Question 1

What is electrical bonding?

Accepted Answer

Electrical bonding is the practice of connecting metallic items (pipework, structural steelwork, metal enclosures) together and to earth, so that all metalwork is held at the same electrical potential (voltage). The purpose is to prevent a dangerous potential difference appearing between metalwork items that a person might touch simultaneously. For example, if a gas pipe became accidentally energised (due to a fault in an appliance connected to it) but was not bonded, touching the gas pipe and the earthed metalwork simultaneously would expose the person to the full fault voltage. With bonding, both items are at the same potential and no current flows through the person. Bonding does not prevent faults - it limits the voltage that can appear between exposed conductive parts during fault conditions.

Question 2

What is the difference between main bonding and supplementary bonding?

Accepted Answer

Main equipotential bonding connects the main incoming services to the installation's earthing terminal near the consumer unit. The services that require main bonding in a domestic property are: the incoming gas pipe (at the consumer's side of the meter, as close to the entry point as practicable); the incoming water supply pipe (at the stopcock or as near to the entry as possible); and any structural steelwork. The bonding conductor must be at least 10mm2 copper for a TN-S or TN-C-S system. Supplementary bonding is additional bonding within a specific room (typically a bathroom or shower room) between exposed conductive parts (metal bath, metal pipework) and extraneous conductive parts (other metalwork) within that location. Under the 18th Edition, supplementary bonding in bathrooms is no longer required if all circuits serving the bathroom are protected by a 30mA RCD - which is now standard practice. Main bonding is always required regardless of RCD provision.

Question 3

What size cable is used for bonding?

Accepted Answer

Main equipotential bonding conductors must be at least half the cross-sectional area of the installation's earthing conductor, and never less than 6mm2 copper. In most domestic properties on a TN-C-S (PME) supply, the earthing conductor is 16mm2, so main bonding conductors must be 10mm2 minimum. In practice, 10mm2 green-and-yellow single-core cable (sometimes in green sleeving on an older installation) is the standard for main bonding. Supplementary bonding conductors (where still required) must be at least 2.5mm2 mechanically protected or 4mm2 unprotected. Bonding connections must be made with proprietary bonding clamps that are accessible for inspection and testing - not concealed within walls. A label 'Safety Electrical Connection - Do Not Remove' must be fixed at each bonding connection.

Question 4

Does replacing pipes or adding a bathroom extension need new bonding?

Accepted Answer

Yes - any work that extends or modifies the metallic service pipework may require new or extended bonding connections. If a plastic section is inserted into the gas or water supply pipe (which is common with modern plastic push-fit pipework), the bonding connection must be re-made on the metal section closest to the meter/entry point, upstream of the plastic break. A plastic section in the pipework creates a break in the bonding continuity - metalwork downstream of the break is no longer bonded. Bathroom additions must have the circuits assessed to determine whether supplementary bonding is needed (not required if all circuits in the bathroom are RCD-protected). An electrician must inspect and certify any significant additions to the electrical installation under Part P of the Building Regulations.

Electrical Bonding

Just so you know...