Earthing vs Bonding — Two Different Functions That Are Often Confused

Earthing and bonding are the two most confused terms in electrical engineering. Many electricians and even some engineers use them interchangeably, but they serve fundamentally different purposes. Getting the distinction wrong leads to installations that look safe but fail dangerously when something goes wrong.

Here is the simplest way to think about it:

• Earthing provides a path for fault current to flow back to the source, so that the protective device (circuit breaker or fuse) can detect the fault and disconnect the supply. Earthing is about clearing faults.
• Bonding connects metallic parts together so they are all at the same electrical potential. Bonding is about preventing touch voltages — ensuring that if you touch two metal surfaces simultaneously, there is no dangerous voltage difference between them.

An analogy: imagine a house with a gas leak. Earthing is the gas detector that triggers the automatic shutoff valve — it detects the problem and removes the hazard. Bonding is the ventilation system that ensures gas never accumulates to a dangerous concentration in any room — it manages the risk even if the shutoff fails. You need both, and neither substitutes for the other.

When a live conductor contacts an exposed metal part (say, the metal case of a washing machine due to a damaged cable inside), fault current must flow from that metal part back to the source (the transformer) to complete the circuit. Without a return path, the circuit breaker sees no fault current and does not trip. The metal case stays live at mains voltage, waiting for someone to touch it.

The earthing system provides this return path. In a typical domestic installation:

1. A circuit protective conductor (CPC) — the green/yellow wire in every cable — connects the metal case of each appliance back to the consumer unit.
2. At the consumer unit, all CPCs terminate at the main earthing terminal (MET).
3. The MET connects to the means of earthing — this could be the supply cable sheath (TN-C-S), a separate earth conductor from the substation (TN-S), or an earth electrode driven into the ground (TT).
4. The fault current flows through this path back to the transformer neutral, completing the circuit and allowing the protective device to detect the overcurrent and trip.

The critical requirement is that this path has low enough impedance for sufficient fault current to flow. If the earth path impedance is too high, the fault current is too low to trip the circuit breaker quickly, and the exposed metalwork remains live for an extended period. This is why AS/NZS 3000 and BS 7671 both require earth fault loop impedance verification on every circuit.

What Happens When Earthing Fails?

If the earth conductor is broken or disconnected, a fault to an exposed metal part cannot be cleared. The metal case sits at full mains voltage (230 V) indefinitely. The circuit breaker does not trip because no fault current can flow. The next person who touches the case while standing on a conductive floor receives a potentially lethal shock. This is why periodic earth continuity testing is not optional — it is life-critical.

Bonding takes a completely different approach to safety. Instead of clearing faults, it manages the voltage difference between metallic parts that a person might touch simultaneously.

Consider a bathroom: you might touch a metal tap (connected to the water pipes) while standing in a metal bathtub (connected to the waste pipes) while the towel rail (connected to the heating pipework) is within arm's reach. If a fault occurs somewhere in the building, one of these metal services could rise to a dangerous voltage relative to the others. Bonding connects them all together so they rise and fall in voltage together — if everything is at 100 V, the voltage between them is 0 V, and there is no shock hazard.

This concept is called an equipotential zone — a space where all simultaneously accessible metallic parts are at the same electrical potential.

Main Protective Bonding

Main protective bonding conductors connect incoming metallic services (water, gas, oil, structural steel, ducting) to the MET at the point where they enter the building. This is required by AS/NZS 3000 Clause 5.6.2 and BS 7671 Section 411.3.1.2. The purpose is to bring all incoming metallic services to the same potential as the electrical earth, creating a building-wide equipotential zone.

The minimum conductor size for main bonding is typically 6 mm² Cu for domestic installations, but it must be at least half the cross-sectional area of the main earthing conductor, subject to minimum sizes in the relevant standard table.

Supplementary Bonding

Supplementary bonding provides additional connections between simultaneously accessible metallic parts within specific high-risk locations — most commonly bathrooms, swimming pools, and medical locations (operating theatres). It is a second line of defence for areas where the consequences of a touch voltage are most severe (wet skin dramatically reduces body resistance, making much lower voltages lethal).

BS 7671 Section 701 requires supplementary bonding in bathrooms connecting: exposed-conductive-parts of Class I equipment, extraneous-conductive-parts (pipes, structural metal), and the protective conductor of each circuit. However, BS 7671 also allows supplementary bonding to be omitted if all circuits in the bathroom are RCD-protected and all extraneous-conductive-parts are connected to the main protective bonding. In practice, most modern installations meet this exception.

Understanding the difference between earthing and bonding becomes clearest when you consider what happens when each fails:

Scenario 1: Broken Earth Conductor

A fault develops in a washing machine (live conductor contacts the metal case). The earth conductor is broken. Result: the metal case sits at 230 V. The circuit breaker does not trip. The RCD may trip if the fault current finds an alternative path to earth (through pipework, for instance), but this is not guaranteed. The situation is immediately dangerous.

Scenario 2: Broken Bonding Conductor

The same fault develops, but this time the earth conductor is intact. The circuit breaker trips normally and clears the fault within milliseconds. However, the bonding conductor to the water pipes is broken. During the brief moment before the breaker trips, the washing machine case was at 230 V while the nearby tap (connected to unbonded pipework) was at a different potential. If someone was touching both simultaneously, they could receive a shock for that fraction of a second. With intact bonding, both the case and the pipes would have risen to the same voltage together, so no current would flow through the person.

The lesson: earthing protects against sustained faults by clearing them. Bonding protects against transient voltage differences during the brief time before the protective device operates. Both are essential, and neither replaces the other.

Certain locations are classified as requiring enhanced equipotential bonding because the risk of electric shock is elevated:

• Bathrooms and shower rooms: Wet skin reduces body resistance from roughly 1,000–2,000 Ω (dry) to as low as 100–200 Ω. This means a touch voltage of just 25 V can drive a dangerous current through a wet person, compared to the 50 V threshold for dry conditions. BS 7671 Section 701 and AS/NZS 3000 Section 6.2 both impose enhanced requirements.
• Swimming pools and saunas: Full body immersion in water creates extremely low body resistance. BS 7671 Section 702 requires supplementary equipotential bonding connecting all metallic parts in Zones 0, 1, and 2 of swimming pools. The safe touch voltage limit drops to 12 V AC in immersion conditions.
• Medical locations (operating theatres): Patients under anaesthesia cannot react to a shock, and invasive procedures can create direct electrical paths to the heart. IEC 60364-7-710 requires a medical IT system with insulation monitoring and supplementary equipotential bonding connecting all metallic parts in Group 2 medical locations, with a maximum equipotential bonding conductor resistance of 0.2 Ω.
• Agricultural and horticultural premises: Animals are more sensitive to electric shock than humans (lower body resistance, larger contact area through hooves). BS 7671 Section 705 reduces the disconnection time requirements and mandates supplementary bonding.

In all these locations, the combination of reduced body resistance, increased contact area, and inability to let go means that even small touch voltages can be lethal. Equipotential bonding is the primary defence.

Both earthing and bonding systems must be verified during commissioning and periodically thereafter:

• Earth continuity (R1+R2): Measured on every circuit to verify the CPC provides a continuous low-impedance path from each socket/accessory back to the MET. Typical acceptable values are fractions of an ohm for domestic circuits.
• Earth fault loop impedance (Zs): Measured to verify that the total impedance of the fault loop (supply transformer, line conductor, CPC, and external earth path) is low enough for the protective device to trip within the required time — typically 0.4 seconds for socket circuits and 5 seconds for distribution circuits.
• Bonding continuity: Each bonding conductor is tested to verify it provides a continuous connection from the bonded service to the MET, with resistance typically < 0.05 Ω for main bonding and < 0.05 Ω for supplementary bonding connections.
• RCD operation: While not a substitute for earthing and bonding, RCDs provide additional protection. They are tested at rated residual current (30 mA for personal protection) to verify trip time < 300 ms (instantaneous types < 40 ms).

Testing is not a formality. A broken earth conductor or disconnected bonding connection creates a hidden hazard that is invisible until a fault occurs — and by then, someone may already be touching the live metalwork.

The relationship between earthing and bonding can be summarised in one sentence: earthing clears faults, bonding prevents shocks. Both are required for a safe installation. Neither substitutes for the other. And both must be verified by testing, not assumed from visual inspection.

Key takeaways:

• Earthing = fault current return path = protective device can detect and clear the fault
• Bonding = equipotential connection = no dangerous voltage between simultaneously accessible parts
• Main bonding connects incoming services (water, gas) to the MET at the building entry
• Supplementary bonding connects metallic parts within high-risk zones (bathrooms, pools, medical)
• Broken earth = sustained shock hazard (fault not cleared)
• Broken bond = transient shock hazard (voltage difference during fault clearance)
• Both must be tested — continuity, loop impedance, and RCD operation