As discussed within the 'fluorescent basics' section, ionisation
within a fluorescent lamp results in the release of electrons and
the creation of positive ions. This action causes an increase in
lamp current and may cause more collisions, more ionisation etc.
If this process is not controlled, the lamp current will rise to
a level that would result in the destruction of the lamp. For this
reason, a standard fluorescent lamp must be connected to some form
of current limiting device. A starting device is also necessary
to provide the characteristics required to strike the lamp.
This is the term given to the form of control gear which for many
years has been the industry standard. The lamp current is limited
by a wire wound magnetic choke or ballast. The choke, which is basically
a coil of wire, makes the circuit inductive, stepping the out of
phase with the voltage. To reduce this phase difference and improve
the efficiency of the circuit it is necessary to include a power
factor correction capacitor. Starting is achieved by the use of
a glow starter which consists of a sealed glass vessel containing
either two bi-metallic contacts or one fixed and one bi-metallic
contact. When power is applied to the lamp circuit, an arc produced
between the two starter contacts heats them until they close. While
the contacts are closed, the lamp cathodes are heated and so emit
electrons. When the starter contacts have cooled sufficiently they
open, causing the magnetic field in the choke to collapse. This
creates a high voltage between the cathodes (in the order of 800V)
which strikes the lamp. The standard switch start circuit cannot
starters can be used as a direct replacement for the glow starter.
Through the use of solid state electronics, these starters perform
the same function as the glow starter but are less damaging to the
lamp cathodes and therefore extend the useful life of the lamp.
If there is insufficient cathode emitter material to sustain the
discharge, a glow starter will continue making attempts to strike
the lamp until either the lamp is changed, or the starter or some
other component in the circuit fails. Most electronic starters employ
a safety shut down system for failed lamps preventing the constant
strike/re-strike cycle associated with older standard switch start
installations. Most glow starters cannot be used in temperatures
below 0ºC. Standard Electronic starters will reliably strike
a lamp at much lower temperatures (down to -20ºC) with special
variants available for even colder installations (down to -40ºC).
It should be noted however, that at these temperatures the lumen
output from a fluorescent lamp is very poor. Although more expensive
to purchase than the more common glow starter, electronic starters
need not be replaced when lamps are changed and will usually continue
to function for more than 10 years.
High frequency electronic control gear consists of a single ballast
unit that performs the functions of all of the major components
in a standard switch start circuit. Through the use of solid state
electronics the HF ballast creates a discharge frequency of 25-40kHz.
This is far higher than standard switch start circuits (100Hz) and
results in a vast improvement in the quality of the light produced.
Lamp flicker is eliminated and as HF operation is more efficient
than normal operation, significant savings can be made in energy
costs. Most HF control gear also incorporates a warm start facility
to extend the useful life of the lamp.
Frequency Dimming (HFD)
Dimming is not possible when using standard switch start or constant
output high frequency control gear. If the dimming of fluorescent
lamps is required, special control gear must be used. Several types
of high frequency dimming system exist but the most common types
employ a 1-10V low voltage analogue control line which is totally
independent of the mains supply and links each luminaire and the
dimmer unit. The dimmer unit is basically a variable resistance
which is placed across the analogue control line. As this system
is analogue, it is both limited in the scope of operation and susceptible
to interference. Other systems exist that use a digital code to
communicate between the ballast units. This system is less affected
by interference and offers the user greater functionality. The latest
HFD units use the relatively new DALI industry standard for digital
control. This new system will allow building management and lighting
control systems to communicate with luminaires from different manufacturers
using the same language. It is even possible for the luminaires
to report operating conditions such as lamp failures etc. When dimmed
using HFD control gear, fluorescent luminaires require less energy
than equivalent non-dimmable circuits further increasing the potential
for energy savings.
Switching Facility (DS)
If a luminaire is wired so that it contains two or more separate
fluorescent circuits within the same body then each circuit can
be switched independently. This allows the output of a fluorescent
luminaire to be controlled without the need for a dimming system.
Part of each luminaire can be switched off when not required therefore
reducing wasted energy. The separate switching principle can be
applied to most types of control gear at a fraction of the cost
of upgrading to high frequency dimming.
In the event of a complete power failure, some form of emergency
lighting must be installed to provide adequate illumination for
the evacuation of a building. This emergency lighting can either
take the form of dedicated emergency luminaires which illuminate
only when required, or standard luminaires which are supplied with
factory fitted emergency units. Emergency units fitted to standard
luminaires can be supplied equipped with a dedicated emergency lamp
(Non-maintained) or a system that illuminates one of the existing
lamps within the luminaire in the event of a power failure (Maintained).
The type of system employed is dependant upon the type of luminaire
but generally, standard fluorescent luminaires are fitted with maintained
emergency systems. The power for emergency operation is normally
supplied by a rechargeable battery system. This may be a single
central battery system supplying the whole building or self contained
battery packs within each luminaire.