Earthing, Bonding & Circuit Design

01Recap: earthing systems and Ze

Ze only measures the external earth path, from the transformer to the incoming MET or consumer unit. Under a fault the circuit breaker should trip in 0.4 s and a 30 mA RCD in 300 ms.

The tutor said about 40 minutes at the start of yesterday's (Day 2) afternoon recording was missed, which matches the note on that page. Older recordings of the mains position are available on the portal.

02Exposed vs extraneous conductive parts

Rule of thumb from the tutor: metal and you installed it means exposed. Metal and you didn't install it (it comes in from outside) means extraneous. If it's plastic it isn't conductive, so it's neither.

BS 7671 · Part 2 Definitions · "Extraneous-conductive-part" and "Exposed-conductive-part" Highlight both definitions. They're also in the Electrician's Guide and the On-Site Guide.

Myth: radiators don't automatically need bonding. They aren't something you install, and they don't come in from the ground. An electric towel rail is different: it's part of the installation, so it's an exposed conductive part.

03Main protective bonding

• Make the bond on the consumer's side of the meter or stopcock, within 600 mm of the gas meter outlet union or the water point of entry, before any branch.
• Plastic incoming service pipe? No main bonding needed, e.g. yellow plastic gas pipe or blue plastic water pipe. If even a short length of the incoming pipe is metal where it leaves the ground, you must bond it.
• CPCs (the green/yellow in your final circuits) go to the earth bar in the consumer unit, not to an external MET.
• Bonding clamps are BS 951. The conductor goes under the plate (lugged, crimped or wrapped round the screw). They carry a "Safety electrical connection – do not remove" label, and a label also goes at the MET. Labels are required by the regulations.

Electrician's Guide to the Building Regs · 3.3.2 Earthing conductor and main protective bonding conductors: list of services to bond · p.58 new · p.52 old

Electrician's Guide to the Building Regs · Plastic supply pipes: no bonding needed. Bonding within 600 mm of meter or entry · p.59 new · p.52–53 old Electrician's Guide to the Building Regs · BS 951 earth clamp label image · p.63 new · p.57 old

On-Site Guide · Section 4 Bonding and earthing: earthing and main bonding figure · p.61

04Sizing earthing and bonding conductors

CSA (cross-sectional area) is the size of the copper only, not including the insulation. A 10 mm² bonding conductor has 7 strands.

Values as shown on the session slides (highlighted rows = typical domestic). Always check the tables in your own book: Tables 3.3.2a and 3.3.2b. For TT, BS 7671 has extra conditions for buried earthing conductors.

Electrician's Guide to the Building Regs · Tables 3.3.2a and 3.3.2b: earthing and main protective bonding conductor sizes · p.59 new · p.53 old

05Why bond? Touch voltage and 1667 Ω

Body resistance ≈ 1000 Ω (less when wet)
I = V ÷ R = 230 ÷ 1000 = 0.23 A = 230 mA

About 50 mA (0.05 A) can stop your heart
(cardiac arrest). 230 mA is far more.
→ it's the CURRENT that kills.

30 mA RCD → max 1667 Ω (BS 7671 Table 41.5)
V = I × R = 0.03 × 1667 = 50.01 V

So with a 30 mA RCD, keeping the resistance
of extraneous parts to earth ≤ 1667 Ω keeps
touch voltage at about 50 V.

A 30 mA RCD is set below the ~50 mA that can cause cardiac arrest. It gives additional protection by cutting off before serious harm.

BS 7671 · Table 41.5: max earth fault loop impedance for RCDs (30 mA → 1667 Ω) · p.77 new · p.70 old

06Supplementary bonding in bathrooms

Supplementary bonding links exposed and extraneous conductive parts locally, e.g. the metal pipes under a bath to a metal shower, so they stay at the same potential. You'll often see it as a loop of green/yellow between copper pipes, such as at a boiler. Somewhere along those pipes, a main bonding conductor goes back to the MET.

BS 7671 · Section 701: locations containing a bath or shower · Reg 701.415.2 supplementary bonding · p.257 new · p.242 old

Supplementary bonding conductor sizes

This depends on what you're linking and whether the conductor is mechanically protected, e.g. in conduit. A single-core green/yellow conductor has only one layer of insulation. The grey sheath on twin and earth is mechanical protection, and an RCD is additional protection, not mechanical.

Main protective bonding = 10 mm²
Link: electric shower (exposed part) → pipes under the bath (extraneous part)
Not mechanically protected → 6 mm² (still 6 mm² if protected)
Exposed part → exposed part, both mechanically protected with 10 mm² → 10 mm²

Electrician's Guide to the Building Regs · 3.3.3 Supplementary bonding conductors · Table 3.3.3 sizes · p.62 new · p.56 old

07Protective conductors and cable colours

• On single-phase, a blue, black or grey conductor used as a line must be sleeved brown at the terminations. For example, the blue of twin and earth used as a switch line gets brown sleeving. It's cheaper than buying twin brown cable.
• A neutral used as a mid-point or supply neutral must stay blue.
• Sleeve and terminate correctly as you go. Inspecting while you work saves problems when you test.
• UK colours changed in 2004 (from red/black). You'll still find old installations.

BS 7671 · Reg 514: identification of conductors · p.141 new · p.132 old Electrician's Guide to the Building Regs · Chapter 11: identification of conductors, old and new colours The tutor was asked about conductor ID in her own exam

08What design means

Design isn't just picking any cable. It's based on the client's needs and on what's already there. Before adding or altering a circuit, check that the existing installation can:

• carry the additional load
• provide adequate shock protection
• has satisfactory earthing and bonding

Think of design as a chain. The load gives the design current, which decides the protective device, which with the installation method and conditions decides the cable size, which must then pass voltage drop. Example: if a client upgrades a shower from 5 kW to 8 kW, the existing cable may no longer be suitable.

BS 7671 · Appendix 1: British Standards and other standards referred to (about 22 pages) · p.411 new · p.385 old

09Cable types and marks of conformity

Marks of conformity show a product meets the required standards:

• CE is the EU mark.
• After Brexit, UKCA is used in Great Britain and UKNI in Northern Ireland.
• You'll also see BASEC (cables), ENEC, BEAB and the Kitemark.

Electrician's Guide to the Building Regs · 2.2.2 Conformance to equipment standards: UKCA / UKNI marks · p.35 new · p.33 old

10Installation reference methods

The reference method describes how and where a cable is installed. This affects how much current it can carry, because heat has to escape. More insulation around a cable means more heat trapped, which means more resistance and a lower current rating. You may then need a bigger cable.

• 103: less than 5 cm surrounded by insulation means no derating. Totally enclosed for 0.5 m or more means the rating is half that of clipped direct.
• Examples from the table: 1.0 mm² T&E clipped direct = 16 A (enclosed in insulation, about half), and 2.5 mm² clipped direct = 27 A. The rating drops for method B and the 100-series methods.
• PVC-sheathed cable must not touch polystyrene, which degrades it over time.
• Twin and earth can go in conduit or mini-trunking for extra protection. Councils often run mini-trunking along skirting and ceilings instead of chasing. But more cables bunched together means more heat.

Electrician's Guide to the Building Regs · 2.3.1 Cable installation methods A, B, C · pp.38–39 new · pp.34–35 old Electrician's Guide to the Building Regs · Reference methods 100, 101, 102, 103 · Table 2.3.1 T&E current ratings · p.40 new · p.36 old

11Safe zones, depth and metal stud walls

Cables concealed in a wall less than 50 mm deep must be protected in one of these ways:

1. Enclosed in earthed metal conduit, trunking or ducting (or similar).
2. Run in the permitted zones. This also needs 30 mA RCD additional protection.
3. Part of a SELV/PELV circuit (extra-low voltage, covered later).

• Why the zones? Nobody is likely to hang a picture 150 mm from the ceiling or right in a corner. Cables in line with an accessory are where people expect them.
• Running across into a different room through a partition isn't "in line with an accessory", so it needs extra protection.
• Metal or plastic capping over cables in a chase is good practice, because it stops nails and screws. Capping doesn't count as conduit or trunking.
• Under floors and in ceilings: unarmoured cables through joists must be at least 50 mm from the top or bottom of the joist, or be protected (earthed steel conduit, etc.).

Electrician's Guide to the Building Regs · Permitted cable routes / safe zones figure and <50 mm depth rules · 2.3.2 Floors and ceilings · p.43 new

BS 7671 · Regs 522.6.202, 522.6.203, 522.6.204: cables in walls and metal stud partitions · p.152 new In the 18th Edition exam you find regs by number, not page. Practise.

12Socket circuits: rings and radials

• This table is for normal socket-outlet circuits. Lighting (usually 6 A), cookers, showers and immersion heaters go through the full design process instead.
• Floor area means the room area served, not metres of cable. On a 20 A radial, 50.02 m² is already too much.
• Many kitchens now use a 32 A radial in 4 mm² instead of a ring. Rings go on a 32 A device. If you need smaller breakers, use two 20 A radials instead.

Electrician's Guide to the Building Regs · 4.2 Final circuits using 13 A socket-outlets · Table 4.2.1 · p.77 new · p.72 old On-Site Guide · Appendix H · Table H2.1: final circuits using socket-outlets · p.218 new · p.210 old "Worth highlighting"

On-Site Guide · Appendix A: maximum demand

13Cable selection: the 7 steps

1. Work out the design current, Ib. Ib = P ÷ V. Also note the installation reference method now, from the question.
2. Select the protective device rating, In (e.g. B40), so that In ≥ Ib.
3. Identify the installation method (C, B, A, 100–103…).
4. Apply any correction factors, e.g. ambient temperature, grouping or thermal insulation.
5. Calculate the tabulated current, It, that the cable must be able to carry.
6. Select the cable size from the tables (BS 7671 or the On-Site Guide).
7. Check the voltage drop.

Ib

Design current of the circuit: the load. "Ib", not "ID".

In

Nominal rating of the protective device in amps, e.g. the "6" on a B6.

Iz

Current-carrying capacity of the cable under normal conditions once correction factors have been applied. The Z here has nothing to do with impedance.

It

Tabulated current-carrying capacity: the value straight from the tables.

VD = (mV/A/m × Ib × L) ÷ 1000

÷1000 because mV/A/m is in millivolts

VD = (mV/A/m × Ib × L) ÷ 4000

÷4000: the ring has 4 legs
(2 lines + 2 neutrals)

If the voltage drop is too high, upsize the cable. You never downsize.

On-Site Guide · Appendix F: voltage drop explanation, radial formula (and ring formula, new edition only) · p.184 new · p.168 old Highlight the formula and the 3% / 5% paragraph

14Worked example: 8.5 kW shower

An 8.5 kW electric shower is to be installed in a domestic property.
• Wired in PVC/PVC flat twin and earth
• Reference method 102
• Cable run 20 m
• Protective device: BS EN 60898 circuit breaker
• No rating factors
Select a suitable cable and protective device. Show all working.

1) Design current
   Ib = P ÷ V = 8500 ÷ 230 = 36.96 A      (37 A is fine)

2) Protective device (domestic, so Type B)
   Next standard size ≥ 36.96 A → In = 40 A, Type B, BS EN 60898
   Check: Ib ≤ In → 36.96 ≤ 40 ✓

3–4) Method 102, no correction factors → Iz = It, and it must be ≥ In = 40 A

5–6) On-Site Guide Table F6 (70 °C flat T&E), method 102 column:
   6 mm²  → 35 A  ✗ (less than 40)
   10 mm² → 47 A  ✓  → select 10 mm² twin and earth

7) Voltage drop (not lighting, so max 5% = 11.5 V)
   Table F6, 10 mm² → 4.4 mV/A/m
   VD = (4.4 × 36.96 × 20) ÷ 1000 = 3.25 V
   3.25 V ≤ 11.5 V ✓

Answer: 10 mm² T&E on a 40 A Type B circuit breaker

On-Site Guide · Appendix B · Table B6: circuit-breaker ratings (BS EN 60898 / RCBOs) · p.159 new · p.145 old

On-Site Guide · Appendix F · Tables F4–F6: current-carrying capacity and voltage drop · from p.187 new · p.171 old On-Site Guide · Table F6: 70 °C thermoplastic flat twin and earth (It and mV/A/m) · p.193 new · p.177 old

15Calculator tips

• Milli = ×10⁻³, so dividing by 1000 is the same as using ×10ˣ −3.
• Newer Casio (fx-991CW, black with round keys): it has an EXE key instead of "=" and a FORMAT key. To get decimals instead of fractions every time, go to Settings → Calc Settings → Input/Output → MathI/DecimalO.
• Older fx-85GT / fx-83GT: use S⇔D to switch fraction ↔ decimal.
• You may need brackets round the top line of the voltage drop formula.
• Bring your own non-programmable scientific calculator to the exam. The ones provided are basic, and phones aren't allowed. Practise with a scientific calculator app at home until yours arrives.

16All book references from this day

Tab these pages. "New" = orange / green-orange editions, "old" = brown / green-brown.

★ = the tutor specifically said to highlight it, or went through it in detail. Page numbers are as given in the session. Check them against your own copy.

17Self-test

Unofficial student notes written from the Module 2 Day 1 webinar recordings (16/09/2026), shared free for fellow learners. Not produced or endorsed by the training provider. Bonding table values are as shown on the session slides. Always check against your course books, BS 7671 and your tutor. Spotted a mistake? Let us know so it can be fixed for everyone.

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