ETL 1110-2-553
30 May 97
f.
Conclusions.
As with all solid-state devices, PLC's must be oper-
ated within their temperature and humidity limits and
(1) RBS disadvantages. For design of modern
protected from the accumulation of dust and from
lock control systems, there is no reason to consider
lightning- or power-system-induced voltage surges.
RBS's, just as there is no reason to design a mechani-
Manufacturers have developed enclosures and devices
cal adding machine. Compared to PLC systems, they
to compensate for these limitations and pro-vide pro-
are extremely difficult and expensive to incorporate
tection in most industrial environments. PLC's are
changes and quickly become unwieldy and less reli-
suitable for application in the typical lock and dam
able as their complexity increases. While it is still
environment when properly specified.
possible to design an RBS to control a lock, it will be
more expensive to fabricate, it will be limited to the
(c) PLC's have been programmed with expert
capabilities of 1960's technology, and, with current
self-diagnostic programs to assist maintenance per-
safety requirements and levels of interlocking, it has
sonnel when problems occur. Because of their modu-
become obsolete technology. There will eventually
lar construction, repair usually involves the simple
be no knowledge or skill base to implement changes,
replacement of a modular component which should be
and finally, it may become an electrician's nightmare
available from a spare parts inventory onsite. More
to obtain and maintain parts.
commonly, the source of the problem is external to
the PLC and involves one of the field devices, such
(2) Current technology. PLC systems provide
as a limit switch which has vibrated loose. Often the
the opportunity to do things differently; to do things
problem can be conveniently identified by trouble-
which have been heretofore impossible with relay
shooting the system from the comfort of a computer
systems. PLC systems are limited only by the
terminal. Through the use of a modem, field person-
creativity of their designers and the budget or policy
nel can tap into resources from the electrical engineer
constraints under which the designers function.
at the district office to assist in the troubleshooting
effort.
(3) Training. The need for training of mainte-
e.
Comparative costs.
nance personnel and the development of in-house
electronic engineering expertise to design and modify
(1) First costs. There is a decided economic
PLC systems are essential for these systems to suc-
advantage in favor of the PLC systems. Most of the
cessfully function as intended. This is a relatively
higher cost of the RBS is due to several factors: the
new, more complex technology requiring higher
large amounts of cable and raceways, the cost to
levels of technological skills from operators, main-
furnish, mount, wire, and enclose the numerous
tainers, and designers. It is unrealistic to assume that
relays, and the costs to hardwire the control consoles.
a PLC system can be successfully designed, built, and
There are also structural costs associated with the
turned over to personnel experienced on the older sys-
increased space requirements necessary to accom-
tems without significant training for the electri-
modate the additional relays compartments, intercon-
cians for upgrade to electronic technicians and with-
necting wiring, conduit, cable trays, etc. These costs
out qualified electronic engineering support during the
could be significant.
transitional period. Without a management commit-
ment to personnel, funding, training, and support, the
(2) Life-cycle costs. The economic advantage of
system will not be successfully implemented. Policy
the PLC would be expected to increase if life-cycle
for the implementation of the PLC system should be a
costs were calculated. The PLC would require less
top down decision.
maintenance and would increase system reliability.
A-3.
Guidance
Its flexibility would reduce costs to implement
changes to the system. One of its biggest assets,
however, is the ability to monitor, detect, and protect
a. General. Control systems and associated
against abnormalities and malfunctions in machinery
structures for operating navigation locks can be
and gate operation.
designed for different levels of technology, depending
A-6
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