Earth Bonding in Domestic Property

Electricity is something we all take for granted. It is the unseen force, a stream of moving protons and electrons, which powers our machines and appliances. Perhaps the only time we give it much thought is when we receive the bill for consuming it.

However, this force, safely restrained within insulation cables, can be extremely dangerous if it is allowed to behave naturally. Electricity continually seeks to dissipate its energy by escaping from a circuit and flowing to earth.

This natural tendency is all well and good. So long as a person does not form part of a pathway allowing it to do so. A flow of electricity passing through a person to earth is commonly referred to as an electric shock.

In passing through any material towards earth, including a person, the electricity encounters resistance. Overcoming the resistance generates heat. This heat causes severe internal and external burns as it passes through a person. The flow of electrons also interferes with the natural electrical activity in the body causing severe damage and possible death.

Electricity will always find a route to earth that offers the least resistance. Faced with the option of a copper earth wire and human flesh, the electricity will choose the earth wire, plus a bit of the flesh.

In an attempt to prevent electrical shock, or damage to appliances caused by a sudden surge in electrical current, all circuits carry a fuse system. A surge in electricity will break the resistance tolerance of the fuse causing it to melt and cut the circuit. The fuse acts as the first line of protection in electrical circuits.

Domestic electrical circuits have to incorporate an earth circuit (PE).

In electrical terminology, earth is represented as ‘T’. This ‘T’ can take a number of forms. An earthing rod buried into the external ground of a property and forming an earth circuit through the electrical appliances would be referred to as ‘TT’. External earthing rods can be poor earth devices. A good earth requires a good contact with earth. The soil around earthing rods can dry out, shrinking away from the rods and causing poor earth contact.

Most homes utilise a TNC-S earthing system. ( N=nuetral, C=Combined, S=Separate) or better known as PME (Protective Multiple Earth).

With PME, earthing is carried by the supply company’s main earth back to the nearest sub-station. This earth is particularly good and reliable.

Bonding, on the other hand, is designed to prevent a disparity between electrical voltages if a fault occurs. Where a sudden release of electricity flows from an appliance, prior bonding of the metal in that appliance to other circuits that could contribute to creating alternative earth paths limit’s the voltage change potential.

There are two types of bonding, main bonding, and supplementary bonding.

Main bonding provides an interconnection between incoming metallic services such as gas and water. It is on these services that the householder will generally find the usual earth bonding yellow and green wires clamped to the pipe-work. There will also be main bonding joined to any metal fabric of the home, such as supporting iron and steel building construction materials. This earth bonding also provides protection in the event that it is the supplier’s earth that is causing the problem.

Supplementary or cross-bonding joins together metal components that could provide a circuit to earth, for instance if a fault developed on a towel rail in a bathroom and resulted in its surface becoming ‘live’, a person touching it and also touching a tap at the same time would form a circuit. Supplementary bonding links the earth across all these metal surfaces to reduce the destructive force of an electric shock. These bonding clamps can be seen on pipes and other metallic connections in bathrooms etc.

The final safety device in the home is performed by Residual Current Circuit Breakers (RCB’s). These devices monitor and detect changes in the steady and balanced flow of electrical current through the positive and negative wiring supply to appliances. A sudden leakage to earth, whether it be by an electric shock or other means of dissipation, will be detected almost instantaneously by the RCB. This will cause the RCB to operate (trip) and immediately break the circuit, significantly reducing the potential for harm or damage.

Although the complexities of earthing and bonding are possibly beyond the scope of DIY enthusiasts, the importance of supplying and maintaining them is not. It is most important that where a homeowner undertakes any remedial work to the property, or to the plumbing and electrical components, that earthing is provided and maintained.

When installing plastic pipework into a copper plumbing network, it is important to ensure that electrical bonds are maintained. This may entail building a bonding bridge between the plastic pipes and the continuation of the copper network to facilitate continuity.

Likewise, when working on pipe-work, cutting into a section of copper pipe during plumbing work will interfere with the bonding circuit. It is a wise precaution to temporarily provide a bonding bridge for electrical safety reasons.

Installing supplementary and cross bonding wiring is a simple procedure. Clamps and regulation-sized earth wire are easily obtainable from DIY and electrical suppliers. Bonding cables must run unbroken and continuous to the main earth block in the mains domestic consumer box, or the dedicated earth block located near it. It is possible to link supplementary bonding cables across platforms, such as bath taps to shower to towel rail, but main bonding must be continuous.

Homeowners should periodically check main and supplementary bonding cables and clamps to ensure that they continue to provide secure and serviceable operation. Any earth or bonding cable that becomes detached must be immediately replaced.

Although attaching earth and bonding connections does not carry any restrictions under the Building Regulations, anyone undertaking such work should make themselves acquainted with BS 7671 of the Wiring Regulations.

As always, any electrical work must be performed competently. Insurance cover may be affected by problems caused by substandard work. It is most important to seek professional advice when attempting to work on electrical installations. Building Regulations state that only certified persons can carry out electrical installation work.