Main equipotential bonding — which extraneous-conductive-parts must be connected (§411.3.1.2)

The guide on protective earthing (§411) lists as a common mistake that PE was once (before 1980) connected to the water or gas pipe — a practice now prohibited as a protective measure. That is not the same thing as main equipotential bonding under §411.3.1.2: connecting extraneous-conductive-parts to the main earthing terminal, not to carry fault current away through that pipe, but to prevent a pipe entering the building from outside from sitting at a different potential from the rest of the installation during a fault. This article covers which parts that involves, and where the connection must be made.

What is an extraneous-conductive-part?

An extraneous-conductive-part is a conductive part that is not part of the electrical installation, but that is liable to introduce a potential — generally earth potential — into the building. Typical examples:

Metallic water pipe at the point where it enters the building.
Metallic gas pipe at the point where it enters the building.
Metallic central-heating pipework and air-handling ductwork that runs through the building.
Exposed metallic structural parts of the building (steel frame, reinforcement, where accessible).
Metallic lift guide rails, where accessible to touch.

For the requirements on the cross-section of the bonding conductor itself (½ the cross-section of the largest protective conductor, min. 6 mm², max. 25 mm² Cu) — see the guide on main bonding for greenhouse structures, which applies that same §544 cross-section rule to a steel greenhouse frame. That article covers the thickness of the connection; this article covers which parts need that connection and where.

What does not need to be bonded

A plastic pipe entering the building carries no earth potential and does not need bonding.
A pipe section downstream of an insulated coupling (for example a plastic transition piece in an otherwise metallic gas pipe) is electrically interrupted from the incoming section and does not require separate bonding, provided that interruption remains reliable.
An insulated metal part that is not connected to earth and cannot simultaneously be touched together with an earthed part is not an extraneous-conductive-part in the sense of §411.3.1.2 — although this assessment calls for caution in practice: apparent insulation can become a conductive path over time (moisture, corrosion).

Where the connection must be made

The main bonding connection must be made as close as practicable to the point of entry of the service into the building, and before any branch, isolating valve or water meter — so that the entire internal pipework, regardless of later branches, is brought to the same potential as the main earthing terminal. A bonding clamp fitted only after a branch may leave the section before that branch outside the bonding.

Practical relevance

At handover of a new installation, or during a NEN 3140 inspection of an existing building, checking main bonding is a recurring inspection point: is every service entering from outside (water, gas, central heating) bonded, is the clamp fitted before the first branch, and is the bonding conductor itself sized correctly? A metallic pipe added later — for example a new gas boiler with a metallic flue, or new air-handling ductwork — must still be added to the bonding scheme; this is regularly forgotten during renovations.

Common mistakes

Bonding only the water pipe and skipping the gas pipe or central-heating pipework — every extraneous-conductive-part entering the building requires its own bonding connection, not just the most obvious one.
Fitting the bonding clamp after a branch, isolating valve or water meter instead of before it — the section before the clamp then remains outside the bonding.
Confusing main bonding with supplementary bonding — main bonding is done once, at the origin of the installation, for the whole building; supplementary bonding is local and applies to specific higher-risk locations (such as bathrooms) when the basic- and fault-protection conditions are not otherwise guaranteed there.
Not adding a later-installed metallic pipe section to the main bonding — for example new air-handling ductwork or a new metallic flue added during a renovation but not incorporated into the existing bonding scheme.