protective conductor which does not form part of a cable or which is not in a common enclosure with the line conductor shall be not less than 2,5 mm2 Cu or 16 mm2 Al if protection against mechanical damage is provided, 4 mm2 Cu or 16 mm2 Al if protection against mechanical damage is not provided.
543.1.3 [2]
A supplementary bonding conductor connecting two exposed-conductive-parts shall have a conductance, if sheathed or otherwise provided with mechanical protection, not less than that of the smaller protective conductor connected to the exposed-conductive-parts. If mechanical protection is not provided, its crosssectional area shall be not less than 4 mm2.
A supplementary bonding conductor connecting an exposed-conductive-part to an extraneous-conductivepart shall have a conductance, if sheathed or otherwise provided with mechanical protection, not less than half that of the protective conductor connected to the exposed-conductive-part. If mechanical protection is not provided, its cross-sectional area shall be not less than 4 mm2.
544.2.2 [4]
A supplementary bonding conductor connecting two extraneous-conductive-parts shall have a crosssectional area not less than 2.5 mm2 if sheathed or otherwise provided with mechanical protection or 4 mm2 if mechanical protection is not provided, except that where one of the extraneous-conductive-parts is connected to an exposed-conductive-part in compliance with Regulation 544.2.2 [4], that regulation shall apply also to the conductor connecting the two extraneous-conductive-parts.
Except where Regulation 544.2.5 [4] applies, supplementary bonding shall be provided by a supplementary conductor, a conductive part of a permanent and reliable nature, or by a combination of these.
Where supplementary bonding is to be applied to a fixed appliance which is supplied via a short length of flexible cable from an adjacent connection unit or other accessory, incorporating a flex outlet, the circuit protective conductor within the flexible cable shall be deemed to provide the supplementary bonding connection to the exposed-conductive-parts of the appliance, from the earthing terminal in the connection unit or other accessory.
544.2.5 [4]
Protective bonding conductors shall be protected against mechanical damage and corrosion, and shall be selected to avoid electrolytic effects. For example, the following may be used:
- Hot-dip galvanized steel strip with dimensions of at least 30 mm × 3 mm
- Hot-dip galvanized round steel of at least 8 mm diameter
- Copper conductor having a minimum cross-sectional area of 4 mm2.
Other suitable materials may be used.
Figure 1 – Examples of earthing arrangements for foundation earth electrode, protective conductors and protective bonding conductors [Figure B.54.1, IEC 60364-5-54]
NOTE. Functional earthing conductors are not shown in Figure B.54.1.
| Symbol | Name |
| C | Extraneous-conductive-part. |
| C1 | Water pipe, metal from outside. Or district heating pipe. |
| C2 | Waste water pipe, metal from outside. |
| C3 | Gas pipe with insulating insert, metal from outside. |
| C4 | Air-conditioning. |
| C5 | Heating system. |
| C6 | Water pipe, metal e.g. in a bathroom. |
| C7 | Waste water pipe, metal e.g. in a bathroom. |
| D | Insulating insert. |
| MDB | Main distribution board. |
| DB | Distribution board. Supplied from the main distribution board. |
| MET | Main earthing terminal. |
| SEBT | Supplementary equipotential bonding terminal. |
| T1 | Concrete-embedded foundation earth electrode or soilembedded foundation earth electrode. |
| T2 | Earth electrode for LPS if necessary. |
| LPS | Lightning protection system (if any). |
| PE | PE terminal(s) in the distribution board. |
| PE/PEN | PE/PEN terminal(s) in the main distribution board. |
| M | Exposed-conductive-part. |
| 1 | Protective earthing conductor (PE). |
| 1a | Protective conductor, or PEN conductor, if any, from supplying network. |
| 2 | Protective bonding conductor for connection to the main earthing terminal. |
| 3 | Protective bonding conductor for supplementary bonding. |
| 4 | Down conductor of a lightning protection system (LPS) if any. |
| 5 | Earthing conductor. |
Table 2 – Keys for figure 1 
Figure 2 – Illustration of earthing and protective conductor terms (Figure 2.1, BS7671)
| Symbol | Name |
| 1,2,3,4 | Protective conductors. |
| 1 | Circuit protective conductor. |
| 2 | Main protective bonding conductor. |
| 3 | Earthing conductor. |
| 4 | Supplementary protective bonding conductors (where required). |
| B | Main earthing terminal. |
| M | Exposed-conductive-part. |
| C | Extraneous-conductive-part. |
| P | Main metallic water pipe (extraneous-conductive-part). |
| T | Earth electrode (TT and IT systems). |
| E | Other means of earthing (TN systems). |
Table 3- Keys for figure 2
A protective bonding conductor shall be identified by the bi-colour combination GREEN-AND-YELLOW as specified in 6.3.2 [3].
The alphanumeric identification of a protective bonding conductor shall be “PB”.
A protective bonding conductor will in most cases be an earthed protective bonding conductor.
In those cases where a distinction between an earthed protective bonding conductor and an unearthed protective bonding conductor is necessary (for example, within electro-medical installations), the earthed protective bonding conductor shall be identified as “PBE” and the unearthed protective bonding conductor shall be identified as “PBU”.
If it is necessary to distinguish between an earthed protective bonding conductor and an unearthed protective bonding conductor, the alphanumeric identification of the earthed protective bonding conductor shall be “PBE”.
If it is necessary to distinguish between an earthed protective bonding conductor and an unearthed protective bonding conductor, the alphanumeric identification of the unearthed protective bonding conductor shall be “PBU”.
The main protective bonding conductor is used to provide a low impedance path between the main earthing terminal and any accessible conductive parts of the electrical installation. This ensures that fault currents can flow freely, allowing the fuse or circuit breaker to operate and clear the fault.
The correct answer is c) the gas and water service pipes
- IEC 60050-195-2021
- IEC 60364-5-54
- IEC TS 62257-5-2015
- BS7671
