Earthing and Bonding Explained: TN-C-S, TN-S, TT Systems & BS 7671 Requirements
Earthing and bonding are the foundation of electrical safety. Every protective device in an installation — MCBs, RCDs, fuses — relies on a properly designed earth fault path to operate correctly. If the earthing or bonding is wrong, protection against electric shock fails.
This is also one of the most heavily tested topics in the IET exam. This guide covers the three earthing systems, main protective bonding, circuit protective conductors, conductor sizes, and the key BS 7671 regulations you need to know.
Why Do We Need Earthing?
The purpose of earthing is simple: if a fault occurs — say the live conductor touches the metal casing of an appliance — the fault current needs a low-resistance path back to the source so that the protective device (MCB, fuse, or RCD) can detect it and disconnect the supply quickly.
Without this path, the metal casing would sit at a dangerous voltage. Anyone touching it would become the path to earth — through their body.
Regulation 411.3.1.1 of BS 7671 requires that, under earth fault conditions, the protective device must disconnect the circuit within:
- 0.4 seconds for circuits supplying socket outlets and portable equipment (up to 32A)
- 5 seconds for fixed equipment and distribution circuits
The Three Earthing Systems
BS 7671 recognises three earthing systems, identified by a letter code:
- T = terre (earth) — connection to earth
- N = neutral — connection to the supply neutral
- C = combined — neutral and earth are the same conductor
- S = separate — neutral and earth are separate conductors
The first letter describes the supply side (how the source is earthed). The second letter describes the installation side (how the installation is earthed).
TN-C-S (PME — Protective Multiple Earthing)
This is the most common earthing system in the UK. The supply company provides the earth via the PEN conductor — a combined protective earth and neutral conductor in the supply cable.
- Ze (external earth fault loop impedance): typically 0.35 Ω
- The neutral and earth are combined in the supply cable and separated at the consumer unit
- Risk: If the PEN conductor breaks (open PEN fault), all exposed metalwork connected to the earthing system can rise to a dangerous voltage
Because of the open PEN risk, there are restrictions on using PME earthing for certain situations — for example, it’s generally not permitted as the sole earth for swimming pools or caravan sites.
TN-S
In a TN-S system, the supply company provides the earth via the metal sheath of the supply cable. The neutral and earth are separate conductors throughout.
- Ze: typically 0.8 Ω
- More commonly found in areas with older underground cable networks
- Considered more reliable than TN-C-S because there’s no PEN conductor to fail
- The earthing terminal is usually a clamp on the supply cable sheath
TT
In a TT system, the supply company provides no earth at all. The installation must provide its own earth via a local earth electrode (typically a driven earth rod).
- Ze: typically 20 Ω or more (much higher than TN systems)
- Common in rural areas with overhead supply lines
- Because Ze is so high, fault currents are low — overcurrent devices (MCBs) may not trip fast enough
- RCD protection is essential on TT systems — it’s the primary means of fault protection
Main Protective Bonding
Main protective bonding connects extraneous conductive parts — metallic services entering the building that could introduce an earth potential — to the Main Earthing Terminal (MET).
The purpose is to ensure that all metalwork in the building is at the same potential. If a fault occurs, bonding prevents a dangerous voltage difference between, say, a gas pipe and an earthed appliance.
What Must Be Bonded?
Under Regulation 411.3.1.2, the following extraneous conductive parts must be connected to the MET via main protective bonding conductors:
- Water service pipes
- Gas service pipes
- Oil supply pipes
- Structural steelwork (if it forms an extraneous conductive part)
- Central heating systems
- Lightning protection systems
- Other metallic services entering the building
Where to Connect
Main bonding conductors must be connected within 600mm of the point of entry of the service into the building — and before any branch in the pipework. For gas, the connection must be on the consumer’s side of the meter, as close as practicable to the meter.
Conductor Sizes
One of the most commonly tested topics in the exam is knowing the minimum conductor sizes for different types of earthing and bonding conductors.
Quick Reference: Minimum Conductor Sizes
| Conductor | Minimum Size (Cu) | BS 7671 Reference |
|---|---|---|
| Main protective bonding | 10 mm² | Reg. 544.1.1 |
| Supplementary bonding | 4 mm² | Reg. 544.2.3 |
| Earthing conductor (bare, in contact with soil) | 16 mm² | Table 54.8 |
| Earthing conductor (mechanically protected) | 2.5 mm² | Table 54.8 |
| CPC (within cable) | Per Table 54.7 | Reg. 543.1.1 |
Note: For supplies with a line conductor up to 35 mm², the main bonding conductor is 10 mm² Cu. For larger supplies, it must be proportionally larger (see Table 54.8).
The Main Earthing Terminal (MET)
The MET is the central connection point for the installation’s earthing system. Everything connects here:
- The earthing conductor (from the means of earthing — supply earth or earth rod)
- All main protective bonding conductors
- The circuit protective conductors (CPCs) via the distribution board
Regulation 542.4.1 requires that the MET must be accessible for testing and inspection, and must allow individual conductors to be disconnected for measurement. In practice, this is often an earth bar inside or adjacent to the consumer unit.
Circuit Protective Conductors (CPCs)
The CPC is the earth conductor within each circuit cable (the bare copper wire in T&E cable, for example). Its job is to carry fault current from the point of the fault back to the MET and then to the source.
The size of the CPC depends on the size of the line conductor and is selected using Table 54.7 of BS 7671:
| Line conductor (mm²) | Minimum CPC (mm²) |
|---|---|
| Up to 16 | Same as line conductor |
| 16 to 35 | 16 |
| Over 35 | Half of line conductor |
In practice, the CPC size in standard T&E cable is often smaller than the line conductor (e.g., 2.5 mm² T&E has a 1.5 mm² CPC). This is important for calculating R1+R2 values during testing.
Common Exam Questions
| Question Type | What to Remember |
|---|---|
| ”What is the minimum size of main bonding?“ | 10 mm² Cu (for supplies up to 35 mm²) |
| “On a TT system, what provides fault protection?” | RCD protection (overcurrent devices can’t clear faults quickly enough) |
| “What does the first letter in TN-S mean?” | T = the source (transformer) is directly connected to earth |
| ”Where must main bonding be connected on gas?” | Consumer’s side of the meter, within 600mm of entry |
| ”What is the typical Ze for a TN-C-S system?“ | 0.35 Ω |
| ”Why is an open PEN fault dangerous?” | All bonded metalwork can rise to supply voltage |
| ”What disconnection time for a 32A socket circuit?“ | 0.4 seconds |
Key Regulations
For quick reference, here are the main BS 7671 regulations for earthing and bonding:
- Reg. 411.3.1.1 — Maximum disconnection times for fault protection
- Reg. 411.3.1.2 — Main protective bonding requirements
- Reg. 411.4.2 — TT system fault protection (RCD required)
- Reg. 542.1 — Earthing arrangements general requirements
- Reg. 542.4.1 — Main earthing terminal requirements
- Reg. 543.1.1 — CPC sizing (Table 54.7)
- Reg. 544.1.1 — Main bonding conductor sizing
- Table 54.7 — Minimum CPC sizes related to line conductor
- Table 54.8 — Minimum earthing conductor sizes
Practice and Further Study
Earthing and bonding falls under Part 4: Protection for Safety of BS 7671. Test your knowledge with our practice quizzes:
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