Fuses and CBs react to short circuits and thus provide protection against their
consequences. However, their time-current characteristics are such that they do not
provide real protection against sustained low-level overloads. For example, a 30A HBC
fuse will carry 40A for a long period. This may harm the cables of the permanent wiring
if they have been sized closely to the protective device rating. An electric motor
overloaded to this extent would however burn out.
The IEE Regulations require overload protection which will operate before the current
exceeds 1.45 times the current-carrying capacity of the smallest conductor in the circuit.
If protection against smaller overloads than this is needed it has to be supplied with the
equipment which needs the protection. The most usual case is that of an electric motor
which is protected against overload by an overload relay in the motor starter. The usual
type of overload relay contains a heater element and a bi-metal strip in each motor line
conductor. An excessive current causes the bi-metal strip to deflect, the amount of
deflection depending on the magnitude of the current and the time for which it flows.
When the deflection reaches a predetermined amount the bi-metal operates the tripping
mechanism, which opens the coil circuit, and this in turn causes the main contacts to
open.
Such an overload protection has a time-current characteristic of the form shown in
Figure 9.15. It will be seen that the starter with this overload device
Figure 9.15 Motor starter
characteristics
will carry a starting current eight times the full load current for 6s. A fuse of the same
rating would not do this.
A motor starter must be capable of operating several thousand times in its working
life. A design which enables it to do so places a limit on the magnitude of the current it
can break. Thus while it can deal with overloads it cannot safely break a short circuit. For
this reason, the starter which protects the motor against overloads must itself be protected
by a fuse to deal with short circuits. The fuse will at the same time protect the permanent
wiring against short circuits.
Now a fuse of the same rating as the motor will not carry the starting current for long
enough for the motor to run up to speed. The fuse backing up the starter must, therefore,
have a rating higher than the normal full load running current of the motor. Motor
manufacturers and starter manufacturers provide information which enables the fuse to be
correctly chosen. Table 9.1 is a selection from such data. It is also now common practice
for manufacturers to quote a motor rating for fuses; this is the running current for which
the fuse should be used.
The arrangement of fuse, starter and motor is shown in single line diagrammatic form
in Figure 9.16. Logically, cables ab and cd are protected against
Table 9.1 Fuse ratings for motor circuits
Type of starter Overload release rating-amps Fusing rating amps
Direct on 0.6 to 1.2 5
1.0 to 2.0 10
1.5 to 3.0 10
2.0 to 4.0 15
3.0 to 6.0 20
5.6 to 10.0 30
9.0 to 15.0 40
13.0 to 17.0 50
Star-delta 4.0 to 7.0 15
6.0 to 10.0 20
9.0 to 17.0 30
1 6.0 to 26.0 40
22.0 to 28.0 50
Figure 9.16 Motor protection
overload by the overload device in the circuit. For example, we can see from Table 9.1
that a motor with a running current of 3A and a starter overload set at 3A will need a
back-up fuse of 15A. The cable is protected against short circuit by the fuse. The time to
raise the temperature of the cable to its limit is given by t=K2S2/I2. Therefore we can use
the formula to determine if the cable is protected against short circuit, and to determine if
the starting current and starting time will raise the temperature of the cable above the
limit value.
One can, however, take advantage of the fact that the starter protects the cable as well
as the motor. In theory, if a very high resistance fault developed on the cable between a
and b neither the fuse nor the starter would open the circuit, and damage might result. In
practice, the chances of this happening are so small that they may be neglected. The usual
practice is to make cables ab and cd such that their current rating is equal to the
maximum setting of the overload protection device fitted in the starter. This is probably
one of the few cases in which it is in order for the cable to have a rating less than that of
the fuse protecting it, and it may be done only where there is a motor starter incorporating
Design of electrical services for buildings 150
an overload relay. One other case is where an overload cannot occur, such as on an
immersion heater circuit, where the heater is working or it is not. The circuit must
however be protected against short circuit.
Protection of persons
People using the building have to be protected against electric shock. They would get a
shock if they came into contact with live parts, and in considering protection a distinction
is made between direct and indirect contact. Direct contact is contact by a person with a
live conductor which is intended to carry current in normal operation. The normal
protection against this is the provision of insulation on all current-carrying cables, and
enclosing terminals and connections
Indirect contact is contact with exposed conductive parts which are not intended to
carry current but have become live as a result of a fault. Such a fault is indicated in
Figure 9.12a. When it occurs, the metal case of an appliance which a person is likely to
handle is raised towards line potential and will cause a shock if it is touched by someone
using the appliance. Protection is provided by the fact that the case is earthed and that a
protective device will disconnect the circuit as soon as a fault current flows to earth.
It is possible that a fault of this type will occur while a person is holding an appliance.
In that case he will be subjected to a dangerous potential during the time it takes for the
protective device to operate and disconnect the supply. For this reason BS 7671 stipulates
the time in which the device must operate, which is 0.4s for 230V circuits serving socket
outlets and 5s for circuits serving only fixed equipment. The reason for the difference is
that a person is likely to have a firmer grip on a portable appliance plugged into a socket
outlet than on a fixed appliance which he is only likely to touch casually. There is a
circumstance where the time to disconnect a socket circuit may be increased to 5s.
A current would also flow through a person if he came into contact with two separate
pieces of metal at different potentials. In the event of a fault such a potential difference
could exist between the exposed metal case of an electrical appliance and earthed
metalwork which is not part of the electrical system, termed extraneous conductive parts,
such as structural steelwork, or a central heating radiator. To avoid danger arising in this
way all extraneous conductive parts must be linked (bonded) to the main earthing
terminal of the installation. The conductors by which this is done are known as main
equipotential bonding conductors and methods for determining its size are given in the
IEE Regulations. This causes all extraneous conductive parts connected to the main
earthing terminal to raise in potential, when a fault occurs on a circuit, creating an
equipotential zone.
Earth protection
As we have seen, fuses and CBs react to short circuits. If they are to provide protection
against faults to exposed conductive parts, the wiring must be such that the fault produces
the same conditions as a short circuit, namely a large excess current in the line conductor.
This will happen only if the impedance of the path taken by the fault current is low
Protection 151
enough. The path is indicated by a-b-c-d-f-g-a in Figure 9.12a/b and is known as the earth
fault loop path; its impedance is known as the earth fault loop impedance.
An earth fault occurs when a phase conductor touches exposed metalwork of an
installation termed an exposed conductive part, and so raises the metal to a dangerous
potential. If a fuse or CB is to be used to clear a fault of this nature, then the fault must
immediately produce a current large enough to operate the fuse or breaker quickly. To
achieve this, all exposed metalwork which, in the event of some fault, could conceivably
become live, is earthed and the earth path is designed to have low impedance. When a
fault occurs, current flows through circuit protective conductors to earth, and because of
the low impedance in the earth fault loop path, the current is large enough to operate the
protective device. It must also be large enough to operate the device within the time
stipulated above. The Regulations give details of the maximum earth fault loop
impedance which can be allowed to ensure operation within the required times for fuses,
CBs and RCBOs.
Since an earth fault raises exposed conductive parts above earth potential it creates the
possibility of a voltage existing between such metal, and nearby metalwork such as
structural metalwork, which introduces earth potential, and is not part of the electrical
installation, and is termed an extraneous-conductive-part, and not affected by the fault
and therefore still at earth potential. Although the fuse or CB will clear the fault, that is to
say remove it, the operating time of the fuse or CB may be long enough to cause danger
of electric shock. One way to prevent danger is to bond the two sets of metalwork
together. We have already mentioned this in Chapter 6. In any event the IEE Regulations
require the main earth terminal at the incoming supply intake to be bonded to the
metalwork as shown in Figure 9.12b, of any gas or water services as near as possible to
the point at which those services enter the building.
The policy of protecting against earth faults by making sure that they produce a short
circuit and, therefore, operate the fuse or circuit breaker has been described as ‘chasing
the ampere’. With ever-increasing demand for electricity, circuit ratings and their fuse
ratings are becoming ever higher and the short circuit current needed to operate the fuse
or circuit breaker becomes higher. Thus ever-lower values are required for earth loop
impedances and the fault currents which the fuses have to break become higher. The
opinion has been expressed that this method of protection will not be able to keep pace
with the consequences of increasing electrical loading.
However, at the present time in urban areas where the electricity company provides an
effective earth to the sheath of the service cable (TN-S), or by connecting the main
earthing terminal to the supply neutral (TN-C-S) there is no difficulty in achieving an
earth loop impedance of less than 1.0ohm. It may perhaps not be possible to do this as
long as steel conduit is relied on as the earth return path, but it can certainly be done by
the use of a separate circuit protective conductor. Because of the need for low earth
impedances separate protective conductors should nowadays always be used on new
installations. Although not strictly necessary, since the cross-sectional area of conduit is
more than adequate to satisfy the regulations.
If for any reason the earth loop impedance cannot be made low enough it becomes
necessary to use a residual current circuit breaker. An RCD which will operate on an
earth fault current of 30mA will react on a 230V circuit even if the earth loop impedance
Design of electrical services for buildings 152
is 480ohms and it is inconceivable that values anything like as high as this would ever be
found on a practical installation.
Temporary installations are likely to have higher earth loop impedance paths than
permanent ones, and it is very common to use earth leakage protection for the temporary
wiring on building sites, particularly in rural areas.
It should be noticed that earthing of exposed metalwork is still necessary when an
RCD is used. This can be shown by reference to Figure 9.12a/b. Suppose the earth return
path f-g did not exist, then normal current would flow through a-b-c-e-g-a and there
would be no imbalance between line and neutral for a current operated CB to detect.
Nevertheless the exposed metalwork, connected through the fault to the conductor at d,
would be at a potential given by
and depending on the relative impedances of the various parts of the earth fault loop
impedance path, this could be anything up to almost full line voltage.
We refer yet again to Figure 9.12a. If we assume that the impedance of the transformer
winding is negligible, the value of the earth fault loop path Zs, is 0.1+0.1+0.2+0.2=0.6Ω.
If=Uoc/Zs
where Uoc is the open circuit voltage of the transformer, the fault current would be
If=240/0.6=400A
and the potential drop from a to c is
Ua−c=If×Za−c
Ua−c=400×(0.1+0.1)=80V
The voltage of the exposed conductive part will be
240–80=160V above earth
If the faulted piece of equipment is touched by a person simultaneously with the central
heating radiator, the person would receive a 160V shock.
Now consider Figure 9.12b, with the main equipotential bonding in place. Assuming
the earth fault current remained the same as without bonding, the volts drop d-f would be
Ud−f=If×Zd−f
Ud−f=400×0.2=80V
The potential of the main earthing terminal will be
Uoc−(Ua−c+Ud−f)
240−(80+80)=80V above earth potential
Now with the main equipotential bonding in place, the voltage of the central heating
radiator will be the same as the main earthing terminal, that is 80V. Therefore anyone in
contact with the faulted piece of equipment and the central heating radiator would receive
a shock voltage equal to the potential difference between the faulted piece of the
Protection 153
equipment and the radiator, which is 160V−80V=80V. Bonding has reduced this from a
value without bonding of 160V. The reason for equipotential bonding is to create an
equipotential zone. Exposed conductive parts are earthed to cause rapid disconnection. It
is important to differentiate between earthing and equipotential bonding. It would be
counter-productive to bond plastic pipework and items of metalwork such as aluminium
window frames that do not introduce a potential in the worst case.
The values of impedance of the earth fault loop path are for explanation only and are
not meant to be typical.
If the conditions prevent the use of fuses and circuits breakers, for shock protection,
such as high earth loop impedances, is possible to use RCDs since these devices do not
require a high current to operate quickly. BS 7671 require an RCD to be used to protect a
socket outlet which is intended for portable equipment to be used outside the building.
This is for additional direct contact protection They also require all socket outlets to be
protected by RCDs when earthing is provided by the user’s own electrode (a TT system)
and not by connection to an earth provided at the supply authorities’ transformer. It is left
to the designer’s discretion whether a separate RCD is provided for each socket outlet,
whether each circuit is protected by an RCD or whether all circuits on a distribution
board are protected by an RCD on the supply side of the board. Although BS 7671
requires that due account be taken of the operation of a single protective device, an RCD
on the main supply of a distribution board would trip the whole board should a fault
occur on one circuit.
It is the designer’s responsibility to use experience and professional judgement in
selecting the scheme of protection.
Earth monitoring
Protection through earthing of exposed metalwork will fail if there is a break in the earth
continuity conductor. It is possible to add an extra cable which will monitor the earth
continuity conductor and break the main circuit if the earth continuity fails. The basic
scheme of such a circuit is shown in Figure 9.17.
The monitoring circuit incorporates a low-voltage transformer, a relay, and a
protective circuit breaker. An additional pilot lead is required to the appliance or portable
tool. The low voltage is used to drive a current round the loop formed by the earth
conductor, a section of the metallic housing of the appliance, the pilot conductor and the
relay coil. This current holds in the relay and thus keeps the coil of the circuit breaker
energized.
If there is a break in the earth continuity conductor, or indeed anywhere in the pilot
circuit, the low-voltage current fails, the relay is de-energized, the main contactor coil
becomes de-energized and the circuit breaker opens
Design of electrical services for buildings 154
Figure 9.17 Monitored earth leakage
protection
the main circuit and cuts off the supply to the appliance. It will be noticed that this
monitoring circuit merely checks that the earth continuity conductor is sound; it does not
add to the basic earth leakage protection.
Earth electrodes
In normal earthing, the earth and the neutral are quite separate. The load current flowing
through the neutral must cause a potential difference between the two ends of the neutral.
Since the end at the supply transformer is earthed, the end at the consumer’s service
terminal must inevitably be at some potential above earth. It cannot, therefore, be used as
an earth point.
Nevertheless, an effective earth has to be found for the earth continuity conductors of
the permanent installation in a building. In urban areas the sheath of the electricity
company’s service cable is normally used for this purpose, but there is no obligation on
the company to provide an earth, and in rural areas where the supply may be by overhead
cable, it may not be possible for them to do so. In such cases, the consumers must provide
their own earth electrodes and the design of these become part of the design of the
building installation.
An earth electrode is a metal rod, which makes effective contact with the general mass
of earth. A common type consists of a small diameter copper rod which can be easily
driven to a depth of 6m or more into ground reasonably free of stones or rock. The soil
remains practically undisturbed and in very close contact with the electrode surface.
Since resistivity is lower in the deeper strata of earth and not very affected by seasonal
conditions, deep driving gives a good earth. Rods of this type are practically incorrodible.
Also it is easy to get access to the connection at the top of the electrode. A typical
arrangement is illustrated in Figure 9.18.
Protection 155
Where the ground is shallow but has low resistivity near the surface, a plate electrode,
either of copper or, of cast iron, can be used. When the soil resistivity is high, a cast iron
plate can be used with a coke surround. This method is illustrated in Figure 9.19.
Standard cast iron plates are made for use as earth electrodes. They are complete with
terminals for the earth continuity conductor. These terminals consist of two copper
sockets each secured by a drift-pin, the two being joined by a tinned copper strand to
which the earth conductor is bound and soldered. The completed connection is sealed and
covered in bitumen before the electrode is buried. It is, therefore, not as accessible as the
connection of the rod type electrode.
Long copper strip can also be used as an earth electrode, and the method of doing this
is shown in Figure 9.20. It will be seen from this that the strip is a useful type of electrode
for shallow soil overlying rock. Strip may be arranged in single lengths, parallel lengths
or in radial groups. Standard strip is commercially available for use as earth electrodes.
Figure 9.18 Copper rod electrode
Figure 9.19 Cast iron plate electrode
Design of electrical services for buildings 156
When current flows from the electrode into the soil, it has to overcome the resistance of
the soil immediately adjacent to the electrode. The path of the current is shown in Figure
9.20. The effect is equivalent to a resistance between the electrode and the general mass
of earth, and this resistance is the resistance of the electrode. Furthermore, the surface of
the ground near the electrode becomes live when current flows from the electrode to earth
and Figure 9.21 shows a typical surface distribution near a rod electrode. It can be seen
that an animal standing near such an electrode could have a substantial voltage applied
between its fore and hind legs, and in fact fatal accidents to livestock from this cause
have been known. The earth electrode should, therefore, be positioned well out of harm’s
way. It should
Figure 9.20 Copper strip electrode
Figure 9.21 Current from electrode
into earth
perhaps be noted that the deeper the electrode is below the ground, the smaller will be the
voltage gradient at the surface.
The effectiveness of earth protection depends on the low resistance of the electrode
when current flows through the electrode into the soil. This resistance cannot be
accurately predicted in advance and must be checked by testing. After installation, the
Protection 157
electrode should be periodically examined and tested to ensure that its initial low
resistance is being maintained. The scheme for testing an electrode is shown in Figure
9.23. The electrode under test is indicated by X; two auxiliary electrodes, Y and Z are
driven in for the test. Y must be placed sufficiently far from X for the resistance areas not
to overlap, and Z is placed approximately halfway between X and Y. The test electrode X
is disconnected from its normal continuity conductor and connected to the test instrument
as shown in Figure 9.23. A low-voltage alternating current is passed between X and Y.
The current is measured and so is the potential between X and Z. The resistance of the
earth electrode is given by the dividend of voltage and current. Check readings are taken
with the electrode Z nearer to and further from electrode X, and the results are
Figure 9.22 Voltage at surface of
ground due to rod electrode
Design of electrical services for buildings 158
Figure 9.23 Earth electrode resistance
test
accepted only if all three readings are substantially the same. If they are not, the test must
be repeated with a greater distance between X and Y.
Protective multiple earthing
This is an alternative method of earthing in which the neutral of the incoming supply also
forms the earth return path. In other words, instead of the neutral and earth of the
incoming supply being separate, they are combined to form a TN-C-S system (definitions
in BS 7671 explains the systems in use). The supply authority is required to maintain the
resistance between the neutral conductor and earth to a maximum of 10Ω To do this the
supply authority earth the supply neutral at various multiple points, protective multiple
earthing (PME).
The installation within the building is carried out in exactly the same way as for any
other system, and separate earth continuity conductors are used. The main earthing
terminal at the intake is not, however, connected to a separate earth return, but connected
to the neutral of the incoming service cable.
Because with this system the neutral is relied on as the earth, there must be no fuses,
cut-outs, circuit breakers or switches anywhere in the neutral. In the UK an area
electricity company may not adopt PME without the permission of the Secretary of State
for the Environment, and stringent requirements are made to ensure that the neutral
conductor is adequate to carry earth fault currents, that it is truly kept at earth potential
and that it is protected against breaks in continuity. The permission of British Telecom is
also required. This is because the currents into and through the ground at the points of
multiple earthing could cause interference to adjacent telephone and telegraph cables in
the ground.
After early hesitations, PME is becoming increasingly widespread in the UK.
Experience has shown that it is in practice as safe in certain conditions as previously used
methods and it has important advantages. In rural areas it makes it unnecessary for
consumers to have their own earth electrodes and therefore removes the risks of earth
Protection 159
electrodes in the care of unqualified persons. In urban areas it makes the electricity
companies’ distribution network cheaper. This system however must not be offered to
boat or caravan supplies, since there is a small risk of the neutral becoming disconnected.
This would cause the shell of the caravan to become live under earth fault conditions. It is
also not to be used on petrol station supplies.
Double insulation
Electrical appliances connected to the permanent wiring of a building, whether through
plugs into socket outlets or by means of permanent connections, must themselves be
protected against faults. There is need for protection against a fault developing on the
appliance itself.
When protection of the permanent wiring depends on earthing, the same principle can
be used for the appliance. The previous discussion has assumed this, and has proceeded
on the basis that the metal casing of an appliance is effectively connected through the
earth pin of the plug to the earth connection in the socket. There is, however, an
alternative method of achieving safety of appliances which does not depend on earthing
the appliances, and this is known as double insulation. For purposes of exposition
appliances of the class known as all insulated may be considered as special cases of
double insulation.
Double insulation consists of two separate sets of insulation. The first is the functional
insulation, which is the ordinary insulation of the conductors needed to confine the
current to the conductors and to prevent electrical contact between the conductors and
parts not forming part of the circuit. The supplementary insulation is additional to and
independent of the functional insulation. It is an entirely separate insulation which
provides protection against shock in the event of the functional insulation’s breaking
down.
Another term which we have to explain is reinforced insulation. This is an improved
functional insulation with such mechanical and electrical properties that it gives the same
degree of protection against shock as does double insulation.
An all-insulated appliance is one which has the entire enclosure made of substantial
and durable insulating material. In effect, it is a double-insulated appliance in which the
supplementary insulation forms the enclosure.
Both earthing and double insulation provide protection against the breakdown of the
primary functional insulation. Earthing depends, in the ways already described, on
ensuring that if the functional insulation fails, exposed metalwork will be prevented from
rising significantly above earth potential. If the case of the appliance can be made of
insulating material which is robust enough to withstand all conditions in which it is to be
used, then there is no danger of shock even if the functional insulation fails. Such an
appliance is an all-insulated one, and this form of construction gives adequate safety, but
some appliances must have exposed metal; for example, hedge clippers and portable
drills. Other appliances are so large that it is impracticable to make an insulating case
strong enough to withstand ordinary usage without making the whole appliance too heavy
and cumbersome; for example, a vacuum cleaner. In these cases, double insulation can be
used. It provides a second barrier of insulating material between conductors and exposed
Design of electrical services for buildings 160
metal parts. The presence of this additional barrier is a protection against the failure of
the functional insulation and makes it unnecessary to earth the exposed metal. Doubleinsulated
equipment is designed so that in general two independent sections of insulation
must both fail before any exposed metal can become live. The functional and protective
insulation must be so arranged that a failure of either is unlikely to spread to the other.
They ought, therefore, to be mechanically distinct, so that there is a surface of
discontinuity between them.
The principle of double insulation is illustrated in Figure 9.24, in which the phase and
neutral conductors each consist of a cable with ordinary functional insulation. The casing
is itself of insulating material and forms the supplementary insulation. If a fault develops
on the functional insulation, a
Figure 9.24 Double insulation
short circuit may develop between the live and neutral conductors but the supplementary
insulation will prevent the metal handle on the outside from becoming live.
An appliance which is double insulated by the use of either supplementary or
reinforced insulation should not be earthed and is not provided with an earth terminal.
When it is connected to a standard three-pin plug the earth pin of the plug is left
unconnected. Double insulation gives the same degree of protection against shocks as
earthing, and makes that protection independent of the earth loop impedance. It also gives
protection against high impedance faults to earth on the appliance itself, and thus guards
against fires caused by local overheating at the appliance.
Double insulation is a means of making an appliance safe. It cannot give protection
against faults on the permanent wiring in the fabric of a building. Thus, if there are any
metal parts used in the wiring installation, they must still be earthed. Such parts would be
conduit, switches, distribution boards, control panels and so on. It is hardly practicable to
carry out an installation of any size without the use of some metal components so that the
principle of double insulation cannot be applied to the permanent wiring. Moreover, the
designer and erector of the services in a building cannot control what appliances may be
connected to the service during the life of the building. Even if all the appliances in a
building when it is first put into use are of the double-insulated pattern, so long as
Protection 161
appliances depending on earth protection (and all electric kettles do) remain in existence,
one of them could at some time be connected to the system. Therefore, the system must
have an effective earth for the appliances to be linked to.
One of the strongest arguments for the use of double-insulated and all-insulated
appliances is that it makes the safety of the portable appliance independent of the
installation to which it is connected. The manufacturer knows that the user is protected by
the design of the appliance and this safety is given without reliance on the earthing
system of the building in which the appliance is to be used, over which the appliance
manufacturer has, of course, no control. The manufacturer has thus gone a long way
towards protecting the user against the latter’s own ignorance of the safe way of
connecting an appliance to a defective system, which could happen either through
ignorance or through inadvertence.
At the same time, if all appliances were known to be double insulated, the designer of
the electrical services could concentrate on protective devices for the system being
designed without having to consider what protection to leave for appliances which the
occupants will bring along later. Unfortunately, this is not the case, and the designer must
consider the interaction of other people’s appliances and the system. In particular,
designers should perhaps reflect on what kind of flexible cords and extension flexes are
likely to bridge the gap between their system and a well-made and safe appliance.
The IEE Regulations do not allow the designer to assume that equipment connected to
socket outlets will be double insulated and they must therefore be provided with earth
continuity conductors.
Portable tools
We shall conclude this chapter by considering the special problems of portable tools.
These are a very important class of appliances, especially in some factories which use
them in large numbers, and they are subject to certain difficulties of their own. When a
tool stalls, it is likely to blow a fuse, and after this has happened a few times the operator
or maintenance engineer decides that to avoid replacing the fuse every time the tool is
momentarily overloaded, a larger fuse will be substituted, capable of carrying the
overload. Unfortunately, it is also capable of carrying a substantial earth fault current, for
an increased length of time, and the operator can be electrocuted before the oversize fuse
clears the fault.
The flexible cables of portable tools come in for exceptionally rough usage and are
therefore particularly liable to develop faults. At the same time people using a portable
tool in a factory are likely to be either standing on, or else touching or close to,
substantial metal parts, so that they have a low impedance to earth. Thus an earth fault is
more probable on a portable tool and its effects are more likely to be immediate and
serious than on almost any other kind of appliance. It ought, therefore, to receive extra
care in protection but in fact may receive less than average attention.
To overcome these difficulties, factories which use portable tools in large numbers
often install a special low-voltage supply to serve them. Typical voltages used are 110V
and 55V to earth. Of course, the tools have to be wound for this supply and it is a
disadvantage that the motors are bulkier and heavier.
Design of electrical services for buildings 162
Another solution is to provide protection against sustained overload by means of
circuit breakers with time-delay characteristics such that they will not operate on
temporary overloads, and separate protection against earth faults by residual current
circuit breakers. A fuse is then needed only for back-up protection, if at all, and can be
large enough not to blow when the tool temporarily stalls. It is also advisable to use earth
monitoring on the earth conductors to portable tools. This makes it necessary for the
flexible cables to have an extra conductor and for the tools themselves to have an extra
connection.
None of these precautions is very suitable for the home handyman, who may not have
enough understanding of electrical theory to appreciate the need for them. It is probably
better for portable tools to be protected by double insulation rather than by reliance on
effective earthing. A similar case can be argued for making domestic appliances double
insulated. Many modern appliances, of which we may quote vacuum cleaners and
hairdryers as examples, are nowadays made with double insulation.
Standards relevant to this chapter are:
BS 88 Cartridge fuses for voltages up to and including 1000V a.c. and 1500V d.c.
BS 2950 Specification for cartridge fuse links for telecommunications and light
current applications
BS 1361 Cartridge fuses for a.c. circuits in domestic and similar premises
BS 1362 General purpose fuse links for domestic and similar purposes (primarily for
use in plugs)
BS 3036 Semi-enclosed electric fuses (ratings up to 100A and 240V)
BS 3535/BS EN
60742
Safety isolating transformers
BS 4293/BS EN
61008–1
Residual current operated circuit breakers
BS 4752 Switchgear and controlgear up to 1000V a.c. and 1200V d.c.
BS 5486 Factory built assemblies of switchgear and controlgear
Protection 163
consequences. However, their time-current characteristics are such that they do not
provide real protection against sustained low-level overloads. For example, a 30A HBC
fuse will carry 40A for a long period. This may harm the cables of the permanent wiring
if they have been sized closely to the protective device rating. An electric motor
overloaded to this extent would however burn out.
The IEE Regulations require overload protection which will operate before the current
exceeds 1.45 times the current-carrying capacity of the smallest conductor in the circuit.
If protection against smaller overloads than this is needed it has to be supplied with the
equipment which needs the protection. The most usual case is that of an electric motor
which is protected against overload by an overload relay in the motor starter. The usual
type of overload relay contains a heater element and a bi-metal strip in each motor line
conductor. An excessive current causes the bi-metal strip to deflect, the amount of
deflection depending on the magnitude of the current and the time for which it flows.
When the deflection reaches a predetermined amount the bi-metal operates the tripping
mechanism, which opens the coil circuit, and this in turn causes the main contacts to
open.
Such an overload protection has a time-current characteristic of the form shown in
Figure 9.15. It will be seen that the starter with this overload device
Figure 9.15 Motor starter
characteristics
will carry a starting current eight times the full load current for 6s. A fuse of the same
rating would not do this.
A motor starter must be capable of operating several thousand times in its working
life. A design which enables it to do so places a limit on the magnitude of the current it
can break. Thus while it can deal with overloads it cannot safely break a short circuit. For
this reason, the starter which protects the motor against overloads must itself be protected
by a fuse to deal with short circuits. The fuse will at the same time protect the permanent
wiring against short circuits.
Now a fuse of the same rating as the motor will not carry the starting current for long
enough for the motor to run up to speed. The fuse backing up the starter must, therefore,
have a rating higher than the normal full load running current of the motor. Motor
manufacturers and starter manufacturers provide information which enables the fuse to be
correctly chosen. Table 9.1 is a selection from such data. It is also now common practice
for manufacturers to quote a motor rating for fuses; this is the running current for which
the fuse should be used.
The arrangement of fuse, starter and motor is shown in single line diagrammatic form
in Figure 9.16. Logically, cables ab and cd are protected against
Table 9.1 Fuse ratings for motor circuits
Type of starter Overload release rating-amps Fusing rating amps
Direct on 0.6 to 1.2 5
1.0 to 2.0 10
1.5 to 3.0 10
2.0 to 4.0 15
3.0 to 6.0 20
5.6 to 10.0 30
9.0 to 15.0 40
13.0 to 17.0 50
Star-delta 4.0 to 7.0 15
6.0 to 10.0 20
9.0 to 17.0 30
1 6.0 to 26.0 40
22.0 to 28.0 50
Figure 9.16 Motor protection
overload by the overload device in the circuit. For example, we can see from Table 9.1
that a motor with a running current of 3A and a starter overload set at 3A will need a
back-up fuse of 15A. The cable is protected against short circuit by the fuse. The time to
raise the temperature of the cable to its limit is given by t=K2S2/I2. Therefore we can use
the formula to determine if the cable is protected against short circuit, and to determine if
the starting current and starting time will raise the temperature of the cable above the
limit value.
One can, however, take advantage of the fact that the starter protects the cable as well
as the motor. In theory, if a very high resistance fault developed on the cable between a
and b neither the fuse nor the starter would open the circuit, and damage might result. In
practice, the chances of this happening are so small that they may be neglected. The usual
practice is to make cables ab and cd such that their current rating is equal to the
maximum setting of the overload protection device fitted in the starter. This is probably
one of the few cases in which it is in order for the cable to have a rating less than that of
the fuse protecting it, and it may be done only where there is a motor starter incorporating
Design of electrical services for buildings 150
an overload relay. One other case is where an overload cannot occur, such as on an
immersion heater circuit, where the heater is working or it is not. The circuit must
however be protected against short circuit.
Protection of persons
People using the building have to be protected against electric shock. They would get a
shock if they came into contact with live parts, and in considering protection a distinction
is made between direct and indirect contact. Direct contact is contact by a person with a
live conductor which is intended to carry current in normal operation. The normal
protection against this is the provision of insulation on all current-carrying cables, and
enclosing terminals and connections
Indirect contact is contact with exposed conductive parts which are not intended to
carry current but have become live as a result of a fault. Such a fault is indicated in
Figure 9.12a. When it occurs, the metal case of an appliance which a person is likely to
handle is raised towards line potential and will cause a shock if it is touched by someone
using the appliance. Protection is provided by the fact that the case is earthed and that a
protective device will disconnect the circuit as soon as a fault current flows to earth.
It is possible that a fault of this type will occur while a person is holding an appliance.
In that case he will be subjected to a dangerous potential during the time it takes for the
protective device to operate and disconnect the supply. For this reason BS 7671 stipulates
the time in which the device must operate, which is 0.4s for 230V circuits serving socket
outlets and 5s for circuits serving only fixed equipment. The reason for the difference is
that a person is likely to have a firmer grip on a portable appliance plugged into a socket
outlet than on a fixed appliance which he is only likely to touch casually. There is a
circumstance where the time to disconnect a socket circuit may be increased to 5s.
A current would also flow through a person if he came into contact with two separate
pieces of metal at different potentials. In the event of a fault such a potential difference
could exist between the exposed metal case of an electrical appliance and earthed
metalwork which is not part of the electrical system, termed extraneous conductive parts,
such as structural steelwork, or a central heating radiator. To avoid danger arising in this
way all extraneous conductive parts must be linked (bonded) to the main earthing
terminal of the installation. The conductors by which this is done are known as main
equipotential bonding conductors and methods for determining its size are given in the
IEE Regulations. This causes all extraneous conductive parts connected to the main
earthing terminal to raise in potential, when a fault occurs on a circuit, creating an
equipotential zone.
Earth protection
As we have seen, fuses and CBs react to short circuits. If they are to provide protection
against faults to exposed conductive parts, the wiring must be such that the fault produces
the same conditions as a short circuit, namely a large excess current in the line conductor.
This will happen only if the impedance of the path taken by the fault current is low
Protection 151
enough. The path is indicated by a-b-c-d-f-g-a in Figure 9.12a/b and is known as the earth
fault loop path; its impedance is known as the earth fault loop impedance.
An earth fault occurs when a phase conductor touches exposed metalwork of an
installation termed an exposed conductive part, and so raises the metal to a dangerous
potential. If a fuse or CB is to be used to clear a fault of this nature, then the fault must
immediately produce a current large enough to operate the fuse or breaker quickly. To
achieve this, all exposed metalwork which, in the event of some fault, could conceivably
become live, is earthed and the earth path is designed to have low impedance. When a
fault occurs, current flows through circuit protective conductors to earth, and because of
the low impedance in the earth fault loop path, the current is large enough to operate the
protective device. It must also be large enough to operate the device within the time
stipulated above. The Regulations give details of the maximum earth fault loop
impedance which can be allowed to ensure operation within the required times for fuses,
CBs and RCBOs.
Since an earth fault raises exposed conductive parts above earth potential it creates the
possibility of a voltage existing between such metal, and nearby metalwork such as
structural metalwork, which introduces earth potential, and is not part of the electrical
installation, and is termed an extraneous-conductive-part, and not affected by the fault
and therefore still at earth potential. Although the fuse or CB will clear the fault, that is to
say remove it, the operating time of the fuse or CB may be long enough to cause danger
of electric shock. One way to prevent danger is to bond the two sets of metalwork
together. We have already mentioned this in Chapter 6. In any event the IEE Regulations
require the main earth terminal at the incoming supply intake to be bonded to the
metalwork as shown in Figure 9.12b, of any gas or water services as near as possible to
the point at which those services enter the building.
The policy of protecting against earth faults by making sure that they produce a short
circuit and, therefore, operate the fuse or circuit breaker has been described as ‘chasing
the ampere’. With ever-increasing demand for electricity, circuit ratings and their fuse
ratings are becoming ever higher and the short circuit current needed to operate the fuse
or circuit breaker becomes higher. Thus ever-lower values are required for earth loop
impedances and the fault currents which the fuses have to break become higher. The
opinion has been expressed that this method of protection will not be able to keep pace
with the consequences of increasing electrical loading.
However, at the present time in urban areas where the electricity company provides an
effective earth to the sheath of the service cable (TN-S), or by connecting the main
earthing terminal to the supply neutral (TN-C-S) there is no difficulty in achieving an
earth loop impedance of less than 1.0ohm. It may perhaps not be possible to do this as
long as steel conduit is relied on as the earth return path, but it can certainly be done by
the use of a separate circuit protective conductor. Because of the need for low earth
impedances separate protective conductors should nowadays always be used on new
installations. Although not strictly necessary, since the cross-sectional area of conduit is
more than adequate to satisfy the regulations.
If for any reason the earth loop impedance cannot be made low enough it becomes
necessary to use a residual current circuit breaker. An RCD which will operate on an
earth fault current of 30mA will react on a 230V circuit even if the earth loop impedance
Design of electrical services for buildings 152
is 480ohms and it is inconceivable that values anything like as high as this would ever be
found on a practical installation.
Temporary installations are likely to have higher earth loop impedance paths than
permanent ones, and it is very common to use earth leakage protection for the temporary
wiring on building sites, particularly in rural areas.
It should be noticed that earthing of exposed metalwork is still necessary when an
RCD is used. This can be shown by reference to Figure 9.12a/b. Suppose the earth return
path f-g did not exist, then normal current would flow through a-b-c-e-g-a and there
would be no imbalance between line and neutral for a current operated CB to detect.
Nevertheless the exposed metalwork, connected through the fault to the conductor at d,
would be at a potential given by
and depending on the relative impedances of the various parts of the earth fault loop
impedance path, this could be anything up to almost full line voltage.
We refer yet again to Figure 9.12a. If we assume that the impedance of the transformer
winding is negligible, the value of the earth fault loop path Zs, is 0.1+0.1+0.2+0.2=0.6Ω.
If=Uoc/Zs
where Uoc is the open circuit voltage of the transformer, the fault current would be
If=240/0.6=400A
and the potential drop from a to c is
Ua−c=If×Za−c
Ua−c=400×(0.1+0.1)=80V
The voltage of the exposed conductive part will be
240–80=160V above earth
If the faulted piece of equipment is touched by a person simultaneously with the central
heating radiator, the person would receive a 160V shock.
Now consider Figure 9.12b, with the main equipotential bonding in place. Assuming
the earth fault current remained the same as without bonding, the volts drop d-f would be
Ud−f=If×Zd−f
Ud−f=400×0.2=80V
The potential of the main earthing terminal will be
Uoc−(Ua−c+Ud−f)
240−(80+80)=80V above earth potential
Now with the main equipotential bonding in place, the voltage of the central heating
radiator will be the same as the main earthing terminal, that is 80V. Therefore anyone in
contact with the faulted piece of equipment and the central heating radiator would receive
a shock voltage equal to the potential difference between the faulted piece of the
Protection 153
equipment and the radiator, which is 160V−80V=80V. Bonding has reduced this from a
value without bonding of 160V. The reason for equipotential bonding is to create an
equipotential zone. Exposed conductive parts are earthed to cause rapid disconnection. It
is important to differentiate between earthing and equipotential bonding. It would be
counter-productive to bond plastic pipework and items of metalwork such as aluminium
window frames that do not introduce a potential in the worst case.
The values of impedance of the earth fault loop path are for explanation only and are
not meant to be typical.
If the conditions prevent the use of fuses and circuits breakers, for shock protection,
such as high earth loop impedances, is possible to use RCDs since these devices do not
require a high current to operate quickly. BS 7671 require an RCD to be used to protect a
socket outlet which is intended for portable equipment to be used outside the building.
This is for additional direct contact protection They also require all socket outlets to be
protected by RCDs when earthing is provided by the user’s own electrode (a TT system)
and not by connection to an earth provided at the supply authorities’ transformer. It is left
to the designer’s discretion whether a separate RCD is provided for each socket outlet,
whether each circuit is protected by an RCD or whether all circuits on a distribution
board are protected by an RCD on the supply side of the board. Although BS 7671
requires that due account be taken of the operation of a single protective device, an RCD
on the main supply of a distribution board would trip the whole board should a fault
occur on one circuit.
It is the designer’s responsibility to use experience and professional judgement in
selecting the scheme of protection.
Earth monitoring
Protection through earthing of exposed metalwork will fail if there is a break in the earth
continuity conductor. It is possible to add an extra cable which will monitor the earth
continuity conductor and break the main circuit if the earth continuity fails. The basic
scheme of such a circuit is shown in Figure 9.17.
The monitoring circuit incorporates a low-voltage transformer, a relay, and a
protective circuit breaker. An additional pilot lead is required to the appliance or portable
tool. The low voltage is used to drive a current round the loop formed by the earth
conductor, a section of the metallic housing of the appliance, the pilot conductor and the
relay coil. This current holds in the relay and thus keeps the coil of the circuit breaker
energized.
If there is a break in the earth continuity conductor, or indeed anywhere in the pilot
circuit, the low-voltage current fails, the relay is de-energized, the main contactor coil
becomes de-energized and the circuit breaker opens
Design of electrical services for buildings 154
Figure 9.17 Monitored earth leakage
protection
the main circuit and cuts off the supply to the appliance. It will be noticed that this
monitoring circuit merely checks that the earth continuity conductor is sound; it does not
add to the basic earth leakage protection.
Earth electrodes
In normal earthing, the earth and the neutral are quite separate. The load current flowing
through the neutral must cause a potential difference between the two ends of the neutral.
Since the end at the supply transformer is earthed, the end at the consumer’s service
terminal must inevitably be at some potential above earth. It cannot, therefore, be used as
an earth point.
Nevertheless, an effective earth has to be found for the earth continuity conductors of
the permanent installation in a building. In urban areas the sheath of the electricity
company’s service cable is normally used for this purpose, but there is no obligation on
the company to provide an earth, and in rural areas where the supply may be by overhead
cable, it may not be possible for them to do so. In such cases, the consumers must provide
their own earth electrodes and the design of these become part of the design of the
building installation.
An earth electrode is a metal rod, which makes effective contact with the general mass
of earth. A common type consists of a small diameter copper rod which can be easily
driven to a depth of 6m or more into ground reasonably free of stones or rock. The soil
remains practically undisturbed and in very close contact with the electrode surface.
Since resistivity is lower in the deeper strata of earth and not very affected by seasonal
conditions, deep driving gives a good earth. Rods of this type are practically incorrodible.
Also it is easy to get access to the connection at the top of the electrode. A typical
arrangement is illustrated in Figure 9.18.
Protection 155
Where the ground is shallow but has low resistivity near the surface, a plate electrode,
either of copper or, of cast iron, can be used. When the soil resistivity is high, a cast iron
plate can be used with a coke surround. This method is illustrated in Figure 9.19.
Standard cast iron plates are made for use as earth electrodes. They are complete with
terminals for the earth continuity conductor. These terminals consist of two copper
sockets each secured by a drift-pin, the two being joined by a tinned copper strand to
which the earth conductor is bound and soldered. The completed connection is sealed and
covered in bitumen before the electrode is buried. It is, therefore, not as accessible as the
connection of the rod type electrode.
Long copper strip can also be used as an earth electrode, and the method of doing this
is shown in Figure 9.20. It will be seen from this that the strip is a useful type of electrode
for shallow soil overlying rock. Strip may be arranged in single lengths, parallel lengths
or in radial groups. Standard strip is commercially available for use as earth electrodes.
Figure 9.18 Copper rod electrode
Figure 9.19 Cast iron plate electrode
Design of electrical services for buildings 156
When current flows from the electrode into the soil, it has to overcome the resistance of
the soil immediately adjacent to the electrode. The path of the current is shown in Figure
9.20. The effect is equivalent to a resistance between the electrode and the general mass
of earth, and this resistance is the resistance of the electrode. Furthermore, the surface of
the ground near the electrode becomes live when current flows from the electrode to earth
and Figure 9.21 shows a typical surface distribution near a rod electrode. It can be seen
that an animal standing near such an electrode could have a substantial voltage applied
between its fore and hind legs, and in fact fatal accidents to livestock from this cause
have been known. The earth electrode should, therefore, be positioned well out of harm’s
way. It should
Figure 9.20 Copper strip electrode
Figure 9.21 Current from electrode
into earth
perhaps be noted that the deeper the electrode is below the ground, the smaller will be the
voltage gradient at the surface.
The effectiveness of earth protection depends on the low resistance of the electrode
when current flows through the electrode into the soil. This resistance cannot be
accurately predicted in advance and must be checked by testing. After installation, the
Protection 157
electrode should be periodically examined and tested to ensure that its initial low
resistance is being maintained. The scheme for testing an electrode is shown in Figure
9.23. The electrode under test is indicated by X; two auxiliary electrodes, Y and Z are
driven in for the test. Y must be placed sufficiently far from X for the resistance areas not
to overlap, and Z is placed approximately halfway between X and Y. The test electrode X
is disconnected from its normal continuity conductor and connected to the test instrument
as shown in Figure 9.23. A low-voltage alternating current is passed between X and Y.
The current is measured and so is the potential between X and Z. The resistance of the
earth electrode is given by the dividend of voltage and current. Check readings are taken
with the electrode Z nearer to and further from electrode X, and the results are
Figure 9.22 Voltage at surface of
ground due to rod electrode
Design of electrical services for buildings 158
Figure 9.23 Earth electrode resistance
test
accepted only if all three readings are substantially the same. If they are not, the test must
be repeated with a greater distance between X and Y.
Protective multiple earthing
This is an alternative method of earthing in which the neutral of the incoming supply also
forms the earth return path. In other words, instead of the neutral and earth of the
incoming supply being separate, they are combined to form a TN-C-S system (definitions
in BS 7671 explains the systems in use). The supply authority is required to maintain the
resistance between the neutral conductor and earth to a maximum of 10Ω To do this the
supply authority earth the supply neutral at various multiple points, protective multiple
earthing (PME).
The installation within the building is carried out in exactly the same way as for any
other system, and separate earth continuity conductors are used. The main earthing
terminal at the intake is not, however, connected to a separate earth return, but connected
to the neutral of the incoming service cable.
Because with this system the neutral is relied on as the earth, there must be no fuses,
cut-outs, circuit breakers or switches anywhere in the neutral. In the UK an area
electricity company may not adopt PME without the permission of the Secretary of State
for the Environment, and stringent requirements are made to ensure that the neutral
conductor is adequate to carry earth fault currents, that it is truly kept at earth potential
and that it is protected against breaks in continuity. The permission of British Telecom is
also required. This is because the currents into and through the ground at the points of
multiple earthing could cause interference to adjacent telephone and telegraph cables in
the ground.
After early hesitations, PME is becoming increasingly widespread in the UK.
Experience has shown that it is in practice as safe in certain conditions as previously used
methods and it has important advantages. In rural areas it makes it unnecessary for
consumers to have their own earth electrodes and therefore removes the risks of earth
Protection 159
electrodes in the care of unqualified persons. In urban areas it makes the electricity
companies’ distribution network cheaper. This system however must not be offered to
boat or caravan supplies, since there is a small risk of the neutral becoming disconnected.
This would cause the shell of the caravan to become live under earth fault conditions. It is
also not to be used on petrol station supplies.
Double insulation
Electrical appliances connected to the permanent wiring of a building, whether through
plugs into socket outlets or by means of permanent connections, must themselves be
protected against faults. There is need for protection against a fault developing on the
appliance itself.
When protection of the permanent wiring depends on earthing, the same principle can
be used for the appliance. The previous discussion has assumed this, and has proceeded
on the basis that the metal casing of an appliance is effectively connected through the
earth pin of the plug to the earth connection in the socket. There is, however, an
alternative method of achieving safety of appliances which does not depend on earthing
the appliances, and this is known as double insulation. For purposes of exposition
appliances of the class known as all insulated may be considered as special cases of
double insulation.
Double insulation consists of two separate sets of insulation. The first is the functional
insulation, which is the ordinary insulation of the conductors needed to confine the
current to the conductors and to prevent electrical contact between the conductors and
parts not forming part of the circuit. The supplementary insulation is additional to and
independent of the functional insulation. It is an entirely separate insulation which
provides protection against shock in the event of the functional insulation’s breaking
down.
Another term which we have to explain is reinforced insulation. This is an improved
functional insulation with such mechanical and electrical properties that it gives the same
degree of protection against shock as does double insulation.
An all-insulated appliance is one which has the entire enclosure made of substantial
and durable insulating material. In effect, it is a double-insulated appliance in which the
supplementary insulation forms the enclosure.
Both earthing and double insulation provide protection against the breakdown of the
primary functional insulation. Earthing depends, in the ways already described, on
ensuring that if the functional insulation fails, exposed metalwork will be prevented from
rising significantly above earth potential. If the case of the appliance can be made of
insulating material which is robust enough to withstand all conditions in which it is to be
used, then there is no danger of shock even if the functional insulation fails. Such an
appliance is an all-insulated one, and this form of construction gives adequate safety, but
some appliances must have exposed metal; for example, hedge clippers and portable
drills. Other appliances are so large that it is impracticable to make an insulating case
strong enough to withstand ordinary usage without making the whole appliance too heavy
and cumbersome; for example, a vacuum cleaner. In these cases, double insulation can be
used. It provides a second barrier of insulating material between conductors and exposed
Design of electrical services for buildings 160
metal parts. The presence of this additional barrier is a protection against the failure of
the functional insulation and makes it unnecessary to earth the exposed metal. Doubleinsulated
equipment is designed so that in general two independent sections of insulation
must both fail before any exposed metal can become live. The functional and protective
insulation must be so arranged that a failure of either is unlikely to spread to the other.
They ought, therefore, to be mechanically distinct, so that there is a surface of
discontinuity between them.
The principle of double insulation is illustrated in Figure 9.24, in which the phase and
neutral conductors each consist of a cable with ordinary functional insulation. The casing
is itself of insulating material and forms the supplementary insulation. If a fault develops
on the functional insulation, a
Figure 9.24 Double insulation
short circuit may develop between the live and neutral conductors but the supplementary
insulation will prevent the metal handle on the outside from becoming live.
An appliance which is double insulated by the use of either supplementary or
reinforced insulation should not be earthed and is not provided with an earth terminal.
When it is connected to a standard three-pin plug the earth pin of the plug is left
unconnected. Double insulation gives the same degree of protection against shocks as
earthing, and makes that protection independent of the earth loop impedance. It also gives
protection against high impedance faults to earth on the appliance itself, and thus guards
against fires caused by local overheating at the appliance.
Double insulation is a means of making an appliance safe. It cannot give protection
against faults on the permanent wiring in the fabric of a building. Thus, if there are any
metal parts used in the wiring installation, they must still be earthed. Such parts would be
conduit, switches, distribution boards, control panels and so on. It is hardly practicable to
carry out an installation of any size without the use of some metal components so that the
principle of double insulation cannot be applied to the permanent wiring. Moreover, the
designer and erector of the services in a building cannot control what appliances may be
connected to the service during the life of the building. Even if all the appliances in a
building when it is first put into use are of the double-insulated pattern, so long as
Protection 161
appliances depending on earth protection (and all electric kettles do) remain in existence,
one of them could at some time be connected to the system. Therefore, the system must
have an effective earth for the appliances to be linked to.
One of the strongest arguments for the use of double-insulated and all-insulated
appliances is that it makes the safety of the portable appliance independent of the
installation to which it is connected. The manufacturer knows that the user is protected by
the design of the appliance and this safety is given without reliance on the earthing
system of the building in which the appliance is to be used, over which the appliance
manufacturer has, of course, no control. The manufacturer has thus gone a long way
towards protecting the user against the latter’s own ignorance of the safe way of
connecting an appliance to a defective system, which could happen either through
ignorance or through inadvertence.
At the same time, if all appliances were known to be double insulated, the designer of
the electrical services could concentrate on protective devices for the system being
designed without having to consider what protection to leave for appliances which the
occupants will bring along later. Unfortunately, this is not the case, and the designer must
consider the interaction of other people’s appliances and the system. In particular,
designers should perhaps reflect on what kind of flexible cords and extension flexes are
likely to bridge the gap between their system and a well-made and safe appliance.
The IEE Regulations do not allow the designer to assume that equipment connected to
socket outlets will be double insulated and they must therefore be provided with earth
continuity conductors.
Portable tools
We shall conclude this chapter by considering the special problems of portable tools.
These are a very important class of appliances, especially in some factories which use
them in large numbers, and they are subject to certain difficulties of their own. When a
tool stalls, it is likely to blow a fuse, and after this has happened a few times the operator
or maintenance engineer decides that to avoid replacing the fuse every time the tool is
momentarily overloaded, a larger fuse will be substituted, capable of carrying the
overload. Unfortunately, it is also capable of carrying a substantial earth fault current, for
an increased length of time, and the operator can be electrocuted before the oversize fuse
clears the fault.
The flexible cables of portable tools come in for exceptionally rough usage and are
therefore particularly liable to develop faults. At the same time people using a portable
tool in a factory are likely to be either standing on, or else touching or close to,
substantial metal parts, so that they have a low impedance to earth. Thus an earth fault is
more probable on a portable tool and its effects are more likely to be immediate and
serious than on almost any other kind of appliance. It ought, therefore, to receive extra
care in protection but in fact may receive less than average attention.
To overcome these difficulties, factories which use portable tools in large numbers
often install a special low-voltage supply to serve them. Typical voltages used are 110V
and 55V to earth. Of course, the tools have to be wound for this supply and it is a
disadvantage that the motors are bulkier and heavier.
Design of electrical services for buildings 162
Another solution is to provide protection against sustained overload by means of
circuit breakers with time-delay characteristics such that they will not operate on
temporary overloads, and separate protection against earth faults by residual current
circuit breakers. A fuse is then needed only for back-up protection, if at all, and can be
large enough not to blow when the tool temporarily stalls. It is also advisable to use earth
monitoring on the earth conductors to portable tools. This makes it necessary for the
flexible cables to have an extra conductor and for the tools themselves to have an extra
connection.
None of these precautions is very suitable for the home handyman, who may not have
enough understanding of electrical theory to appreciate the need for them. It is probably
better for portable tools to be protected by double insulation rather than by reliance on
effective earthing. A similar case can be argued for making domestic appliances double
insulated. Many modern appliances, of which we may quote vacuum cleaners and
hairdryers as examples, are nowadays made with double insulation.
Standards relevant to this chapter are:
BS 88 Cartridge fuses for voltages up to and including 1000V a.c. and 1500V d.c.
BS 2950 Specification for cartridge fuse links for telecommunications and light
current applications
BS 1361 Cartridge fuses for a.c. circuits in domestic and similar premises
BS 1362 General purpose fuse links for domestic and similar purposes (primarily for
use in plugs)
BS 3036 Semi-enclosed electric fuses (ratings up to 100A and 240V)
BS 3535/BS EN
60742
Safety isolating transformers
BS 4293/BS EN
61008–1
Residual current operated circuit breakers
BS 4752 Switchgear and controlgear up to 1000V a.c. and 1200V d.c.
BS 5486 Factory built assemblies of switchgear and controlgear
Protection 163
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