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SIL – functional safety

The term safety integrity level (SIL) is becoming more and more significant in the field of process technology. The safety integrity level defines the requirements that a device or a system is expected to fulfill so that the failure probability can be specified.

SIL in Ex areas – standards

The aim of the SIL is to achieve maximum possible operational reliability. If a device or system fails, a defined state is attained. A standard-based inspection is carried out based on statistical probability. Standards IEC 61508 and IEC 61511 are largely used as the basis for this.

SIL is used in many industries within process technology and process engineering, including the chemical industry, refineries, oil and gas production, paper manufacturing, and conventional power supply.

In addition to functional safety requirements, systems in potentially explosive areas are also subject to Ex standards EN 60079-0 ff.

IEC 61508: manufacturer

IEC 61508 describes the requirements that the manufacturer has to bear in mind when producing devices or systems.

Title: "Functional safety of electrical, electronic or programmable electronic safety-related systems"

IEC 61511: installation and operation of systems

IEC 61511 describes the requirements for installing and operating systems with functional safety.

Title: "Functional safety – Safety instrumented systems for the process industry sector"

Compliance with the standard is determined by operators, owners, and planners on the basis of safety plans and national regulations. In addition, the standard also describes the requirements for using a device in an application on the basis of its proven effectiveness (proven in use).

SIL inspection

The complete signal path must be taken into account during the SIL inspection. In a typical safety application, the safety integrity level is calculated using average failure probabilities of individual devices.

Explanation: Table 2 of IEC 61508-1 describes the relationship between the average failure probability and the achievable SIL. The level required determines the overall budget for the sum of all PFD (probability of failure on demand) values.

A system with a single-channel structure and low demand rate is used as an example. In this case, for a SIL of 2 the average PFD value is between 10-3 and 10-2.

Please refer to the failure limit values in the table.

SILOperating mode with low demand rate
(average probability of the specified function failing on demand)
4≥ 10-5 to < 10-4
3≥ 10-4 to < 10-3
2≥ 10-3 to < 10-2
1≥ 10-2 to < 10-1

Application example

Possible distribution of PFD values  

Possible distribution of PFD values in a safety closed-loop control circuit

A sensor and actuator are assembled in the field and are exposed to chemical and physical loads – including the process medium, pressure, temperature, and vibration. Accordingly, these components have a high risk of failure. The sensor therefore represents 25% and the actuator 40% of the total PFD (probability of failure on demand).

15% remains for the failsafe controller and 10% for each of the Interface modules. Neither of these come into contact with the process medium and both are usually located in a protected control cabinet.

Calculation is based on typical values.

Terminology from EN 61508 and EN 61511

SILSafety integrity level
One of four discrete levels for assessing the requirements for the reliability of safety functions in E/E/PE systems. E/E/PE refers to electrical/electronic/programmable electronic systems. SIL 4 is the highest safety integrity level and 1 is the lowest.
EUCEquipment under control
Equipment, machines, devices or systems used in production, materials processing or transport.
MTBFMean Time Between Failures
The expected mean time between failures.
PFDProbability of failure on demand
The probability of a failure on demand. Describes the probability of a safety instrumented system failing to perform its function when required.
PFDavgAverage probability of failure on demand
The average probability of the function failing on demand.
E/E/PESElectrical/electronic/programmable electronic system
All electrical devices and systems that can be used to perform a safety instrumented function. Includes both simple electrical devices and all types of programmable logic controllers (PLCs).
PFHProbability of dangerous failure per hour
Describes the probability of a dangerous failure occurring per hour.
SFFSafe failure fraction
The safe failure fraction is the ratio of the rate of safe failures plus the rate of diagnosed or detected faults in relation to the total failure rate of the system.
SIFSafety instrumented function
SISSafety instrumented system
The safety instrumented system consists of one or more safety instrumented functions. An SIL requirement is applicable for each of these safety instrumented functions.


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