13.2 Earthing System Configurations: TN-S, TN-C-S (PME and PNB) & TT

13.2 Earthing System Configurations: TN-S, TN-C-S & TT

In electrotechnical engineering, earthing is the practice of connecting the non-current-carrying metallic parts of an installation—such as metal enclosures, conduit, consumer units, and appliance casings—to the general mass of earth. The primary function of system earthing is to provide a low-impedance path that facilitates the rapid flow of earth fault current, enabling overcurrent protective devices (fuses, MCBs) or residual current devices (RCDs) to automatically disconnect the supply during an insulation breakdown before lethal shock or fire can develop.

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The BS 7671 Letter Code Classification System

Under BS 7671 Part 2, earthing arrangements are identified by an internationally harmonized lettering system originating from the International Electrotechnical Commission (IEC 60364). The designation uses French and English terminology to define the exact relationship between the power supply source, the installation metalwork, and the configuration of neutral and protective conductors:

1. First Letter: Relationship of the Power Source to Earth

• T (Terre, French for Earth): Direct electrical connection of one point of the power source to earth (in low-voltage distribution, this is almost universally the star point of the supply transformer secondary winding).
• I (Isolated): All live parts of the power source are isolated from earth, or connected to earth through a deliberately high source impedance (rarely used in public UK distribution, found primarily in specialized medical operating theaters and marine systems).

2. Second Letter: Relationship of Exposed-Conductive-Parts of the Installation to Earth

• T (Terre): Direct electrical connection of exposed-conductive-parts to earth, completely independent of the earthing of any point of the power source. The consumer must provide their own earth electrode (such as an earth rod).
• N (Neutral): Direct electrical connection of exposed-conductive-parts to the earthed point of the power source (the supplier provides a dedicated metallic connection back to the transformer star point).

3. Subsequent Letters: Arrangement of Neutral and Protective Conductors

• S (Separate): Neutral and protective functions are provided by separate conductors throughout the system.
• C (Combined): Neutral and protective functions are combined into a single conductor (designated as a PEN conductor — Protective Earth & Neutral, or CNE — Combined Neutral & Earth).

                                EARTHING SYSTEM CODES
                       ┌──────────────────┴──────────────────┐
                       ▼                                     ▼
                 First Letter                          Second Letter
         [Relationship of Source to Earth]     [Exposed Metalwork to Earth]
              T = Terre (Direct Earth)               T = Terre (Earth Rod)
              I = Isolated / High Z                  N = Neutral Connection
                                                             │
                                               ┌─────────────┴─────────────┐
                                               ▼                           ▼
                                        Subsequent: S               Subsequent: C
                                     Separate Neutral & PE      Combined Neutral & PE

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1. The TN-S System (Terre-Neutral-Separate)

In a TN-S system, the supply transformer star point is solidly connected to earth at the substation. From the transformer to the final circuit load, the neutral conductor and the protective conductor run as completely separate, distinct conductors throughout both the DNO distribution network and the consumer installation.

  Substation Transformer                       Consumer Installation Origin
┌────────────────────────┐                    ┌───────────────────────────────┐
│ Star Point (Earthed)   │                    │                               │
│     [Neutral (N)] ─────┼────────────────────┼───> Neutral Conductor         │
│     [Line (L)]    ─────┼────────────────────┼───> Line Conductor            │
│                        │                    │                               │
│   Solid Earth Terminal ┼──[Cable Sheath]────┼───> Main Earthing Terminal    │
└────────────────────────┘                    └───────────────────────────────┘

Construction and Physical Arrangement

• Cable Construction: Historically installed in urban UK networks from the 1930s to the 1970s using paper-insulated lead-covered steel-wire armoured (PILCSWA) underground cables.
• Earth Return Medium: The DNO provides an earthing facility at the consumer service cut-out by sweating or clamping a lead or copper conductor directly onto the metallic lead sheath and steel armouring of the incoming service cable.
• Maximum External Earth Fault Loop Impedance: The standard nominal maximum external loop impedance quoted by UK DNOs for a TN-S supply is $Z_e = 0.8\ \Omega$.

Engineering Advantages and Vulnerabilities

• Advantages: Highly stable, metallic earth return path that is completely segregated from the current-carrying neutral conductor. It presents zero risk of neutral-break voltage rises on bonded metalwork.
• Vulnerabilities: Over decades of service, underground lead cable sheaths can suffer electrolytic corrosion, physical fracture from ground shifting, or high-resistance degradation at joint boxes. Furthermore, when DNOs repair damaged PILCSWA mains with modern plastic-sheathed waveform cables, earth continuity can be inadvertently compromised if proper joint bonding is neglected.

2. The TN-C-S System (Protective Multiple Earthing - PME)

The TN-C-S system is the modern standard for new electrical connections in the United Kingdom. In this arrangement, the supply network utilizes a combined neutral and protective earth conductor (designated as a PEN conductor, or CNE—Combined Neutral and Earth), while the consumer's internal installation separates them into distinct neutral and protective conductors.

How PME Works

• In the DNO distribution main (typically installed using concentric or waveform aluminium/copper cables), a single conductor carries both the normal neutral return load current and the fault current.
• To ensure that this combined conductor remains as close to $0\text{ V}$ true earth potential as possible, the DNO connects the PEN conductor to earth electrodes at multiple points along the distribution route—at street joint boxes, distribution pillars, and the end of each cable branch. This technique is legally termed Protective Multiple Earthing (PME).
• At the consumer's service cut-out (the intake head), the combined PEN conductor terminates. Here, the DNO provides a solid brass link connecting the neutral block directly to the consumer earthing terminal. From this cut-out earth terminal, a separate earthing conductor is routed to the installation's Main Earthing Terminal (MET).
• Inside the building, the neutral and protective conductors are kept separate: the ESQCR (regulation 8(4)) prohibit a consumer from combining the neutral and protective functions in a single conductor within their installation.
• Maximum External Earth Fault Loop Impedance: The standard nominal maximum external loop impedance quoted by UK DNOs for a TN-C-S supply is $Z_e = 0.35\ \Omega$. This is the lowest $Z_e$ of all three systems due to the massive cross-section of the supply conductor and parallel earth paths.

The Broken PEN Conductor Hazard

While TN-C-S provides an exceptionally low-impedance earth path under normal conditions, it introduces a severe, unique safety hazard: the broken or open-circuit PEN conductor.

  Substation Transformer                  DNO Distribution Main                  Consumer Installation
┌────────────────────────┐                ┌──────────────────┐                  ┌──────────────────────┐
│ L1 (230 V) ────────────┼────────────────┼──────────────────┼──────────────────┼─> [230 V Load]       │
│                        │                │                  │                  │         │            │
│ Star Point (0 V) ──────┼──[PEN Cable]───┼─── [BREAK / ✕] ──┼── Cut-out Neutral┼─────────┘            │
│         │              │                │                  │        │         │                      │
│    Solid Earth         │                │                  │     PME Link     │                      │
└─────────┬──────────────┘                └──────────────────┘        │         │                      │
          │                                                           ▼         │                      │
          │                                                          MET ───────┼─> To Metal Casings!  │
          │                                                           │         │   (Cooker / CU)      │
          │                                                 Main Bonding │      └──────────────────────┘
          │                                                           │                    ▲
          │                                                           ▼                    │
          │                                                  [Extraneous Metalwork]        │ 230 V Potential
          │                                                  (Gas/Water Underground Pipe)  │ Rise!
          │                                                           │                    │
          └─────────── Earth Return Path Through Mass of Earth ───────┴────────────────────┘

If the supply PEN conductor is severed or fractures upstream in the street (e.g. from civil excavation damage or joint degradation):

1. The normal low-resistance metallic neutral return path to the substation transformer is broken.
2. When appliances inside the dwelling (such as a refrigerator, heating system, or lighting) are operating, load current flows through the appliance into the consumer unit neutral bar.
3. Because the street neutral is broken, this current cannot return along the street cable. Instead, it flows across the DNO cut-out link into the Main Earthing Terminal (MET).
4. The return current attempts to find its way back to the substation star point through the building's main protective bonding conductors, passing through metallic water, gas, or district heating pipes into the ground.
5. Because the contact resistance of pipes buried in the ground is far higher than a metallic copper conductor, Ohm's law ($V = I \times R$) dictates that the neutral bar, the MET, and ALL connected metallic equipment casings and bonded pipework rise toward the full mains voltage ($230\text{ V}$)!

BS 7671 Special Location Restrictions for PME

Due to this hazard, BS 7671 imposes strict prohibitions and technical requirements on using PME earthing in locations where occupants or animals are in direct electrical contact with true earth potential:

• Electric Vehicle Charging Points (Section 722): PME earthing must not be used for outdoor charging points unless specific safety devices are installed: an approved open-PEN detection device (which disconnects line, neutral, and earth within 5 seconds of voltage deviation), a dedicated independent earth electrode with sufficiently low resistance, or an isolating transformer.
• Caravan Parks & Marinas (Sections 708 & 709): PME earthing is strictly prohibited for supplying caravan pitches and boat berths.
• Construction & Demolition Sites (Section 704): PME supplies are generally prohibited for temporary construction supplies.
• Agricultural & Horticultural Premises (Section 705): PME is restricted due to the extreme electrical sensitivity of livestock to minute touch voltages.
• Petrol Filling Stations: PME earthing must never be extended to forecourt fuel dispensers due to the risk of arcing causing catastrophic fuel vapor ignition.

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TN-C-S with Protective Neutral Bonding (PNB)

PNB is a second form of TN-C-S supply. Amendment 4 includes it in BS 7671 for the first time, the Amendment 4 certificates let you record it, and the 2365 Unit 203 range lists it alongside PME. As with PME, the neutral and protective functions are combined in the supply, but the connection between neutral and earth, and its earth electrode, is made at one point close to the consumer. It is typically used where a single consumer, or a very small group such as a farm or remote property, is supplied from its own transformer.

NICEIC's Amendment 4 guidance gives two quick checks taken from Energy Networks Association guidance: a supply serving more than four consumers cannot be PNB, and nor can one where any consumer is more than 40 m from the earth electrode reference point. If in doubt, ask the DNO, which must provide information about the earthing arrangement under the ESQCR.

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3. The TT System (Terre-Terre)

In a TT system, the supply transformer star point is solidly connected to earth at the substation, exactly as in TN systems. However, the DNO provides no earthing facility whatsoever to the consumer at the service cut-out. Only the live line conductor and neutral conductor are delivered to the property.

Construction and Physical Arrangement

• Common in rural areas, isolated farmsteads, and older overhead line networks fed by wooden poles.
• The consumer is legally responsible for establishing their own connection to the general mass of earth by installing an independent earth electrode—most commonly one or more copper-bonded steel earth rods driven deeply into the ground outside the property.
• An earthing conductor is routed from the earth electrode to the consumer's Main Earthing Terminal (MET).
• Maximum External Earth Fault Loop Impedance: The external loop impedance ($Z_e$) of a TT system is not controlled by a metallic cable sheath; it consists of the resistance of the consumer's earth rod ($R_A$), the mass of soil between the property and the substation, and the substation transformer earth rod. Consequently, typical $Z_e$ values range from $21\ \Omega$ up to $200\ \Omega$ (or higher in rocky/dry terrain).

Why RCD Protection is Mandatory on TT Systems

Because the earth fault return path relies entirely on the resistance of the soil, the total earth fault loop impedance ($Z_s$) of a TT circuit is extremely high compared to TN systems. Under Ohm's Law ($I_f = U_0 / Z_s$), this high impedance severely chokes the prospective earth fault current:

• If a TT installation has an earth fault loop impedance of $Z_s = 50\ \Omega$ on a nominal $230\text{ V}$ supply, the maximum earth fault current is: $I_f = \frac{230\text{ V}}{50\ \Omega} = 4.6\text{ A}$

• A standard $32\text{ A}$ Type B Miniature Circuit Breaker (MCB) requires an instantaneous trip current of $5 \times I_n = 5 \times 32\text{ A} = \mathbf{160\text{ A}}$ to disconnect within $0.1\text{ s}$. Even a modest $6\text{ A}$ Type B MCB requires $5 \times 6\text{ A} = 30\text{ A}$.

• A fault current of just $4.6\text{ A}$ will never trip a $32\text{ A}$ circuit breaker or blow a fuse. The fault current would flow continuously, maintaining the metallic chassis of the faulted appliance at full mains potential ($230\text{ V}$), melting cable insulation, and creating an imminent risk of fatal electrocution and structural building fire.

Earthing System Comparison Matrix