Extraneous and Exposed Conductive Parts: What They Are and Why Bonding Depends on Them

Ask a room of trainee electricians what the difference is between an exposed-conductive-part and an extraneous-conductive-part and you’ll get a lot of hesitation. Ask them why the water pipe in a kitchen gets a 10 mm² green-and-yellow conductor while the cooker doesn’t, and the room goes quiet.

 

These two definitions from Part 2 of BS 7671 are the foundation of the whole of Chapter 54. If you don’t understand which metal parts are which, you can’t explain why main protective bonding exists, when supplementary bonding is needed, or how to size any of the conductors involved. And the 18th Edition exam knows this, which is why the topic turns up repeatedly across Parts 2, 4, 5, and 7.

 

This guide sets out the definitions clearly, explains what bonding is actually doing during a fault, and walks through the sizing tables you’ll need to navigate in the exam.

The Two Definitions That Matter

Both terms are defined in Part 2 of BS 7671, and the exam expects you to know them precisely, not approximately.

 

Term	BS 7671 Definition	The Practical Test	Examples
Exposed-conductive-part	Conductive part of equipment which can be touched and which is not normally live, but which can become live under fault conditions	Does this metalwork belong to something that uses electricity to do its job?	Cooker casing, metal luminaire body, Class I appliance enclosure, steel conduit
Extraneous-conductive-part	Conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation	Does this metalwork come in from the ground or the building structure and carry no electricity at all?	Metallic gas and water service pipes, oil pipes, structural steel, central heating pipework, metal ductwork

 

The simplest way to hold the distinction in your head is this: an exposed-conductive-part is part of the electrical system. It has a supply cable going to it. An extraneous-conductive-part is part of the building. It has nothing to do with electricity but happens to be metal and happens to be connected to the ground.

 

Exam tip: The word “extraneous” means “coming from outside”. An extraneous-conductive-part is outside the electrical installation but inside the building. If a question describes a metal part that is fed by a circuit, it is an exposed-conductive-part, whatever it looks like.

 

One more piece of vocabulary. Because the incoming water pipe is buried in the ground, it sits at Earth potential, which is nominally 0 V. Electricians often describe this as the pipe being “earthy”. It’s not a formal term, but it captures exactly what makes an extraneous-conductive-part dangerous.

 

Why Bonding Works: The Cooker and the Water Pipe

To see why bonding exists, picture a kitchen with a metal-cased electric cooker and a metallic cold water pipe rising from the floor. The cooker casing is an exposed-conductive-part. The pipe is an extraneous-conductive-part.

 

Scenario 1: No fault

Someone touches the cooker and the pipe at the same time. Nothing happens. Neither part is live, so there is no potential difference and no shock, whether bonding is present or not.

 

Scenario 2: Fault, no bonding

A fault inside the cooker puts the casing at 230 V relative to Earth. The water pipe is still at 0 V because it’s connected to the ground. Someone touches both at once.

 

Point of Contact	Potential	Result
Hand on cooker casing	230 V	230 V potential difference across the chest
Hand on water pipe	0 V

 

Until the protective device operates, the person is the conductor between the two. This is the exact scenario that main protective bonding is designed to eliminate.

 

Scenario 3: Fault, bonding in place

Now the water pipe is connected to the main earthing terminal by a main protective bonding conductor, and the cooker has its CPC connected to the same terminal. Electrically, the cooker casing and the pipe are now the same point.

 

Point of Contact	Potential	Result
Hand on cooker casing	230 V	230 V minus 230 V = 0 V across the body
Hand on water pipe	230 V

 

Both parts rise to the same voltage during the fault, so there is no potential difference to drive current through the person. That is what “equipotential” means, and it’s why the correct name in Regulation 411.3.1.2 is protective equipotential bonding.

 

Key point: Bonding does not stop a part becoming live. It ensures that everything a person can touch simultaneously rises to the same voltage, so the touch voltage between them is close to zero until the fault is cleared. Earthing provides the path for fault current; bonding removes the potential difference. They are different jobs.

 

For a fuller treatment of how earthing and bonding divide the work, see our guide to earthing and bonding in TN-C-S, TN-S and TT systems.

 

There Is Only One “Earth” in an Installation

Loose language costs marks. On site, everyone calls the bare conductor in twin-and-earth “the earth”. In BS 7671 terms, and in the exam, every green-and-yellow conductor has a specific name and only one of them is the earthing conductor.

 

Conductor	Runs Between	BS 7671 Term	Regulation
The earthing conductor	Means of earthing (cut-out, cable sheath, or earth electrode) and the main earthing terminal (MET)	Earthing conductor	Reg. 542.3
Main protective bonding conductors	MET and each extraneous-conductive-part entering the building (gas, water, oil, structural steel)	Main protective bonding conductor	Reg. 544.1
Circuit protective conductors (CPCs)	MET and the earth terminal of each item of equipment	Circuit protective conductor	Reg. 543
Supplementary bonding conductors	Exposed and extraneous-conductive-parts in the same location	Supplementary protective bonding conductor	Reg. 544.2

 

The earthing conductor is the only one properly called “the earth”. It’s a single conductor, from the intake to the MET. Everything else is a CPC or a bonding conductor.

 

Remember: All main bonding conductors and all CPCs terminate at the MET. Supplementary bonding conductors do not. They connect locally to the CPC of nearby equipment and rely on that CPC for their path back to the MET.

 

Sizing the Earthing Conductor and Main Bonding

Once you know what you’re connecting, the sizing follows a logical chain. Three regulations do the work.

 

Step 1: Earthing conductor (Reg. 543.1.1 and Table 54.7)

Unless you use the adiabatic equation, the earthing conductor is sized from Table 54.7 against the line conductor (the meter tails).

 

Line Conductor (Tails)	Minimum Earthing Conductor
Up to 16 mm²	Same size as line conductor
16 mm² to 35 mm²	16 mm²
Above 35 mm²	Half the line conductor CSA

 

Typical domestic tails are 25 mm², so the earthing conductor is 16 mm². An older property still on 16 mm² tails also lands on 16 mm².

 

Step 2: Main protective bonding for TN-S (Reg. 544.1.1)

The main bonding conductor must be not less than half the CSA of the earthing conductor, subject to a minimum of 6 mm² and a maximum of 25 mm² copper.

 

Earthing Conductor	Half CSA	Minimum Main Bonding (TN-S)
10 mm²	5 mm²	6 mm² (minimum applies)
16 mm²	8 mm²	10 mm² (next standard size)
25 mm²	12.5 mm²	16 mm²

 

Step 3: Main protective bonding for TN-C-S / PME (Table 54.8)

On a PME supply the bonding conductors may carry a share of neutral current under a broken-PEN fault, so BS 7671 sets a higher floor. Table 54.8 sizes them against the supply neutral.

 

Supply Neutral CSA	Minimum Main Bonding (PME)
35 mm² or less	10 mm²
Over 35 mm² up to 50 mm²	16 mm²
Over 50 mm² up to 95 mm²	25 mm²

 

For a standard domestic TN-C-S intake with 25 mm² tails and a 100 A cut-out fuse, the answer is therefore: earthing conductor 16 mm², main bonding 10 mm².

 

Exam tip: A question asking for the minimum main bonding conductor will often tell you the earthing system. If it says PME or TN-C-S, go to Table 54.8 and the answer is never less than 10 mm². If it says TN-S, use the half-CSA rule with the 6 mm² floor. Candidates who apply the TN-S rule to a PME supply get 6 mm² and lose the mark.

 

TT Systems: Protecting the Earthing Conductor

On a TT system the earth is not supplied by the distributor. The consumer’s own earth electrode provides it, and the earthing conductor to that electrode is often outdoors or buried. Because it has no supply cable around it for protection, Regulation 542.3.1 and Table 54.1 size it according to how well it is protected against corrosion and mechanical damage.

 

Buried Earthing Conductor Condition	Minimum Copper CSA
Protected against corrosion and mechanical damage	As Table 54.7 (typically 2.5 mm² or larger as calculated)
Protected against corrosion but not mechanical damage	16 mm²
Not protected against corrosion	25 mm²

 

The logic is simple: the less protection the conductor has, the bigger it must be so that it stays intact for the life of the installation. Once the earthing conductor is fixed, the main bonding follows the same rule as TN-S: half the CSA, minimum 6 mm².

 

For the RCD and Ze requirements that go with a TT supply, see our TT earthing system guide.

 

Supplementary Bonding: Where It Goes and Where It Stops

Supplementary bonding is additional local bonding in a location where the risk of shock is increased, most commonly a room containing a bath or shower. It links exposed-conductive-parts and extraneous-conductive-parts that are simultaneously accessible, so that the touch voltage between them is kept low even if the main bonding and CPC path is imperfect.

 

Two rules govern where it goes:

 

Rule	What It Means in Practice
Stays local	Supplementary bonding conductors connect parts within the same room or vicinity. They are not run back to the MET.
Relies on CPCs	The connection back to the MET is made through the CPC of the equipment it lands on, such as the shower’s earth terminal or the shaver socket’s earth terminal. The CPCs must already be in place and sound.

 

A typical bathroom arrangement: the radiator is bonded to the earth terminal of the electric shower, and the copper pipework at the basin is bonded to the earth terminal of the shaver socket or luminaire. Each conductor stops there.

 

When can it be omitted?

Regulation 701.415.2 permits supplementary bonding in a bathroom to be omitted where all of the following are met:

 

1. All final circuits of the location comply with the disconnection times in Reg. 411.3.2
2. All final circuits of the location have 30 mA RCD additional protection (Reg. 415.1)
3. All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding (Reg. 411.3.1.2)

 

Important: The RCD condition alone is not enough. Candidates often answer “supplementary bonding isn’t needed if there’s an RCD”. The regulation requires the RCD and the disconnection times and effective main bonding. All three, or supplementary bonding stays.

 

Our article on supplementary bonding and touch voltage covers the touch-voltage calculation behind this rule.

 

Sizing Supplementary Bonding Conductors

Regulation 544.2 sizes supplementary bonding according to what is being connected to what. Decide which of the three cases you’re in, then apply the minimum for mechanical protection.

 

Connection	Minimum CSA Rule	Regulation
Exposed to exposed	Not less than the smaller of the two CPCs connected to those parts	Reg. 544.2.1
Exposed to extraneous	Not less than half the CSA of the CPC connected to the exposed-conductive-part	Reg. 544.2.2
Extraneous to extraneous	2.5 mm² if sheathed or mechanically protected, otherwise 4 mm²	Reg. 544.2.4

 

In every case Regulation 544.2.3 applies an overall minimum: 2.5 mm² copper where the conductor is mechanically protected or sheathed, 4 mm² copper where it is not.

 

Worked example

Supplementary bonding is to be installed between a central heating radiator and the cold water pipe at a sink, and then between the hot and cold pipes at the sink. None of the conductors is mechanically protected. What is the minimum copper CSA?

 

Step	Reasoning
Identify the parts	Radiator, cold pipe, hot pipe: all extraneous-conductive-parts
Select the rule	Extraneous to extraneous, Reg. 544.2.4
Apply protection condition	Not mechanically protected, so 4 mm²
Answer	4 mm² copper

 

Remember: Extraneous-to-extraneous bonding is the easy one. If it’s unprotected, the answer is always 4 mm². If it’s protected, 2.5 mm². There is no CPC to compare against because neither part has a circuit feeding it.

 

Plastic Pipes and the Bonding Trap

Modern properties increasingly have plastic incoming water services and plastic internal pipework, with short metal sections at appliances and taps. This changes the picture completely.

 

If the pipe enters the building in plastic, and any metal sections do not go into the ground or connect to anything else that does, then the metalwork cannot introduce Earth potential. It is not an extraneous-conductive-part, and Regulation 411.3.1.2 does not require it to be bonded. It’s sometimes described as “floating”.

 

Bonding it anyway isn’t harmless. You take a piece of metal that was at no particular potential and deliberately connect it to the installation’s earth. Under a fault or a broken-PEN condition, you’ve now given that metal a potential it would never have had. It’s the same reason nobody bonds a metal shaving mirror or a towel rail on plastic pipes.

 

Situation	Extraneous-Conductive-Part?	Bond?
Metallic water pipe entering from the ground	Yes	Yes, main bonding
Plastic incoming pipe, plastic internal pipework, metal tap tails only	No	No
Plastic incoming pipe, metallic internal pipework confirmed isolated from Earth	No	No
Plastic incoming pipe, metallic internal pipework also connected to a bonded gas pipe or steelwork	Yes, via that connection	Yes

 

Key point: Where it isn’t obvious, the decision is made by measurement, not assumption. A resistance test between the metalwork and the MET of more than approximately 22 kΩ is the widely used threshold for concluding a part is not extraneous. The water inside the pipe is not counted as a conductive path for this purpose.

 

Exam Questions to Expect

This topic is spread across the paper rather than concentrated in one part. Here’s how it tends to appear:

 

Question Style	Where It Sits	What They’re Testing
”Which of the following is an extraneous-conductive-part?”	Part 2	Can you spot the metal water pipe among a list of appliance casings?
”The metal casing of a Class I luminaire is described as…”	Part 2	Recognising an exposed-conductive-part
”Main protective bonding connects extraneous-conductive-parts to…”	Part 4, Reg. 411.3.1.2	The MET, not the consumer unit earth bar of a particular circuit
”Minimum CSA of main bonding on a PME supply with 25 mm² tails”	Part 5, Table 54.8	10 mm², and knowing the PME rule differs from TN-S
”Minimum supplementary bonding between two extraneous parts, not mechanically protected”	Part 5, Reg. 544.2.4	4 mm²
”Supplementary bonding in a bathroom may be omitted where…”	Part 7, Reg. 701.415.2	All three conditions, not just the RCD

 

Tab Chapter 54 and Tables 54.7 and 54.8 in your book before the exam. The definitions themselves should be memorised, because Part 2 is alphabetical and slow to search under time pressure. For more worked questions on this chapter, see Chapter 54 exam questions and answers explained.

 

Practice and Further Study

Earthing and bonding questions reward candidates who understand the reasoning, not just the numbers. Test yourself on the parts of BS 7671 where this topic lives:

• Part 4 — Protection for Safety quiz
• Part 5 — Selection and Erection of Equipment quiz
• Part 6 — Inspection and Testing quiz
• Part 7 — Special Installations quiz

 

Our app includes 690+ practice questions covering all 8 parts of BS 7671, each with a detailed explanation referencing the specific regulation or table, plus topic-specific quizzes on earthing and bonding and timed mock exams that mirror the weighting of the real City & Guilds 2382-26 paper.

 

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