TABLE OF CONTENTS In a safety instrumented function the logic solver provides the intelligence to perform any needed logic as well as many other potential functions such as filtering, averaging, comparison and calculations. There are several types of technologies that have been used including relays, pneumatic logic, solid state logic, intrinsically safe solid state logic, programmable logic controllers, distributed control systems and purpose built safety programmable logic controllers.
Electromechanical relays and pneumatic logic have been used for many decades to perform logical functions in automated safety protection systems. These circuits can be wired (or piped) to perform AND, OR and even 2oo3 voting functions. Time delay relays provide filtering functionality. "Trip Amplifiers" are also available that convert analog inputs into a digital trip signal for use with relay logic. Generally such circuits have a bias toward a fail de-energized mode.
Special relays called "Safety Relays" are even constructed with multiple springs, protection against welded contacts, positive guided contacts, mechanically connected multiple contacts for monitoring and sealed construction. These components are strongly biased to the de-energized failure mode and generally have a safety certification. With such components the designer can create a safety instrumented function with highly predictable "fail-safe" characteristics; a primary advantage of the relay logic approach. A safety relay is shown in Figure 10-1.
Relays are considered a good choice for the logic solver when the I/O count is low (less than 6) and future logic changes are highly unlikely.
Given the availability of a...
