Earthing and Bonding: Why It Is the Last Line of Defence
A flat whose bathroom was renovated months ago. The work looks neat and everything functions: the heater heats, the lights work, no breaker trips. What nobody sees is that an installer removed a bonding conductor from a metal pipe to get his work done, and never put it back. The whole installation runs entirely normally and shows no sign of a problem — until one fault moment, when a metal casing in the bathroom becomes a danger to life. That is the nature of earthing faults: completely silent until the moment they are needed.
1. What Earthing Actually Does
The common idea that earthing "drains excess electricity into the ground where it disappears" is inaccurate and leads to bad decisions. Earthing does two specific things:
- It provides a low impedance path for fault current. When a live conductor contacts an appliance's metal casing, a good conductive path must exist for fault current to return to the source. That path allows a large fault current, and it is precisely that large current which makes the protective device operate quickly. The aim is not to bleed current away quietly, but to create an unambiguous fault that forces protection to act at once.
- It holds exposed metalwork near earth potential. Even in the moments before protection operates, the potential difference between a metal casing and earth, and between different metal parts, is kept as small as possible, reducing the voltage a person could be exposed to.
2. Earthing versus Bonding
| Aspect | Earthing | Bonding |
|---|---|---|
| Purpose | Connects exposed metalwork to the earth reference to carry fault current | Connects different metal parts together to minimise potential difference between them |
| Question answered | How does fault current return so the breaker operates? | If a person touches two metal bodies at once, what is the voltage between them? |
| What is connected | Appliance casings, boards, metal containment, via protective conductors | Incoming metal water and gas pipes, HVAC frames, building-associated metalwork |
| Consequence if missing | Insufficient fault current, so the device does not trip or is delayed | Dangerous potential difference between two metal bodies that both look earthed |
Main Protective Bonding of Incoming Services
At the building's entry, incoming metal services — metal water and gas pipes, and structural metalwork where required — are bonded to the main earthing terminal with conductors of suitable size. These pipes run throughout the building and may sit at a different potential from the electrical installation; bonding at entry brings them all to one reference and removes the dangerous difference.
Supplementary Bonding
In higher-risk locations — bathrooms, wet areas, swimming pools — where wet skin has far lower resistance, local supplementary bonding may be required, connecting simultaneously accessible metal parts to each other and to the protective conductor. The aim is that everything a person can touch at once sits at essentially the same potential, so no current flows through them at all. Requirements and the conditions for omitting it are set by local codes and decided by a qualified person.
3. Why High Resistance Earthing Is More Dangerous Than None
This is the article's central idea. Consider three cases:
- Sound earthing: a fault occurs, large fault current flows, protection disconnects within a fraction of a second, the casing stays safe.
- No earthing at all: plainly hazardous, but detectable by inspection and measurement, and many technicians recognise it.
- Earthing present but high resistance: a corroded clamp, a loose joint, a partly severed conductor, an electrode in degraded dry soil. This is the worst case.
Why is the third worst? Because high resistance limits fault current to a very small value. The consequences compound:
- The protective device never sees enough current, so it does not trip, or trips only after a long delay.
- The metal casing stays at a dangerous potential for a long time because the fault is not cleared.
- Meanwhile, everything in the installation appears completely normal: appliances run, lights work, no breaker trips, nothing draws attention.
That is exactly the danger: bad earthing announces itself with no symptom, and is revealed only by specialist measurement. Hence periodic continuity and earth resistance testing is necessary work, not a luxury.
4. Touch Voltage
Touch voltage is the potential difference a person's body is actually exposed to when contacting faulty metalwork — between hand and the floor they stand on, or between two hands on two bodies. The danger is not that voltage exists in the network, but the difference imposed across the person and how long it lasts.
- That is why we bond parts together: two metal bodies at the same potential produce no difference when touched together.
- That is why disconnection time matters, not just disconnection: a dangerous voltage lasting an instant is far less harmful than the same voltage lasting seconds.
- That is why wet areas are more dangerous: wet skin and a conductive floor raise the current through the body at the same voltage.
5. Earth Electrode Types
| Type | Description | Notes |
|---|---|---|
| Driven rod | A coated metal rod driven vertically, extendable in sections | Most common and easiest; performance improves with depth as it reaches moister, more stable soil |
| Buried tape or strip | A conductor laid horizontally in a trench | Useful where soil is shallow or rocky and deep driving is impossible |
| Mesh or ring electrode | Interconnected conductors surrounding the building or plant | Used on larger installations; distributes potential in the soil, reducing surface gradients |
| Foundation earth | A conductor integrated into the building's foundation steel or concrete | Very stable long term, protected from corrosion; must be installed during construction |
| Plate | A buried metal plate | Used in particular site conditions |
Electrode performance depends heavily on soil type, moisture and salt content, so resistance varies seasonally: severe summer dryness raises it noticeably, which is why measuring under the worst expected conditions is more honest than measuring after rain. The connection between conductor and electrode must remain accessible for inspection, not buried and forgotten.
6. Never Use a Gas or Water Pipe as the Sole Electrode
- Continuity is not guaranteed: large parts of networks have been replaced with plastic pipe, so a visible metal section may be electrically isolated from earth with no outward sign.
- Flexible couplings and water meters: any non-conductive fitting in the path breaks it.
- Future alterations: a plumber replaces a section with plastic to fix a leak, and the electrical installation silently loses its earth.
- Gas pipe is a special hazard: passing fault current through a gas network is never acceptable, and for safety reasons it must not be used as an electrode under any circumstances.
The important distinction: incoming metal water and gas pipes should be bonded to the main earthing terminal, which is entirely different from using them as an electrode. Bonding guards against potential difference; the electrode is a permanent structural responsibility that cannot be entrusted to a network non-electricians may alter.
7. Testing and Common Faults
What Gets Tested
- Protective conductor continuity: confirming a sound path from every point in the installation back to the main earthing terminal, by measurement that reveals loose joints and broken conductors.
- Bonding conductor continuity: actually testing main and supplementary bonding — seeing a wire near a pipe is not enough.
- Electrode resistance and earth fault loop impedance: instrument measurements establishing whether prospective fault current is sufficient to operate protection in the required time.
- RCD operation: periodic testing, since it is an essential partner in the protection system.
These measurements need appropriate test instruments and safety procedures, must be performed by a qualified technician or engineer, and should be recorded for comparison at later inspections.
The Most Frequent Field Faults
- Corroded clamps in the earth pit: a clamp in damp soil corrodes over time, can look sound and still have very high resistance. Appearance never replaces measurement.
- Bonding disconnected after renovation work: the most common and most dangerous fault. A plumber, HVAC technician or builder removes a bonding conductor to work and forgets to restore it. The installation runs exactly as before, with no symptom.
- An unconnected protective conductor in a box or socket: the earth tail is tucked into the box unterminated, and the socket works perfectly normally.
- Undersized conductors: a protective or bonding conductor thinner than required cannot carry fault current safely.
- Relying on metal conduit or trunking as a protective conductor without assured joints: one twisted or loose joint breaks continuity.
- A buried, forgotten electrode: an inaccessible connection point cannot be tested or maintained.
- Parts bonded together without a return to the main earthing terminal: an isolated bonding network gives a false sense of safety.
Conclusion
Earthing is not a reassurance wire; it is an engineered system whose job is to make a fault visible to protection so it disconnects fast, and to keep the metal you touch near earth potential. Bonding is its counterpart, preventing potential difference between two metal bodies within reach. The gravest property of the whole system is its silence: high resistance earthing and disconnected bonding produce no visible symptom, and the installation runs perfectly normally until the moment it is needed and fails. So do not judge earthing by looking at it — judge it by periodic measurement, by a qualified person, under the local code in force.
