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Level 3 Diploma in Electrical Installations (Buildings and Structures) Unit 305 Handout 3
305: Electrical systems design
Handout 3: Earthing
Learning outcome
The learner will:
5 Understand earthing and protection.
Assessment criteria
The learner can:
5.1 explain the purpose of earthing and protective conductors
5.2 identify extraneous and exposed conductive parts.
Earthing
Purpose of earthing
The three main reasons for earthing electrical systems are as follows:
to maintain the potential of any part of the system at a defined value with respect to earth
to allow current to flow to earth in the event of a fault, so that the protective equipment will
operate to isolate the faulty circuit
to ensure that, in the event of a fault, apparatus normally ‘dead’ cannot reach a dangerous
potential with respect to earth (earth is normally taken as 0 volts, ‘no volts’).
Supply system earthing
Local distribution is normally undertaken with a three-phase three-wire system at 11kV. This is
then transformed down to 400V/230V four-wire system, as shown in the diagram below:
The primary is connected in delta, whilst the secondary is connected in star. The star point of the
secondary provides the neutral. Connecting between any two line conductors will give us 400 volts,
whilst connecting between any line and neutral will give us 230 volts.
It is a legal requirement that all electrical systems connected to the public supply system must be
earthed. This is usually achieved by connecting the star point of the supply transformer to earth,
using an earth electrode.
Electrical installations connected to the public supply system will also be earthed in some way. The
method of providing this earth will determine the earthing system used (TN-S, TN-C-S, TT, etc).
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Level 3 Diploma in Electrical Installations (Buildings and Structures) Unit 305 Handout 3
Equipotential bonding
BS 7671:2018 (page 29) defines ‘equipotential bonding’ as:
Electrical connection maintaining various exposed-conductive-parts and
extraneous-conductive-parts at substantially the same potential.
When used (as it usually is) for the purpose of safety, it is referred to as ‘Protective equipotential
bonding’, which is defined as ‘equipotential bonding for the purpose of safety’ (page 35).
BS 7671:2018 (page 29) further defines ‘Exposed-conductive-parts’ as:
Conductive part of equipment which can be touched and which is not normally live,
but which can become live under fault conditions.
This includes:
steel conduit
steel trunking
steel tray
steel enclosures of wiring systems
metal accessories
metallic equipment.
BS 7671:2018 (page 30) defines ‘Extraneous-conductive-parts’ as:
A conductive part liable to introduce a potential, generally Earth potential, and not
forming part of the electrical installation.
This will include:
metallic service pipes (gas, oil, water)
steel duct work
structural steel.
If all conductive parts within an installation are electrically connected together then they will be at
the same electrical potential. If two separate parts that are at the same potential are touched
simultaneously then the potential difference between them will be 0 volts and no current will flow.
This will apply even if the parts are at, say, 230 volts as a result of a fault.
Since the supply system is earthed, generally at the star point of the supply transformer, when a
fault to the equipotential bonding occurs and its potential rises, then current will flow to earth and
this current will cause the protective device(s) to operate and disconnect the supply.
These are the principles of equipotential bonding and protective equipotential bonding.
Main protective bonding
BS 7671:2018 Regulation 411.3.1.2 (page 58) states that, “In each installation, main protective
bonding conductors complying with Chapter 54 shall connect to the main earthing terminal
extraneous-conductive-parts, including the following:
(i) water installation pipes
(ii) gas installation pipes
(iii) other installation pipework and ducting
(iv) central heating and air conditioning systems
(v) exposed metallic structural parts of the building.
Metallic pipes entering the building having an insulating section at their point of entry need not be
connected to the protective equipotential bonding”.
The diagram on the following page shows how this is applied:
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Level 3 Diploma in Electrical Installations (Buildings and Structures) Unit 305 Handout 3
Supplementary equipotential bonding
The IET On-Site Guide (Section 4.6, page 50) states that: ‘The purpose of supplementary
bonding is to reduce the voltage between the various exposed-conductive-parts and
extraneous-conductive-parts of a location during a fault to earth.’
Regulation 411.3.2.5 (page 59) informs us that “Where it is not feasible for an overcurrent
protective device to interrupt the supply in accordance with Regulation 411.3.2 or the use of an
RCD for this purpose is not appropriate, see Section 419 (page 78). However, disconnection may
be required for reasons other than protection against electric shock”
Regulation 419.2 (page 78) states “Except where Regulation 419.2 applies, if automatic
disconnection cannot be achieved in the time required by Regulation 411.3.2.2, 411.3.2.3 or
411.3.2.4 as appropriate, supplementary protective equipotential bonding shall be provided in
accordance with Regulation 415.2 and the voltage between simultaneously accessible exposed-
conductive-parts and/or extraneous-conductive-parts shall not exceed 50 V AC or 120 V DC”.
The sizing of supplementary bonding conductors is dealt with in Regulation 544.2.1 to 544.2.5,
Table 4.6 of the IET On-Site Guide (page 50) gives clearer guidance on this.
There are many myths relating to where and when supplementary equipotential bonding should be
used and Section 4.7 (page 51) of the IET On-Site Guide dispels some of these myths and is
reproduced in full below.
Supplementary equipotential bonding is required in some of the locations and installations falling
within the scope of Part 7 of BS 7671:2018.
If the installation meets the requirements for BS 7671:2018 for earthing and bonding, there is no
specific requirement for supplementary equipotential bonding of:
kitchen pipes, sinks or draining boards
metallic boiler pipework
metal furniture in kitchens
metallic pipes to wash hand basins and WCs
locations containing a bath or shower, providing the requirements of 701.415.2 (page 241)
are met.
Note: Metallic waste pipes deemed to be extraneous-conductive-parts must be connected
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Level 3 Diploma in Electrical Installations (Buildings and Structures) Unit 305 Handout 3
by main protective bonding conductors to the main earthing terminal; see also 4.3
(page 47-48) of the IET On-Site Guide.
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