What is protective bonding?

Glossary · Wiring, circuits and faults

How protective bonding works

Electrical Safety First says earthing and bonding are two distinct methods to prevent electric shock.^{[3]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding} Earthing gives fault current a path to earth so the protective device switches off. Bonding reduces the risk of shock to anyone who touches two separate metal parts when there is a fault, by connecting bonding conductors that reduce the voltage there might have been.^{[3]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding}

The IET puts the difference this way: earthing connects the exposed metal parts of the installation to the main earthing terminal, while protective bonding connects extraneous-conductive-parts to that terminal to prevent a potential difference creating a hazardous touch voltage.^{[2]IET, BS 7671 FAQs - Earthing and Bonding} An extraneous-conductive-part is a conductive part liable to introduce a potential, generally earth potential, that does not form part of the electrical installation.^{[1]IET, Protective bonding habits} A metal water pipe coming in from the ground is the textbook case. See our guide to earthing systems for the earthing side.

Main and supplementary bonding

The IET lists what main bonding conductors connect: water installation pipes, gas installation pipes, other installation pipework and ducting, central heating and air conditioning systems and exposed metallic structural parts, as long as each meets the definition of an extraneous-conductive-part.^{[1]IET, Protective bonding habits} Where a metal pipe has an insulating section at its point of entry, it need not be connected.^{[1]IET, Protective bonding habits}

Supplementary bonding is usually only required in special locations such as rooms containing a bath or shower, or when automatic disconnection cannot be achieved. The current edition of BS 7671 allows it to be omitted provided the circuits are RCD protected and the extraneous parts are connected to the main earthing terminal through the main bonding conductor.^{[1]IET, Protective bonding habits} That is why many modern bathrooms have no visible supplementary bonding.

Where bonding is not needed

The IET says it is a common misconception that all water pipes need bonding. If the incoming service pipe is plastic and nothing makes accidental contact with earth, it cannot introduce earth potential, so it is not an extraneous part.^{[2]IET, BS 7671 FAQs - Earthing and Bonding} The IET also lists metal baths and sinks among items that are not likely to be extraneous-conductive-parts, because they do not introduce earth potential, so they do not need protective bonding.^{[1]IET, Protective bonding habits}

Older homes often have more bonding than today’s rules require. The IET explains that the 15th Edition of the wiring regulations in 1981 called for items such as baths, sinks, taps, tanks and radiators to be bonded, and the 16th Edition in 1991 reduced that to extraneous parts only.^{[1]IET, Protective bonding habits} The IET also warns that bonding things that need no bonding is not harmless, because fault currents can then be exported through the installation.^{[1]IET, Protective bonding habits}

For homes on a PME supply, the IET says bonding requirements are more onerous than for TN-S, to withstand diverted neutral currents from a broken supply neutral.^{[5]IET, Broken PEN}

Example

Joanne is selling a 1930s house in Chapel Allerton. The buyer’s surveyor notes a green-and-yellow wire clamped to the pipe by the gas meter and another to the bath tap. Her electrician explains that the one at the meter is main bonding and should stay, while the one at the bath tap may be older supplementary bonding that current rules would not require. He leaves both as they are and records them on the report.

What people get wrong

Many people assume bonding protects against live wires touching pipes. The IET says that is not its purpose. Bonding keeps metal parts at the same potential, and other regulations protect pipework from live parts.

The IET describes this as a common misunderstanding behind bonding that is not required.^{[1]IET, Protective bonding habits} The second mistake is touching the clamps. The IET warns that before disconnecting any earthing or bonding conductor, an electrician must check that no diverted neutral current is flowing in it, and that this can occur even when the installation is isolated.^{[5]IET, Broken PEN} Leave them alone.

When to ask an electrician

Electrical Safety First says an electrician must check the earthing and bonding before any electrical work, because the safety of new work depends on them.^{[3]Electrical Safety First, Electrical Safety Advice for Earthing and Bonding} GOV.UK says a rented home’s inspection will find out whether there is a lack of earthing or bonding.^{[4]Ministry of Housing, Communities and Local Government, Electrical safety standards in the private and social rented sectors: guidance} If you suspect something is missing, book an EICR.

The bottom line

Protective bonding links metal parts that could bring in a voltage to the main earth, so no dangerous difference can appear in a fault. Main bonding is for the whole installation. Supplementary bonding is usually only for special locations. Plastic pipes need none, and old bonding is not a fault in itself. Fit, test and change it only through a registered electrician.