Chapter 9
Protection
Introduction
It is a truism that electricity is dangerous and can cause accidents, if not treated with
respect. A large part of any system design is concerned with ensuring that accidents will
not happen, or that if they do, their effects will be limited. It might be reasonably argued
that these considerations are the most important part of a design engineer’s task. In the
previous chapters, we have spoken about choice of accessories, selection of cables and
their correct sizing, the arrangement of outlets on a number of separate circuits and the
proper ways of installing cables and we have pointed out the need for protecting cables
against mechanical damage. If these matters are given the care they deserve, the
likelihood of faults on the electrical installation will be small. Nevertheless, it is still
necessary to provide protection against such faults as may happen.
The general principle of protection is that a faulty circuit should be cut off from the
supply and isolated until the fault can be found and repaired. The protective device must
detect that there is a fault and must then isolate the part of the installation in which it has
detected the fault. One could perhaps suggest many theoretical ways of doing this, but it
is also necessary that the method adopted should bear a reasonable proportion to the cost
of the whole installation. Historically, the methods, which could be adopted at any time,
depended on what devices could be economically manufactured at that time, but once a
method has been adopted it tends to remain in use and newer products do not completely
supersede it. Enthusiasts sometimes stress the advantages of a new idea while forgetting
that the older method had some favourable features which the new one does not match.
The result of developments is that at present there are several protective devices available
and except for BS 3036 Fuses (rewireable) there appear to be no overriding grounds for
preferring any one to the others.
The devices available restrict the type of protection that can be given. A logically ideal
system of protection against all possible faults cannot be made economically, and the
protection designed must make use of the equipment commercially available. This can
lead people to argue from the available techniques to the faults to be guarded against, and
in the process become so obsessed with the ease of guarding against an improbable fault
that they forget the importance of protection against a more likely one. It seems more
satisfactory to start by considering the faults that may happen.
Two dangers to be prevented are fire and shock to people and livestock. In turn these
dangers can arise from three kinds of fault, namely a short circuit, an overload and a fault
to earth.
If through a fault in the wiring or in an appliance the line or phase and neutral
conductors become connected, the current that flows is limited only by the sum of the
resistance of the cables of the permanent wiring and the impedance of the accidental
contact between the two cables the latter generally being regarded as negligible. If the
fault connecting the line and neutral has a negligible impedance the two conductors are
effectively short circuited. The current that flows through the conductors is a short-circuit
current, and is very high and if allowed to continue would burn the insulation. The high
conductor temperature resulting from the excessive current could start a fire. If the excess
current continues to flow further after the insulation has been damaged, there is also a
possibility that the conductor might touch exposed metal and give a shock to anyone
touching the metal.
If the fault that connects the line and neutral has some impedance the current flowing
through the fault and conductors is less than the current in a short circuit of negligible
impedance. It is still likely to be higher than the maximum current the circuit can safely
carry and if it persists over a period of time, it can cause serious damage. When an
overcurrent is flowing in a circuit, which is electrically sound, it is described as an
overload. An overload can be caused by an electric motor stalling.
When an excessive mechanical load is imposed on an electric motor it continues to run
but draws a higher than normal current from the supply. The circuit supplying the motor,
therefore, carries a higher current than it has been designed for, and although it is not as
high as a short circuit current, it can still be high enough to be dangerous. A fault in the
internal wiring of a motor can also cause an electrical overload, although if it is serious
enough it is likely to amount to a short circuit.
A fault to earth occurs if through some defect the line conductor becomes connected to
earthed metalwork. The effect is similar to a short circuit, but whereas a short circuit will
not raise exposed metalwork, termed exposed conductive parts, above zero potential, an
earth fault will. We can see this by looking at Figure 9.1 which shows diagrammatically
an electric fire with an earthed metal case. Suppose the fire becomes damaged and the
phase cable touches the case at point A. A current will flow through the case and circuit
protective conductor to earth at point B, which would normally be the earth at the
electricity distribution company’s transformer.
Figure 9.1 Earth fault
Design of electrical services for buildings 132
Now let
Uoc=Open circuit supply transformer voltage
I=Fault current flowing
ZL=total impedance from line connection of supply transformer through line
conductor, fault and the circuit protective conductor to earth connection at supply
transformer
Zc=impedance of earth path from fault back to earth connection
Zs=Total impedance of the fault circuit.
Then the current flowing will be Uoc/Zs, and the voltage drop between A and B will
be IZc=UocZc/Zs. Now Zc/Zs is likely to be of the order of 0.4 to 0.5, so that on a Uoc of
240V the metal case at A will be raised to about 100V.
We cannot explain how electrical circuits in buildings are protected against short
circuits, overloads and earth faults without referring to the various protective devices
which can be used. To make our account intelligible we propose first of all to describe
the devices available and then go on to discuss how they are applied in practice.
Rewirable fuses
The earliest protective device consisted of a thin fuse cable held between terminals in a
porcelain or bakelite holder. It is illustrated in Figure 9.2. It is inserted in the circuit being
protected and the size of fuse cable is matched to the rating of the circuit. The fuse is
designed so that if the current exceeds the rated current of the circuit the fuse cable melts
and interrupts the circuit. Although commonly called rewirable fuses, their correct name
is semi-enclosed fuses, and it is by this name that they are referred to in British Standard
3036 and in the IEE Regulations BS 7671.
Figure 9.2 Rewirable fuse
Protection 133
HRC fuses
The rewirable fuse has limited breaking capacity. If a very large current flows the fuse
cable melts very rapidly and a large amount of energy is released. It can be large enough
to cause serious damage to the fuse carrier. It was found that some of this energy can be
absorbed by a packing of inert fibrous or granular material wound with cable, and this led
to the development of the cartridge fuse, illustrated in Figure 9.3.
The fuse element is mounted between two end caps which form the terminals of the
complete fuse link. The fuse element is surrounded by a closely packed silica filler and
the whole is contained in a ceramic casing. When the fuse element melts, or blows, the
silica filler absorbs the energy. Fuses of this type are known variously as high rupturing
capacity (HRC) or high breaking capacity (HBC) fuses or, less technically, as cartridge
fuses.
Operation of fuses
Both rewirable and cartridge fuses work in a similar way. The current heats the fuse
element until the latter melts, after which there is an arc between the ends of the broken
element, and finally the arc extinguishes and the circuit is completely interrupted.
The time taken for the fuse to melt depends on the magnitude of the current, and a fuse
will have a characteristic curve of time against current. A set of such characteristic curves
is shown in Figure 9.4. The total operating time is the sum of the melting, or pre-arcing,
time and the time during which there is an arc, known as the arcing time. The arcing time
varies with the power factor and transient characteristics of the circuit, the voltage, the
point in the alternating cycle of supply at which the arcing commences and on some other
factors. It is not, however, of significant length except for very large overcurrents when
the total operating time is very short.
Figure 9.3 Cartridge fuse
Design of electrical services for buildings 134
Figure 9.4 Time-current characteristics
of HRC fuses
The minimum fusing current is the minimum current at which a fuse will melt, that is
to say the asymptotic value of the current shown on the time-current characteristics. The
current rating is the normal current. It is the current stated by the manufacturer as the
current which the fuse will carry continuously without deterioration. It is also referred to
as current carrying capacity and other similar terms. The fusing factor is the ratio
When a short circuit occurs, the melting process is adiabatic and the melting energy is
given by
where
W=melting energy
i=instantaneous current
R=instantaneous resistance of that part of element which melts on short circuit
t=time
Protection 135
tm=melting time.
R is assumed to vary in the same manner with i and t for all short circuits and the
quantity
Figure 9.5 Short circuit l2t
characteristics
is approximately constant for the pre-arcing time of a fuse. It is often called the prearcing
I2t. It is this quantity which determines the amount of excess energy passing
through the circuit before the circuit is broken and it is particularly important in the
protection of semiconductor circuits and the reduction of overheating in power circuits.
Typical I2t characteristics are shown in Figure 9.5.
Oscillograms of the operation of a fuse are shown in Figure 9.6.
Design of electrical services for buildings 136
CB
An alternative to a fuse element which melts when overheated is a circuit breaker. A
circuit breaker (CB) is one which has a rating similar to that of a fuse and is about the
same physical size as a fuse carrier of the same rating. A typical circuit breaker is shown
in Figure 9.7.
It has a magnetic hydraulic time delay, and the essential component is a sealed tube
filled with silicone fluid which contains a closely fitted iron slug. Under normal operating
conditions the time delay spring keeps the slug at one end of the tube (a).
When an overload occurs, the magnetic pull of the coil surrounding the tube increases,
and the slug moves through the tube, the speed of travel depending on the magnetic force
and, therefore, on the size of the current, (b). As the slug approaches the other end of the
tube the air gaps in the magnetic circuit are reduced and the magnetic force is increased
until it is great enough to trip the circuit breaker, (c). With this mechanism the time taken
to trip is inversely proportional to the magnitude of the overload.
When a heavy overload or a complete short circuit occurs, the magnetic force is
sufficient to trip the circuit breaker instantaneously in spite of the large air gaps in the
magnetic circuit, (d). In this way, time delay tripping is achieved up to about seven times
rated current, and instantaneous tripping above that level.
An alternative design has a bi-metal element which is heated by the circuit current and
operates the trip catch when it deflects. A simple magnetic coil is included to trip the
catch on short circuit, so that the resulting characteristic is similar to that of the magnetichydraulic
type. The thermal-magnetic type is liable to be affected by the ambient
temperature. If several CBs are mounted inside a closed distribution board, the heat from
the currents passing through all the circuits in the board will raise the temperature inside
the enclosure. Thermally operated CBs used in this way may have to be de-rated to
prevent their tripping before an overload occurs. The effect of ambient temperature can,
however, be reduced and the need for de-rating obviated by designing the bi-metal to run
at a relatively high temperature. Typical time-current characteristics of CBs are given in
Figure 9.8.
The CB has a toggle switch by which it can be operated manually. This switch is
thrown into the off position when the overload device trips the
Protection 137
Figure 9.8 Time-current curves for
CBs
breaker, and the CB is reset by the same switch. CBs can, therefore, combine the
functions of switch and fuse, and in some cases this is a very useful and economic
procedure. In a factory or store, for example, one may want to control the lights for a
large area from a bank of switches at a single point. If a distribution board with CBs is
placed at this point, it is possible to dispense with a separate bank of switches.
RCD
Another device frequently used is the residual current device (RCD). This is a circuit
breaker which detects a current leaking to earth and uses this leakage current to operate
the tripping mechanism. The leakage current is a residual current and gives the device its
name. It should be noted here that this device will not protect against short circuit and
overload. Residual current devices which do incorporate overcurrent protection are
referred to as RCBOs.
The principle of the RCD is shown in Figure 9.9. The load current through the circuit
is fed through two equal and opposing coils wound on a common transformer core. On a
healthy circuit, the line and neutral currents are the same and produce equal and opposing
fluxes in the transformer core. However, if there is an earth fault, more current flows in
Protection 139
the line than returns in the neutral, and the line coil produces a bigger flux than the
neutral coil. There is thus a resultant or residual flux which induces a current in the
search coil, and this in turn operates the relay and trips the breaker. Some RCDs are
manufactured with electronic circuitry which simulates the above operation.
Figure 9.9 Residual current device
The value adopted for the rated tripping current, which is defined as the out-of-balance
current at which the circuit breaker will trip in less than 0.1s, is a maximum of 30mA for
protection against electric shock. The unit always includes a test button which simulates
an out-of-balance condition by injecting a test current bypassing one of the principal
coils. A resistor limits the magnitude of the test current so that the test also checks the
sensitivity of the breaker. It should be noted that the test switch checks the operation of
the residual current circuit breaker, but does not check the soundness of the earth
continuity conductor. It is a requirement of BS 7671 that if an installation incorporates
one of these devices a notice shall be fixed near the origin of the supply stating that the
device shall be tested quarterly, by the operation of the test button. This is to ensure that
the mechanical parts are kept operational. The author recommends that a notice be also
fixed near to the RCD itself. The origin of the supply may be in a seldom-accessed place.
Isolating transformers
In some applications a double-wound transformer can achieve adequate protection for the
system on its secondary side. The secondary is not earthed and the construction of the
transformer is carefully designed to prevent any possibility of contact between the
secondary and primary windings. The voltage between the line and return connections of
the secondary is fixed, but because no part of it is earthed or connected to any other fixed
Design of electrical services for buildings 140
point, the voltage relative to earth is quite undetermined. In other words the line can be at
230V above earth and the return at earth, or the one can be at 120V above earth and the
other at 120V below earth, or the line can be at earth and the return at 230V below earth,
or any other combination.
Thus, if anyone in contact with earthed metal touches the line conductor, the line is
brought immediately to earth potential and the return drops to 240V below earth. The
same thing happens if through a fault in an appliance a conductor touches earthed metal.
This prevents danger of shock. It is important to note that the system will not ensure
safety if more than one appliance is fed from the transformer. Figure 9.11 shows two
appliances near some earthed metalwork. Suppose a fault develops at A. The line
conductor will be held at earth potential and the return conductor will be at mains voltage
below earth. The circuit continues in operation and there is now no protection against a
second fault occurring at another point such as B.
The chief use of isolating transformers is in shaver units fixed in bathrooms to supply
electric razors. There is practically no possibility of two independent faults occurring in
such an application. For other applications the system must be erected so as to prevent
faults, or protect against faults.
We have now reviewed the equipment that is available to protect electrical
installations in buildings and can go on to consider the nature of the protection that is
needed.
Figure 9.11 Wrong use of isolating
transformer
Capacity of circuit
The methods by which a cable is sized for the duty it has to perform have been explained
in Chapter 4. There is no need to be worried about the effects of a fault on the voltage
drop; it is the high current that has to be guarded against. The protection has to safeguard
the circuit against overcurrent, and an overcurrent is any current higher than the rating of
the cable. Now the cable rating (Iz) must be rated equal to or greater than the nominal
current rating of the protective device (In). Also the nominal current rating of the
protective device must be rating equal to or greater than the normal current of the circuit
concerned (Ib), therefore the following expression must be satisfied
Protection 141
Iz≥In≥Ib
The guiding principle to be followed is that every cable in a permanent installation in a
building must be protected if it is liable to be subjected to overload or short circuit. The
protective device must not have a current rating greater than that of the cable, and in most
cases it will have a rating either equal to or only just less than that of the cable. It follows
that at every point at which a smaller cable branches from a larger one there must be a
protective device to safeguard the smaller cable. In conventional systems, this is provided
by the use of switchgear and distribution boards where a main divides into two or more
sub-mains and where a sub-main divides into a number of final circuits. It is also because
every branch has to be protected, that a fused connection unit is installed whenever a
single branch is taken off a ring main, and the fuse at the origin does not protect the spur.
Fault currents
The current normally plotted on the time-current characteristics of fuses and circuit
breakers is that known as the prospective fault current. This is the current, which would
flow in the circuit if the fuse were not there, and a short circuit or earth fault current
occurred. It is indicated as a dotted line in Figure 9.6. In practice, the fuse will open the
circuit before this prospective current is reached; the fuse is said to cut off and the
instantaneous current attained is called the cut off current.
The wave form of the prospective fault current depends on the position of the fault
within the whole of the supply network, the relative loading of the phases within the
network and whether the supply comes through a transformer or directly from a local
generator. These questions can be of importance to the engineer concerned with
protecting a public supply system, but are of less consequence to the designer of the
services within a building. For the designer’s purposes the simple procedure described
here is adequate and more complicated considerations need not be taken into account in
selecting the protection devices to be used within the building.
The prospective current is defined as the RMS value of the alternating component,
whereas the cut-off current is defined as the instantaneous current at cut-off. These
definitions produce the paradox that the numerical value of the cut-off current may be
greater than the numerical value of the prospective current.
The prospective earth fault current is determined by the supply voltage and the
impedance of the path taken by the fault current. In Figure 9.12, the path at the remote
end of the circuit, is indicated by a-b-c-d-f-g-a. The figure shows diagrammatically the
usual situation in urban installations where the consumer’s earth point is connected to the
sheath of the electricity distribution company’s cable, which is earthed at the transformer
end as on a TN-S system. If the supply voltage is Uoc and the impedance of this path is
Zs, then the prospective earth fault current is given by If=Uoc/Zs. The total impedance Zs
is made up of the impedance of the transformer and the impedance of all the cables in the
path. The impedance of a transformer is almost entirely reactive, and is therefore always
referred to as reactance. In practice, it is found that the reactance of the transformer sets
an upper limit to the fault current that can flow. Thus if a normal 500kVA transformer is
short circuited, the current on the secondary side will be 14000A, while the
Design of electrical services for buildings 142
corresponding figure for a 750kVA transformer is 21000A. These are average figures for
typical transformers and ignore the impedance of the supply system on the primary side
of the transformer. They therefore include a small hidden safety factor. The resistance of
the service cable and its sheath is usually quite low, but even a short length of a final
circuit cable makes a great reduction in the prospective fault current. For example 20m of
1.5mm2 cable in an installation supplied from a 750kVA transformer will limit the fault
current to 1000A.
Figure 9.12a Fault currents
Figure 9.12b Fault currents
Data on cable resistance are given by the manufacturers in their catalogues. Information
is also given in the IEE Guidance Note 1, and the On-Site Guide, the leading makers of
fuses and CBs provide in their catalogues tables and graphs showing the prospective fault
currents with different lengths of cable on the secondary side of standard transformers.
Protection 143
The fault itself usually has some impedance, so that the actual fault current is less than
the prospective fault current. The prospective earth fault current is calculated on the
assumption that the impedance of the fault in Figure 9.12 is zero. The actual fault current
could theoretically be calculated by adding the actual impedance of the fault between d
and e to the impedance used for calculating the prospective earth fault current, and as the
total actual impedance will thus be higher than that of the complete short circuit assumed
in calculating the prospective current, the actual fault current will be less than the
prospective one.
When a fault occurs on an appliance the cables of the final circuit play an important
part in limiting the fault current. Nevertheless, there is always the possibility, even if a
small one, of a low impedance fault immediately load side of the fuse or circuit breaker.
It is this fault which, however unlikely it may be that it will happen, will produce the
highest possible fault current and it is this fault with which the protection must be capable
of dealing.
Discrimination
In an installation having the type of distribution described in Chapter 6, there is a series
of fuses and circuit breakers between the incoming supply and the final outlet. Ideally,
the protective devices should be graded so that when a fault occurs, only the device
nearest the fault operates. The others should not react and should remain in circuit to go
on supplying other healthy circuits. Discrimination is said to take place when the smaller
fuse opens before the larger fuse. This is the desired state of affairs, and when the
unwanted converse situation happens it is said that discrimination is lost.
Figure 9.13 shows the time-current characteristic for a CB serving a final circuit and
for a 60A HBC fuse protecting the sub-main leading to the distribution board on which
the CB is mounted. If the fault current is less than 1200A, the CB will open first. If the
fault current is greater than this, the HBC fuse will blow before the CB can operate. The
resistance of the cables of the final circuit reduces the fault current, so that the further
away from the distribution board a fault occurs, the lower will be the fault current. If the
fault current at the distribution board is, say, 2000A, it will fall below this along the final
circuit from the board and will drop to 1200A fairly soon after the board. A fault on the
board would blow the board fuse but a fault on the final circuit any distance from the
board would leave the board intact and open the CB. The discrimination is said to be
acceptable. If the fault current at the supply end of the final circuit is so high that the
HBC fuse always opens before the CB, there is no discrimination. If there is
discrimination at the load end of the final circuit but not for a fault a short way along of
the final circuit, we say the discrimination is not acceptable. With the characteristic
curves of Figure 9.13, the discrimination would probably be acceptable even for a fault
level at the board of 3000A. The possibility of a fault at the board is small and the actual
fault currents to be cleared will almost certainly be due to faults at appliances. Provided
discrimination is maintained for these faults, its loss on the rare occasions when a fault
occurs on the permanent wiring close to the board can probably be accepted.
Design of electrical services for buildings 144
Figure 9.13 Discrimination
In general, discrimination is a problem only when a system uses all the same devices, or
an appropriate mixture of CBs and fuses. It can be seen from Figures 9.4 and 9.5 that a
fuse will always discriminate against another fuse of a larger rating. A 2:1 rule of thumb
for fuses is sometimes used. That is, a fault on a 32A fused final circuit would cause the
32A fuse to operate first, if it was backed up on the distribution circuit by a fuse of at
least 60A. Manufacturers produce ‘lollipop’ or ‘bulrush’ graphs to compare the relative
energies let through by certain devices. As was previously described, there is a time
required for a cartridge fuse element to heat up and start to melt. This is termed the prearcing
time. When the fuse element begins to melt, the overcurrent, being interrupted,
will cause an arc across the melting element. The overcurrent will eventually be
interrupted. The total operating I2t of the
Protection 145
Figure 9.14 Lollipop graph
fuse is the sum of the pre-arcing I2t and the arcing I2t. For proper discrimination to occur
the total operating I2t of the fuse required to operate must not be greater than the prearcing
I2t of the upstream fuse. If fuse ‘b’ in Figure 9.14 was used to supply a fuseboard
in which fuse ‘a’ was fitted, a fault on the circuit supplied by ‘a’ would cause fuse ‘a’ to
operate, but the total energy let through would cause fuse ‘b’ to be in its arcing sector,
thus weakening the fuse ‘b’. If fuse ‘c’ was used to supply a fuseboard, in which either
fuse ‘a’ or ‘b’ was fitted, a fault on either circuit would cause the lesser fuse to operate
without being in the arcing sector I2t of fuse ‘c’. These lollipop graphs must be consulted
on high fault currents where the fuse will operate in 0.1s or less.
Breaking capacity and back-up protection
A certain amount of energy is released when a circuit is broken, whether by a fuse or by a
circuit breaker. This energy must be absorbed by the device breaking the circuit, and the
capacity to do so limits the current which the device can safely break. The breaking
capacity is defined as the maximum current that can be broken at a rated voltage. In
switchgear practice, it is common to refer to breaking capacity in MVA, but for the fuses
and circuit breakers used in building services, it is usual to refer to breaking capacity in
amperes. However, whether MVA or amperes are used, the statement of breaking
capacity is incomplete unless it contains the voltage and power factor at which it applies.
In building services, in practice faults are invariably so close to unity power factor that
it is hardly necessary to specify power factor. Fuses and circuit breakers are rated at the
voltage of the supply which in the UK is either 230V or 400V, and therefore the
Design of electrical services for buildings 146
statement of voltage can be taken for granted. Thus in spite of what has been said in the
last paragraph breaking capacity is often quoted in amperes only, without further
statement.
The cut-off current depends not only on the characteristics of the fuse but also on the
nature of the fault current and the point in the alternating supply cycle at which the fault
occurs and the fuse starts to act. Although in the majority of cases the maximum
prospective fault current will not in fact be reached, for safety the fuse or circuit breaker
must have a breaking capacity at least equal to the maximum prospective fault current.
The breaking capacity of the various devices used in protection is of some importance
and must be taken into account when a selection has to be made between different
devices and schemes of protection.
It is not easy to give an exact figure for the breaking capacity of a rewireable fuse,
because it depends on the type of fuse carrier or fuse holder used and the exact
composition of the fuse cable and also because it is liable to vary with the age of the fuse
cable. As an approximation it can be taken to be of the order of 3000 to 4000A for the
largest types of rewireable fuse.
HRC fuses to BS 1361:1971 have a breaking capacity of 16500A at 0.3 power factor
when rated at 240V and of 33000A at 0.3 power factor when rated at 415V. HRC fuses to
BS EN 88 have a maximum breaking capacity of 80KA.
CBs to BS EN 60898 used on distribution boards to protect final circuits generally
have a two breaking capacities. The higher capacity breaking capacity Icn means that the
device will only interrupt that current once, and will no longer be serviceable. At the
lower value Ics the device will break that current as many times as is stated in the
specification for the device.
It may often be convenient to use a breaking device which has breaking capacity less
than the maximum prospective fault current. As an example, with the arrangement of
Figure 9.13, it could be that the fault current for a dead short circuit on an appliance at the
end of a final circuit is, say, 1500A, whilst the fault current for a short circuit on the
wiring within a short distance of the board is 4000A. The former is the only fault which is
probable, and a CB with a breaking capacity of 3000A (Ics 3.0), might well be considered
a suitable form of protection. If this were installed and the possible fault of 4000A
occurred and the CB were left to clear it, the CB would suffer damage and the wiring
might also be damaged before the circuit was fully broken. In such a case back-up
protection is required, and is provided by the HBC fuse at the supply end of the sub-main
distribution circuit. The fuse limits the maximum fault energy: if the fault current and,
therefore, the fault energy is greater than the CB can handle the back-up fuse blows. If on
the other hand the fault current is within the capacity of the CB the back-up fuse does not
act.
Back-up protection and discrimination are closely connected, but they are not the same
thing. Even if the breaking capacity of the final circuit fuse is such that no back-up
protection is needed, the final circuit fuse must still discriminate against the sub-main
fuse. In other words, discrimination is still needed even when back-up protection is not.
Since discrimination is always needed it must also accompany back-up protection, but it
does not follow that the provision of back-up protection will automatically give
discrimination. It is quite possible to choose the back-up fuse so that it blows, before the
Protection 147
distribution sub-circuit fuse, however small the fault current, and this would be back-up
protection without discrimination. It is of course to be avoided.
Summing up, we can say that fuses and CBs must be so chosen that back-up
protection is provided if it is needed and that discrimination is always provided.
We have described the equipment that is in practice available to give protection to
electrical installations and we have considered the nature and magnitude of fault currents
and the interaction of fuses and circuit breakers in series with each other. Knowing the
probable faults and the equipment available we can now turn to consider how this
equipment can be used to give satisfactory protection against the faults that may arise.
Protection
Introduction
It is a truism that electricity is dangerous and can cause accidents, if not treated with
respect. A large part of any system design is concerned with ensuring that accidents will
not happen, or that if they do, their effects will be limited. It might be reasonably argued
that these considerations are the most important part of a design engineer’s task. In the
previous chapters, we have spoken about choice of accessories, selection of cables and
their correct sizing, the arrangement of outlets on a number of separate circuits and the
proper ways of installing cables and we have pointed out the need for protecting cables
against mechanical damage. If these matters are given the care they deserve, the
likelihood of faults on the electrical installation will be small. Nevertheless, it is still
necessary to provide protection against such faults as may happen.
The general principle of protection is that a faulty circuit should be cut off from the
supply and isolated until the fault can be found and repaired. The protective device must
detect that there is a fault and must then isolate the part of the installation in which it has
detected the fault. One could perhaps suggest many theoretical ways of doing this, but it
is also necessary that the method adopted should bear a reasonable proportion to the cost
of the whole installation. Historically, the methods, which could be adopted at any time,
depended on what devices could be economically manufactured at that time, but once a
method has been adopted it tends to remain in use and newer products do not completely
supersede it. Enthusiasts sometimes stress the advantages of a new idea while forgetting
that the older method had some favourable features which the new one does not match.
The result of developments is that at present there are several protective devices available
and except for BS 3036 Fuses (rewireable) there appear to be no overriding grounds for
preferring any one to the others.
The devices available restrict the type of protection that can be given. A logically ideal
system of protection against all possible faults cannot be made economically, and the
protection designed must make use of the equipment commercially available. This can
lead people to argue from the available techniques to the faults to be guarded against, and
in the process become so obsessed with the ease of guarding against an improbable fault
that they forget the importance of protection against a more likely one. It seems more
satisfactory to start by considering the faults that may happen.
Two dangers to be prevented are fire and shock to people and livestock. In turn these
dangers can arise from three kinds of fault, namely a short circuit, an overload and a fault
to earth.
If through a fault in the wiring or in an appliance the line or phase and neutral
conductors become connected, the current that flows is limited only by the sum of the
resistance of the cables of the permanent wiring and the impedance of the accidental
contact between the two cables the latter generally being regarded as negligible. If the
fault connecting the line and neutral has a negligible impedance the two conductors are
effectively short circuited. The current that flows through the conductors is a short-circuit
current, and is very high and if allowed to continue would burn the insulation. The high
conductor temperature resulting from the excessive current could start a fire. If the excess
current continues to flow further after the insulation has been damaged, there is also a
possibility that the conductor might touch exposed metal and give a shock to anyone
touching the metal.
If the fault that connects the line and neutral has some impedance the current flowing
through the fault and conductors is less than the current in a short circuit of negligible
impedance. It is still likely to be higher than the maximum current the circuit can safely
carry and if it persists over a period of time, it can cause serious damage. When an
overcurrent is flowing in a circuit, which is electrically sound, it is described as an
overload. An overload can be caused by an electric motor stalling.
When an excessive mechanical load is imposed on an electric motor it continues to run
but draws a higher than normal current from the supply. The circuit supplying the motor,
therefore, carries a higher current than it has been designed for, and although it is not as
high as a short circuit current, it can still be high enough to be dangerous. A fault in the
internal wiring of a motor can also cause an electrical overload, although if it is serious
enough it is likely to amount to a short circuit.
A fault to earth occurs if through some defect the line conductor becomes connected to
earthed metalwork. The effect is similar to a short circuit, but whereas a short circuit will
not raise exposed metalwork, termed exposed conductive parts, above zero potential, an
earth fault will. We can see this by looking at Figure 9.1 which shows diagrammatically
an electric fire with an earthed metal case. Suppose the fire becomes damaged and the
phase cable touches the case at point A. A current will flow through the case and circuit
protective conductor to earth at point B, which would normally be the earth at the
electricity distribution company’s transformer.
Figure 9.1 Earth fault
Design of electrical services for buildings 132
Now let
Uoc=Open circuit supply transformer voltage
I=Fault current flowing
ZL=total impedance from line connection of supply transformer through line
conductor, fault and the circuit protective conductor to earth connection at supply
transformer
Zc=impedance of earth path from fault back to earth connection
Zs=Total impedance of the fault circuit.
Then the current flowing will be Uoc/Zs, and the voltage drop between A and B will
be IZc=UocZc/Zs. Now Zc/Zs is likely to be of the order of 0.4 to 0.5, so that on a Uoc of
240V the metal case at A will be raised to about 100V.
We cannot explain how electrical circuits in buildings are protected against short
circuits, overloads and earth faults without referring to the various protective devices
which can be used. To make our account intelligible we propose first of all to describe
the devices available and then go on to discuss how they are applied in practice.
Rewirable fuses
The earliest protective device consisted of a thin fuse cable held between terminals in a
porcelain or bakelite holder. It is illustrated in Figure 9.2. It is inserted in the circuit being
protected and the size of fuse cable is matched to the rating of the circuit. The fuse is
designed so that if the current exceeds the rated current of the circuit the fuse cable melts
and interrupts the circuit. Although commonly called rewirable fuses, their correct name
is semi-enclosed fuses, and it is by this name that they are referred to in British Standard
3036 and in the IEE Regulations BS 7671.
Figure 9.2 Rewirable fuse
Protection 133
HRC fuses
The rewirable fuse has limited breaking capacity. If a very large current flows the fuse
cable melts very rapidly and a large amount of energy is released. It can be large enough
to cause serious damage to the fuse carrier. It was found that some of this energy can be
absorbed by a packing of inert fibrous or granular material wound with cable, and this led
to the development of the cartridge fuse, illustrated in Figure 9.3.
The fuse element is mounted between two end caps which form the terminals of the
complete fuse link. The fuse element is surrounded by a closely packed silica filler and
the whole is contained in a ceramic casing. When the fuse element melts, or blows, the
silica filler absorbs the energy. Fuses of this type are known variously as high rupturing
capacity (HRC) or high breaking capacity (HBC) fuses or, less technically, as cartridge
fuses.
Operation of fuses
Both rewirable and cartridge fuses work in a similar way. The current heats the fuse
element until the latter melts, after which there is an arc between the ends of the broken
element, and finally the arc extinguishes and the circuit is completely interrupted.
The time taken for the fuse to melt depends on the magnitude of the current, and a fuse
will have a characteristic curve of time against current. A set of such characteristic curves
is shown in Figure 9.4. The total operating time is the sum of the melting, or pre-arcing,
time and the time during which there is an arc, known as the arcing time. The arcing time
varies with the power factor and transient characteristics of the circuit, the voltage, the
point in the alternating cycle of supply at which the arcing commences and on some other
factors. It is not, however, of significant length except for very large overcurrents when
the total operating time is very short.
Figure 9.3 Cartridge fuse
Design of electrical services for buildings 134
Figure 9.4 Time-current characteristics
of HRC fuses
The minimum fusing current is the minimum current at which a fuse will melt, that is
to say the asymptotic value of the current shown on the time-current characteristics. The
current rating is the normal current. It is the current stated by the manufacturer as the
current which the fuse will carry continuously without deterioration. It is also referred to
as current carrying capacity and other similar terms. The fusing factor is the ratio
When a short circuit occurs, the melting process is adiabatic and the melting energy is
given by
where
W=melting energy
i=instantaneous current
R=instantaneous resistance of that part of element which melts on short circuit
t=time
Protection 135
tm=melting time.
R is assumed to vary in the same manner with i and t for all short circuits and the
quantity
Figure 9.5 Short circuit l2t
characteristics
is approximately constant for the pre-arcing time of a fuse. It is often called the prearcing
I2t. It is this quantity which determines the amount of excess energy passing
through the circuit before the circuit is broken and it is particularly important in the
protection of semiconductor circuits and the reduction of overheating in power circuits.
Typical I2t characteristics are shown in Figure 9.5.
Oscillograms of the operation of a fuse are shown in Figure 9.6.
Design of electrical services for buildings 136
CB
An alternative to a fuse element which melts when overheated is a circuit breaker. A
circuit breaker (CB) is one which has a rating similar to that of a fuse and is about the
same physical size as a fuse carrier of the same rating. A typical circuit breaker is shown
in Figure 9.7.
It has a magnetic hydraulic time delay, and the essential component is a sealed tube
filled with silicone fluid which contains a closely fitted iron slug. Under normal operating
conditions the time delay spring keeps the slug at one end of the tube (a).
When an overload occurs, the magnetic pull of the coil surrounding the tube increases,
and the slug moves through the tube, the speed of travel depending on the magnetic force
and, therefore, on the size of the current, (b). As the slug approaches the other end of the
tube the air gaps in the magnetic circuit are reduced and the magnetic force is increased
until it is great enough to trip the circuit breaker, (c). With this mechanism the time taken
to trip is inversely proportional to the magnitude of the overload.
When a heavy overload or a complete short circuit occurs, the magnetic force is
sufficient to trip the circuit breaker instantaneously in spite of the large air gaps in the
magnetic circuit, (d). In this way, time delay tripping is achieved up to about seven times
rated current, and instantaneous tripping above that level.
An alternative design has a bi-metal element which is heated by the circuit current and
operates the trip catch when it deflects. A simple magnetic coil is included to trip the
catch on short circuit, so that the resulting characteristic is similar to that of the magnetichydraulic
type. The thermal-magnetic type is liable to be affected by the ambient
temperature. If several CBs are mounted inside a closed distribution board, the heat from
the currents passing through all the circuits in the board will raise the temperature inside
the enclosure. Thermally operated CBs used in this way may have to be de-rated to
prevent their tripping before an overload occurs. The effect of ambient temperature can,
however, be reduced and the need for de-rating obviated by designing the bi-metal to run
at a relatively high temperature. Typical time-current characteristics of CBs are given in
Figure 9.8.
The CB has a toggle switch by which it can be operated manually. This switch is
thrown into the off position when the overload device trips the
Protection 137
Figure 9.8 Time-current curves for
CBs
breaker, and the CB is reset by the same switch. CBs can, therefore, combine the
functions of switch and fuse, and in some cases this is a very useful and economic
procedure. In a factory or store, for example, one may want to control the lights for a
large area from a bank of switches at a single point. If a distribution board with CBs is
placed at this point, it is possible to dispense with a separate bank of switches.
RCD
Another device frequently used is the residual current device (RCD). This is a circuit
breaker which detects a current leaking to earth and uses this leakage current to operate
the tripping mechanism. The leakage current is a residual current and gives the device its
name. It should be noted here that this device will not protect against short circuit and
overload. Residual current devices which do incorporate overcurrent protection are
referred to as RCBOs.
The principle of the RCD is shown in Figure 9.9. The load current through the circuit
is fed through two equal and opposing coils wound on a common transformer core. On a
healthy circuit, the line and neutral currents are the same and produce equal and opposing
fluxes in the transformer core. However, if there is an earth fault, more current flows in
Protection 139
the line than returns in the neutral, and the line coil produces a bigger flux than the
neutral coil. There is thus a resultant or residual flux which induces a current in the
search coil, and this in turn operates the relay and trips the breaker. Some RCDs are
manufactured with electronic circuitry which simulates the above operation.
Figure 9.9 Residual current device
The value adopted for the rated tripping current, which is defined as the out-of-balance
current at which the circuit breaker will trip in less than 0.1s, is a maximum of 30mA for
protection against electric shock. The unit always includes a test button which simulates
an out-of-balance condition by injecting a test current bypassing one of the principal
coils. A resistor limits the magnitude of the test current so that the test also checks the
sensitivity of the breaker. It should be noted that the test switch checks the operation of
the residual current circuit breaker, but does not check the soundness of the earth
continuity conductor. It is a requirement of BS 7671 that if an installation incorporates
one of these devices a notice shall be fixed near the origin of the supply stating that the
device shall be tested quarterly, by the operation of the test button. This is to ensure that
the mechanical parts are kept operational. The author recommends that a notice be also
fixed near to the RCD itself. The origin of the supply may be in a seldom-accessed place.
Isolating transformers
In some applications a double-wound transformer can achieve adequate protection for the
system on its secondary side. The secondary is not earthed and the construction of the
transformer is carefully designed to prevent any possibility of contact between the
secondary and primary windings. The voltage between the line and return connections of
the secondary is fixed, but because no part of it is earthed or connected to any other fixed
Design of electrical services for buildings 140
point, the voltage relative to earth is quite undetermined. In other words the line can be at
230V above earth and the return at earth, or the one can be at 120V above earth and the
other at 120V below earth, or the line can be at earth and the return at 230V below earth,
or any other combination.
Thus, if anyone in contact with earthed metal touches the line conductor, the line is
brought immediately to earth potential and the return drops to 240V below earth. The
same thing happens if through a fault in an appliance a conductor touches earthed metal.
This prevents danger of shock. It is important to note that the system will not ensure
safety if more than one appliance is fed from the transformer. Figure 9.11 shows two
appliances near some earthed metalwork. Suppose a fault develops at A. The line
conductor will be held at earth potential and the return conductor will be at mains voltage
below earth. The circuit continues in operation and there is now no protection against a
second fault occurring at another point such as B.
The chief use of isolating transformers is in shaver units fixed in bathrooms to supply
electric razors. There is practically no possibility of two independent faults occurring in
such an application. For other applications the system must be erected so as to prevent
faults, or protect against faults.
We have now reviewed the equipment that is available to protect electrical
installations in buildings and can go on to consider the nature of the protection that is
needed.
Figure 9.11 Wrong use of isolating
transformer
Capacity of circuit
The methods by which a cable is sized for the duty it has to perform have been explained
in Chapter 4. There is no need to be worried about the effects of a fault on the voltage
drop; it is the high current that has to be guarded against. The protection has to safeguard
the circuit against overcurrent, and an overcurrent is any current higher than the rating of
the cable. Now the cable rating (Iz) must be rated equal to or greater than the nominal
current rating of the protective device (In). Also the nominal current rating of the
protective device must be rating equal to or greater than the normal current of the circuit
concerned (Ib), therefore the following expression must be satisfied
Protection 141
Iz≥In≥Ib
The guiding principle to be followed is that every cable in a permanent installation in a
building must be protected if it is liable to be subjected to overload or short circuit. The
protective device must not have a current rating greater than that of the cable, and in most
cases it will have a rating either equal to or only just less than that of the cable. It follows
that at every point at which a smaller cable branches from a larger one there must be a
protective device to safeguard the smaller cable. In conventional systems, this is provided
by the use of switchgear and distribution boards where a main divides into two or more
sub-mains and where a sub-main divides into a number of final circuits. It is also because
every branch has to be protected, that a fused connection unit is installed whenever a
single branch is taken off a ring main, and the fuse at the origin does not protect the spur.
Fault currents
The current normally plotted on the time-current characteristics of fuses and circuit
breakers is that known as the prospective fault current. This is the current, which would
flow in the circuit if the fuse were not there, and a short circuit or earth fault current
occurred. It is indicated as a dotted line in Figure 9.6. In practice, the fuse will open the
circuit before this prospective current is reached; the fuse is said to cut off and the
instantaneous current attained is called the cut off current.
The wave form of the prospective fault current depends on the position of the fault
within the whole of the supply network, the relative loading of the phases within the
network and whether the supply comes through a transformer or directly from a local
generator. These questions can be of importance to the engineer concerned with
protecting a public supply system, but are of less consequence to the designer of the
services within a building. For the designer’s purposes the simple procedure described
here is adequate and more complicated considerations need not be taken into account in
selecting the protection devices to be used within the building.
The prospective current is defined as the RMS value of the alternating component,
whereas the cut-off current is defined as the instantaneous current at cut-off. These
definitions produce the paradox that the numerical value of the cut-off current may be
greater than the numerical value of the prospective current.
The prospective earth fault current is determined by the supply voltage and the
impedance of the path taken by the fault current. In Figure 9.12, the path at the remote
end of the circuit, is indicated by a-b-c-d-f-g-a. The figure shows diagrammatically the
usual situation in urban installations where the consumer’s earth point is connected to the
sheath of the electricity distribution company’s cable, which is earthed at the transformer
end as on a TN-S system. If the supply voltage is Uoc and the impedance of this path is
Zs, then the prospective earth fault current is given by If=Uoc/Zs. The total impedance Zs
is made up of the impedance of the transformer and the impedance of all the cables in the
path. The impedance of a transformer is almost entirely reactive, and is therefore always
referred to as reactance. In practice, it is found that the reactance of the transformer sets
an upper limit to the fault current that can flow. Thus if a normal 500kVA transformer is
short circuited, the current on the secondary side will be 14000A, while the
Design of electrical services for buildings 142
corresponding figure for a 750kVA transformer is 21000A. These are average figures for
typical transformers and ignore the impedance of the supply system on the primary side
of the transformer. They therefore include a small hidden safety factor. The resistance of
the service cable and its sheath is usually quite low, but even a short length of a final
circuit cable makes a great reduction in the prospective fault current. For example 20m of
1.5mm2 cable in an installation supplied from a 750kVA transformer will limit the fault
current to 1000A.
Figure 9.12a Fault currents
Figure 9.12b Fault currents
Data on cable resistance are given by the manufacturers in their catalogues. Information
is also given in the IEE Guidance Note 1, and the On-Site Guide, the leading makers of
fuses and CBs provide in their catalogues tables and graphs showing the prospective fault
currents with different lengths of cable on the secondary side of standard transformers.
Protection 143
The fault itself usually has some impedance, so that the actual fault current is less than
the prospective fault current. The prospective earth fault current is calculated on the
assumption that the impedance of the fault in Figure 9.12 is zero. The actual fault current
could theoretically be calculated by adding the actual impedance of the fault between d
and e to the impedance used for calculating the prospective earth fault current, and as the
total actual impedance will thus be higher than that of the complete short circuit assumed
in calculating the prospective current, the actual fault current will be less than the
prospective one.
When a fault occurs on an appliance the cables of the final circuit play an important
part in limiting the fault current. Nevertheless, there is always the possibility, even if a
small one, of a low impedance fault immediately load side of the fuse or circuit breaker.
It is this fault which, however unlikely it may be that it will happen, will produce the
highest possible fault current and it is this fault with which the protection must be capable
of dealing.
Discrimination
In an installation having the type of distribution described in Chapter 6, there is a series
of fuses and circuit breakers between the incoming supply and the final outlet. Ideally,
the protective devices should be graded so that when a fault occurs, only the device
nearest the fault operates. The others should not react and should remain in circuit to go
on supplying other healthy circuits. Discrimination is said to take place when the smaller
fuse opens before the larger fuse. This is the desired state of affairs, and when the
unwanted converse situation happens it is said that discrimination is lost.
Figure 9.13 shows the time-current characteristic for a CB serving a final circuit and
for a 60A HBC fuse protecting the sub-main leading to the distribution board on which
the CB is mounted. If the fault current is less than 1200A, the CB will open first. If the
fault current is greater than this, the HBC fuse will blow before the CB can operate. The
resistance of the cables of the final circuit reduces the fault current, so that the further
away from the distribution board a fault occurs, the lower will be the fault current. If the
fault current at the distribution board is, say, 2000A, it will fall below this along the final
circuit from the board and will drop to 1200A fairly soon after the board. A fault on the
board would blow the board fuse but a fault on the final circuit any distance from the
board would leave the board intact and open the CB. The discrimination is said to be
acceptable. If the fault current at the supply end of the final circuit is so high that the
HBC fuse always opens before the CB, there is no discrimination. If there is
discrimination at the load end of the final circuit but not for a fault a short way along of
the final circuit, we say the discrimination is not acceptable. With the characteristic
curves of Figure 9.13, the discrimination would probably be acceptable even for a fault
level at the board of 3000A. The possibility of a fault at the board is small and the actual
fault currents to be cleared will almost certainly be due to faults at appliances. Provided
discrimination is maintained for these faults, its loss on the rare occasions when a fault
occurs on the permanent wiring close to the board can probably be accepted.
Design of electrical services for buildings 144
Figure 9.13 Discrimination
In general, discrimination is a problem only when a system uses all the same devices, or
an appropriate mixture of CBs and fuses. It can be seen from Figures 9.4 and 9.5 that a
fuse will always discriminate against another fuse of a larger rating. A 2:1 rule of thumb
for fuses is sometimes used. That is, a fault on a 32A fused final circuit would cause the
32A fuse to operate first, if it was backed up on the distribution circuit by a fuse of at
least 60A. Manufacturers produce ‘lollipop’ or ‘bulrush’ graphs to compare the relative
energies let through by certain devices. As was previously described, there is a time
required for a cartridge fuse element to heat up and start to melt. This is termed the prearcing
time. When the fuse element begins to melt, the overcurrent, being interrupted,
will cause an arc across the melting element. The overcurrent will eventually be
interrupted. The total operating I2t of the
Protection 145
Figure 9.14 Lollipop graph
fuse is the sum of the pre-arcing I2t and the arcing I2t. For proper discrimination to occur
the total operating I2t of the fuse required to operate must not be greater than the prearcing
I2t of the upstream fuse. If fuse ‘b’ in Figure 9.14 was used to supply a fuseboard
in which fuse ‘a’ was fitted, a fault on the circuit supplied by ‘a’ would cause fuse ‘a’ to
operate, but the total energy let through would cause fuse ‘b’ to be in its arcing sector,
thus weakening the fuse ‘b’. If fuse ‘c’ was used to supply a fuseboard, in which either
fuse ‘a’ or ‘b’ was fitted, a fault on either circuit would cause the lesser fuse to operate
without being in the arcing sector I2t of fuse ‘c’. These lollipop graphs must be consulted
on high fault currents where the fuse will operate in 0.1s or less.
Breaking capacity and back-up protection
A certain amount of energy is released when a circuit is broken, whether by a fuse or by a
circuit breaker. This energy must be absorbed by the device breaking the circuit, and the
capacity to do so limits the current which the device can safely break. The breaking
capacity is defined as the maximum current that can be broken at a rated voltage. In
switchgear practice, it is common to refer to breaking capacity in MVA, but for the fuses
and circuit breakers used in building services, it is usual to refer to breaking capacity in
amperes. However, whether MVA or amperes are used, the statement of breaking
capacity is incomplete unless it contains the voltage and power factor at which it applies.
In building services, in practice faults are invariably so close to unity power factor that
it is hardly necessary to specify power factor. Fuses and circuit breakers are rated at the
voltage of the supply which in the UK is either 230V or 400V, and therefore the
Design of electrical services for buildings 146
statement of voltage can be taken for granted. Thus in spite of what has been said in the
last paragraph breaking capacity is often quoted in amperes only, without further
statement.
The cut-off current depends not only on the characteristics of the fuse but also on the
nature of the fault current and the point in the alternating supply cycle at which the fault
occurs and the fuse starts to act. Although in the majority of cases the maximum
prospective fault current will not in fact be reached, for safety the fuse or circuit breaker
must have a breaking capacity at least equal to the maximum prospective fault current.
The breaking capacity of the various devices used in protection is of some importance
and must be taken into account when a selection has to be made between different
devices and schemes of protection.
It is not easy to give an exact figure for the breaking capacity of a rewireable fuse,
because it depends on the type of fuse carrier or fuse holder used and the exact
composition of the fuse cable and also because it is liable to vary with the age of the fuse
cable. As an approximation it can be taken to be of the order of 3000 to 4000A for the
largest types of rewireable fuse.
HRC fuses to BS 1361:1971 have a breaking capacity of 16500A at 0.3 power factor
when rated at 240V and of 33000A at 0.3 power factor when rated at 415V. HRC fuses to
BS EN 88 have a maximum breaking capacity of 80KA.
CBs to BS EN 60898 used on distribution boards to protect final circuits generally
have a two breaking capacities. The higher capacity breaking capacity Icn means that the
device will only interrupt that current once, and will no longer be serviceable. At the
lower value Ics the device will break that current as many times as is stated in the
specification for the device.
It may often be convenient to use a breaking device which has breaking capacity less
than the maximum prospective fault current. As an example, with the arrangement of
Figure 9.13, it could be that the fault current for a dead short circuit on an appliance at the
end of a final circuit is, say, 1500A, whilst the fault current for a short circuit on the
wiring within a short distance of the board is 4000A. The former is the only fault which is
probable, and a CB with a breaking capacity of 3000A (Ics 3.0), might well be considered
a suitable form of protection. If this were installed and the possible fault of 4000A
occurred and the CB were left to clear it, the CB would suffer damage and the wiring
might also be damaged before the circuit was fully broken. In such a case back-up
protection is required, and is provided by the HBC fuse at the supply end of the sub-main
distribution circuit. The fuse limits the maximum fault energy: if the fault current and,
therefore, the fault energy is greater than the CB can handle the back-up fuse blows. If on
the other hand the fault current is within the capacity of the CB the back-up fuse does not
act.
Back-up protection and discrimination are closely connected, but they are not the same
thing. Even if the breaking capacity of the final circuit fuse is such that no back-up
protection is needed, the final circuit fuse must still discriminate against the sub-main
fuse. In other words, discrimination is still needed even when back-up protection is not.
Since discrimination is always needed it must also accompany back-up protection, but it
does not follow that the provision of back-up protection will automatically give
discrimination. It is quite possible to choose the back-up fuse so that it blows, before the
Protection 147
distribution sub-circuit fuse, however small the fault current, and this would be back-up
protection without discrimination. It is of course to be avoided.
Summing up, we can say that fuses and CBs must be so chosen that back-up
protection is provided if it is needed and that discrimination is always provided.
We have described the equipment that is in practice available to give protection to
electrical installations and we have considered the nature and magnitude of fault currents
and the interaction of fuses and circuit breakers in series with each other. Knowing the
probable faults and the equipment available we can now turn to consider how this
equipment can be used to give satisfactory protection against the faults that may arise.
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