As a result of the devastating poison gas accident in Seveso, Italy, in 1976, it was decided to tighten the laws and regulations for the protection of people, living beings and the environment. The first result was the standard IEC 61508 "Functional safety of electrical/electronic programmable safety-related systems", published in 1998, which has been applied as EN 61508 since 2002. This standard defined the safety requirements in automation technology comprehensively for the first time. While IEC 61508 is primarily aimed at manufacturers of components for protective devices, IEC 61511 "Functional safety - Safety instrumented systems in the process industry" is relevant for operators and planners of protective devices.

The aim of all these standards is to reduce the risk to an acceptable level.
In the case of a piping and instrumentation flow diagram in plant and process engineering for the safety assessment of a process, for example, the individual components of sensors, evaluation electronics and actuators must be considered separately in order to find a general solution approach. The sensors measure physical process variables such as temperature, level or pressure and convert the measured value into a standard signal. Control units are generally used for the evaluation electronics, which convert, linearize, store and process the process variables according to rules and then forward them to the actuator for execution. Actuators can then intervene in the process sequence by adjusting functional variables of various kinds.

Two approaches

Safety-related variants can be implemented in the evaluation electronics using two approaches: One approach is for users to rely on a programmable safety controller. This solution is particularly suitable for special plant construction with a process control system and is characterized by extensive functional applications as well as a high signal density and number of signals. However, complex programming applications are required here; in addition, the inputs and outputs are tied to card types and multi-channel capability. The investment costs vary depending on the number of channels and the amount of software required. In addition, each application must be calculated and evaluated separately according to SIL. Solutions up to SIL 4 can be realized in this way.

An alternative to the programmable safety controller is the use of safety
use of safety monitors or limiters, such as those offered by Jumo with the 'SafetyM' series. Such a solution is particularly suitable for smaller applications such as special machines and individual applications with a low signal density and number of signals. In addition to low investment costs, the advantages include less parameterization work for each application. The application works with standard signals and relies on a redundant internal structure with redundant sensor inputs. Three different function outputs (analog/binary) are available. In principle, solutions up to SIL 3 are possible.

In conjunction with defined sensors from Jumo, the entire SIL chain has already been calculated and the company can issue the corresponding certificates. However, sensors from other manufacturers can also be easily connected to the safety monitors or limiters. There are also no manufacturer restrictions with regard to the actuators.