UFC 3-535-01
17 November 2005
15-3.6
Several factors should be considered in deciding between a PLC or a PC
based system. Some considerations are:
15-3.6.1
Monitoring AGL equipment the PLC system must be customized for each
type of CCR, since to date there is no PLC software combination that is available or
easily configured for this task.
15-3.6.2
For PC based systems, software has been developed and tested, and is
easily configured to monitor AGL equipment. Software developed for PC systems is
specific for the task and works with all types of CCRs and other related AGL equipment.
15-3.6.3
With PLC systems there are established industrial standards and criteria
for mean-time-between-failure (MTBF).
15-4
CONSTANT CURRENT SERIES CIRCUITS
A constant current series circuit is an ungrounded system, where circuit elements are
connected in a string with the same current flowing to each element. The circuit is one
continuous loop starting and ending at the power source (the constant current regulator
for the AGL circuit). The CCR maintains a constant current independent of the load on
the circuit.
15-4.1
With constant current, a short-circuit across the output of the CCR is a no-
load condition, and an open-circuit is an overload. In a simple series circuit, a lamp
failure would cause an open-circuit. For this reason a bypass device such as an
isolating transformer is installed between the primary side and secondary side as part of
each light fixture connection, or for connecting other loads such as guidance signs. In
certain cases, such as for approach lights where 5 steady-burning lights are on a single
secondary circuit isolated from the primary series circuit by one or more isolation
transformers, a film disc cutout may be used. Film disc cutouts operate in such a way
that when a lamp burns out, the film-disc cutout shorts the failed lamp, thereby allowing
current to flow to the remaining connected fixtures.
15-4.2
Advantages of series lighting circuits are:
15-4.2.1
All lamps operate at the same current, hence same intensity.
15-4.2.2
Single-conductor cable of one size and insulation voltage rating can be
used throughout the circuit.
15-4.2.3
Intensity control of lights can be obtained over a wide range.
15-4.2.4
A single ground fault along the circuit will not affect light operation.
15-4.2.5
Ground faults are easily located.
15-4.3
Major disadvantages of series lighting circuits are:
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