Under certain conditions, electrical thermometers can be used in a safety related system in accordance with EN 61508. The version of the electrical resistance thermometer or thermocouple as well as the features of the temperature transmitter, all have to be taken into account for the evaluation of the safety related system.
Need for risk reduction
Due to rising expectations of society on the level of safety in processing plants, the risks presented from technical systems have been ever more reduced over time. Guidelines and standards have been created to help every plant owner to operate to the highest levels of safety. Conducting accident analyses and risk assessments is the basis for this. The aim is to reduce the risk presented by a technical system by means of safety measures.
To prevent failure in a plant, electrical/electronic/ programmable electronic systems (E/E/PE systems) are employed. The totality of all the required safety functions which serve towards maintaining the safe state of a plant is referred to as a safety instrumented system SIS or safety related system. An example is a temperature monitoring system that, when the temperature limits are exceeded, reliably shuts down the power supply of a plant, placing it in a safe state and thus preventing a hazardous event.
Architecture of a safety related system
An E/E/PE system basically consists of the elements of sensor, controller and actuator. In this case one refers to a single-channel architecture of the safety system (1oo1 system). The architecture describes the specific configuration of hardware and software elements in a system. A 1oo1 system (1 out of 1) consists of a channel that has to work safely so that the safety function can be performed. For safety systems with multi-channel architecture, hardware or software elements are implemented with redundancy.
The probability that a safety function on demand is carried out (i.e. when a system fault occurs) is defined by the safety integrity. To obtain a measure of the requirements for safety integrity, these are divided into four safety integrity levels (SIL). If SIL 4 is achieved, the probability that the safety function is executed is at its maximum, thus the maximum possible risk reduction is ensured.
The safety integrity level always refers to the entire safety system. An element has no SIL, it may only be suitable for a SIL application. For example, the model T32.xS temperature transmitter alone does not form a safety instrumented system. The operator is responsible for defining and maintaining the required safety integrity level as well as the entire safety system and the individual elements.
WIKA, as a manufacturer of electrical thermometers, supports the user in this by confirming that the requirements of EN 61508 have been met, such as during the development of the T32.xS.
To find out more about the fundamentals of functional safety in accordance with EN61508 and the safety design for a temperature measuring point, the full White Paper can be found at http://instrumentation.co.za/+C18200
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