ETL 1110-2-553
30 May 97
$hardwired# system because of the many wires and
greatly increasing the cost of fabrication. The indus--
cables which must be routed and terminated. How-
try has established a standard enclosure for motor
ever, this term will be avoided, since a simple manual
control equipment which is referred to as a motor
off-on switch with no relays or logic at all can also
control center. These centers are constructed of
be referred to as a hardwired system, and, even in the
standardized modular units which can be quickly
PLC system, there is some hardwiring.
assembled and wired in the factory. However,
because most of today's control systems of any
complexity are PLC-based, the standard size of a
c.
Relay-based control systems.
given motor starter cubicle is not large enough to
house all the relays required. Thus, if a complex
(1) History. Primitive relay systems were first
system is designed using relay logic, custom fabrica-
used commercially by telegraph companies in the
tion is required to house the equipment. RBS's have
1800's. This simple technology has been in use for
been the 20th century workhorse of industry. Electri-
decades to control the operating equipment at locks
cians have worked with this equipment over the years
and dams. The typical locks and dams constructed
and are familiar with the operating principles and the
prior to the 1960's utilized RBS's which were housed
required troubleshooting and maintenance procedures.
in custom-made switchboards. The control systems
were operated at 480 volts to reduce voltage drops
(b) An RBS requires a significant amount of
and capacitive coupling effects encountered in long
maintenance. As these systems age, relay coils fail,
control circuits. This control voltage is considered
contacts freeze together, wires vibrate loose, and ter-
extremely dangerous and has been eliminated from
minals corrode. System reliability is inversely pro-
standard design practice since the early 1960's.
portional to complexity; the more relays, coils, wires,
Recently installed RBS's are usually operated at
and contacts, the greater the probability one of these
120 volts.
components will fail. Interlocks are routed all over
the lock, and a fault can occur anywhere, thus
(2) Description. An RBS performs logic opera-
requiring considerable time to locate. Troubleshoot-
tions through the use of numerous electrical/
ing a relay-based control system requires that an
mechanical relays with coils, armatures, springs, and
electrician take a meter from point to point, some-
contacts. When the relay is energized, a magnetic
times over the whole breadth of the structure, to find
field produced by the current flowing through the coil
the problem. This takes time and could expose the
moves the armature which, in turn, causes the con-
electrician to energized electrical circuits.
tacts of the relay to change state. When the relay is
de-energized, the contacts again change state by
(c) RBS's are also very inflexible to upgrades.
spring action. Through the use of multiple relays, a
Logic changes, when necessary, require major modifi-
logical pattern can be created through the intercon-
cations to the components. Relays must be physically
nection of the relay coils and contacts. Each relay
removed and replaced, requiring time-consuming and
coil and/or contact is connected to another contact or
expensive control panel modification or replacement.
relay coil through a pair of wires. Since most relays
Changes in the logic can also require modification
have only three or four sets of contacts, large num-
and addition to the cable and raceway systems.
bers of relays and contacts are required to create a
complex logic system.
d.
PLC systems.
(3) Evaluation. While safer to operate and main-
(1) History.
tain, a 120-volt system requires the addition of more
hardwiring and relays. Therefore, adhering to
(a) Around 1970, the PLC was developed by the
modern safety standards for control voltages requires
automotive industry to replace the inflexible,
a much more costly and complex relay system just to
expensive, high-maintenance relay systems. By the
duplicate the functioning of the earlier locks. The
mid to late 1970's, PLC's were finding their way into
design is further complicated by requirements for
industrial control systems nationwide. With the
additional interlocking features to prevent damage to
PLC's came a new age in control system design.
operating equipment and machinery.
Initially, PLC processor memories were limited, but,
for the first time, complex control systems could be
(a) The switchboards necessary to contain the
created and changed by simple programming
relay-based equipment need to be custom made,
A-4
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