When comparing the life cycle costs of one
system with those of another, the greater ini-
tial cost of an energy-effective system will
almost always be recouped after a period of
time because of the saving in energy costs.
Thispayback periodvaries with different sys-
tems.
In comparing dissimilar systems, it is
impossible to place a dollar value on the
quality of light. A direct system, for example,
is usually less costly than an indirect one
that produces the same quantity of light on a
horizontal workplane, but thequalityof light
is vastly different.
Cost comparisons are made on equal
illuminance values of equivalent quality. If
there is a difference in the connected load,
the additional air-conditioning required to
handle the larger load must also be counted.
Switch Control
An electric current is the flow of electrons
between two points along a path. If the path
is interrupted, the current cannot flow. A
switchbreaksthe flow of electricity in a cir-
cuit when it is open (“off”) and it allows
unimpeded flow when closed (“on”).
Manual Switches
The manually operatedtoggle switchmakes
contact by snapping one metal piece against
another.Mercury switchescontain a vial of
mercury; contact is made between two elec-
trodes when the vial is tripped to the “on”
position. These switches operate silently.
The toggle designates “on” in the up position
and “off” in the down position. A rocker
switchand apush-button switchoperate in
the same manner (figure 11.6).
A single-pole, single-throw switch is
connected at any point between the
luminaire and the power supply. It opens
only one side of the circuit and is therefore
called a “single-pole”; it moves only between
an open and a closed position and is there-
fore called a “single-throw.” This is the
switch most frequently used to control elec-
tric luminaires and wall receptacles.
A single-pole, double-throw switch
directs the current in either of two directions.
It is used to alternately turn on two different
luminaires with a single switch action, such
as a safelight and the general light in a dark-
room. The up position will designate “on” for
one luminaire, the down position “on” for theELECTRICITYFigure 11.6 Toggle switch and rocker switch.