Efficient Plant September 2021 - 9

feature | automation safety
higher upfront price diff erence in just one
year of energy cost savings, while improving
sustainability by reducing demand on the
power grid. Th e situation is similar with
safety-control systems. Some manufacturers
are hesitant to incorporate what they believe
to be the more expensive technology represented
in safety PLCs, despite guaranteed
and measurable operations and maintenance
lifecycle savings. Beyond the cost
savings, the technology dramatically speeds
troubleshooting, reducing downtime.
RELAY SHORTCOMINGS
Plant maintenance staff oſt en appreciate the
straightforward simplicity of safety relays
and contactors. However, these components
don't have the fl exibility to meet many of
the requirements of a modern automation
solution.
Th e nature of how a safety-relay-based
system is wired makes it diffi cult to modify
without compromising the safety of the
personnel and equipment it's protecting.
Signifi cant engineering and rewiring time
is required to make changes, with lots of
opportunity for errors.
When these types of modifi cations are
implemented, electrical enclosures originating
as a well-organized collection of control
components and cables can become a confusing
tangle of crisscrossed jumpers and
rewires. Future changes become increasingly
diffi cult, and any issues become more
challenging to resolve.
All motors, switches, and other control
devices that must be accommodated in the
safety system typically require an individually
hardwired safety relay, creating many
potential points of failure to examine when
the need for troubleshooting arises. Th ese
electro-mechanical relays are more prone
to wear and failure than non-mechanized
components.
Additionally, safety-relay protection
systems may be mistakenly bypassed by
SEPTEMBER 2021
Safety relays require signifi cant wiring, making modifi cations and troubleshooting more diffi cult.
Figures courtesy Siemens
operators in the event of a malfunction,
compromising the intended protective
function.
A MODERN SOLUTION
Safety PLCs address these and other concerns.
Contrasted with the past, there is a
need to monitor many more input/output
(I/O) points, including those performing
safety-related functions. A single safety
PLC-also referred to as a safety controller
or fail-safe PLC-can control hundreds of
safety I/O points, whereas, with a safetyrelay
system, each point requires a distinct
and dedicated relay device.
For today's complex control and safety
systems, safety PLCs are the most capable
and cost-effi cient devices for performing
required safety tasks and complying with
regulations. Th ese controllers can provide
standard and safety control, simplify system
architectures, decrease design time, and
reduce electrical-panel footprints.
With soſt ware and program version
control built into most modern safety PLCs,
system changes are easily implemented
and traced. Th ese changes can be made by
simply modifying a programming routine,
and safety function libraries-pre-certifi ed
to meet IEC 61508 and ISO 26262 standards
for functional safety management, and
automotive functional safety management,
respectively-reduce testing time, increase
fl exibility, and ensure proper functionality.
Using safe-ladder logic as one means
of safety programming, safety functionality
changes can be easily made through
programming, rather than relay device
replacement and related rewiring. Modifying
an application is as simple as entering
appropriate credentials for access to the
safety program, updating the program, and
then loading the updated program to the
safety controller.
Additionally, OEMs can create custom
libraries for safety functionalities commonly
used in their solutions. Th is provides a
high degree of scalability and easy transfer
of safety standards to new and retrofi tted
applications.
When safety PLCs are used in systems
instead of standard PLCs, the need for safety
relays is eliminated. Safety PLCs incorporate
embedded diagnostics for identifying faults
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Efficient Plant September 2021

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