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Chapter 54: earthing arrangements and the METTN-S, TN-C-S, TT. Which can you install?

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Chapter 54 covers earthing arrangements and protective conductors. If a question mentions main protective bonding or supplementary bonding, you're going to 544. If it's about the earthing arrangement itself, you're earlier in the chapter.

The earthing systems — including the two you won't install

System	Notes
TT	The one that needs an earth electrode
TN-S	Separate earth throughout — the old lead-sheathed supply
TN-C-S	Combined then separated. PME is a version of TN-C-S
IT	High-impedance system. You are unlikely to meet one
TN-C	Terra Neutral Combined — earth and neutral combined all the way in and never separated. Not permitted in UK consumer installations (ESQCR Reg 8(4): no PEN conductor in a consumer's installation). You may meet it working abroad

Why TN-C is worth five seconds of your time. The question "which combination of those letters haven't we used?" is exactly how an exam writer builds a distractor. Knowing TN-C exists, and that it's not allowed here, stops you second-guessing the three that are.

Table 54.1 — a buried earthing conductor

This table gives the minimum cross-sectional area of an earthing conductor that is buried in the ground, and it splits on two questions: is it protected against mechanical damage, and is it protected against corrosion by a sheath?

Condition	Minimum CSA
Protected against mechanical damage and against corrosion by a sheath e.g. buried inside conduit or ducting	2.5 mm² copper · 10 mm² steel
Not protected against mechanical damage, but protected against corrosion by a sheath	16 mm² copper · 16 mm² coated steel
Not protected against corrosion — mechanical protection irrelevant	25 mm² copper · 50 mm² steel

Highlight the last row in Table 54.1 (p.208). With no protection against corrosion, a buried earthing conductor must be at least 25 mm² copper or 50 mm² steel, whatever its mechanical protection.

542.4 — the main earthing terminal

Part 1's fundamental principles said every installation must have an earthing arrangement. Part of that arrangement is a main earthing terminal (MET) or earth bar, and every installation must have one, because five different things have to land on it:

The earthing conductor
Circuit protective conductors
Main protective bonding conductors
Functional earthing conductors, where required
Lightning protection system bonding, where there is one

≡The earthing terminal may be external or internal. You need one or the other, not both — as long as it lets you get the earthing conductor from the supplier's head through to your final circuits.

Chapter 54 · earthing arrangements and protective conductors Table 54.1 · p.208 · minimum CSA of a buried earthing conductor ★ 542.4 · main earthing terminal · what connects to it ★ 544 · protective bonding conductors

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