Ask three people on a site what the green and yellow wire does and you will often get three different answers. The words earthing and bonding are used as if they meant the same thing. They do not, and the difference is not academic.
Two different jobs
Earthing provides a deliberate low-impedance path from exposed metalwork back to the source of supply. Its purpose is not to make things safe by draining electricity into the ground — it is to let enough fault current flow that a fuse blows or a breaker trips, fast. Earthing is about disconnection time.
Bonding connects metalwork together so that everything a person can touch at once sits at substantially the same potential. It does not carry fault current back to source and it does not operate any protective device. Its purpose is to make sure that when a fault does raise the voltage on one piece of metal, it raises it on the neighbouring pipe too, so there is no difference for a body to bridge.
How the confusion causes harm
An installation with excellent earthing and no supplementary bonding can still present a lethal touch voltage between a metal pipe and an appliance case during the fraction of a second before the breaker operates. In a plant room with wet floors, that fraction of a second is the whole story.
The reverse failure is quieter. Bonding conductors installed beautifully, earth path degraded somewhere out of sight by a corroded joint or a connection loosened by vibration, and nothing announces it. Everything measures fine at the socket. The circuit works. Nobody discovers the disconnection time is now four seconds instead of four hundred milliseconds until the day it matters.
What actually needs checking
- Earth fault loop impedance, measured, against the value the protective device needs to operate in the required time — not assumed from the design.
- Continuity of protective and bonding conductors, end to end, including through equipment enclosures and across flexible connections.
- Bonding at services entry — incoming water, gas and any structural metalwork — and at the point where any of it was cut and re-joined by another trade.
- Plastic repairs in metal pipework. A short plastic insert in a bonded run silently breaks the bond and is invisible behind lagging.
- Earth electrode resistance where the installation depends on one, measured seasonally if the ground dries out.
The pattern worth remembering
Most earthing and bonding faults are not design errors. They are maintenance errors: a joint opened for a repair and not remade, a bond removed to fit a new valve, a conductor left disconnected because the work is not finished. The design was fine. The installation stopped matching it, and nothing on the panel says so.
That is why periodic verification exists, and why it has to be measured rather than looked at.
When Was Your Earthing Last Measured Rather Than Inspected?
Our inspection and monitoring team carries out earth fault loop impedance testing, continuity verification and electrode resistance measurement, with findings reported against the disconnection times your protective devices actually need.
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