BS 7671 Guide · Section 2
Before you start: supply, earthing and demand
Every design decision you make later depends on a handful of facts about the supply and the load. Get them right first and the cable sizes, protective devices and test results fall into place.
Why Part 3 comes first
Part 3, “Assessment of general characteristics”, is the planning stage. Establish what the installation is for, what feeds it, how it is earthed, what load it carries and what conditions it faces. Only then choose protection (Part 4) and equipment (Part 5).
Chapter 31 covers the supply, earthing and demand. Chapters 32 to 36 cover external influences, compatibility, maintainability, safety services and continuity of service.
The same thinking applies to small jobs. Before any addition or alteration, Regulation 132.16 expects you to check that the existing equipment, including the DNO’s, can take the extra load, and that the earthing and bonding are adequate.
Regulation references: Part 3, Chapters 31–36, 132.2, 132.16
What to find out about the supply
Regulation 313.1 lists the supply characteristics you need. In practice that means five numbers or facts. Record them; they go straight onto the certificate.
| Characteristic | What it tells you | Where to get it |
|---|---|---|
| Nominal voltage and frequency | 230 V single-phase or 400 V three-phase, 50 Hz. Statutory limits allow 230 V to run 10% high or 6% low. | Supply type and number of phases at the intake |
| Ze (external earth fault loop impedance) | The part of the fault loop outside your installation. It sets how fast your devices can disconnect. | Measure, or ask the DNO |
| Prospective fault current (Ipf) | The worst-case current at the origin. Every device must be able to break it safely. | Measure, or use the DNO’s declared value |
| Earthing arrangement | TN-S, TN-C-S or TT. It drives your shock protection, bonding sizes and RCD strategy. | Inspect the intake, then confirm |
| DNO fuse type and rating | Caps the available demand and provides back-up protection for your meter tails and consumer unit. | Read the cut-out carrier; do not break the seal |
Under the Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR), the distributor must give you the maximum fault current, the maximum external loop impedance, the type and rating of its protective device and the earthing arrangement if you have a reasonable need for them.
Without anything more specific from the DNO, designers commonly use the long-standing maximum values for a single-phase supply up to 100 A: 0.8 Ω for TN-S, 0.35 Ω for TN-C-S and 21 Ω for the DNO’s part of a TT loop. The IET suggests assuming 16 kA prospective fault current for such supplies where the service cable is longer than about 2 m, unless the DNO advises otherwise. These are design assumptions, not guarantees, which is why you still measure.
Regulation references: 313.1, 434.1, 434.5.1; ESQCR regulations 27 and 28
Earthing systems and how to spot them
The first letter is how the source is earthed (T: direct; I: isolated or via an impedance). The second is how your metalwork is earthed (T: own electrode; N: via the supply). S and C-S say whether neutral and earth are separate or combined. Regulation 312.2 describes each type.
TN-S
The DNO provides a separate earth, usually the lead sheath or armour of an older underground cable. On site you will typically see an earth clamp on the cable sheath or a separate earth terminal at the cut-out, with no link to the neutral.
No open-PEN risk and a fairly low Ze. But old sheaths corrode, and Ze can creep up.
TN-C-S (PME)
The most common arrangement in the UK. The supply uses a combined neutral and earth (PEN) conductor that the DNO earths at several points. Your earth terminal is taken from the neutral at or beside the cut-out, and there is usually a PME warning label at the intake.
It gives the lowest Ze and is cheap to provide. The risk is a broken PEN conductor: every earthed metal part in the building can then rise towards line voltage. That is why main protective bonding must be larger on PME, sized against the supply neutral using Table 54.8 (10 mm² copper for a typical house).
PNB: new in Amendment 4
A4 adds protective neutral bonding (PNB) to Regulation 312.2.1.1 as a variant of TN-C-S, with a new figure. The supply is still a PEN conductor, but it serves a single connection point, typically a dedicated transformer for one customer or a pole transformer feeding a remote property. The PEN is earthed once, at an electrode placed as close as practicable to the consumer’s terminals, to limit voltage rise if the PEN breaks.
For restrictions, treat a PNB earth terminal as PME. Check bonding sizes with the DNO.
TT
No earth comes from the DNO. Your installation is earthed by its own electrode, usually a rod, and you will see an earthing conductor running out to it. Ze readings are in tens or hundreds of ohms.
A broken supply neutral cannot put a dangerous voltage on your metalwork. But the loop is far too high for fuses or MCBs to clear in time, so RCDs do the work. Regulation 411.5.3 links electrode resistance to RCD rating: RA multiplied by the RCD’s rated residual current must not exceed 50 V. Keep electrode resistance well below that limit, because soil dries and freezes; the IET’s guidance treats readings above 200 Ω as possibly unstable.
IT
The source is isolated from earth or earthed through a high impedance. A first fault raises an alarm on an insulation monitor rather than tripping. You will meet it in Group 2 medical locations and some process plants, never on public supplies.
Regulation references: 312.2, 312.2.1.1, 411.4, 411.5.3, 411.6, Table 54.8, 544.1.1
When a PME earth is restricted
ESQCR forbids a distributor from connecting a combined neutral and earth to metalwork in a caravan or boat, and forbids a consumer from combining neutral and protective functions inside their own installation. BS 7671 points you to the ENA’s Engineering Recommendation G12 (Issue 5) for the full list of places where PME is not permitted or not recommended. The usual ones are:
- Caravans, motor caravans and boats: the pitch or berth supply must not use the PME earth. TT with RCD protection is normal.
- Fuel filling stations: the dispensing area is normally TT, segregated from any PME used elsewhere on site.
- Construction and demolition sites: PME is generally not offered for site supplies; expect TN-S from a dedicated transformer, or TT.
- Farms: livestock react to small touch voltages, so take care extending PME to outbuildings.
- Pools and campsite shower blocks: may need special earthing or TT.
- EV charge points: Regulation 722.411.4.1 does not allow a PME earth for a charge point that is outdoors, or could charge a vehicle parked outdoors, unless one of its listed measures is used, such as a device that disconnects on a detected open-PEN condition. The alternative is to make the charge point circuit TT.
Regulation references: 312.2.1.1, 708, 709, 722.411.4.1; ESQCR regulations 8 and 9; ENA G12 Issue 5
Maximum demand and diversity
Regulation 311.1 asks you to work out the maximum demand, and lets you take account of diversity. Diversity simply recognises that not every load runs flat out at the same moment.
The IET On-Site Guide gives a conventional method for typical homes; check its current figures. For anything unusual, base allowances on real use and write the assumptions down.
A worked example
A house has a 100 A single-phase supply. Connected loads are a 9.5 kW shower, a 10 kW cooker, a 3 kW immersion heater, two ring finals, two lighting circuits and a new 7.4 kW EV charger. At full rating that is close to 200 A. These illustrative allowances are our own, not from a table:
- Shower: when on, it runs at full load. Allow 41 A.
- Cooker: hobs and oven cycle on thermostats and rarely all run together. Allow 18 A.
- Immersion: can run for hours. Allow its full 13 A.
- Two ring finals: allow 20 A between them for kettles, washing and general use.
- Lighting: allow 3 A.
That gives 95 A before the charger. Regulation 722.311 does not allow diversity on EV charging circuits unless load control is used, so the charger adds its full 32 A, giving 127 A. The supply will not carry that.
Fit a charger that monitors supply current and throttles charging below the fuse rating, upgrade the supply, or both. Record your choice.
Regulation references: 311.1, 132.16, 536.4.202, 722.311
Notification: Part P and its equivalents
In England, Part P of the Building Regulations applies to electrical work in dwellings. Approved Document P makes three kinds of work notifiable:
- installing a new circuit;
- replacing a consumer unit;
- adding to or altering a circuit in a special location (a room with a bath or shower, within the zones, or a room with a swimming pool or sauna heater).
Other work must still comply but need not be notified. Registered competent person scheme members self-certify. Anyone else must use building control or a registered third-party certifier before or during the work.
Wales has its own Building Regulations and a wider list of notifiable work, so check the Welsh guidance. In Scotland, electrical work falls under the building standards; where a building warrant applies, an Approved Certifier of Construction (schemes run by SELECT and NICEIC) can certify the electrical installation. Northern Ireland has its own Building Regulations and technical booklets.
References: Building Regulations 2010 Part P (England); Approved Document P
Next, see how these facts shape protection against electric shock and earthing and bonding.
Important. This guide is written by Tradefox to help qualified electricians understand BS 7671:2018+A4:2026. It is a plain-English summary for learning and reference only. It is not the standard, it does not reproduce it, and it is not professional advice. Always work to the current edition of BS 7671, the IET guidance notes, manufacturers’ instructions, Building Regulations and your scheme provider’s requirements. Electrical work must be carried out by a competent person.
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