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IEE Wiring Regulations:
Design and Verification of Electrical
17th Edition IEE Wiring Regulations: Inspection, Testing and Certification,
Electric Wiring: Domestic, ISBN 978-0-7506-8735-5
Department in Oxford, UK: phone (44) (0) 1865 843830; fax (44) (0) 1865 853333;
email: [email protected] Alternatively you can submit your request online
17th edition IEE wiring regulations : design and verification of electrical
installations. – 6th ed. 1. Electric wiring, Interior – safety regulations – Great Britain
2. Electric wiring, Interior – Handbooks, manuals, etc. I. Title II. Scaddan, Brian.
16th edition IEE wiring regulations III. Institution of Electrical Engineers
IV. Seventeenth edition IEE wiring regulations
621.3’1924’0941
Library of Congress Control Number: 2008926538
ISBN: 978-0-7506-8721-8
Ted Stocks
17th Edition IEE Wiring Regulations vii
CHAPTER 1 Design ............................................................................1
Assessment of General Characteristics ..................................................1
Protection for Safety ..............................................................................6
Protection Against Electric Shock ...........................................................7
Protection Against Thermal Effects (IEE Regulations Chapter 42) .........15
Protection Against Overcurrent ............................................................16
Protection Against Overload .................................................................18
Protection Against Fault Current ..........................................................19
Protection Against Undervoltage (IEE Regulations Section 445) ...........23
(IEE Regulations Sections 442 and 443) ...............................................23
Isolation and Switching ........................................................................23
Design Calculations .............................................................................25
CHAPTER 2 Inspection and Testing ..................................................53
Initial Verification .................................................................................53
Inspection ...........................................................................................54
Testing ................................................................................................54
Approved Test Lamps and Indicators ...................................................55
Calibration, Zeroing/Nulling and Care of Instruments ...........................57
The Tests ............................................................................................58
Continuity of Protective Conductors .....................................................59
Continuity of Ring Final Circuit Conductors ..........................................61
Insulation Resistance ..........................................................................70
Polarity ...............................................................................................72
Ring Final Circuits ...............................................................................72
Radial Circuits .....................................................................................73
Earth Electrode Resistance ..................................................................73
External Loop Impedance Ze ................................................................77
Earth Fault Loop Impedance Zs ...........................................................78
Additional Protection ...........................................................................78
CHAPTER 3 Special Locations IEE Regulations Part 7 .......................85
Introduction ........................................................................................85
BS 7671 Section 701: Bathrooms, etc. . ...............................................86
BS 7671 Section 702: Swimming Pools ...............................................89
BS 7671 Section 703: Hot Air Saunas .................................................92
BS 7671 Section 704: Construction Sites .............................................93
BS 7671 Section 705: Agricultural and Horticultural Locations .............95
BS 7671 Section 706: Restrictive Conductive Locations .......................97
BS 7671 Section 708: Caravan and Camping Parks .............................97
BS 7671 Section 709: Marinas ............................................................99
BS 7671 Section 711: Exhibitions, Shows and Stands ........................100
BS 7671 Section 712: Solar Photovoltaic (PV) Supply Systems ..........101
BS 7671 Section 717: Mobile or Transportable Units .........................102
BS 7671 Section 721: Caravans and Motor Caravans ........................102
BS 7671 Section 740: Amusement Devices, Fairgrounds,
Circuses, etc. ....................................................................................104
BS 7671 Section 753: Floor and Ceiling Heating Systems ..................104
BS 7671 Appendices ................................................107
Sample Questions .....................................................109
Suggested Solutions to Sample Questions ..................115
INDEX ...............................................................................................123
There are many electrical operatives who, quite innocently I am
sure, select wiring systems based on the old adage of ‘that’s the
way it’s always been done’ or ‘we always use that size of cable for
that circuit’ etc. Unfortunately this approach, except for a few
standard circuits, is quite wrong. Each wiring system should be
designed to be fit for purpose and involves more than arbitrary
The intention of this book is to illustrate the correct procedure for
basic design of installations from initial assessment to final commissioning. It will also be of use to candidates studying for a C&G
2391-20 Design qualification.
This edition has been revised to serve as an accompaniment to the
new City & Guilds scheme and has been brought fully up-to-date
with the 17th Edition IEE Wiring Regulations.
Any design to the 17th Edition of the IEE Wiring Regulations
BS 7671 must be primarily concerned with the safety of persons,
property and livestock. All other considerations such as operation,
maintenance, aesthetics, etc., while forming an essential part of
the design, should never compromise the safety of the installation.
The selection of appropriate systems and associated equipment
and accessories is an integral part of the design procedure, and as
such cannot be addressed in isolation. For example, the choice of a
particular type of protective device may have a considerable effect
on the calculation of cable size or shock risk, or the integrity of
conductor insulation under fault conditions.
Perhaps the most difficult installations to design are those involving
additions and/or alterations to existing systems, especially where no
original details are available, and those where there is a change of
usage or a refurbishment of a premises, together with a requirement
to utilize as much of the existing wiring system as possible.
So, let us investigate those parts of the Wiring Regulations that
need to be considered in the early stages of the design procedure.
Regardless of whether the installation is a whole one, an addition, or an alteration, there will always be certain design criteria
to be considered before calculations are carried out. Part 3 of the
2 IEE Wiring Regulations: Design and Verification
17th Edition, ‘Assessment of General Characteristics’, indicates
six main headings under which these considerations should be
addressed. These are:
Recognized safety services
Assessment of continuity of service.
Let us look at these headings in a little more detail.
For a new design, will the installation be suitable for its
For a change of usage, is the installation being used for its
If not, can it be used safely and effectively for any other
Has the maximum demand been evaluated?
Can diversity be taken into account?
Are the supply and earthing characteristics suitable?
Are the methods for protection for safety appropriate?
If standby or safety supplies are used, are they reliable?
Are the installation circuits arranged to avoid danger and
facilitate safe operation?
Appendix 5 of the IEE Regulations classifies external influences
which may affect an installation. This classification is divided
into three sections, the environment (A), how that environment is
Examples of Classifications of External Influences.
AD6 Waves
BA3 Handicapped
CA1 Non-combustible
utilized (B) and construction of buildings (C). The nature of any
influence within each section is also represented by a number.
Table 1.1 gives examples of the classification.
With external influences included on drawings and in specifications, installations and materials used can be designed accordingly.
It is of great importance to ensure that damage to, or mal-operation
of, equipment cannot be caused by harmful effects generated by
other equipment even under normal working conditions. For example, MIMS cable should not be used in conjunction with discharge
lighting, as the insulation can break down when subjected to the
high starting voltages; the operation of residual current devices
(RCDs) may be impaired by the magnetic fields of other equipment;
computers, PLCs, etc. may be affected by normal earth leakage
currents from other circuits.
The Electricity at Work Regulations 1989 require every system to
be maintained such as to prevent danger; consequently, all installations require maintaining, some more than others, and due account
of the frequency and quality of maintenance must be taken at the
design stage. It is usually the industrial installations that are mostly
affected by the need for regular maintenance, and hence, consultation with those responsible for the work is essential in order to
4 IEE Wiring Regulations: Design and Verification
ensure that all testing, maintenance and repair can be effectively
and safely carried out. The following example may serve to illustrate an approach to consideration of design criteria with regard to a
change of usage.
A vacant two-storey light industrial workshop, 12 years old, is to be taken over and
used as a Scout/Guide HQ. New shower facilities are to be provided. The supply is
three-phase 400/230 V and the earthing system is TN-S.
The existing electrical installation on both floors comprises steel trunking at a height of
2.5 m around all perimeter walls, with steel conduit, to all socket outlets and switches
(metal-clad), to numerous isolators and switch-fuses once used to control single- and
three-phase machinery, and to the lighting which comprises fluorescent luminaires
suspended by chains from the ceilings. The ground floor is to be used as the main
activity area and part of the top floor at one end is to be converted to house separate
male and female toilet and shower facilities accommodating two 8 kW/230 V shower
units in each area.
If the existing electrical installation has been tested and inspected and shown to be safe:
1. Outline the design criteria, having regard for the new usage, for
(a) The existing wiring system and
(b) The wiring to the new showers.
2. What would be the total assumed current demand of the shower units?
Suggested approach/solution
1(a) Existing system
Clearly the purpose for which the installation was intended has changed; however, the
new usage is unlikely, in all but a few instances, to have a detrimental effect on the
existing system. It will certainly be under-loaded; nevertheless this does not preclude
the need to assess the maximum demand.
The supply and earthing arrangements will be satisfactory, but there may be a need
to alter the arrangement of the installation, in order to rebalance the load across the
phases now that machinery is no longer present.
The new shower area will probably have a classification AD3 or 4 and will be subject to Section 701, IEE Regulations. Ideally all metal conduit and trunking should be
removed together with any socket outlets within 3 m of the boundary of zone 1. The
trunking could be replaced with PVC; alternatively it could be boxed in using insulating material and screw-on lids to enable access. It could be argued that no action
is necessary as it is above 2.25 m and therefore outside of all the zones. Suspended
fluorescent fittings should be replaced with the enclosed variety, with control switches
preferably located outside the area.
The activities in the ground-floor area will almost certainly involve various ball games,
giving it a classification of AG2 (medium impact). Conduit drops are probably suitable,
but old isolators and switch-fuses should be removed, and luminaires fixed to the ceiling and caged, or be replaced with suitably caged spotlights on side walls at high level.
As the whole building utilization can now be classified as BA2 (children), it is probably
wise to provide additional protection against shock by installing 30 mA RCDs on all
Unlikely to be any compatibility problems with the new usage.
Mainly periodic test and inspection with some maintenance of lighting, hence suitable
access equipment should be available, together with spare lamps and tubes. Lamp
disposal facilities should be considered. A maintenance programme should be in place
and all safety and protective measures should be effective throughout the intended life
1(b) New shower area (BS 7671 Section 701)
As this is a new addition, the installation will be designed to fulfil all the requirements for
which it is intended. The supply and earthing system should be suitable, but a measurement of the prospective fault current (PFC) and Ze should be taken. The loading of
the showers will have been accounted for during the assessment of maximum demand.
In the unlikely event of original design and installation details being available, it may
be possible to utilize the existing trunking without exceeding space factors or de-rating
cables due to the application of grouping factors. However, it is more probable that
6 IEE Wiring Regulations: Design and Verification
a re-evaluation of the trunking installation would need to be undertaken, or alternatively, install a completely separate system. Whichever the method adopted, a distribution circuit supplying a four-way distribution board located outside the area would be
appropriate, the final circuits to each shower being run via individual control switches
also outside, and thence to the units using a PVC conduit system. Protection against
shock would be by basic protection (insulation and barriers and enclosures) and fault
protection (protective earthing, protective equipotential bonding and automatic disconnection); additional protection would be provided by RCDs/RCBOs’.
These have already been addressed in 1(a) above.
There will be no incompatibility between any equipment in this area.
Afforded by the individual switches and/or circuit breakers allowing isolation to maintain or repair/replace defective units.
2 Total assumed current demand
Design current Ib for each unit 8000/230 35 A applying diversity:
100% of 35
25% of 35
Total assumed current demand 87.5 A
As an answer to a C&G 2400 examination question, this suggested approach is more
comprehensive than time constraints would allow, and hence an abbreviated form is
acceptable. The solutions to the questions for Chapter 3 of this book illustrate such
shortened answers.
Part 4 of the 17th Edition details the methods and applications of
protection for safety, and consideration of these details must be
made as part of the design procedure. Areas that the designer needs
to address are: protection against shock, thermal effects, overcurrent,
undervoltage, overvoltage, and the requirements for isolation and
switching. Let us now deal, in broad terms, with each of these areas.
There are two ways that persons or livestock may be exposed to
the effects of electric shock; these are (a) by touching live parts of
electrical equipment or (b) by touching exposed-conductive parts
of electrical equipment or systems, which have been made live
by a fault. Table 1.2 indicates the common methods of protecting
against either of these situations.
Insulation or barriers and enclosures (Basic protection)
One method used to protect against contact with live parts is to
insulate or house them in enclosures and/or place them behind
barriers. In order to ensure that such protection will be satisfactory, the enclosures/barriers must conform to BS EN 60529, commonly referred to as the Index of Protection (IP) code. This details
the amount of protection an enclosure can offer to the ingress of
mechanical objects, foreign solid bodies and moisture. Table 1.3
(see page 10) shows part of the IP code. The X in a code simply
means that protection is not specified; for example, in the code
IP2X, only the protection against mechanical objects is specified,
not moisture. Also, protection for wiring systems against external
mechanical impact needs to be considered. Reference should be
made to BS EN 62262, the IK code (Table 1.4, see page 11).
Protective earthing, protective equipotential bonding and
automatic disconnection in case of a fault (Fault protection)
As Table 1.2 indicates, this method is the most common method
of providing Fault protection, and hence it is important to expand
8 IEE Wiring Regulations: Design and Verification
Common Methods of Protection Against Shock.
SELV (separated
extra low voltage)
Basic and fault
Used for circuits in environments
such as bathrooms, swimming pools,
restrictive conductive locations,
agricultural and horticultural situations,
and for 25 V hand lamps in damp
situations on construction sites. Also
useful for circuits in schools, or college
Insulation of live
This is simply ‘basic insulation’.
Except where otherwise specified, such
as swimming pools, hot air saunas,
etc., placing LIVE PARTS behind
barriers or in enclosures to at least
IP2X is the norm. Two exceptions to
1. Accessible horizontal top surfaces
of, for example, distribution boards
or consumer units, where the
protection must be to at least IP4X
2. Where a larger opening than IP2X
is necessary, for example entry to
lampholders where replacement of
lamps is needed.
Access past a barrier or into an
enclosure should only be possible by
the use of a tool, or after the supply
has been disconnected, or if there is
an intermediate barrier to at least IP2X.
This does not apply to ceiling roses or
ceiling switches with screw-on lids.
Restricted to areas only accessible
to skilled persons, for example substations with open fronted busbar
Restricted to areas only accessible to
skilled persons, e.g. sub-stations with
open fronted busbar chambers, etc.
Overhead travelling cranes or overhead
RCDs (residual
current devices)
These may only be used as additional
protection, and must have an operating
current of 30 mA or less, and an
operating time of 40 ms or less at a
residual current of 5 IΔn.
Used where the loop impedance
requirements cannot be met or for
protecting socket outlet circuits
supplying portable equipment used
Preferred method of earth fault
protection for TT systems.
The most common method in use.
Relies on the co-ordination of the
characteristics of the earthing,
impedance of circuits, and operation of
protective devices such that no danger
is caused by earth faults occurring
anywhere in the installation.
Sometimes referred to as double
insulated equipment and marked with
the BS symbol ⵧ .
Rarely used – only for very special
installations under strict supervision.
Earth-free local
10 IEE Wiring Regulations: Design and Verification
No protection of persons against contact with live or moving parts inside the
Protection against accidental or inadvertent contact with live or moving parts inside the
enclosure by a large surface of the human body, for example, a hand, not for protection
against deliberate access to such parts. Protection against ingress of large solid foreign
Protection against contact with live or moving parts inside the enclosure by fingers.
Protection against ingress of medium-sized solid foreign bodies.
Protection against contact with live or moving parts inside the enclosure by tools,
wires or such objects of thickness greater than 2.5 mm. Protection against ingress of
small foreign bodies.
Complete protection against contact with live or moving parts inside the enclosures.
prevented, but dust cannot enter in an amount sufficient to interfere with satisfactory
operation of the equipment enclosed.
enclosure shall have no effect.
2. Protection against drops of liquid. Drops of falling liquid shall have no harmful effect
when the enclosure is tilted at any angle up to 15° from the vertical.
3. Protection against rain. Water falling in rain at an angle equal to or smaller than 60°
with respect to the vertical shall have no harmful effect.
6. Protection against conditions on ships’ decks (deck with watertight equipment).
8. Protection against indefinite immersion in water under specified pressure. It must not
be possible for water to enter the enclosure.
IK Codes Protection Against Mechanical Impact.
Impact 10 joules
Impact 5 joules
Impact 2 joules
Impact 20 joules
There are two basic ways of receiving an electric shock by contact
with conductive parts made live due to a fault:
1. Via parts of the body and the general mass of earth (typically
hands and feet) or
2. Via parts of the body and simultaneously accessible exposed
and extraneous conductive parts (typically hand to hand) –
12 IEE Wiring Regulations: Design and Verification
Clearly, the conditions shown in Figure 1.1 would provide no
protection, as the installation is not earthed. However, if it can
be ensured that protective devices operate fast enough by providing low impedance paths for earth fault currents, and that main
protective bonding is carried out, then the magnitude and duration of earth faults will be reduced to such a level as not to cause
The disconnection times for final circuits not exceeding 32A is
0.4 s and for distribution circuits and final circuits over 32A is 5 s.
For TT systems these times are 0.2 s and 1 s.
The connection of protective bonding conductors has the effect
of creating a zone in which, under earth fault conditions, all
exposed and extraneous conductive parts rise to a substantially
equal potential. There may be differences in potential between
Link for E
General mass of earth or other
bonding to gas, water, etc.
simultaneously accessible conductive parts, but provided the
design and installation are correct, the level of shock voltage will
not be harmful.
Figure 1.2 shows the earth fault system which provides Fault
The low impedance path for fault currents, the earth fault loop
path, comprises that part of the system external to the installation, i.e. the impedance of the supply transformer, distributor and
service cables Ze, and the resistance of the line conductor R1 and
circuit protective conductor (cpc) R2, of the circuit concerned.
The total value of loop impedance Zs is therefore the sum of these
Zs Ze (R1 R2 )Ω
Provided that this value of Zs does not exceed the maximum value
given for the protective device in question in Tables 41.2, 41.3
14 IEE Wiring Regulations: Design and Verification
or 41.4 of the Regulations, the protection will operate within the
It must be noted that the actual value of (R1 R2) is determined
Tabulated value of (R1 R2 ) Circuit length Multiplier
The multiplier corrects the resistance at 20°C to the value at conductor operating
The designer obviously has some measure of control over the values of R1 and R2, but the value of Ze can present a problem when
the premises, and hence the installation within it, are at drawing
board stage. Clearly Ze cannot be measured, and although a test
made in an adjacent installation would give some indication of a
likely value, the only recourse would either be to request supply
network details from the Distribution Network Operator (DNO)
and calculate the value of Ze, or use the maximum likely values
quoted by the DNOs, which are:
These values are pessimistically high and may cause difficulty in
even beginning a design calculation. For example, calculating the
size of conductors (considering shock risk) for, say, a distribution
circuit cable protected by a 160 A, BS 88 fuse and supplied via a
TNC-S system, would present great difficulties, as the maximum
value of Zs (Table 41.4(a)) for such a fuse is 0.25 Ω and the quoted
likely value of Ze is 0.35 Ω. In this case the DNO would need to be
This still remains a contentious issue even though the Regulations
are quite clear on the matter. Supplementary bonding is used as
Additional protection to Fault protection and required under the
1. When the requirements for loop impedance and associated
disconnection times cannot be met (RCDs may be installed
as an alternative) and
2. The location is an area of increased risk such as detailed
in Part 7 of the Regulations, e.g. bathrooms, etc. and
swimming pools (see also Chapter 3).
(IEE REGULATIONS CHAPTER 42)
The provision of such protection requires, in the main, a commonsense approach. Basically, ensure that electrical equipment
that generates heat is so placed as to avoid harmful effects on
surrounding combustible material. Terminate or join all live conductors in approved enclosures, and where electrical equipment
contains in excess of 25 litres of flammable liquid, make provision
to prevent the spread of such liquid, for example a retaining wall
round an oil-filled transformer.
16 IEE Wiring Regulations: Design and Verification
In order to protect against burns from equipment not subject to a
Harmonized Document limiting temperature, the designer should
conform to the requirements of Table 42.1, IEE Regulations.
Section 422 of this chapter deals with locations and situations
where there may be a particular risk of fire. These would include
locations where combustible materials are stored or could collect
and where a risk of ignition exists. This chapter does not include
locations where there is a risk of explosion.
The term overcurrent may be sub-divided into:
1. Overload current and
2. Fault current.
The latter is further sub-divided into:
(a) Short-circuit current (between live conductors) and
(b) Earth fault current (between line and earth).
Overloads are overcurrents occurring in healthy circuits and caused
by, for example, motor starting, inrush currents, motor stalling,
connection of more loads to a circuit than it is designed for, etc.
Fault currents, on the other hand, typically occur when there is
mechanical damage to circuits and/or accessories causing insulation failure or breakdown leading to ‘bridging’ of conductors. The
impedance of such a ‘bridge’ is assumed to be negligible.
Clearly, significant overcurrents should not be allowed to persist
for any length of time, as damage will occur to conductors and
Table 1.5 indicates some of the common types of protective device
used to protect electrical equipment during the presence of over
currents and fault currents.
Commonly Used Protective Devices.
Semi-enclosed re-wireable
Gradually being replaced
by other types of
protection. Its high fusing
factor results in lower cable
or, conversely, larger cable
Does not offer good shortcircuit current protection.
Ranges from 5 A to 200 A.
HBC fuse links BS 88-6 and
Mainly commercial and
Give excellent short-circuit
current protection. Does
not cause cable de-rating.
‘M’ types used for motor
protection. Ranges from
2 A to 1200 A.
HBC fuse links BS 1361
House service and
Not popular for use in
consumer units; however,
gives good short-circuit
current protection, and
does not result in cable
Ranges from 5 A to 100 A.
MCBs and CBs (miniature
circuit breakers) BS 3871,
now superseded by BS EN
60898 CBs
and commercial/industrial
MCCBs (moulded case
circuit breakers) BS EN
Industrial situations where
high current and breaking
capacities are required
Very popular due to ease of
operation. Some varieties
have locking-off facilities.
Range from 1 A to 63 A
single and three phase. Old
types 1, 2, 3 and 4 now
replaced by types B, C and
D with breaking capacities
from 3 kA to 25 kA.
Breaking capacity, 22 kA
to 50 kA in ranges 16 A to
1200 A. 2, 3 and 4 pole
18 IEE Wiring Regulations: Design and Verification
Protective devices used for this purpose have to be selected to conform with the following requirements:
1. The nominal setting of the device In must be greater than or
equal to the design current Ib:
In I b
2. The current-carrying capacity of the conductors Iz must
be greater than or equal to the nominal setting of the
I z In
3. The current causing operation of the device I2 must be less
than or equal to 1.45 times the current-carrying capacity of
the conductors Iz:
For fuses to BS 88 and BS 1361, and MCBs or CBs, compliance
with (2) above automatically gives compliance with (3). For fuses
to BS 3036 (re-wireable) compliance with (3) is achieved if the
nominal setting of the device In is less than or equal to 0.725 Iz:
In 0.725 Iz
This is due to the fact that a re-wireable fuse has a fusing factor of
2, and 1.45/2 0.725.
Overload devices should be located at points in a circuit where there
is a reduction in conductor size or anywhere along the length of a
conductor, providing there are no branch circuits. The Regulations
indicate circumstances under which overload protection may be
omitted; one such example is when the characteristics of the load
are not likely to cause an overload, hence there is no need to provide
protection at a ceiling rose for the pendant drop.
When a ‘bridge’ of negligible impedance occurs between live conductors (remember, a neutral conductor is a live conductor) the
short-circuit current that could flow is known as the ‘prospective
short-circuit current’ (PSCC), and any device installed to protect
against such a current must be able to break and in the case of a
circuit breaker, make the PSCC at the point at which it is installed
without the scattering of hot particles or damage to surrounding
materials and equipment. It is clearly important therefore to select
protective devices that can meet this requirement.
It is perhaps wise to look in a little more detail at this topic. Figure
1.3 shows PSCC over one half-cycle; t1 is the time taken to reach
‘cut-off ’ when the current is interrupted, and t2 the total time
taken from start of fault to extinguishing of the arc.
During the ‘pre-arcing’ time t1, electrical energy of considerable
proportions is passing through the protective device into the conductors. This is known as the ‘pre-arcing let-through’ energy and
is given by (If)2tl where If is the short-circuit current at ‘cut-off ’.
The total amount of energy let-through into the conductors is given
by (If)2tl in Figure 1.4.
For faults up to 5 s duration, the amount of heat and mechanical energy that a conductor can withstand is given by k2s2, where
k is a factor dependent on the conductor and insulation materials
(tabulated in the Regulations), and s is the conductor csa. Provided
the energy let-through by the protective device does not exceed the
20 IEE Wiring Regulations: Design and Verification
Short-circuit current (amperes)
Arc being extinguished
Pre-arcing let-through.
Energy let-through lf2t
energy withstand of the conductor, no damage will occur. Hence,
the limiting situation is when (If)2t k2s2. If we now transpose
this formula for t, we get t k2s2/(If)2, which is the maximum disconnection time (t in seconds).
When an installation is being designed, the PSCC at each relevant
point in the installation has to be determined, unless the breaking
capacity of the lowest rated fuse in the system is greater than the
PSCC at the intake position. For supplies up to 100 A the supply
authorities quote a value of PSCC, at the point at which the service cable is joined to the distributor cable, of 16 kA. This value will
decrease significantly over only a short length of service cable.
We have already discussed this topic with regard to shock risk, and
although the protective device may operate fast enough to prevent
shock, it has to be ascertained that the duration of the fault,
however small, is such that no damage to conductors or insulation
will result. This may be verified in two ways:
1. If the protective conductor conforms to the requirements of
Table 54.7 (IEE Regulations), or if
2. The csa of the protective conductor is not less than that
calculated by use of the formula:
which is another rearrangement of I2t k2S2.
For flat, twin and three-core cables the formula method of verification will be necessary, as the cpc incorporated in such cables
is always smaller than the associated line conductor. It is often
desirable when choosing a cpc size to use the calculation, as
invariably the result leads to smaller cpcs and hence greater economy. This topic will be expanded further in the section ‘Design
22 IEE Wiring Regulations: Design and Verification
(If)2t Characteristics: 2–800 A Fuse Links. Discrimination is
Achieved if the Total (If)2t of the Minor Fuse Does Not Exceed the
Pre-arcing (If)2t of the Major Fuse.
It2t Pre-arcing
It2t Total at 415 V
It is clearly important that, in the event of an overcurrent, the protection associated with the circuit in question should operate, and
not other devices upstream. It is not enough to simply assume that
a device one size lower will automatically discriminate with one a
size higher. All depends on the ‘let-through’ energy of the devices.
If the total ‘let-through’ energy of the lower rated device does not
exceed the pre-arcing ‘let-through’ energy of the higher rated device,
then discrimination is achieved. Table 1.6 shows the ‘let-through’
values for a range of BS 88 fuse links, and illustrates the fact that
devices of consecutive ratings do not necessarily discriminate. For
example, a 6 A fuse will not discriminate with a 10 A fuse.
(IEE REGULATIONS SECTION 445)
In the event of a loss of or significant drop in voltage, protection
should be available to prevent either damage or danger when the
supply is restored. This situation is most commonly encountered in
motor circuits, and in this case the protection is provided by the contactor coil via the control circuit. If there is likely to be damage or danger due to undervoltage, standby supplies could be installed and, in
the case of computer systems, uninterruptible power supplies (UPS).
(IEE REGULATIONS SECTIONS 442 AND 443)
This chapter deals with the requirements of an electrical installation to withstand overvoltages caused by 1. transient overvoltages
of atmospheric origin and 2. switching surges within the installation. It is unlikely that installations in the UK will be affected by
the requirements of item 1. as the number of thunderstorm days
per year is not likely to exceed 25.
Let us first be clear about the difference between isolators and
switches. An isolator is, by definition, ‘A mechanical switching device which provides the function of cutting off, for reasons
of safety, the supply to all or parts of an installation, from every
24 IEE Wiring Regulations: Design and Verification
Common Types of Isolators and Switches.
Isolator or
Not designed to be operated on load.
Isolation can also be achieved by the
removal of fuses, pulling plugs, etc.
Any situation where a load
needs to be frequently
operated, i.e. light switches,
switches on socket outlets,
A functional switch could be used
as a means of isolation, i.e. a oneway light switch provides isolation
for lamp replacement provided the
switch is under the control of the
person changing the lamp.
At the origin of an installation
or controlling sub-mains or
These can perform the function of
isolation while housing the circuit
As for switch-fuse
Mainly used for higher current ratings
and have their fuses as part of the
Main switch on consumer
units and distribution fuse
These are ON LOAD devices but can
still perform the function of isolation.
source. A switch is a mechanical switching device which is capable
of making, carrying and breaking normal load current, and some
overcurrents. It may not break short-circuit currents.’
So, an isolator may be used for functional switching, but not usually vice versa. Basically an isolator is operated after all loads are
switched off, in order to prevent energization while work is being carried out. Isolators are off-load devices, switches are on-load devices.
The IEE Regulations (Section 537) deal with this topic and in
particular Isolation, Switching off for mechanical maintenance,
Emergency switching, and Functional switching.
Tables 1.7 and 1.8 indicate some of the common devices and their
Ditto but over 32 A
Basically, all designs follow the same procedure:
2. Determination of design current Ib
3. Selection of protective device having nominal rating or
4. Selection of appropriate rating factors
5. Calculation of tabulated conductor current It
6. Selection of suitable conductor size
7. Calculation of voltage drop
8. Evaluation of shock risk
9. Evaluation of thermal risks to conductors.
Let us now consider these steps in greater detail. We have already
dealt with ‘assessment of general characteristics’, and clearly one
result of such assessment will be the determination of the type and
disposition of the installation circuits. Table 1.9 gives details of
commonly installed wiring systems and cable types. Having made
the choice of system and cable type, the next stage is to determine
the design current.
Common Wiring Systems and Cable Types.
System/Cable Type
1 Flat twin and three-core Domestic and commercial
cable with cpc; PVC
sheathed, PVC insulated,
Used clipped direct to
surface or buried in plaster
either directly or encased
in oval conduit or tophat section; also used
in conjunction with PVC
mini-trunking.
2 PVC mini-trunking
Used with (1) above for
neatness when surface
3 PVC conduit with singlecore PVC insulated
Commercial and light
Easy to install, high
impact, vermin proof,
self-extinguishing, good in
corrosive situations. When
used with ‘all insulated’
accessories provides a
degree of Fault protection
4 PVC trunking: square,
rectangular, skirting,
dado, cornice, angled
bench. With single-core
When used with all
Fault protection on the
system. Some forms come
pre-wired with copper
busbars and socket outlets.
Segregated compartment
type good for housing
different band circuits.
5 Steel conduit and
trunking with single-core
Light and heavy industry,
Black enamelled conduit
and painted trunking
used in non-corrosive, dry
environments. Galvanized
finish good for moist/damp
or corrosive situations. May
be used as cpc, though
separate one is preferred.
6 Busbar trunking
rising mains in tall
Overhead plug-in type ideal
for areas where machinery
may need to be moved.
Arranged in a ring system
with section switches,
provides flexibility where
7 Mineral insulated copper
sheathed (MICS) cable
exposed to touch or
PVC covered. Clipped
direct to a surface or
perforated tray or in
trunking or ducts
All industrial areas,
especially chemical works,
boiler houses, petrol filling
stations, etc.; where harsh
conditions exist such as
extremes of heat, moisture,
corrosion, etc., also used
for fire alarm circuits
Very durable, long-lasting,
can take considerable
impact before failing.
capacity greater than same
in other cables. May be run
with circuits of different
categories in unsegregated
trunking. Cable reference
CC – Bare copper sheathed
V – PVC covered
M – Low smoke and fume
(LSF) material covered
L – Light duty (500 V)
H – Heavy duty (750 V).
Hence a two-core 2.5 mm2
light duty MI cable with
PVC oversheath would be
shown: CCV 2L 2.5.
8 F.P. 200. PVC sheathed
aluminium screened
silicon rubber insulated,
Clipped direct to surface
or on perforated tray or
run in trunking or ducts
Fire alarm and emergency
withstand fire. May
be run with circuits of
different categories in nonsegregated trunking.
28 IEE Wiring Regulations: Design and Verification
9 Steel wire armoured.
PVC insulated, PVC
underground supplies, etc.
conductors, clipped
direct to a surface or on
cable tray or in ducts or
10 As above but insulation For use in high
is XLPE. Cross (X) linked temperature areas
(L) poly (P) ethylene (E)
All areas where there is a
risk of damage by heat, oil
11 HOFR sheathed
cables (heat, oil, flame
Combines a certain
amount of flexibility with
These are usually flexible
This is defined as ‘the magnitude of the current to be carried by a
circuit in normal service’, and is either determined directly from
manufacturers’ details or calculated using the following formulae:
V Eff% PF
3 VL Eff % PF
V line to neutral voltage in volts
VL line to line voltage in volts
Eff% efficiency
The application of diversity to an installation permits, by assuming that not all loads will be energized at the same time, a
reduction in main or distribution circuit cable sizes. The IEE
Regulations Guidance Notes or On-Site Guide tabulate diversity in
the form of percentages of full load for various circuits in a range
of installations. However, it is for the designer to make a careful
judgement as to the exact level of diversity to be applied.
Nominal rating or setting of protection In
We have seen earlier that the first requirement for In is that it
should be greater than or equal to Ib. We can select for this condition from IEE Regulations Tables 41.2, 41.3 or 41.4. For types and
sizes outside the scope of these tables, details from the manufacturer will need to be sought.
There are several conditions which may have an adverse effect on
conductors and insulation, and in order to protect against this,
rating factors (CFs) are applied. These are:
Factor for ambient air and ground
(From IEE Regulations Tables 4B1, 4B2
or 4B3)
Factor for groups of cables
(From IEE Regulations Table 4C1 to 4C5)
if BS 3036 re-wireable fuse is used
if cable is buried direct in the ground
if both are involved
(Factor is 0.725)
(Factor is 0.9)
(Factor is 0.9 0.725 0.653)
Factor if cable is surrounded by
(IEE Regulations Table 52.2)
30 IEE Wiring Regulations: Design and Verification
The factors are applied as divisors to the setting of the protection
In; the resulting value should be less than or equal to the tabulated
current-carrying capacity It of the conductor to be chosen.
It is unlikely that all of the adverse conditions would prevail at the
same time along the whole length of the cable run and hence only
the relevant factors would be applied. A blanket application of correction factors can result in unrealistically large conductor sizes,
1. If the cable in Figure 1.5 ran for the whole of its length,
grouped with others of the same size in a high ambient
temperature, and was totally surrounded with thermal
insulation, it would seem logical to apply all the CFs, as
they all affect the whole cable run. Certainly the factors for
the BS 3036 fuse, grouping and thermal insulation should
be used. However, it is doubtful if the ambient temperature
will have any effect on the cable, as the thermal insulation,
if it is efficient, will prevent heat reaching the cable. Hence
apply Cg, Cc and Ci.
2. In Figure 1.6(a) the cable first runs grouped, then leaves the
group and runs in high ambient temperature, and finally
is enclosed in thermal insulation. We therefore have three
different conditions, each affecting the cable in different
areas. The BS 3036 fuse affects the whole cable run and
therefore Cc must be used, but there is no need to apply all
of the remaining factors as the worst one will automatically
compensate for the others. The relevant factors are shown
in Figure 1.6(b) and apply only if Cc 0.725 and Ci 0.5.
If protection was not by BS 3036 fuse, then apply only
Ci 0.5.
Factor 0.97
32 IEE Wiring Regulations: Design and Verification
Grouping 0.7
Thermal insulation 0.5
3. In Figure 1.7 a combination of cases 1 and 2 is considered.
The effect of grouping and ambient temperature is
0.7 0.97 0.679. The factor for thermal insulation is
still worse than this combination, and therefore Ci is the
only one to be used.
Tabulated conductor current-carrying capacity It
Ca Cg Cc Ci
Remember, only the relevant factors are to be used!
As we have seen when discussing overload protection, the IEE
Regulations permit the omission of such protection in certain circumstances (433.3.1); in these circumstances, In is replaced by Ib
Selection of suitable conductor size
During the early stages of the design, the external influences will
have been considered, and a method of circuit installation chosen.
Appendix 4, IEE Regulations Table 4A2 gives examples of installation methods, and it is important to select the appropriate method
in the current rating tables. For example, from IEE Regulations
Table 4D2A the tabulated current ratings It for reference method B
are less than those for method C. Having selected the correct cable
rating table and relevant reference method, the conductor size is
determined to correspond with It.
In many instances this may well be the most onerous condition
to affect cable sizes. The Regulations require that the voltage at
the terminals of fixed equipment should be greater than the lower
limit permitted by the British Standard for that equipment, or
in the absence of a British Standard, that the safe functioning
of the equipment should not be impaired. These requirements
are fulfilled if the voltage drop between the origin of the installation and any load point does not exceed the following values (IEE
Regulations, Appendix 12) (Table 1.10).
Accompanying the cable current rating tables are tabulated values
of voltage drop based on the milli-volts (mV) dropped for every
ampere of design current (A), for every metre of conductor length
(m), i.e.
Volt drop mV/A/m
Voltage drop values.
34 IEE Wiring Regulations: Design and Verification
or fully translated with Ib for A and L (length in metres):
Volt drop mV I b length
For conductor sizes in excess of 16 mm2 the impedance values
of volt drop in the IEE Regulations tables, Appendix 4 (columns
headed z) should be used. The columns headed r and x indicate
the resistive and reactive components of the impedance values.
Evaluation of shock risk
This topic has been discussed earlier; suffice to say that the calculated value of loop impedance should not exceed the tabulated
value quoted for the protective device in question.
Evaluation of thermal constraints
As we know, the ‘let-through’ energy of a protective device under
fault conditions can be considerable and it is therefore necessary
to ensure that the cpc is large enough, either by satisfying the
requirements of IEE Regulations Table 54.7 or by comparing its
size with the minimum derived from the formula:
s minimum csa of the cpc
I fault current
t disconnection time in seconds
k factor taken from IEE Regulations Tables 54.2 to 54.6.
The following examples illustrate how this design procedure is put
A consumer has asked to have installed a new 9 kW/230 V shower unit in a domestic premises. The existing eight-way consumer unit houses BS 3871 MCBs and supplies two ring final circuits, one cooker circuit, one immersion heater circuit and two
lighting circuits, leaving two spare ways. The earthing system is TN–C–S with a measured value of Ze of 0.18 Ω, and the length of the run from consumer unit to shower is
approximately 28 m. The installation reference method is method C, and the ambient
temperature will not exceed 30°C. If flat twin cable with cpc is to be used, calculate
the minimum cable size.
In this case, the major concern is the maximum demand. It will need to be ascertained
whether or not the increased load can be accommodated by the consumer unit and
the supplier’s equipment.
Design current Ib (based on rated values)
Choice and setting of protection
The type of MCB most commonly found in domestic installations over 10 years old is a
BS 3871 Type 2, and the nearest European standard to this is a BS EN 60898 Type B.
So from IEE Regulations Table 41.3, the protection would be a 40 A Type B CB with a
corresponding maximum value of loop impedance Zs of 1.15 Ω.
As a shower is unlikely to cause an overload, Ib may be used instead of In:
C a C g C c Ci
but as there are no rating factors,
I t Ib
I t 39 A
As the cable is to be PVC Twin with cpc, the conductor size will be selected from IEE
Regulations Table 4D5 column 6. Hence It will be 47 A and the conductor size 6.0 mm2.
36 IEE Wiring Regulations: Design and Verification
From IEE Regulations Table 4D5 column 7, the mV drop is 7.3, so:
mV Ib L
Volt drop 7 . 3 39 28
7 . 97 V (acceptable)
Whilst this may satisfy BS 7671, such a high value could cause inefficiency and the
Evaluation for shock risk
The line conductor of the circuit has been calculated as 6.0 mm2, and a twin cable of
this size has a 2.5 mm2 cpc. So, using the tabulated values of R1 and R2 given in the
On-Site Guide, 28 m of cable would have a resistance under operating conditions of:
28 (3 . 08 7 . 41) 1 . 2
0 . 35 Ω
(1.2 multiplier for 70°C conductor operating temperature) and as Ze is 0.18, then:
Z s Z e R1 R2
0 . 18 0 . 35
0 . 53 Ω
Which is clearly less than the maximum value of 1.15.
Fault current I is found from:
U0 nominal line voltage to earth
Zs calculated value of loop impedance
t for 434 A from IEE Regulations curves, Figure 3.4 for a 40 A CB is less than 0.1 s.
k from IEE Regulations Table 54.3 is 115.
4342 0 . 1
1 . 2 mm2
Which means that the 2.5 mm2 cpc is perfectly adequate. It does not mean that a
1.2 mm2 cpc could be used.
Hence, provided the extra demand can be accommodated, the new shower can be
wired in 6.0 mm2 flat twin cable with a 2.5 mm2 cpc and protected by a 40 A BS EN
60898 Type B CB.
Four industrial single-phase fan assisted process heaters are to be installed adjacent
to each other in a factory. Each one is rated at 50 A/230 V. The furthest heater is some
32 m from a distribution board, housing BS 88 fuses, located at the intake position.
It has been decided to supply the heaters with PVC singles in steel trunking (reference method B), and part of the run will be through an area where the ambient temperature may reach 35°C. The earthing system is TN–S with a measured Ze of 0.3 Ω.
There is spare capacity in the distribution board, and the maximum demand will not
be exceeded. Calculate the minimum size of live conductors and cpc.
Calculations will be based on the furthest heater. Also, only one common cpc needs to
be used (IEE Regulation 543.1.2).
Ib 50 A
Type and setting of protection In
In Ib so, from IEE Regulations Table 41.4, a BS 88 50 A fuse would be used, with a
corresponding maximum value of Zs of 1.04 Ω (Figure 1.8).
38 IEE Wiring Regulations: Design and Verification
Ze 0.3 32 m
Singles in trunking
As the circuits will be grouped and, for part of the length, run in a high ambient temperature, both Ca and Cg will need to be used.
Ca for 35°C
Cg for four circuits
0.94 (Table 4B1)
0.65 (Table 4C1)
Tabulated current-carrying capacity It
As the heaters are fan assisted, they are susceptible to overload, hence In is used:
0 . 94 0 . 65
From IEE Regulations Table 4D1A column 4, It 101 A, and the conductor size is
From IEE Regulations Table 4D1B, the mV drop for 25.0 mm2 is 1.8 mV.
Volt drop 1 . 8 50 32
2 . 88 V (acceptable)
In this case, as the conductors are singles, a cpc size has to be chosen either from IEE
Regulations Table 54.7, or by calculation. The former method will produce a size of
16 mm2, whereas calculation tends to produce considerably smaller sizes. The calculation involves the rearrangement of the formula:
Zs Ze (R1 R2 ) L 1 . 2
to find the maximum value of R2 and selecting a cpc size to suit. The value of Zs used
will be the tabulated maximum, which in this case is 1.04. The rearranged formula is:
R2 Z e ) 1000]
[(1 . 04 0 . 3) 1000]
R1 L 1.2
32 1 . 2
0 . 727 (frrom R1 R2 tables)
[(Z s
18 . 54 mΩ
The nearest value to this maximum is 18.1 mΩ (from R1 R2 tables) giving a cpc size of
1.0 mm2. This will satisfy the shock risk requirements, but we will still have to know the
actual value of Zs, so:
Z s 0.3 (0 . 727 18 . 1) 1 . 2 32
Fault current I Uo
t from 50 A BS 88 curve 3 s
k 115 (IEE Regulations Table 54.3)
3 . 46 mm2
Hence, our 1.0 mm2 cpc is too small to satisfy the thermal constraints, and hence a
2.5 mm2 cpc would have to be used. So the heaters would be supplied using 25.0 mm2
live conductors, a 2.5 mm2 cpc and 50 A BS 88 protection.
40 IEE Wiring Regulations: Design and Verification
Part of the lighting installation in a new warehouse is to comprise a distribution circuit
to a three-phase lighting distribution board from which nine single-phase final circuits
are to be fed. The distribution circuit, protected by BS 88 fuses, is to be four-core PVC
SWA cable and is 22 m long. The armouring will provide the function of the cpc. The
distribution board will house BS EN 60898 Type C CBs, and each final circuit is to
supply fourteen 300 W discharge luminaires. The longest run is 45 m, and the wiring
system will be singles in trunking, the first few metres of which will house all nine final
circuits. The earthing system is TN-C-S and the value of Zs calculated to be 0.2 Ω. The
ambient temperature will not exceed 30°C.
Determine all relevant cable/conductor sizes
14 300 W
Four-core PVC SWA
Ze 0.2 22 m
Design current of each final circuit Ib
As each row comprises fourteen 300 W/230 V discharge fittings:
Ib 14 300 1 . 8
32 . 8 A
(the 1.8 is the multiplier for discharge lamps)
As the nine circuits will be balanced over three phases, each phase will feed three
rows of fittings:
Ib per phase 3 32 . 8 98 . 4 A
Distribution circuit design current Ib
Distribution circuit Ib per phase 98.4 A
Nominal rating of protection In
In Ib so, from IEE Regulations Table 41.4, the protection will be 100 A with a maximum loop impedance Zs of 0.42 Ω.
Discharge units do cause short duration overloads at start-up, so it is perhaps best to
use In rather than Ib:
From IEE Regulations Table 4D4A column 3, It 102 A, giving a cable size of 25 mm2.
From IEE Regulations Table 4D4B column 4, the mV drop is 1.5.
Volt drop 1 .5 98 .4 22
3 . 23 V (acceptable)
This is the three-phase drop, the single phase being:
1 . 87 V
Cable manufacturer’s information shows that the resistance of the armouring on a
25 mm2 four-core cable is 2.1 mΩ/m. Hence,
R1 0 .727 mΩ / m and R2 2 . 1 mΩ / m
Z s 0.2 (0 . 727 2 . 1) 22 1 . 2
0 . 274 Ω
Clearly ok, as Zs maximum is 0.42 Ω.
42 IEE Wiring Regulations: Design and Verification
0 . 274
t 0.7 from BS 88 (curve for 100 A)
k 51 (IEE Regulations Table 54.4)
8392 0 . 7
13 . 76 mm2
Manufacturer’s information gives the gross csa of 25 mm2 four-core SWA cable as
76 mm2. Hence the armouring provides a good cpc.
If we had chosen to use IEE Regulations Table 54.7 to determine the minimum size it
which still results in a smaller size than will exist.
Final circuits design current Ib
Ib 32.8 A (calculated previously).
Setting of protection In
From IEE Regulations Table 41.3, In Ib 40 A with a corresponding maximum value
for Zs of 0.57 Ω.
Only grouping needs to be considered:
Cg for nine circuits 0.5 (IEE Regulations Table 4C1).
From IEE Regulations Table 4D1A, It 80 A 101 A and conductor size will be 25 mm2.
The assumption that the whole of the design current of 32.8 A will flow in the circuit
would be incorrect, as the last section will only draw:
2 . 34 A
the section previous to that 4.68 A, the one before that 7.02 A and so on, the total volt
drop being the sum of all the individual volt drops. However, this is a lengthy process and
for simplicity the volt drop in this case will be based on 32.8 A over the whole length.
From IEE Regulations Table 4D1B column 3, the mV drop for a 25 mm2 conductor is
1.8 mV.
Volt drop 1 . 8 32 . 8 45
Add this to the sub-main single-phase drop, and the total will be:
1 . 87 2 . 6 4 . 47 V (acceptable)
Shock risk constraints
In this case Ze will be the Zs value for the distribution circuit.
Rearranging as before, to establish a minimum cpc size, we get:
Z e ) 1000]
R1 (for 25 mm2 )
(0 . 57 0 . 274) 1000
0 . 727 4 . 75 mΩ
45 1 . 2
R2 [(Z s
Therefore, the nearest value below this gives a size of 4.0 mm2:
Total Z s 0 . 274 0.57 Ω)
(0 . 727 4 . 61)
45 12 0 . 56 Ω (less than the maximum of
44 IEE Wiring Regulations: Design and Verification
t from Type C CB curve for 32 A is less than 0.1 s. k 115 (IEE Regulations Table
4102 0 . 1
1 . 12 mm2
Hence our 4.0 mm2 cpc is adequate. So, the calculated cable design details are as
Distribution circuit protection 100 A BS 88 fuses, Distribution circuit cable 25 mm2
four-core SWA with armour as the cpc.
Final circuit protection 32 A Type C, BS EN 60898 MCB, Final circuit cable 25 mm2
singles with 4.0 mm2 cpc.
Part of the design procedure is to select the correct size of conduit or trunking. The
basic requirement is that the space factor is not exceeded and, in the case of conduit,
that cables can be easily drawn in without damage.
For trunking, the requirement is that the space occupied by conductors should not
exceed 45% of the internal trunking area. The IEE Regulations Guidance Notes/OnSite Guide give a series of tables which enable the designer to select appropriate sizes
by the application of conductor/conduit/trunking terms. This is best illustrated by the
What size of straight conduit 2.5 m long would be needed to accommodate ten
2.5 mm2 and five 1.5 mm2 stranded conductors?
Tabulated cable term for 1.5 mm2 stranded 31
Tabulated cable term for 2.5 mm2 stranded 43
31 5 155
43 10 430
The corresponding conduit term must be equal to or greater than the total cable term.
Hence the nearest conduit term to 585 is 800, which gives a conduit size of 25 mm.
How many 4.0 mm2 stranded conductors may be installed in a straight 3 m run of
25 mm conduit?
Tabulated conduit term for 25 mm 800
Tabulated cable term for 4.0 mm2 58
Number of cables 800
Hence thirteen 4.0 mm2 conductors may be installed.
What size conduit 6 m long and incorporating two bends would be needed to house
eight 6.0 mm2 conductors?
Tabulated cable term for 6.0 mm2 58
Overall cable term 58 8 464
Nearest conduit term above this is 600, giving 32 mm conduit.
What size trunking would be needed to accommodate twenty-eight 10 mm2
Tabulated cable term for 10 mm2 36.3
Overall cable term 36.3 28 1016.4
Nearest trunking term above this is 1037, giving 50 mm 50 mm trunking.
46 IEE Wiring Regulations: Design and Verification
What size of trunking would be required to house the following conductors:
201.5 mm2 stranded
352.5 mm2 stranded
284.0 mm2 stranded
Tabulated cable term for 1.5 mm2 8.1
Tabulated cable term for 2.5 mm2 11.4
Tabulated cable term for 4.0 mm2 15.2
Hence 8.1 20 162
11.4 35 399
15.2 28 425.6
Total 986.6
The nearest trunking term is 993, giving 100 mm 225 mm trunking, but it is more
likely that the more common 50 mm 250 mm would be chosen (Figure 1.10).
(ring main)
TP&N Busbar
switch- chamber
(usually PVC
Layout of industrial installation.
It is often desirable to make allowance for future additions to trunking systems, but care
must be taken to ensure that extra circuits do not cause a change of grouping factor
which could then de-rate the existing conductors below their original designed size.
Having designed the installation it will be necessary to record the design details either
in the form of a schedule for small installations or on drawings for the more complex
installation. These drawings may be of the block, interconnection, layout, etc., type.
The following figures indicate some typical drawings (see Figures 1.11 and 1.12).
Note the details of the design calculations shown in Figure 1.12, all of which is essential information for the testing and inspection procedure.
With the larger types of installation, an alphanumeric system is very useful for crossreference between block diagrams and floor plans showing architectural symbols.
Figure 1.13 shows such a system.
Distribution system, block type.
FS fuse switch
48 IEE Wiring Regulations: Design and Verification
69.5 67 38
50 29 DB 1C
0.15 6.32 kA
0.16 6.03 kA
4C 16 mm2
3C 4 mm2 PVC/SWA/PVC
11.75 5.5
0.19 4.39 kA
DB 1D
3C 23 mm2 XLPE/SWA/PVC
4C 35 mm2 XLPE/SWA/PVC
4C 16 mm2 XLPE/SWA/PVC
0.2 4.09 kA
AMD 40 25 35
VD 1.92 V
ELI 0.17 M
PFC 4.8 kA
4C 25 mm2 XLPE/SWA/PVC
150.2 152.7
3C 4 mm2
3C 16 mm2
4C 70 mm2 XLPE/SWA/PVC
1C 35 mm2 PVC/SWA/PVC
CC 4.4
AMD 4.4
VD 0.5 V
ELI 0.21 PFC 3.7 kA
0.093 8.5 kA
FIGURE 1.12 Distribution system, interconnection type. CC: Circuit current;
AMD: Assumed maximum demand; VD: Volt drop; ELI: Earth loop impedance;
PFC: Prospective fault or short-circuit current.
Distribution board 3 (DB3) under the stairs would have appeared on a diagram such
as Figure 1.13, with its final circuits indicated. The floor plan shows which circuits are
fed from DB3, and the number and phase colour of the protection. For example, the
fluorescent lighting in the main entrance hall is fed from fuse or MCB 1 on the brown
phase of DB3, and is therefore marked DB3/Br1. Similarly, the water heater circuit in
the female toilets is fed from fuse or MCB 2 on the black phase, i.e. DB3/Bk2.
and 30 A SP&N
isolator mounted
DB3/Gr1
DB3/Bk2
DB3/Gr2
DB3/Br2
DB3/Br4
DB3/Gr3
DB3/Br1
2 DB3/Br4 DB3/Br4
DB3/Bk1
70 W DB3/Gr4
Figures 1.14, 1.15 and 1.16 illustrate a simple but complete scheme for a small
garage/workshop. Figure 1.14 is an isometric drawing of the garage and the installation, from which direct measurements for materials may be taken. Figure 1.15 is the
associated floor plan, which cross-references with the DB schedule and interconnection details shown in Figure 1.16.
50 IEE Wiring Regulations: Design and Verification
Isometric drawing for garage/workshop.
1 – Three-phase supply to ramp: 20 mm conduit
2 – Single-phase supply to double sockets: 20 mm conduit. Also 3, 5, 6, 9, 11, 13
4 – Single-phase supply to light switch in store: 20 mm conduit
7 – Single-phase supply to light switch in compressor: 20 mm conduit
8 – Three-phase supply to compressor: 20 mm conduit
10 – Single-phase supply to heater in WC: 20 mm conduit
12 – Single-phase supply to light switch in WC: 20 mm conduit
14 – Single-phase supply to light switch in office: 20 mm conduit
15 – Main intake position
16 – Single-phase supplies to switches for workshop lights: 20 mm conduit
17 – 50 mm 50 mm steel trunking
18 – Supplies to fluorescent fittings: 20 mm conduit
BrBkGr1
Floor plan for garage/workshop.
BrBkGr2
Br1 C 10 A
Bk1 C 10 A
Gr1 C 10 A
Br2 C 30 A
Gr2 C 30 A
Br3 B 10 A
WS lighting 4
Bk3 B 10 A
WS lighting 2
WS lighting 3
TN–S Gr3 B 10 A
Office, WC, store and
Ip 3 kA Br4 B 10 A
compressor room lighting
Ze 0.4 WS, water heater
Bk4 B 15 A
Gr4 B 30 A
Br5 B 30 A
SOs 2 and 3, radial
SOs 5 and 6, radial
Bk5 B 30 A SOs 9, 11 and 13, radial
3 10 mm singles 1.5 mm cpc
3 125 W 2000 mm doubles
2 1.5 mm singles 1 mm cpc
3 125 W 2000 mm and
8 80 W 1200 mm doubles
Fused spur box
2 2.5 mm singles 1 mm cpc
2 6.0 mm singles 1.5 mm cpc
100 A DB with main switch
protection by MCB
Bk2 C 30 A
3 1.5 mm singles 1 mm cpc
Details of connection diagram for garage/worksho p.
52 IEE Wiring Regulations: Design and Verification
This is the subject of Part 6 of the IEE Regulations, and opens with
the statement to the effect that it must be verified that all installations, before being put into service, comply with the Regulations,
i.e. BS 7671. The author interprets the comment ‘before being put
into service by the user ’ as before being handed over to the user,
not before the supply is connected. Clearly a supply is needed to
conduct some of the tests.
The opening statement also indicates that verification of compliance be carried out during the erection of the installation and after it
has been completed. In any event, certain criteria must be observed:
1. The test procedure must not endanger persons, livestock or
2. Before any inspection and testing can even start, the person
carrying out the verification must be in possession of all
the relevant information and documentation. In fact the
installation will fail to comply without such information
(IEE Regulations 611.3 xvi). How, for example, can a verifier
accept that the correct size conductors have been installed,
without design details (IEE Regulations 611.3 iv)?
So, let us start, as they say, at the beginning. Armed with the
results of the Assessment of General Characteristics, the designer’s
54 IEE Wiring Regulations: Design and Verification
details, drawings, charts, etc., together with test instruments, the
verification process may proceed.
Usually referred to as a visual inspection, this part of the procedure is carried out against a check list as detailed in the IEE
Regulations, Section 611, Appendix 6 and in the Guidance Notes
3 for inspection and testing. Much of the initial inspection will
involve access to enclosures housing live parts; hence, those parts
of the installation being tested should be isolated from the supply.
Naturally, any defects found must be rectified before instrument
This involves the use of test equipment and there are several
important points to be made in this respect:
1. Electronic instruments must conform to BS 4743 and
electrical instruments to BS 5458 and their use to HSE
GS 38.
2. The Electricity at Work Regulations 1989 state an absolute
requirement in Regulation 4(4), that test equipment be
maintained in good condition. Hence it is important to
ensure that regular calibration is carried out.
3. Test leads used for measurement of voltages over 50 V
should have shrouded/recessed ends and/or when one end is
a probe, it should be fused and be insulated to within 2 mm
to 4 mm of the tip and have finger guards.
4. Always use approved voltmeters, test lamps, etc.
Inspection and Testing 55
As has been mentioned, instruments must comply with the relevant British Standard, and provided they are purchased from established bona fide instrument manufacturers, this does not present a
There is a range of instruments needed to carry out all the standard installation tests, and some manufacturers produce equipment
with dual functions; indeed there are now single instruments capable of performing all the fundamental tests. Table 2.1 indicates the
basic tests and the instruments required.
APPROVED TEST LAMPS AND INDICATORS
Search your tool boxes: find, with little difficulty one would suspect, your ‘neon screwdriver ’ or ‘testascope’; locate a very deep
pond; and drop it in!
1 Continuity of ring final
0.05–08 Ω
2 Continuity of protective
2–0.005 Ω or less
3 Earth electrode
Any value over about 3–4 Ω
Special ohmmeter
Infinity to less than 1 MΩ
Ohmmeter, bell, etc.
6 Earth fault loop
0–2000 Ω
7 Operation of RCD
5–500 mA
8 Prospective short-circuit
2 A to 20 kA
56 IEE Wiring Regulations: Design and Verification
Imagine actually allowing electric current at low voltage (50–
1000 V AC) to pass through one’s body in order to activate a test
lamp! It only takes around 10–15 mA to cause severe electric
shock, and 50 mA (1/20th of an ampere) to kill.
Apart from the fact that such a device will register any voltage
from about 5 V upwards, the safety of the user depends entirely on
the integrity of the current-limiting resistor in the unit. An electrician received a considerable shock when using such an instrument
after his apprentice had dropped it in a sink of water, simply wiped
it dry and replaced it in the tool box. The water had seeped into
the device and shorted out the resistor. There are, however, some
modern devices which are proximity devices and give some indication of the presence of voltage whatever the value. These should
never be accepted as proof that a circuit is not energized!!
An approved test lamp should be of similar construction to that
Inspection and Testing 57
The following procedure is recommended when using approved
test lamps to check that live parts have been made dead:
1. Check that the test lamp is in good condition and the leads
are undamaged. (This should be done regardless of the
purpose of use.)
2. Establish that the lamp is sound by probing onto a known
supply. This is best achieved by using a proving unit. This is
simply a pocket-sized device which electronically produces
230 V DC.
3. Carry out the test to verify the circuit is dead.
4. Return to the proving unit and check the lamp again.
It has long been the practice when using a test lamp to probe
between line and earth for an indication of a live supply on the
phase terminal. However, this can now present a problem where
RCDs exist in the circuit, as of course the test is applying a deliberate line-to-earth fault.
Some test lamps have LED indicators, and the internal circuitry
of such test lamps limits the current to earth to a level below that
at which the RCD will operate. The same limiting effect applies to
multi-meters. However, it is always best to check that the testing
device will have no effect on RCDs.
CALIBRATION, ZEROING/NULLING AND
Precise calibration of instruments is usually well outside the province of the electrician, and would normally be carried out by the
manufacturer or local service representative. A check, however, can
be made by the user to determine whether calibration is necessary
by comparing readings with an instrument known to be accurate,
58 IEE Wiring Regulations: Design and Verification
or by measurement of known values of voltage, resistance, etc. on
one of the many ‘check boxes’ currently marketed. However, as we
have already seen, regular calibration is a legal requirement.
It may be the case that readings are incorrect simply because the
instrument is not zeroed/nulled before use, or because the internal
battery needs replacing. Most modern instruments have battery
condition indication, and of course this should never be ignored.
Always adjust any selection switches to the off position after testing. Too many instrument fuses are blown when, for example, a
multi-meter is inadvertently left on the ohms range and then used
to check for mains voltage.
The following set procedure may seem rather basic but should
ensure trouble-free testing:
1. Check test leads for obvious defects.
2. Zero/null the instrument.
3. Select the correct range for the values anticipated. If in
doubt, choose the highest range and gradually drop down.
4. Make a record of the test results.
5. When an unexpected reading occurs, make a quick check on
the test leads just to ensure that they are not open circuited.
6. Return switches/selectors to the off position.
7. Replace instrument and leads in carrying case.
The IEE Regulations indicate a preferred sequence of tests and
state that if, due to a defect, compliance cannot be achieved, the
defect should be rectified and the test sequence started from the
beginning. The tests for ‘Site applied insulation’, ‘Protection by
separation’, and ‘Insulation of non-conducting floors and walls’ all
Inspection and Testing 59
require specialist high voltage equipment and in consequence will
not be discussed here. The sequence of tests for an initial inspection and test is as follows:
Prospective fault current (PFC) between live conductors
10. Phase sequence
11. Functional testing
12. Voltage drop.
One other test not included in Part 7 of the IEE Regulations but
which nevertheless has to be carried out is external earth fault
loop impedance (Ze).
These include the cpcs of radial circuits, main and supplementary
Protective bonding conductors. Two methods are available: either
can be used for cpcs, but bonding can only be tested by the second.
At the distribution board, join together the line conductor and its
associated cpc. Using a low resistance ohmmeter, test between
line and cpc at all the outlets in the circuit. The reading at the
farthest point will be (R1 R2) for that circuit. Record this value,
60 IEE Wiring Regulations: Design and Verification
as after correction for temperature it may be compared with the
designer’s value (more about this later).
Connect one test instrument lead to the main earthing terminal,
and a long test lead to the earth connection at all the outlets in
the circuit. Record the value after deducting the lead resistance.
An idea of the length of conductor is valuable, as the resistance
can be calculated and compared with the test reading. Table 2.2
gives resistance values already calculated for a range of lengths and
It should be noted that these tests are applicable only to ‘all insulated’ systems, as installations using metallic conduit and trunking, MICC and SWA cables will produce spurious values due to
the probable parallel paths in existence. This is an example of
where testing needs to be carried out during the erection process
and before final connections and bonding are in place.
Inspection and Testing 61
If conduit, trunking or SWA is used as the cpc, then the verifier has the option of first inspecting the cpc along its length for
soundness then conducting the long-lead resistance test.
CONTINUITY OF RING FINAL CIRCUIT
The requirement of this test is that each conductor of the ring
is continuous. It is, however, not sufficient to simply connect an
ohmmeter, a bell, etc., to the ends of each conductor and obtain a
reading or a sound.
So what is wrong with this procedure? A problem arises if an interconnection exists between sockets on the ring, and there is a break
in the ring beyond that interconnection. From Figure 2.2 it will
be seen that a simple resistance or bell test will indicate continuity via the interconnection. However, owing to the break, sockets
4–11 are supplied by the spur from socket 12 – not a healthy situation. So how can one test to identify interconnections?
There are three methods of conducting such a test. Two are based
on the principle that resistance changes with a change in length
62 IEE Wiring Regulations: Design and Verification
or CSA; the other, predominantly used, relies on the fact that the
resistance measured across any diameter of a circular loop of conductor is the same. Let us now consider the first two.
If we were to take a length of conductor XYZ and measure the
resistance between its ends (Figure 2.3), then double it over at
Y, join X and Z, and measure the resistance between XZ and Y
(Figure 2.4), we would find that the value was approximately a
quarter of the original. This is because the length of the conductor
is halved and hence so is the resistance, and the CSA is doubled
and so the resistance is halved again.
In order to apply this principle to a ring final circuit, it is necessary to know the position of the socket nearest the mid-point of
Inspection and Testing 63
the ring. The test procedure is then as follows for each of the conductors of the ring:
1. Measure the resistance of the ring conductor under test
between its ends before completing the ring in the fuse
board. Record this value, say R1.
2. Complete the ring.
3. Using long test leads, measure between the completed
ends and the corresponding terminal at the socket nearest
the mid-point of the ring. Record this value, say R2. (The
completed ends correspond to point XZ in Figure 2.4, and
the mid-point to Y.)
4. Measure the resistance of the test leads, say R3, and subtract
this value from R2,
i.e. R2
R3 R4 say
5. A comparison between R1 and R4 should reveal, if the ring is
healthy, that R4 is approximately a quarter of R1.
The second method tests two ring circuit conductors at once, and
is based on the following.
Take two conductors XYZ and ABC and measure their resistances
(Figure 2.5). Then double them both over, join the ends XZ and
AC and the mid-points YB, and measure the resistance between
XZ and AC (Figure 2.6). This value should be a quarter of that for
XYZ plus a quarter of that for ABC.
If both conductors are of the same length and CSA, the resultant
value would be half that for either of the original resistances.
64 IEE Wiring Regulations: Design and Verification
R2 艐 ½ R1
Applied to a ring final circuit, the test procedure is as follows:
1. Measure the resistance of both line and neutral conductors
before completion of the ring. They should both be the
same value, say R1.
2. Complete the ring for both conductors, and bridge together
line and neutral at the mid-point socket (this corresponds
to point YB in Figure 2.6). Now measure between the
completed line and neutral ends in the fuse board (points
XZ and AC in Figure 2.6). Record this value, say R2.
3. R2 should be, for a healthy ring, approximately half of
R1 for either line or neutral conductor. When testing the
continuity of a cpc which is a different size from either line
or neutral, the resulting value R2 should be a quarter of R1
for line or neutral plus a quarter of R1 for the cpc.
Inspection and Testing 65
Method 3 (generally used)
The third method is based on the measurement of resistance
at any point across the diameter of a circular loop of conductor
As long as the measurement is made across the diameter of the
ring, all values will be the same. The loop of conductor is formed
by crossing over and joining the ends of the ring circuit conductors
at the fuse board. The test is conducted as follows:
1. Identify both ‘legs’ of the ring.
2. Join one line and one neutral conductor of opposite legs of
3. Obtain a resistance reading between the other line and
neutral (Figure 2.8). (A record of this value is important.)
4. Join these last two conductors (Figure 2.9).
5. Measure the resistance value between L and N at each
socket on the ring. All values should be the same,
approximately a quarter of the reading in (3) above.
The test is now repeated but the neutral conductors are replaced by
the cpcs. If the cable is twin with cpc, the cpc size will be smaller
66 IEE Wiring Regulations: Design and Verification
than the line conductor, and although the readings at each socket
will be substantially the same, there will be a slight increase in
values towards the centre of the ring, decreasing back towards the
start. The highest reading represents R1 R2 for the ring.
The basic principle of this method is that the resistance measured
between any two points, equidistant around a closed loop of conductor, will be the same.
Such a loop is formed by the line and neutral conductors of a ring
final circuit (Figure 2.10).
Inspection and Testing 67
Let the resistance of conductors be as shown.
R measured between L and N on socket A will be:
0.2 0.5 0.2 0.3 0.4 0.1 0.3
R measured between L and N at socket B will be:
0.3 0.2 0.5 0.2 0.3 0.4 0.1 2
Hence all sockets on the ring will give a reading of 1 Ω between
If there were a break in the ring in, say, the neutral conductor, all
measurements would have been 2, incorrectly indicating to the
tester that the ring was continuous. Hence the relevance of step 3
in the test procedure, which at least indicates that there is a continuous L–N loop, even if an interconnection exists. Figure 2.11
shows a healthy ring with interconnection.
68 IEE Wiring Regulations: Design and Verification
0.1815 A
0.1815 0.109 X
0.109 FIGURE 2.12
Here is an example that shows the slight difference between measurements on the line/cpc test. Consider a 30 m ring final circuit
wired in 2.5 mm2 with a 1.5 mm2 cpc. Figure 2.12 illustrates this
arrangement when cross-connected for test purposes.
From the resistance tables, 1.5 mm2 conductor is seen to have a
resistance of 12.1 mΩ/m, and 2.5 mm2, 7.27 mΩ/m. This gives the
resistance from X to A as 15 12.1/1000 0.1815 Ω and from X
to B as 15 7.27/1000 0.109. The same values apply from Y to
A and Y to B.
So measuring across X and Y we have 2 0.1815 0.363, in
parallel with 2 0.109 [(0.363 0.218)/(0.363 0.218)] Ω
(product over sum) 0.137 Ω.
Inspection and Testing 69
40–50 V dc
Measuring across A and B (the mid-point) gives 0.1815 0.109 0.29 Ω, in parallel with the same value, i.e. 0.29 Ω, which
gives 0.29/2 0.145 Ω.
While there is a difference 0.008 Ω the amount is too small to suggest any faults on the ring.
If an enclosure/barrier is used to house or obscure live parts, and is
not a factory-built assembly, it must be ascertained whether or not
it complies with the requirements of the IP codes IP2X or IPXXB,
or IP4X or IPXXD. For IP2X or IPXXB, the test is made using the
British Standard Finger, which is connected in series with a lamp
and a supply of not less than 40 V and not more than 50 V. The
test finger is pushed into or behind the enclosure/barrier and the
lamp should not light (Figure 2.13).
The test for IP4X or IPXXD is made with a 1.0 mm diameter wire
with its end bent at right angles. The wire should not enter the
70 IEE Wiring Regulations: Design and Verification
Test Voltage and Minimum MΩ Values for Insulation
ELV Circuits (SELV
and PELV)
LV Circuits up to
LV Circuits above
An insulation resistance tester, which is a high resistance ohmmeter, is used for this test. The test voltages and minimum MΩ
values are shown in Table 2.3.
Clearly with voltages of these levels, there are certain precautions
to be taken prior to the test being carried out. Persons should
be warned, and sensitive electronic equipment disconnected or
unplugged. A common example of this is the dimmer switch. Also,
as many accessories have indicator lamps, and items of equipment
such as fluorescent fittings have capacitors fitted, these should be
disconnected as they will give rise to false readings.
Poles to earth (Figure 2.14)
2. Ensure that all protective devices are in place and all
switches are closed
3. Link all poles of the supply together (where appropriate)
4. Test between the linked poles and earth.
Between poles (Figure 2.15)
1. As previous test
2. As previous test
Inspection and Testing 71
L1–N
L2–N
L3–N
3. Remove all lamps, equipment, etc.
4. Test between poles.
Test results on disconnected equipment should conform to the
relevant British Standard for that equipment. In the absence of a
British Standard, the minimum value is 0.5 MΩ.
For small installations, the tests are performed on the whole system, whereas for larger complex types, the installation may be
72 IEE Wiring Regulations: Design and Verification
sub-divided into sections and tests performed on each section. The
reason for this is that as conductor insulation and the circuits they
supply are all in parallel, a test on the whole of a large installation
would produce pessimistically low readings even though no faults
Although for standard 400 V/230 V installations the minimum
value of insulation resistance is 1 MΩ, a reading of less than 2 MΩ
should give rise to some concern. Circuits should be tested individually to locate the source/s of such a low reading.
It is required that all fuses and single-pole devices such as singlepole circuit breakers and switches are connected in the line conductor only. It is further required that the centre contact of Edison
screw lampholders be connected to the line conductor (E14 and E27
ES types are exempt as the screwed part is insulated) and that socket
outlets and similar accessories are correctly connected.
If method 3 for testing ring circuit conductor continuity was performed, then any cross-polarity would have shown itself and been
rectified. Hence no further test is necessary. However, if method
1 or 2 were used, and the mid-point socket was correct, reversals
elsewhere in the ring would not be detected and therefore two tests
1. Link completed line and cpc loops together at the fuse board
and test between L and E at each socket. A no reading result
will indicate a reversed polarity (Figure 2.16).
2. Repeat as in 1, but with L and N linked.
Inspection and Testing 73
For radial circuits, the test method 1 for continuity of protective conductors will have already proved correct polarity. It just
remains to check the integrity of the neutral conductor for socket
outlet circuits, and that switch wires and neutrals are not mixed at
lighting points. This is done by linking L and N at the fuse board
and testing between L and N at each outlet and between N and
switch wire at each lighting point.
Also for lighting circuits, to test for switches in line conductors, etc.,
link L and E at the fuse board and test as shown in Figure 2.17.
If we were to place an electrode in the earth and measure the
resistance between the electrode and points at increasingly larger
distances from it, we would notice that the resistance increased
74 IEE Wiring Regulations: Design and Verification
Continuity tester or
Polarity on ES lampholder.
with distance until a point was reached (usually around 2.5 m)
beyond which no increase in resistance was noticed (Figure 2.18).
It is a requirement of the Regulations that for a TT system, exposed
conductive parts be connected via protective conductors to an earth
electrode, and that the protection is by either an RCD or an overcurrent device, the RCD being preferred. Conditional on this is the
requirement that the product of the sum of the resistances of the
earth electrode and protective conductors, and the operating current
of the protective device, shall not exceed 50 V, i.e. Ra Ia 50 V.
(Ra is the sum of the resistances of the earth electrode and the protective conductors connecting it to the exposed conductive part.)
Clearly then, there is a need to measure the resistance of the earth
electrode. This may be done in either of two ways.
Based on the principle of the potential divider (Figure 2.19, page
76), an earth resistance tester is used together with test and
Inspection and Testing 75
No further increase in resistance
Resistance increases up to
approximately 2.5 m
dependent on size of
electrode and type of soil
Electrode resistance areas.
auxiliary electrodes spaced as shown in Figure 2.20. This spacing
ensures that resistance areas do not overlap.
The method of test is as follows:
1. Place the current electrode (C2) away from the electrode
under test, approximately 10 times its length, i.e. 30 m for a
3 m rod.
76 IEE Wiring Regulations: Design and Verification
Place the potential electrode mid-way.
Connect test instrument as shown.
Record resistance value.
Move the potential electrode approximately 3 m either side
of the mid position, and record these two readings.
6. Take an average of these three readings (this is the earth
electrode resistance).
Inspection and Testing 77
7. Determine the maximum deviation or difference of this
average from the three readings.
8. Express this deviation as a percentage of the average
9. Multiply this percentage deviation by 1.2.
10. Provided this value does not exceed a figure of 5% then the
accuracy of the measurement is considered acceptable.
Three readings obtained from an earth electrode resistance test
were 181 Ω, 185 Ω and 179 Ω. What is the value of the electrode
resistance and is the accuracy of the measurement acceptable?
Average value 181 185 179
181.67 Ω
Maximum deviation 185 − 181.67 3.33
3.33 × 100
Measurement accuracy 1.83% 1.2 2.2%
(which is acceptable).
For TT systems the result of this test will indicate compliance if
the product of the electrode resistance and the operating current of
the overcurrent device does not exceed 50 V.
On TT systems protected by an RCD, a loop impedance tester is
used and effectively measures Ze, which is taken as the earth electrode resistance.
This is carried out by connecting an earth fault loop impedance
tester between the line conductor and the main earthing terminal at
78 IEE Wiring Regulations: Design and Verification
the intake position with the main protective bonding conductors disconnected. This ensures that parallel resistance paths will not affect
the reading. Great care must be taken when conducting this test, as
the installation has to be energized and probes and/or clips are used
to make contact; also the installation is not earthed for the duration
of the test. When the test is completed reconnect the bonding.
EARTH FAULT LOOP IMPEDANCE Zs
This has to be measured in order to ensure that protective devices
will operate in the specified time under fault conditions. As the
value of (R1 R2)Ω for a particular circuit will have already been
established, Zs may be found by simply adding the R1 R2 value
to Ze. Alternatively, it may be measured directly at the extremity of
a particular circuit. Whichever method is used, the value obtained
will need to be corrected to compensate for ambient and conductor
operating temperatures before a comparison is made with the
tabulated values of Zs in the Regulations.
In practice, it would be unusual to take temperature measurements on site and in consequence a ‘rule of thumb’ method is
adopted. This simply requires that measured values of loop impedance do not exceed 80% (0.8) of tabulated values.
All main protective and supplementary bonding must be in place during this test.
Only the basic type of RCD will be considered here. Clearly, such
devices must operate to their specification; an RCD tester will
Inspection and Testing 79
establish this. As with loop impedance testing, care must be taken
when conducting this test as an intentional earth fault is created
in the installation. In consequence, a loop impedance test must
be conducted first to confirm that an earth path exists or the RCD
test could prove dangerous.
It is important to know why an RCD has been installed as this has
direct effect on the tests performed. The tests are as follows:
1. With the tester set to the RCD rating, half the rated current
is passed through the device. It should not trip.
2. With full rated current passed through the device, it should
trip within 200 ms (300 ms for RCBOs).
3. For RCDs having a residual current rating of 30 mA or less,
a test current of 5 IΔn should be applied and the device
should operate in 40 ms or less.
4. All RCDs have a test button which should be operated to
ensure the integrity of the tripping mechanism. It does not
check any part of the earthing arrangements or the devices
sensitivity. As part of the visual inspection, it should be
verified that a notice, indicating that the device should be
tested via the test button quarterly, is on or adjacent to the
There seems to be a popular misconception regarding the ratings
and uses of RCDs in that they are the panacea for all electrical ills
and the only useful rating is 30 mA!
Firstly, RCDs are not fail-safe devices, they are electromechanical in operation and can malfunction. Secondly, general purpose
RCDs are manufactured in ratings from 5 mA to 1000 mA (30 mA,
100 mA, 300 mA and 500 mA being the most popular) and have
many uses. The following list indicates residual current ratings
and uses as mentioned in BS 7671.
80 IEE Wiring Regulations: Design and Verification
All socket outlets rated at not more than 20 A and for
un-supervised general use.
Mobile equipment rated at not more than 32 A for use
All circuits in a bath/shower room.
Preferred for all circuits in a TT system.
All cables installed less than 50 mm from the surface of a
wall or partition (in the safe zones) if the installation is
un-supervised, and also at any depth if the construction of
the wall or partition includes metallic parts.
In zones 0, 1 and 2 of swimming pool locations.
All circuits in a location containing saunas, etc.
Socket outlet final circuits not exceeding 32 A in agricultural
Circuits supplying Class II equipment in restrictive
conductive locations.
Each socket outlet in caravan parks and marinas and final
circuit for houseboats.
All socket outlet circuits rated not more than 32 A for show
All socket outlet circuits rated not more than 32 A for
construction sites (where reduced low voltage, etc. is not used).
All socket outlets supplying equipment outside mobile or
transportable units.
All circuits in caravans.
All circuits in circuses, etc.
A circuit supplying Class II heating equipment for floor and
ceiling heating systems.
Inspection and Testing 81
Any circuit supplying one or more socket outlets of rating
exceeding 32 A, on a construction site.
At the origin of a temporary supply to circuses, etc.
Where there is a risk of fire due to storage of combustible
All circuits (except socket outlets) in agricultural locations.
Socket outlets of rating exceeding 32 A in agricultural
Where loop impedance values cannot be met, RCDs of an appropriate rating can be installed. Their rating can be determined from
I n 50/ Zs
where IΔn is the rated operating current of the device, 50 is the
touch voltage and Zs is the measured loop impedance.
A PFC tester, usually incorporated with a loop impedance tester,
is used for this. When testing at the intake position, probes and/or
clips will be needed and hence great care needs to be taken when
connecting to live terminals, etc. Measurements are taken between
L and N, and L and E.
For multi-phase circuits, e.g. supplies to three-phase motors, etc, it
is important to check that the phase sequence is correct to ensure
82 IEE Wiring Regulations: Design and Verification
correct direction of rotation. A phase sequence instrument is used
which is basically a small three-phase motor.
Tests on assemblies
These are carried out on a switchgear, interlock, control gear, etc.,
to ensure that they are mounted and installed according to the
Requirements of the 17th Edition.
There may be occasions when verification of voltage drop is
required. This would be achieved by calculation or by reference to
charts or tables.
After an installation has had an initial verification and been put
into service, there is a requirement for regular periodic verification
to take place. In some cases where, for example, a Local Authority
is involved, the interval between tests is mandatory. In other cases
the interval is only a recommendation. For example, the recommended time between tests on domestic installations is 10 years,
whereas places of public entertainment have a mandatory interval
Clearly, periodic tests may prove difficult, as premises are usually
occupied and in full service, and hence careful planning and consultation are needed in order to minimize any disruption. A thorough
visual inspection should be undertaken first, as this will indicate
to the experienced inspector the depth to which he or she need go
with the instrument tests, and an even more rigorous investigation may be required if drawings/design data are not available.
Inspection and Testing 83
The visual inspection will need to take into account such items as
safety, wear and tear, corrosion, signs of overloading, mechanical
damage, etc. In many instances, a sample of items inspected may
be taken, for example a minimum of 10% of switching devices
may be taken. If, however, the sample indicates considerable deterioration then all items must be inspected.
The test sequence where relevant and where possible should be the
same as that for an initial verification. This is not essential:
As with visual inspection, sample tests may be made, usually
10%, with the proviso that this is increased in the event of faults
being found. In the light of previous comments regarding sampling, it is clear that periodic verification is subjective, varying
from installation to installation. It is also more dangerous and difficult and hence requires the inspector to have considerable experience. Accurate and coherent records must be made and given to
the person/s ordering the work. Such records/reports must indicate
any departures from or non-compliances with the Regulations, any
restrictions in the testing procedure, any dangerous situations,
etc.; if the installation was erected according to an earlier edition
of the Regulations, it should be tested as far as possible to the
requirements of the 17th Edition, and a note made to this effect
on the test report.
It should be noted that if an installation is effectively supervised in
normal use, then Periodic Inspection and Testing can be replaced
by regular maintenance by skilled persons. This would only apply
to, say, factory installations where there are permanent maintenance staff.
IEE Regulations Part 7
The bulk of BS 7671 relates to typical, single- and three-phase,
installations. There are, however, some special installations or
locations that have particular requirements. Such locations may
present the user/occupant with an increased risk of death or injuries from electric shock.
BS 7671 categorizes these special locations in Part 7 and they comprise the following:
Bathrooms, shower rooms, etc.
Rooms containing sauna heaters
Conducting locations with restrictive movement
Exhibitions shows and stands
Amusement devices, fairgrounds, circuses, etc.
86 IEE Wiring Regulations: Design and Verification
Let us now briefly investigate the main requirements for each of
these special locations.
BS 7671 SECTION 701: BATHROOMS, ETC.
This section deals with rooms containing baths, shower basins or
areas where showers exist but with tiled floors, for example leisure/
recreational centres, sports complexes, etc.
Each of these locations are divided into zones 0, 1 and 2, which
give an indication of their extent and the equipment/wiring, etc.
that can be installed in order to reduce the risk of electric shock.
So! Out with the tape measure, only to find that in a one-bedroom
flat, there may be no zone 2. How can you conform to BS 7671?
The stark answer (mine) is that you may not be able to conform
exactly. You do the very best you can in each particular circumstance to ensure safety. Let us not forget that the requirements of
BS 7671 are based on reasonableness.
above the floor out to a radius of 600 mm from a fixed shower
head or 1200 mm radius for a demountable head (Figure 3.1a).
Special Locations 87
88 IEE Wiring Regulations: Design and Verification
radius of 600 mm from a fixed shower head or 1200 mm radius for
a demountable head (Figure 3.1b).
above floor level (Figure 3.1c).
Special Locations 89
the cpcs, exposed and extraneous conductive parts within the
However, this bonding may be omitted if the premises has a protective earthing and automatic disconnection system in place; all
BS 7671 SECTION 702: SWIMMING POOLS
In a similar fashion to bathrooms and shower rooms, etc., swimming pool locations are also divided into zones 0, 1, and 2:
Zone 0 is in the pool/basin or fountain.
Zone 1 extends 2.0 m horizontally from the rim of zone 0 and
2.5 m vertically above it regardless of the pool being above
or below ground level. If there are diving boards, shutes or
90 IEE Wiring Regulations: Design and Verification
viewing galleries, etc. the height extends to a point 2.5 m
from their top surface and 1.5 m horizontally either side of
such shutes, etc.
Zone 2 extends a further 1.5 m horizontally from the edge of
zone 1 and 2.5 m above ground level.
Now, what can we install in these zones?
Only SELV to be used.
Only systems supplying equipment in these zones are permitted.
Metal cable sheaths or metallic covering of wiring systems shall
be connected to the supplementary equipotential bonding. Cables
should preferably be enclosed in PVC conduit.
Switchgear, control gear and socket outlets
None permitted except for locations where there is no zone 2. In
this case a switch or socket outlet with an insulated cap or cover
may be installed beyond 1.25 m from the edge of zone 0 at a height
of no less than 300 mm. Additionally, the circuits must be protected by:
1. SELV or
2. Automatic disconnection using a 30 mA RCD or
3. Electrical separation.
Only that which is designed for these locations.
Special Locations 91
Other equipment may be used when the pool/basin is not in use
(cleaning, maintenance, etc.) provided the circuits are protected by:
Socket outlets and control devices should have a warning notice
indicating to the user that they should not be used unless the location is unoccupied by persons.
Zone 2 (there is no zone 2 for fountains)
Socket outlets and switches, provided they are protected by:
Zone 0 IPX8 (submersion)
Zone 1 IPX4 (splashproof) or IPX5 (where water jets are used
for cleaning)
Zone 2 IPX2 (drip proof) indoor pools
IPX4 (splashproof) outdoor pools
IPX5 (where water jets are used for cleaning).
All extraneous conductive parts in zones 0, 1 and 2 must be connected by supplementary bonding conductors to the protective
conductors of exposed conductive parts in these zones.
92 IEE Wiring Regulations: Design and Verification
BS 7671 SECTION 703: HOT AIR SAUNAS
Once again a zonal system, that is, 1, 2 and 3, has been used as
per Figure 3.2. In this case the zones are based on temperature.
All circuits in the location should have additional protection
against shock by 30 mA RCDs except sauna heater circuits unless
It is preferred that the wiring systems for the sauna will be
installed outside. However, any wiring inside should be heat resisting and any metal sheaths or conduit must be inaccessible in
Special Locations 93
All should be at least IPX4 and IPX5 if water jets are to be used for
Zone 1 only the sauna equipment
Zone 2 no restriction regarding temperature resistance
Zone 3 must be suitable for 125°C and cable sheaths for 175°C.
Only that which is associated with the sauna heater equipment
may be installed in zone 2 and in accordance with the manufacturer’s instructions. All other should be outside.
BS 7671 SECTION 704: CONSTRUCTION SITES
Not as complicated as one may think. The only areas that require
special consideration are where construction work is being carried
out, not site huts, etc.
So, let us keep all this as simple as possible. Clearly, construction
sites are hazardous areas and in consequence the shock risk is
For socket outlet circuits of rating up to and including 32 A and
circuits supplying equipment of rating up to and including 32 A,
the means of protection shall be:
1. Reduced low voltage (preferred for portable hand tools and
hand lamps up to 2 kW)
2. Automatic disconnection of supply with additional
protection by 30 mA RCDs
94 IEE Wiring Regulations: Design and Verification
3. Electrical separation
4. SELV or PELV (SELV being preferred for portable hand
lamps in damp locations).
For socket outlet circuits rated above 32 A, a 500 mA RCD is
These are not addressed in BS 7671, presumably as there are so
many different possibilities. So common sense must prevail and
equipment used with an appropriate degree of protection in accordance with the severity of the influence.
Apart from some requirements for flexible cables, the only comment relates to ensuring that cables that pass under site roads, etc.
are protected against mechanical damage.
An Assembly for Construction Sites (ACS), which is basically the
main intake supply board, should comprise a lockable isolator and,
for current using equipment and socket outlets:
1. Overcurrent devices
2. Fault protective devices
3. Socket outlets if required.
Plugs and sockets/cable couplers
All should be to BS EN 60309-2.
Special Locations 95
BS 7671 SECTION 705: AGRICULTURAL AND
HORTICULTURAL LOCATIONS
The requirements apply only to locations that do not include the
main farmhouse outside of which the environment is hazardous
and where, of course, livestock is present (animals are susceptible
to lethal shock levels at 25 V AC).
Protection against shock may be provided by:
1. Automatic disconnection of supply with additional RCD
(a) Final circuits supplying socket outlets rated at 32 A or
less (30 mA)
(b) Final circuits supplying socket outlets rated more than
32 A (100 mA)
(c) All other circuits (300 mA).
2. SELV or PELV.
Protection against thermal effects:
1. Heating appliances should be mounted at appropriate
distances from combustible materials and livestock, with
radiant heaters at a minimum distance of 0.5 m.
2. For fire protection an RCD rated at 300 mA or less should
Wherever livestock is housed, supplementary bonding must be
carried out connecting all exposed and extraneous conductive parts
that can be touched by livestock. All metal grids in floors must be
connected to the supplementary equipotential bonding.
96 IEE Wiring Regulations: Design and Verification
1. All equipment must be to at least IP44 and luminaires
exposed to dust and moisture Ingress, IP54.
2. Appropriate protection for socket outlets where influences
are greater than AD4 AE3 and/or AG1.
3. Appropriate protection where corrosive substances are
The user of the installation should be provided with plans and
diagrams showing the location of all equipment, concealed cable
routes, distribution and the equipotential bonding system.
Any, just as long as it is suitable for the environment and fulfils
the required minimum degrees of protection.
A high impact PVC conduit/trunking system would be appropriate
in many cases as it is not affected by corrosion, is rodent proof and
has no exposed conductive parts. However, the system would be
designed to suit the particular environmental conditions.
Wiring systems should be erected so as to be, where possible,
inaccessible to livestock. Overhead lines should be insulated and,
where vehicles/mobile equipment are used, underground cables
should be at least 0.6 m deep and mechanically protected and
1.0 m deep in arable land.
Self-supporting suspended cables should be at a height of at
least 6 m.
Special Locations 97
Whatever! As long as it is suitable for the conditions and that emergency switching is placed in a position inaccessible to livestock and
can be accessed in the event of livestock panic (Stampede!!!).
BS 7671 SECTION 706: RESTRICTIVE
CONDUCTIVE LOCATIONS
These are very rare locations which could include metal tanks,
boilers, ventilation ducts, etc., where access is required for maintenance, repair or inspection. Bodily movement will be severely
restricted and in consequence such areas are extremely dangerous.
This section deals with the installation inside the location and the
requirements for bringing in accessories/equipment from outside.
For fixed equipment in the location, one of the following methods
of protection shall be used:
1. Automatic disconnection of supply but with additional
2. The use of Class II equipment backed up by a 30 mA RCD
4. SELV.
For hand-held lamps and tools and mobile equipment, SELV or
electrical separation should be used.
BS 7671 SECTION 708: CARAVAN AND
We drive into a caravan/camping park for our holiday and need to
connect to a supply of electricity for all our usual needs. This is
98 IEE Wiring Regulations: Design and Verification
accommodated by the provision of suitably placed socket outlets,
supplied via distribution circuits.
Equipment should have at least the following protection codes:
1. IPX4 for the presence of splashes (AD4)
2. IP3X for presence of small objects (AE2)
3. IK08 for presence of high severity mechanical stress (AG3)
(The IK codes are for impact and 08 is an impact of
5 joules).
The distribution circuits are erected either underground or
1. Underground cable (preferred) should be suitably protected
against mechanical damage, tent pegs, steel spikes, etc. and
at a depth of no less than 0.6 m.
2. If overhead, then 6 m above ground where there is vehicle
movement and 3.5 m elsewhere.
Switchgear and socket outlets
1. Supply equipment should be adjacent to, or within 20 m of,
2. Socket outlets should be: to BS EN 60309-2; IP44, at
between 0.5 m and 1.5 m above ground, rated not less than
16 A and have individual overcurrent and 30 mA RCD
3. If the supply is TN-C-S the protective conductor of each
socket needs to be connected to an earth rod.
Special Locations 99
BS 7671 SECTION 709: MARINAS
This location is basically a camping park for boats and has similar
requirements to those of caravan and camping parks.
It is where you arrive in your 40 ft 8 berth cruiser (some hope)
looking for a place to park!!!
However, the environment is a little more harsh than the caravan
park due to the the possibilities of corrosion, mechanical damage, structural movement and flammable fuels, together with the
increased risk of electric shock.
Due to the harsh conditions mentioned the classification of influences would include:
solid foreign bodies
Distribution circuits, like those in caravan parks, can be either
underground or overhead, as well as PVC covered mineral insulated, cables in cable management systems, etc.
However, overhead cables on or incorporating a support wire,
cables with aluminium conductors or mineral insulated cables
shall not be installed above a jetty or pontoon, etc.
Underground cables should have additional mechanical protection
and be installed 0.5 m deep.
Overhead cables should be at the same heights as in caravan parks.
100 IEE Wiring Regulations: Design and Verification
Isolation, switching and socket outlets
Generally the same as caravan parks.
Socket outlets should be installed not less than 1 m above the
highest water level except that for floating pontoons, walkways,
etc. this height may be reduced to 300 mm.
BS 7671 SECTION 711: EXHIBITIONS, SHOWS
This section deals with the protection of the users of temporary
structures erected in or out of doors and is typical of antique fairs,
motorbike shows, arts and craft exhibitions, etc.
It does not cover public or private events that form part of entertainment activities, which are the subject of BS 7909.
None particularly specified. Clearly they must be considered and
Armoured or mechanically protected cables where there is a risk of
Cables shall have a minimum conductor size of 1.5 mm2.
Against shock:
1. Supply cables to a stand or unit, etc. must be protected at
the cable origin by a time-delayed RCD of residual current
rating not exceeding 300 mA.
Special Locations 101
2. All socket outlet circuits not exceeding 32 A and all other
final circuits, excepting emergency lighting, shall have
additional protection by 30 mA RCDs.
3. Any metallic structural parts accessible from within the unit
stand, etc. shall be connected by a main protective bonding
conductor to the main earthing terminal of the unit.
Against thermal effects:
1. Clearly in this case all luminaires, spot lights, etc. should
be placed in such positions as not to cause a build-up of
excessive heat that could result in fire or burns.
Every unit, etc. should have a readily accessible and identifiable
Tongue in cheek here!! Every installation should be inspected and
tested on site in accordance with Part 6 of BS 7671.
BS 7671 SECTION 712: SOLAR PHOTOVOLTAIC (PV)
These are basically solar panels generating DC which is then converted to AC via an invertor. Those dealt with in BS 7671 relate to
those systems that are used to ‘top up’ the normal supply.
There is a need for consideration of the external influences that
may affect cabling from the solar units outside to control gear
There must be protection against overcurrent and a provision
made for isolation on both the DC and AC sides of the invertor.
102 IEE Wiring Regulations: Design and Verification
As the systems can be used in parallel with or as a switched alternative to the public supply, reference should be made to Chapter 55
of BS 7671.
BS 7671 SECTION 717: MOBILE OR
Medical facilities units, mobile workshops, canteens, etc. are
the subject of this section. They are self-contained with their
own installation and designed to be connected to a supply by, for
instance, a plug and socket.
The standard installation protective measures against shock are
required with the added requirement that the automatic disconnection of the supply should be by means of an RCD.
Also, all socket outlets for the use of equipment outside the unit
should have additional protection by 30 mA RCDs.
The supply cable should be HO7RN-F, oil and flame resistant
heavy duty rubber with a minimum copper conductor size of
Socket outlets outside should be to a minimum of IP44.
BS 7671 SECTION 721: CARAVANS AND MOTOR
These are the little homes that people tow behind their cars or
that are motorized, not those that tend to be located on a fixed
site. It would be unusual for the general Electrical Contractor to
wire new, or even rewire old units. How many of us ever rewire
our cars? In consequence, only the very basic requirements are
Special Locations 103
These units are small houses on wheels and subject to the basic
requirements of protection against shock and overcurrent. Where
automatic disconnection of supply is used this must be provided
by a 30 mA RCD.
The wiring systems should take into account the fact that the
structure of the unit is subject to flexible/mechanical stresses and,
therefore, our common flat twin and three-core cables should not
Unless the caravan demand exceeds 16 A, the inlet should conform to the following:
(a) To BS EN 60309-1 or 2 if interchangeability is required
(b) No more than 1.8 m above ground level
(c) Readily accessible and in a suitable enclosure outside the
(d) Identified by a notice that details the nominal voltage,
frequency and rated current of the unit.
Also, inside the caravan, there should be an isolating switch and
a notice detailing the instructions for the connection and disconnection of the electricity supply and the period of time between
inspection and testing (3 years).
Accessories and luminaires should be arranged such that no damage can occur due to movement, etc.
104 IEE Wiring Regulations: Design and Verification
There should be no compatibility between sockets of low and extra
Any accessory exposed to moisture should be IP55 rated (jet proof
and dust proof).
BS 7671 SECTION 740: AMUSEMENT DEVICES,
FAIRGROUNDS, CIRCUSES, ETC.
This is not an area that is familiar to most installation electricians
and hence will only be dealt with very briefly.
The requirements of this section are very similar to those of
Section 711 Exhibitions, shows, etc. and parts of Section 706
Agricultural locations (because of animals) regarding supplementary bonding.
For example, additional protection by 30 mA is required for:
1. Lighting circuits, except those that are placed out of arms
reach and not supplied via socket outlets.
2. All socket outlet circuits rated up to 32 A.
3. Mobile equipment supplied by a flexible cable rated
up to 32 A.
Automatic disconnection of supply must be by an RCD.
Equipment should be rated to at least IP44.
The installation between the origin and any equipment should be
inspected and tested after each assembly on site.
BS 7671 SECTION 753: FLOOR AND CEILING
Systems referred to in this section are those used for thermal storage heating or direct heating.
Special Locations 105
1. Automatic disconnection of supply with disconnection
achieved by 30 mA RCD.
2. Additional protection for Class II equipment by 30 mA
3. Heating systems provided without exposed conductive parts
shall have a metallic grid of spacing not more than 300 mm
installed on site above a floor system or below a ceiling
system and connected to the protective conductor of the
1. Where skin or footwear may come into contact with floors
the temperature shall be limited, for example to 30°C.
2. To protect against overheating of these systems the
temperature of any zone should be limited to a maximum
of 80°C.
Minimum of IPX1 for ceilings and IPX7 for floors.
The designer must provide a comprehensive and detailed plan of
the installation which should be fixed on or adjacent to the system
BS 7671 Appendices
There are 15 appendices in BS 7671, only one of which is a requirement, that is number 1; all the rest are for information.
British Standards referred to in BS 7671
A list of Statutory Regulations, etc.
Time/current curve for protective devices
Tables of cable current-carrying capacities and voltage
drops for busbar and powertrack systems
Definitions – multiple source, DC and other systems
Protection of conductor’s in parallel against overcurrent
Effect of harmonic current on balanced three-phase
Voltage drop in consumer’s installations
Measurement of resistance of non-conducting floors and
Earth fault loop impedances using the 0.8 factor
Arrangement of ring and radial final circuits.
This appendix looks at typical C&G 2392-30 examination questions and Appendix 3 gives suggested solutions expected by the
examiners. Clearly, in many instances there is not always one correct answer, and the examiner will have a range of alternatives from
which to award marks. Owing to the time constraints, approximately 18 minutes per question, the candidate is not expected
to, and nor can he or she, write an essay in answer to descriptive
questions. All that is required are reasoned statements which indicate a knowledge and understanding of the subject matter, and if
time allows, specific reference to relevant parts of the 17th Edition,
although this is not essential unless asked for. It is not sufficient to
simply quote Regulation numbers or Parts in answer to a question.
In fact, no marks are awarded for such answers.
A factory manufacturing chemical products is situated close to the supply transformer
feeding an industrial estate. The earthing system is TN-C-S with a measured loop
impedance of 0.015 Ω and PFC of 16 kA. It is required to increase the level of lighting
by installing 26 400 W/230 V high-bay discharge luminaires. The existing wiring system is a mixture of PVC/SWA cables and galvanized trunking and conduit. There is no
spare capacity in any of the existing distribution fuse boards.
Outline the design considerations for the new lighting, with regards to:
1. Maximum demand and diversity
5. Control and protective devices.
110 IEE Wiring Regulations: Design and Verification
A consumer is having major alterations to their premises, one part of which is to convert an existing kitchen extension to a pottery room housing a 9 kW/230 V fan-assisted
kiln and it is proposed to utilize the existing cooker circuit to supply it. The cabling is
6.0 mm2 twin with 2.5 mm2 cpc, clipped direct throughout its 25 m run and protected
by a 32 A BS EN 60898 Type B CB and there are no adverse conditions prevailing.
The external value of loop impedance has been measured as 0.3 Ω. Show by calculation what changes, if any, are required to enable the existing system to be used.
You are to provide the temporary electrical installation for a construction site on
which the site huts and offices together with the main supply point are on the opposite
side of the access road to the building under construction. The services required are
1. The site huts and offices
2. Portable tools
(a) Indicate a suitable method of running supplies from the site hut area to the
construction area. What type of sockets and cable couplers should be used?
(b) State the voltages and disconnection times for 1 and 2 above.
(c) If one of the circuits for the portable tools is protected by a 16 A Type B CB,
what is the maximum value of the loop impedance Zs for that circuit?
Appendix 2: Sample Questions 111
Part of a farm complex supplied by a TT system is to be converted for use as a poultry
incubation area. The existing wiring is some 30 years old and incorporates a voltageoperated earth leakage circuit breaker. Outline the design criteria to be considered with
1. The wiring system
2. Protection against shock
3. Protection against thermal effects.
EXAMPLE A2.5
A single-phase distribution circuit to a distribution board housing BS 88 fuses is wired
in 6.0 mm2 SWA/XLPE cable.
A radial lighting circuit wired in 1.5 mm2 PVC copper cable with a 1.5 mm2 cpc and
protected by a 10 A BS 88 fuse is fed from the board. The length of the lighting circuit
is 40 m.
The measured value of Zs at the distribution board is 2.1 Ω, and the ambient temperature at the time of measurement was 20°C.
(a) What would be the minimum gross size of the distribution cable armouring if it is to
be used as the cpc?
(b) Calculate the value of Zs at the extremity of the lighting circuit.
Is this value acceptable?
EXAMPLE A2.6
During a periodic test and inspection of the installation in a butcher’s shop, it is revealed
that the circuit supplying an electrically operated compressor does not meet the maximum earth fault loop impedance requirements. The circuit is protected by a 16 A
Type C CB, and the unit is situated 1 m from a steel sink. Explain how, under certain conditions, this situation may be resolved by the use of supplementary bonding.
Support your answer with calculation.
112 IEE Wiring Regulations: Design and Verification
EXAMPLE A2.7
A 2.5 mm2 ring final circuit 60 m long is wired in singles in a PVC conduit; the cpc is
1.5 mm2. A ring circuit continuity test is performed involving measurements at each
1. What is the purpose of this test?
2. Explain a method of identifying the opposite ‘legs’ of the ring.
3. What would be the reading between L and E at the socket nearest the mid-point of
4. What is the significance of this mid-point reading?
EXAMPLE A2.8
1. Give three examples for the use of an RCD, indicating residual operating currents
and operating times.
2. How often should a consumer operate an RCD via its test button. What does this
test achieve?
3. Give one example for the use of a time-delayed RCD.
EXAMPLE A2.9
Figure A2.1 shows a ring final circuit wired in flat twin with cpc cable
2.5 mm2 1.5 mm2. The protection is by 32 A Type B CB. If a test for continuity was
performed at 15°C using the measurement at each socket method, calculate:
1. The reading at each socket between L and N.
2. The value of R1 R2.
3. The value of Zs for comparison with the tabulated maximum value.
(2.5 mm2 copper has a resistance of 7.41 mΩ/m, 1.5 mm2 is 1.21 mΩ/m and the value
of Zs is measured as 0.28 Ω.)
Appendix 2: Sample Questions 113
Zs 0.28 10 m
EXAMPLE A2.10
A small three-storey commercial office complex is due to have a periodic test and
inspection. Outline the major steps you would take regarding:
1. Disturbance to office routine.
2. Meeting the requirements of the Electricity at Work Regulations 1989.
3. Measuring the continuity of main protective bonding conductors.
4. Reporting defects and issuing certificates.
EXAMPLE A2.11
1. Give two reasons, when conducting an insulation resistance test on a large complex
installation, for breaking it down into smaller sections. What precautions should be
taken before commencing the tests?
2. The test results for each section of such an installation are 50 MΩ, 20 MΩ, 100 MΩ
and 4 MΩ. Show by calculation the expected overall insulation resistance at the
intake position.
114 IEE Wiring Regulations: Design and Verification
EXAMPLE A2.12
1. A wiring system employing the use of singles in steel trunking is to be installed.
Outline the main design and installation considerations with regards to this
2. The trunking at one point will accommodate the following single-stranded
Determine the minimum size of trunking to be used.
1. Determine the new maximum demand by calculating the increase in load and adding to the existing maximum demand. Check that suppliers’ equipment and main
switchgear/busbars, etc., can accommodate the extra load.
power (1 . 8 for discharge lamps)
26 400 1 . 8
81 . 4 A
Increase in load No diversity would be allowed as it is likely that the lamps will be on all the time.
2. Luminaires need to be accessible for cleaning, repair, lamp replacement, etc.
Access equipment should be available
Spare lamps, chokes, etc., should be kept
Luminaires supplied via plug and socket arrangement to facilitate easy removal,
and without losing supply to the other lamps
3. As chemicals are being produced the atmosphere could be corrosive, and there
may be a fire risk; external influences classification would be AF2 and BE2.
4. If valid documentation exists it is possible for a decision to be made to use, at least in
part, the existing trunking system. If not, and this is most likely, a new system should
be installed using either singles in galvanized steel trunking and conduit, or PVC
sheathed material insulated cable, or PVC sheathed SWA cable, with circuits spread
over three phases. Protect against shock by automatic disconnection of supply.
5. Control by switch operating a three-phase 80 A contactor feeding a three-phase
distribution board housing BS 88 fuses to cater for the high PFC at the intake.
Contactor and DB must also be able to handle the PFC.
116 IEE Wiring Regulations: Design and Verification
Kiln design current CB setting In, such that In Ib 40 A
No correction factors, hence It 40 A
I t 46 A
Cable size 6.0 mm2
71 . 18 V ok
Volt drop Shock risk Zs Ze (R1 R2)
0.3 (3 . 08 7 . 41) 25 1 . 2
0.615 Ω ok, as Zs maximum is 1.44 Ω
Thermal constraints I U0
0 . 615
t 0 . 1s for a 40 A T ype B CB
1 .03 mm2
So the 2.5 mm2 cpc is ok. The only change to the existing installation would be to
uprate the 32 A CB to 40 A.
Appendix 3: Suggested Solutions to Sample Questions 117
(a) PVC sheathed SWA cable supported over the access road on a catenary wire
at a minimum height of 5.8 m. Cable couplers and sockets should be to BS EN
(b) Site huts and offices
0.4 s and 5 s
110 V CTE (preferred) 5 s
(c) From Table 41.6, IEE Regulations, Zs 0.67 Ω or, calculate from:
Ia for a 16 A Type B CB from Figure 3.4 in IEE Regulation curves
So Z s 55
0 . 6875 Ω
EXAMPLE A3.4
1. It is unlikely that the existing wiring will meet the new requirements, and due to its
age it would be best to replace with a new all-insulated system, for example singles
in PVC conduit out of reach of livestock and supplied by a manufacturer who specifies resistance to the onerous conditions found on farms.
2. Remove the voltage-operated ELCB. These are not permitted. RCD protection will
be needed for: socket outlet circuits up to 32 A (30 mA), over 32 A (100 mA) and all
other final circuits (300 mA).
Protection against shock would be by automatic disconnection of supply with
3. Protection against fire may be achieved by using an RCD rated up to 300 mA,
except where equipment essential to the welfare of livestock is involved.
Incubation and subsequent hatching involves the use of infrared lamps to maintain a
stable temperature. The enclosures of such lamps may become hot and hence must
be located in positions that will not cause fire or burns. Radiant heaters should not be
located less than 0.5 m from livestock or combustible materials.
118 IEE Wiring Regulations: Design and Verification
EXAMPLE A3.5
(a) Referring to Table 54.7:
Sp k1S
(Sp is the CSA of the protective conductor)
As XLPE is thermosetting:
143 and k2 46
18 . 6 mm2
Sp (b) Distribution circuit Zs 2.1 Ω.
(12 . 1 12 . 1) 40 1 . 2
1 . 16 Ω
Final circuit (R1 R2 ) So total Zs
3.26 Ω
Zs maximum for a 10 A BS 88 fuse for a final circuit not exceeding 32 A
is 5.11 Ω.
EXAMPLE A3.6
Provided that the value of loop impedance allows a fast enough disconnection time to
protect against thermal effects, then compliance with the Regulations may be achieved
by connecting a supplementary bonding conductor between the exposed conductive
parts of the compressor and the sink. The resistance of such a conductor must be less
Ia, the current causing operation of the protection within 5 s, is 160 A for a Type C CB.
0 . 313 Ω
So R Appendix 3: Suggested Solutions to Sample Questions 119
EXAMPLE A3.7
1. To identify breaks in the ring and/or interconnections across the ring.
2. Test with a low reading ohmmeter between each L, N and E leg and the corresponding terminal at the nearest socket. A low value indicates the short leg, a high
value the long leg.
30 m of 2.5 mm2 30 m of 1.5 mm2
3. Reading at mid-point 2
30 0 . 00741 30 0 . 0121
0 . 293 Ω
4. This value is (R1 R2) for the ring.
1.5 mm2 cpc
2.5 mm2 line
EXAMPLE A3.8
1. (a) When the loop impedance value for an overcurrent device cannot be met. The
product of the residual operating current of the device and the loop impedance should not exceed 50 V. The device should trip within 200 ms at the rated
(b) If additional protection against shock is required. RCD should be rated at 30 mA
or less, and trip within 40 ms at a residual operating current of 150 mA.
(c) In agricultural situations, for protection against fire. The RCD should not be
rated above 300 mA, and used for circuits other than those essential for the
welfare of livestock. The tripping time would be within 200 ms at a rated residual current.
120 IEE Wiring Regulations: Design and Verification
2. The RCD should be tested quarterly via the test button. This only checks the operating mechanism not any earthing arrangements.
3. On a TT system where the whole installation is protected by, say, a 100 mA device
and the sockets by a 30 mA device. A time delay on the 100 mA RCD will give discrimination with the 30 mA RCD.
EXAMPLE A3.9
1. The ring is 40 m long, so the L to N reading at each socket would be:
2 40 7 . 41
0 . 148 Ω
2. The mid-point R1 R2 is 20 m of 2.5 mm 20 m of 1.5 mm in parallel:
20 (7 . 41 12 . 1)
0 . 195 Ω
3. (R1 R2) corrected for 15°C 0.195 1.02 0.199 Ω
Correction for operating temperature 0.199 1.2 0.239 Ω
so Z s 0 . 28 0 . 239
0 . 52 Ω This is ok as tabulated maximum valu e is 1 . 44 Ω
EXAMPLE A3.10
1. Careful planning and consultation will be required before any work commences. It
may be the case that access to the premises is better suited to a weekend or evenings when no staff are present. If this is not possible, then the installation should
be tested in small sections, all the tests required in each section being done at that
time. Clearly, in the modern office, computers play a major role, and unless UPS
are present, advice should be sought before isolating any supplies.
2. The inspector is a duty holder and as such must take all precautions to safeguard
himself and others. Visual inspection can involve entry into enclosures housing
live parts, and unless it is completely impracticable, supplies must be isolated and
locked off. Testing on or near live equipment is prohibited unless it is unreasonable
for it to be dead, for example loop impedance and RCD tests. All test equipment
Appendix 3: Suggested Solutions to Sample Questions 121
must be suitable for the use intended and should be in a safe condition. All test
results must be recorded.
3. It is usual for the bonding conductor to be disconnected for test purposes. Unless
all supplies to the complete installation can be isolated, bonding conductors must
4. All test results and details of the inspection must be entered on to schedules and
a periodic report given to the person ordering the work. The report should include
details of the extent of the work, any dangerous situations prevailing, restrictions to
the inspection and test, and serious defects.
Any certificate issued should indicate and explain departures from the 17th Edition,
especially those due to installations constructed before the current Regulations.
EXAMPLE A3.11
1. Large installations may have circuits in parallel which can result in pessimistically
low values of insulation resistance even though there are no defects. Dividing the
installation into smaller sections will overcome these low readings.
Subdivision of the installation, especially on periodic inspections, will enable minimum disruption of work processes.
All persons must be informed that the test is to take place, all supplies isolated
from the part of the installation in question, all electronic devices, capacitors, neon
indicators, etc., should be disconnected, and ensure that no electrical connection
exists between any live conductor and earth.
2. The overall value will be the sum of the individual insulation resistances in parallel,
50 20 100 4
0 . 02 0 . 05 0 . 01 0 . 25
122 IEE Wiring Regulations: Design and Verification
EXAMPLE A3.12
1. The design should embrace grouping of circuits, space factor if trunking sizes are
outside the scope of tabulated sizes, and external influences which may affect the
choice of trunking finish.
With regard to the installation, supports must be at the correct spacing, joints
should be bridged with an earth strap, and where trunking passes through walls,
ceilings, etc., it should be externally and internally sealed to the level of the fire
resistance required for the building construction.
2. Using the tabulated conductor and trunking terms, we have:
28 8.1 226.8
20 11.4 228
12 22.9 274.8
10 36.3 363
Hence trunking size is 75 375 mm2.
Resistance of Copper Conductors at 20°C.
Conductor CSA (mm2)
Resistance (m/m)
Agricultural locations, 104
Alphanumeric system, 47, 49
Amusement devices, fairgrounds etc., 104
Approved test lamp, 55
Barriers and enclosures, 7, 8
Basic protection, 8, 9
Bathrooms, 86
British Standard ﬁnger, 69
BS 7671 Appendices, 107
Cable types, 26
Calibration, 57
Caravans, 102
Caravan/camping parks, 97
Characteristics of overload devices, 18
Class II equipment, 9
Compatibility, 3
Conducting location with restricted
movement, 97
Conductor energy withstand, 19
Conductor size, 32, 35
Conduit capacities, 44
Construction sites, 93
Continuity of protective conductors, 59
Continuity of ring ﬁnal circuit
conductors, 61
Design calculations, 25
Design current Ib, 28
Disconnection times, 12
Disconnector, 24
Discrimination, 22
Distribution diagram, 47, 48
Diversity, 29
Drawings, 47
Earth electrode resistance, 73−77
Earth fault current, 21
Earth fault loop impedance, 14, 59, 77
Earth fault loop path, 13
Earth free bonding, 9
Electrical separation, 9
Exhibitions, 100
Exposed conductive parts, 7
Extraneous conductive parts, 11
External inﬂuences, 2
External loop impedance, 41, 77
Fault current, 19
Fault protection, 8, 9
Floor and ceiling heating systems, 104
Functional switch, 24
Functional tests, 82
Fuse-switch, 24
Inspection, 54
Inspection and testing, 53
Insulation of live parts, 8
Insulation resistance, 70
Interconnection diagram, 48
IK codes, 11
IP codes, 10
Isolation and switching, 23
Isolator, 23
Layout drawing, 46
Let-through energy, 19, 34
Main protective bonding, 12
Maintainability, 3
Marinas, 99
Mobile or transportable units, 102
Non-conducting location, 9
Obstacles, 8
Operation of RCDs, 55
Overcurrent, 16
Periodic inspection and testing, 82
Placing out of reach, 9
Polarity, 59, 72
Prospective short circuit current, 19, 55
Protection against fault current, 19
Protection against overcurrent, 16
Protection against overload, 18
Protection against overvoltage, 23
Protection against shock, 7
Protection against thermal effects, 15
Protection against undervoltage, 23
Protection by barriers and enclosures, 59,
Protection for safety, 6
Protection rating In, 41, 42
Protective conductor size, 21
Protective devices, 17
Proving unit, 57
Purpose supplies and structure, 2
RCDs, 78
Resistance of copper conductors, 60, 122
Rooms containing saunas, 92
SELV, 8
Shock risk, 7, 34
Short-circuit current, 16, 19
Solar photovoltaic systems, 101
Supplementary bonding, 15
Swimming pools, 89
Switch disconnector, 24
Switch-fuse, 24
Systems, TT, TN-s, TN-C-S, 14
Tabulated conductor current, 25, 32
Test equipment, 55
Testing, 54
Tests on assemblies, 82
Thermal risk to conductors, 19, 25, 34
Trunking capacities, 44
Voltage drop, 25, 33
Wiring systems, 26
BS 7671 2001 IEE Wiring Regs Explained
supplementaryquestionseasyeng-180302223528
ITU Computer Organization Syllabus 2017-2018
fundamentals of electrıcıty and electronıcs - Bilgi Paketi