Chapter 15
Emergency supplies
Introduction
There are rare occasions when the public electricity supply fails and a building is left
without electricity. In some buildings, the risk of being totally without electricity cannot
be taken, and some provision must be made for an alternative supply to be used in an
emergency. What form this provision should take is an economic matter which depends
on the magnitude of the risk of failure and the seriousness of the consequences of failure.
In this chapter, we shall say something about the available methods of providing an
alternative supply.
Standby service cable
The Electricity Supply Authority can be asked to bring two separate service cables into
the building. They will normally make a charge for this, but it provides security against a
fault in one of the cables. It does not, of course, give security against a failure of the
public supply altogether.
In heavily built up areas, such as London and other large cities, the public distribution
system is in the form of a network and each distribution cable in the streets is fed from a
sub-station at each end. The supply system itself thus contains its own standby provision.
The only addition the building developer can make is to duplicate the short length of
cable from the distributor in the road into the building, and it may be doubted whether the
risk of this cable failing is sufficiently great to justify the cost of duplicating it. In rural
areas the service cable to individual buildings may be quite long, and may take the form
of an overhead line rather than an underground cable. The risk of damage is thus greater
than in urban areas and there is much more reason for installing a duplicate cable.
Battery systems
Central battery and individual battery systems have been discussed in Chapter 7 as means
of providing emergency lighting. A central battery system can also provide d.c. power.
An alternative is for the battery to feed a thyristor inverter which then gives a.c. power.
It is difficult to install and keep charged a battery large enough to give the quantities
of power needed in a whole building. In building services, in practice, batteries are used
for emergency lighting but seldom for emergency power.
A battery system will give an emergency supply only for as long as the battery charge
lasts. It then becomes dependent on a restoration of the mains supply for long enough to
recharge the battery. Thus we can see that this system does not protect against long
interruptions of the public supply.
Standby generators
A diesel or gas turbine generator set can be installed in a building to provide electricity
when the public supply fails. This is a complete form of protection against all possible
interruptions of the main supply. The generator can be large enough to supply all the
needs of the building and its output can be connected to the ordinary mains immediately
after the supply authority’s meters and it then provides standby facilities for the entire
building. It is cheaper, and may be adequate for the risk to be guarded against, to have a
smaller generator serving only the more important outlets. In this case, the distribution
must be arranged so that these outlets can be switched from main to emergency supply at
one point and so that there is no unintentional path from the emergency generator to
outlets not meant to be served by it. In effect the building is divided at the main intake
into two distribution systems and only one of them is connected to the emergency
changeover switch. It is also possible to install a completely separate system of wiring
from the emergency generator to outlets quite distinct from the normal ones. This may be
the simplest thing to do in a small building or when the emergency supply is required to
serve only one or two outlets. It has the disadvantage that individual pieces of equipment
have to be disconnected from one outlet and reconnected to another. Whilst this may not
be acceptable in a hospital it may be quite in order in a large residence or hostel to have
one or two emergency power points into which vacuum cleaners and other domestic
equipment can be plugged when the main power supply is interrupted.
Buildings in which standby generators have been installed include poultry farms,
chemical process plants, hospitals, telephone exchanges, computer rooms and prisons.
An emergency generator can be started either manually or automatically. A manual
start is simple, but it involves a delay during which the building is without power. This
delay can be avoided by automatic starting, initiated by a sensing unit which detects a
drop in the mains voltage. Figure 15.1 shows the circuit of a typical mains failure control
panel.
When the mains fail, relay 1CC/6 is de-energized and opens the main circuit breaker.
It also completes the circuit to the operating coil of relay 2CC/6, thus preparing the
circuit for shut down when the mains are restored. Relays VS1, VS2 and VS3 are
separately operated by each of the three phases, and each has the effect of de-energizing
the main relay ICC/6, so that the system is brought into operation on the failure of any
one phase.
When the mains fail, relay VS 1/2 is also de-energized and its contacts then bring into
operation relays T1, R1, R2 and R3. Relay R1 starts the run solenoid of the diesel engine,
relay R2 energizes the starter motor and relay R3 temporarily disconnects the battery
Emergency supplies 229
charger. If the engine has not started after 10s, relay T1 de-energizes relays R1, R2 and
R3 and lights a fail-to-start warning lamp. It also energizes relay R4, one of whose
contacts breaks the circuit of relay R1 which has the effect of making a restart
impossible. Thus, if the engine does not start within 10s it locks out and nothing further
can happen until it receives some manual attention.
If, or perhaps we should say when, the engine starts, relay VS4/1 is energized. This
interrupts the operating coil circuits of relays R2 and R3 and prevents relay Ti from
energizing relay R4. The starting sequence is thus brought to an end, the battery charger
is reconnected and the starter motor stopped.
Relay T2 is now energized and in turn energizes relay R5 which completes the circuit
to the operating coil of relay 2CC/6. The latter closes the standby generator. At the same
time, it lights the indicator to show that the standby generator is on load and puts a break
in the operating coil circuit of relay 1CC/6. This ensures that the mains circuit breaker
will not close while the standby circuit breaker is closed. The plant is now running on
standby.
When the mains are restored relays VS2 and VS3 are energized and in turn energize
relay VS1. The circuit to complete the supply to relay ICC/6 is thus prepared. Also the
circuits to relays T1 and R1 are broken. Relay Ti then breaks the circuit to relay R4 and
energizes relays R2 and R3. The starter circuit is kept open by relay R1 which is kept deenergized
by relay VS1.
With R1 de-energized the run solenoid is de-energized and the standby set stops. As
soon as it shuts down relays VS4 and T2 open. The latter breaks the circuit to relay R5
and the normally open contact of R5 breaks the circuit to relay 2CC/6. This opens the
standby circuit breaker and simultaneously completes the circuit to relay 1CC/6, which
then closes the main circuit breaker. The standby set is now shut down and the load is
back on the mains.
It takes 8 to 10s for a diesel generator to come to full speed. With the system just
described this period is needed to bring the emergency supply into action after the mains
have failed and, therefore, during this period there is no supply to the load. In some
applications an interruption even of this short
Design of electrical services for buildings 230
Figure 15.1 Automatic starting circuit
duration is not acceptable, and a more complex arrangement is necessary. In one system
the diesel engine is coupled to a clutch the other side of which is connected to a squirrel
cage induction motor. The induction motor drives an alternator through a flywheel, and
the alternator supplies the load. Under normal conditions the induction motor is
connected to the mains and the set operates as a motor alternator supplied from the mains.
When the mains fail the motor is disconnected from the mains and the diesel engine is
started. As soon as it reaches its running speed, the clutch operates and the alternator is
driven through the shaft of the motor by the diesel engine. During the time it takes for the
engine to come up to speed the alternator is kept going by the flywheel. The automatic
controls required for this arrangement are similar to those already described.
Clearly this scheme is much more expensive and involves some permanent losses in
the motor alternator set. It is used only for comparatively small power outputs for special
purposes, such as telecommunications and power for aircraft landing systems.
Standby generators are normally supplied as complete units on a stand. Figure 15.2 is
a picture of a typical set. The diesel engine is a normal engine with a governor, and it
would be outside the scope of this book to enter on
Emergency supplies 231
Figure 15.2 Standby generator
a description of diesel engines. The alternator is directly coupled to the engine and has an
automatic voltage regulator. The commonest type of alternator used is the screen
protected, provided with brushless excitation. It is directly coupled to the engine and in
smaller sizes may be overhung. In larger sizes it is supported at both ends from the set
base plate. A separate exciter is mounted within the casing on the main shaft. In most
modern sets the automatic voltage regulator is one of the static types. Finally, there is a
control panel with voltmeters, ammeters, battery charger, incoming and outgoing
terminals and the relays and circuits for the automatic start and stop control. A fuel tank
is needed for the diesel engine, but this is normally supplied as a separate item and fixed
independently of the generator set, with a short fuel pipe between them.
Diesel engines are noisy and it is prudent to arrange some form of sound-attenuating
enclosure. The enclosure must have openings for fresh air to the engine and for the
engine exhaust, and these openings will be found to limit the degree of silencing that can
be achieved. Several manufacturers supply diesel generator sets complete in an enclosure
which provides silencing and is also weatherproof, so that the set can be installed
outdoors.
Similar generating sets can, of course, be used to supply power to a building under
normal conditions. In the UK it is not economic for consumers to generate their own
power, but there are still parts of the world where it is a reasonable proposition.
There are, however, cases in industrial countries where it is economic for consumers
to use their own generating plant to supply peak loads. Many factories are supplied on a
tariff which includes a charge for the peak instantaneous load. A factory may have a
Design of electrical services for buildings 232
process which takes a fairly steady load during most of the day with a high peak for one
or two hours. It may then be economic to limit the power taken from the public supply to
rather less than the maximum needed and to make up the deficiency at peak times with
the factory’s own plant. Figure 15.3 shows a load diagram to illustrate this. The public
mains are used at all times and are used by themselves so long as the load is less than
300kVA. As soon as the load exceeds this figure the factory’s own generating plant is
started and is run in parallel with the public mains. The power taken from the public
mains is limited to a maximum of 300kVA at all times.
The same type of diesel generator is used for this application as for standby purposes.
The start up and shut down sequences are initiated automatically by a kVA meter instead
of by a voltage detector, but are otherwise similar to those already described.
Uninterruptible power supplies
The requirements of computers have led to the development of uninterruptible power
supply units, generally referred to as UPS. In essence they
Figure 15.3 Load diagram
depend on a battery and inverter to provide an emergency supply. Because computers are
sensitive to voltage and frequency fluctuations it is useful to feed them through a network
which smoothes out fluctuations in the supply and suppresses surges caused by switching
of other equipment connected to the same supply. A UPS generally incorporates such a
circuit and therefore the complete unit contains a battery, charger, inverter, changeover
switch and smoothing circuit. In another arrangement, shown in block diagram form in
Figure 15.4, the mains supply is rectified and then inverted, with the battery connected
Emergency supplies 233
between the rectifier and inverter. This avoids a separate charger and eliminates the
changeover switch. The battery is automatically kept charged and any drop in mains
voltage results in current being taken from the battery with no effect on the output. The
capacity of the UPS will depend on the application. For a general office application the
capacity may supply the computer for one hour. In large offices such as in an insurance
company head office, the inverter is supplied from a very large battery bank giving many
hours of back-up.
Figure 15.4 Scheme of UPS unit
Figure 15.5 Standby system
Most UPS have batteries which will maintain the output for at least an hour. Insurance
companies and the like are likely to have very large UPS systems because of the large
number of computers they use on their premises. The one-hour UPS may be adequate for
a small computer in an industrialized country with a very reliable public electricity
supply. An interruption in supply is not likely to last for long, and there is adequate time
to complete work in hand, save data on disc and switch off in an orderly way before the
battery runs down. If this is not considered sufficient protection, the UPS can be
combined with a standby generator as shown in Figure 15.5. The UPS battery provides
the supply while the generator set runs up to speed. This arrangement ensures secure
supply to the computer without the mechanical complication and losses of the motor
alternator system described previously. Standards relevant to this chapter are:
BS 5266 Code of practice for the emergency lighting of premises
Design of electrical services for buildings 234
Emergency supplies
Introduction
There are rare occasions when the public electricity supply fails and a building is left
without electricity. In some buildings, the risk of being totally without electricity cannot
be taken, and some provision must be made for an alternative supply to be used in an
emergency. What form this provision should take is an economic matter which depends
on the magnitude of the risk of failure and the seriousness of the consequences of failure.
In this chapter, we shall say something about the available methods of providing an
alternative supply.
Standby service cable
The Electricity Supply Authority can be asked to bring two separate service cables into
the building. They will normally make a charge for this, but it provides security against a
fault in one of the cables. It does not, of course, give security against a failure of the
public supply altogether.
In heavily built up areas, such as London and other large cities, the public distribution
system is in the form of a network and each distribution cable in the streets is fed from a
sub-station at each end. The supply system itself thus contains its own standby provision.
The only addition the building developer can make is to duplicate the short length of
cable from the distributor in the road into the building, and it may be doubted whether the
risk of this cable failing is sufficiently great to justify the cost of duplicating it. In rural
areas the service cable to individual buildings may be quite long, and may take the form
of an overhead line rather than an underground cable. The risk of damage is thus greater
than in urban areas and there is much more reason for installing a duplicate cable.
Battery systems
Central battery and individual battery systems have been discussed in Chapter 7 as means
of providing emergency lighting. A central battery system can also provide d.c. power.
An alternative is for the battery to feed a thyristor inverter which then gives a.c. power.
It is difficult to install and keep charged a battery large enough to give the quantities
of power needed in a whole building. In building services, in practice, batteries are used
for emergency lighting but seldom for emergency power.
A battery system will give an emergency supply only for as long as the battery charge
lasts. It then becomes dependent on a restoration of the mains supply for long enough to
recharge the battery. Thus we can see that this system does not protect against long
interruptions of the public supply.
Standby generators
A diesel or gas turbine generator set can be installed in a building to provide electricity
when the public supply fails. This is a complete form of protection against all possible
interruptions of the main supply. The generator can be large enough to supply all the
needs of the building and its output can be connected to the ordinary mains immediately
after the supply authority’s meters and it then provides standby facilities for the entire
building. It is cheaper, and may be adequate for the risk to be guarded against, to have a
smaller generator serving only the more important outlets. In this case, the distribution
must be arranged so that these outlets can be switched from main to emergency supply at
one point and so that there is no unintentional path from the emergency generator to
outlets not meant to be served by it. In effect the building is divided at the main intake
into two distribution systems and only one of them is connected to the emergency
changeover switch. It is also possible to install a completely separate system of wiring
from the emergency generator to outlets quite distinct from the normal ones. This may be
the simplest thing to do in a small building or when the emergency supply is required to
serve only one or two outlets. It has the disadvantage that individual pieces of equipment
have to be disconnected from one outlet and reconnected to another. Whilst this may not
be acceptable in a hospital it may be quite in order in a large residence or hostel to have
one or two emergency power points into which vacuum cleaners and other domestic
equipment can be plugged when the main power supply is interrupted.
Buildings in which standby generators have been installed include poultry farms,
chemical process plants, hospitals, telephone exchanges, computer rooms and prisons.
An emergency generator can be started either manually or automatically. A manual
start is simple, but it involves a delay during which the building is without power. This
delay can be avoided by automatic starting, initiated by a sensing unit which detects a
drop in the mains voltage. Figure 15.1 shows the circuit of a typical mains failure control
panel.
When the mains fail, relay 1CC/6 is de-energized and opens the main circuit breaker.
It also completes the circuit to the operating coil of relay 2CC/6, thus preparing the
circuit for shut down when the mains are restored. Relays VS1, VS2 and VS3 are
separately operated by each of the three phases, and each has the effect of de-energizing
the main relay ICC/6, so that the system is brought into operation on the failure of any
one phase.
When the mains fail, relay VS 1/2 is also de-energized and its contacts then bring into
operation relays T1, R1, R2 and R3. Relay R1 starts the run solenoid of the diesel engine,
relay R2 energizes the starter motor and relay R3 temporarily disconnects the battery
Emergency supplies 229
charger. If the engine has not started after 10s, relay T1 de-energizes relays R1, R2 and
R3 and lights a fail-to-start warning lamp. It also energizes relay R4, one of whose
contacts breaks the circuit of relay R1 which has the effect of making a restart
impossible. Thus, if the engine does not start within 10s it locks out and nothing further
can happen until it receives some manual attention.
If, or perhaps we should say when, the engine starts, relay VS4/1 is energized. This
interrupts the operating coil circuits of relays R2 and R3 and prevents relay Ti from
energizing relay R4. The starting sequence is thus brought to an end, the battery charger
is reconnected and the starter motor stopped.
Relay T2 is now energized and in turn energizes relay R5 which completes the circuit
to the operating coil of relay 2CC/6. The latter closes the standby generator. At the same
time, it lights the indicator to show that the standby generator is on load and puts a break
in the operating coil circuit of relay 1CC/6. This ensures that the mains circuit breaker
will not close while the standby circuit breaker is closed. The plant is now running on
standby.
When the mains are restored relays VS2 and VS3 are energized and in turn energize
relay VS1. The circuit to complete the supply to relay ICC/6 is thus prepared. Also the
circuits to relays T1 and R1 are broken. Relay Ti then breaks the circuit to relay R4 and
energizes relays R2 and R3. The starter circuit is kept open by relay R1 which is kept deenergized
by relay VS1.
With R1 de-energized the run solenoid is de-energized and the standby set stops. As
soon as it shuts down relays VS4 and T2 open. The latter breaks the circuit to relay R5
and the normally open contact of R5 breaks the circuit to relay 2CC/6. This opens the
standby circuit breaker and simultaneously completes the circuit to relay 1CC/6, which
then closes the main circuit breaker. The standby set is now shut down and the load is
back on the mains.
It takes 8 to 10s for a diesel generator to come to full speed. With the system just
described this period is needed to bring the emergency supply into action after the mains
have failed and, therefore, during this period there is no supply to the load. In some
applications an interruption even of this short
Design of electrical services for buildings 230
Figure 15.1 Automatic starting circuit
duration is not acceptable, and a more complex arrangement is necessary. In one system
the diesel engine is coupled to a clutch the other side of which is connected to a squirrel
cage induction motor. The induction motor drives an alternator through a flywheel, and
the alternator supplies the load. Under normal conditions the induction motor is
connected to the mains and the set operates as a motor alternator supplied from the mains.
When the mains fail the motor is disconnected from the mains and the diesel engine is
started. As soon as it reaches its running speed, the clutch operates and the alternator is
driven through the shaft of the motor by the diesel engine. During the time it takes for the
engine to come up to speed the alternator is kept going by the flywheel. The automatic
controls required for this arrangement are similar to those already described.
Clearly this scheme is much more expensive and involves some permanent losses in
the motor alternator set. It is used only for comparatively small power outputs for special
purposes, such as telecommunications and power for aircraft landing systems.
Standby generators are normally supplied as complete units on a stand. Figure 15.2 is
a picture of a typical set. The diesel engine is a normal engine with a governor, and it
would be outside the scope of this book to enter on
Emergency supplies 231
Figure 15.2 Standby generator
a description of diesel engines. The alternator is directly coupled to the engine and has an
automatic voltage regulator. The commonest type of alternator used is the screen
protected, provided with brushless excitation. It is directly coupled to the engine and in
smaller sizes may be overhung. In larger sizes it is supported at both ends from the set
base plate. A separate exciter is mounted within the casing on the main shaft. In most
modern sets the automatic voltage regulator is one of the static types. Finally, there is a
control panel with voltmeters, ammeters, battery charger, incoming and outgoing
terminals and the relays and circuits for the automatic start and stop control. A fuel tank
is needed for the diesel engine, but this is normally supplied as a separate item and fixed
independently of the generator set, with a short fuel pipe between them.
Diesel engines are noisy and it is prudent to arrange some form of sound-attenuating
enclosure. The enclosure must have openings for fresh air to the engine and for the
engine exhaust, and these openings will be found to limit the degree of silencing that can
be achieved. Several manufacturers supply diesel generator sets complete in an enclosure
which provides silencing and is also weatherproof, so that the set can be installed
outdoors.
Similar generating sets can, of course, be used to supply power to a building under
normal conditions. In the UK it is not economic for consumers to generate their own
power, but there are still parts of the world where it is a reasonable proposition.
There are, however, cases in industrial countries where it is economic for consumers
to use their own generating plant to supply peak loads. Many factories are supplied on a
tariff which includes a charge for the peak instantaneous load. A factory may have a
Design of electrical services for buildings 232
process which takes a fairly steady load during most of the day with a high peak for one
or two hours. It may then be economic to limit the power taken from the public supply to
rather less than the maximum needed and to make up the deficiency at peak times with
the factory’s own plant. Figure 15.3 shows a load diagram to illustrate this. The public
mains are used at all times and are used by themselves so long as the load is less than
300kVA. As soon as the load exceeds this figure the factory’s own generating plant is
started and is run in parallel with the public mains. The power taken from the public
mains is limited to a maximum of 300kVA at all times.
The same type of diesel generator is used for this application as for standby purposes.
The start up and shut down sequences are initiated automatically by a kVA meter instead
of by a voltage detector, but are otherwise similar to those already described.
Uninterruptible power supplies
The requirements of computers have led to the development of uninterruptible power
supply units, generally referred to as UPS. In essence they
Figure 15.3 Load diagram
depend on a battery and inverter to provide an emergency supply. Because computers are
sensitive to voltage and frequency fluctuations it is useful to feed them through a network
which smoothes out fluctuations in the supply and suppresses surges caused by switching
of other equipment connected to the same supply. A UPS generally incorporates such a
circuit and therefore the complete unit contains a battery, charger, inverter, changeover
switch and smoothing circuit. In another arrangement, shown in block diagram form in
Figure 15.4, the mains supply is rectified and then inverted, with the battery connected
Emergency supplies 233
between the rectifier and inverter. This avoids a separate charger and eliminates the
changeover switch. The battery is automatically kept charged and any drop in mains
voltage results in current being taken from the battery with no effect on the output. The
capacity of the UPS will depend on the application. For a general office application the
capacity may supply the computer for one hour. In large offices such as in an insurance
company head office, the inverter is supplied from a very large battery bank giving many
hours of back-up.
Figure 15.4 Scheme of UPS unit
Figure 15.5 Standby system
Most UPS have batteries which will maintain the output for at least an hour. Insurance
companies and the like are likely to have very large UPS systems because of the large
number of computers they use on their premises. The one-hour UPS may be adequate for
a small computer in an industrialized country with a very reliable public electricity
supply. An interruption in supply is not likely to last for long, and there is adequate time
to complete work in hand, save data on disc and switch off in an orderly way before the
battery runs down. If this is not considered sufficient protection, the UPS can be
combined with a standby generator as shown in Figure 15.5. The UPS battery provides
the supply while the generator set runs up to speed. This arrangement ensures secure
supply to the computer without the mechanical complication and losses of the motor
alternator system described previously. Standards relevant to this chapter are:
BS 5266 Code of practice for the emergency lighting of premises
Design of electrical services for buildings 234
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