With the exception of the incandescent ones, all the lamps we have described go on and
off 100 times a second when working on a 50Hz supply. This is too rapid a flicker to be
noticeable to the eye, but it could with older types of fluorescent tube produce a
stroboscopic effect on rotating machinery, which may appear either to be stationary or to be moving at a much slower speed than it really is. Such an illusion can obviously be a
source of danger and should be avoided. If adjacent lamps go on and off at different
times, that is to say if they are out of step with each other, the effect is reduced, and it is
possible to obtain this breaking of step with twin lamp luminaires. The luminaire is made
with one choke for each lamp but only one capacitor. The capacitor is arranged in series
with only one of the lamps, and the chokes and capacitor are so sized that although the
combined current through both lamps has the usual power factor of about 0.8, the current
through one lamp leads the voltage while the current through the other lags. The two
lamp currents are neither in step nor 180° out of phase and, therefore, extinguish and relight
at different instants. Luminaires of this kind are known as lead/lag luminaires and
are often specified for workshops and factories. Tri-phosphor fluorescent with electronic
ballast operates at 20kHz.
Circuits and controls
The way in which luminaires are connected in circuits has been described in Chapter 5.
We showed in that chapter how a switch could control each light or each group of lights.
The switch would be one of the types described in Chapter 1. There are, however, other
methods of control possible and they are particularly important for street-lighting and
other external lights for which some form of automatic control is almost essential.
The first method and one still widely used is by means of a time switch. Time switches
are made with an enormous variety of dials, or digital displays, for almost any
conceivable application can be found from some manufacturer’s standard range of
products, but in exceptional cases special dials or digital display can be made to order.
For street or external lights, the usual requirement is for the lamps to be turned on at
sunset and off either at midnight or at sunrise. The times of sunset and sunrise vary
throughout the year, and solar compensating dials are made for time switches to follow
the annual variation in sunset and sunrise. As the variation depends on latitude, different
dials/displays are made for different zones of latitude.
In the UK street-lighting is the responsibility of the local authority. The usual
arrangement is for the electricity company to supply each column directly off its main
running through the street, or if necessary off a main laid specially for the lights. A time
switch is fixed inside each column, and switches the current to the luminaire at the top of
the column. The supply is not metered because one knows the total hours in a year during
which a solar dial switch is on and also the rating of the lamp, and can thus calculate the
number of units of electricity consumed in a year. The column also contains a service
head fuse, on the supply side of the time switch.
A new housing development may often include estate roads which are constructed by
the developer, but which will ultimately be taken over by the local authority. The same
system of road-lighting can be adopted, but it is advisable to agree the details with the
street-lighting department of the local authority beforehand. The system can also be used
where the roads are to remain private, but the agreement of the electricity company must
be obtained in advance because the method of supply may affect the tariff they will want
to apply.
The design engineer should make sure to select and specify columns which have a
space inside them large enough to contain the time switch and fuse; there are light duty
columns which do not have a wide enough base.
If the external lights are not too far from buildings, final circuits can serve them from
distribution boards within the buildings. The cable sizes should be checked for voltage
drop, and may need to be greater than ordinary lighting circuits. For running underground
PVCSWAPVC or MICC, cable is probably the most suitable. A circuit to several lights is
taken through a time switch and the time switch can be conveniently mounted next to the
distribution board. It is also possible to obtain a modular-type distribution board which
has a time switch to be mounted on the DIN rail inside the board. Internal lights may also
be controlled by a time switch, and this may be a convenient way of switching, say,
corridor lights in an office block.
When lights, whether internal or external, are supplied through a time switch served
from a distribution board, the supply is of course part of the metered supply to the
building. Whether this is more or less desirable than an unmetered but timed supply
depends on the agreement between the consumer and the supply authority.
The other method of automatic control of lights is the use of photoelectric cells. In this
method, a photoelectric cell is arranged to make a circuit when the illumination falls
below a set value and to break the circuit when the illumination rises. The advantage over
a time switch is that the control takes account of weather conditions. There are summer
evenings which, because of storm clouds, are almost as dark as winter evenings, but a
time switch cannot distinguish between a cloudy day and a clear one. Furthermore, in the
UK at least, a time switch must usually be reset twice a year because of British Summer
Time, whereas a photoelectric cell needs no alteration.
When photoelectric cells are used, the lighting circuits can be arranged in the same
way as when time switches are used. There can be one cell to each light, or there may be
one cell for a circuit on which there are several lights. In the latter case, if the circuit is
switched directly by the cell, the number of lights on the circuit is limited by the current
which the cell can switch. The maximum current which a given cell can switch may be
lower if the current is inductive than if it is resistive, so that a cell may be able to switch
fewer fluorescent lights than incandescent ones. Nevertheless, a small cell can be used to
control many lights, or even several different circuits, if it is used with a relay. The cell is
placed in the operating coil circuit of the relay and the relay main contacts switch the
lights. A multipole relay makes it possible for one photoelectric cell to control several
lighting circuits.
The position of the photoelectric cell requires a little thought. It must be close to the
area illuminated by the lights it controls, in order to react to the daylight in that area. At
the same time, it must not receive direct light from any of the lamps it controls; if it did,
as soon as they came on, it would switch them off again, so that they would flicker on
and off continuously. Nor must it receive direct light from any other lamps which may be
turned on at night, otherwise switching on one set of lights will immediately switch off
the other set. In general, the illumination on the photocell during the hours of darkness
when the lights are on must be less than the level at which the cell switches the lights.
Most outdoor artificial lighting produces illumination levels much lower than daylight
and a photocell set at the minimum acceptable daylight level will not turn the lights off
until daylight is again adequate, but there are areas, such as the forecourts of garages,
Design of electrical services for buildings 122
supermarkets and cinemas, where the artificial lighting approaches full daylight level,
and in these cases it is certainly difficult, and may even be impossible, to design a
photoelectric scheme of control. For ordinary street lighting, a cell placed in the top of a
column above the lantern will respond adequately to natural light falling on the road and
can be shielded from the light of the lamp.
The number of hours in a year during which a photoelectric cell will be in the ‘on’
position cannot be known exactly. The supply authority may, therefore, be unwilling to
agree to the use of an unmetered supply. Nevertheless, several local authorities are using
photoelectric cells to control street lighting and have negotiated a suitable method of
payment with the area electricity company.
off 100 times a second when working on a 50Hz supply. This is too rapid a flicker to be
noticeable to the eye, but it could with older types of fluorescent tube produce a
stroboscopic effect on rotating machinery, which may appear either to be stationary or to be moving at a much slower speed than it really is. Such an illusion can obviously be a
source of danger and should be avoided. If adjacent lamps go on and off at different
times, that is to say if they are out of step with each other, the effect is reduced, and it is
possible to obtain this breaking of step with twin lamp luminaires. The luminaire is made
with one choke for each lamp but only one capacitor. The capacitor is arranged in series
with only one of the lamps, and the chokes and capacitor are so sized that although the
combined current through both lamps has the usual power factor of about 0.8, the current
through one lamp leads the voltage while the current through the other lags. The two
lamp currents are neither in step nor 180° out of phase and, therefore, extinguish and relight
at different instants. Luminaires of this kind are known as lead/lag luminaires and
are often specified for workshops and factories. Tri-phosphor fluorescent with electronic
ballast operates at 20kHz.
Circuits and controls
The way in which luminaires are connected in circuits has been described in Chapter 5.
We showed in that chapter how a switch could control each light or each group of lights.
The switch would be one of the types described in Chapter 1. There are, however, other
methods of control possible and they are particularly important for street-lighting and
other external lights for which some form of automatic control is almost essential.
The first method and one still widely used is by means of a time switch. Time switches
are made with an enormous variety of dials, or digital displays, for almost any
conceivable application can be found from some manufacturer’s standard range of
products, but in exceptional cases special dials or digital display can be made to order.
For street or external lights, the usual requirement is for the lamps to be turned on at
sunset and off either at midnight or at sunrise. The times of sunset and sunrise vary
throughout the year, and solar compensating dials are made for time switches to follow
the annual variation in sunset and sunrise. As the variation depends on latitude, different
dials/displays are made for different zones of latitude.
In the UK street-lighting is the responsibility of the local authority. The usual
arrangement is for the electricity company to supply each column directly off its main
running through the street, or if necessary off a main laid specially for the lights. A time
switch is fixed inside each column, and switches the current to the luminaire at the top of
the column. The supply is not metered because one knows the total hours in a year during
which a solar dial switch is on and also the rating of the lamp, and can thus calculate the
number of units of electricity consumed in a year. The column also contains a service
head fuse, on the supply side of the time switch.
A new housing development may often include estate roads which are constructed by
the developer, but which will ultimately be taken over by the local authority. The same
system of road-lighting can be adopted, but it is advisable to agree the details with the
street-lighting department of the local authority beforehand. The system can also be used
where the roads are to remain private, but the agreement of the electricity company must
be obtained in advance because the method of supply may affect the tariff they will want
to apply.
The design engineer should make sure to select and specify columns which have a
space inside them large enough to contain the time switch and fuse; there are light duty
columns which do not have a wide enough base.
If the external lights are not too far from buildings, final circuits can serve them from
distribution boards within the buildings. The cable sizes should be checked for voltage
drop, and may need to be greater than ordinary lighting circuits. For running underground
PVCSWAPVC or MICC, cable is probably the most suitable. A circuit to several lights is
taken through a time switch and the time switch can be conveniently mounted next to the
distribution board. It is also possible to obtain a modular-type distribution board which
has a time switch to be mounted on the DIN rail inside the board. Internal lights may also
be controlled by a time switch, and this may be a convenient way of switching, say,
corridor lights in an office block.
When lights, whether internal or external, are supplied through a time switch served
from a distribution board, the supply is of course part of the metered supply to the
building. Whether this is more or less desirable than an unmetered but timed supply
depends on the agreement between the consumer and the supply authority.
The other method of automatic control of lights is the use of photoelectric cells. In this
method, a photoelectric cell is arranged to make a circuit when the illumination falls
below a set value and to break the circuit when the illumination rises. The advantage over
a time switch is that the control takes account of weather conditions. There are summer
evenings which, because of storm clouds, are almost as dark as winter evenings, but a
time switch cannot distinguish between a cloudy day and a clear one. Furthermore, in the
UK at least, a time switch must usually be reset twice a year because of British Summer
Time, whereas a photoelectric cell needs no alteration.
When photoelectric cells are used, the lighting circuits can be arranged in the same
way as when time switches are used. There can be one cell to each light, or there may be
one cell for a circuit on which there are several lights. In the latter case, if the circuit is
switched directly by the cell, the number of lights on the circuit is limited by the current
which the cell can switch. The maximum current which a given cell can switch may be
lower if the current is inductive than if it is resistive, so that a cell may be able to switch
fewer fluorescent lights than incandescent ones. Nevertheless, a small cell can be used to
control many lights, or even several different circuits, if it is used with a relay. The cell is
placed in the operating coil circuit of the relay and the relay main contacts switch the
lights. A multipole relay makes it possible for one photoelectric cell to control several
lighting circuits.
The position of the photoelectric cell requires a little thought. It must be close to the
area illuminated by the lights it controls, in order to react to the daylight in that area. At
the same time, it must not receive direct light from any of the lamps it controls; if it did,
as soon as they came on, it would switch them off again, so that they would flicker on
and off continuously. Nor must it receive direct light from any other lamps which may be
turned on at night, otherwise switching on one set of lights will immediately switch off
the other set. In general, the illumination on the photocell during the hours of darkness
when the lights are on must be less than the level at which the cell switches the lights.
Most outdoor artificial lighting produces illumination levels much lower than daylight
and a photocell set at the minimum acceptable daylight level will not turn the lights off
until daylight is again adequate, but there are areas, such as the forecourts of garages,
Design of electrical services for buildings 122
supermarkets and cinemas, where the artificial lighting approaches full daylight level,
and in these cases it is certainly difficult, and may even be impossible, to design a
photoelectric scheme of control. For ordinary street lighting, a cell placed in the top of a
column above the lantern will respond adequately to natural light falling on the road and
can be shielded from the light of the lamp.
The number of hours in a year during which a photoelectric cell will be in the ‘on’
position cannot be known exactly. The supply authority may, therefore, be unwilling to
agree to the use of an unmetered supply. Nevertheless, several local authorities are using
photoelectric cells to control street lighting and have negotiated a suitable method of
payment with the area electricity company.
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