Centro Barbecue 6500 Safe use Specifications download pdf (Page 11)

Rockwell Automation Publication 2094-RM002B-EN-P - May 2012 11

Safety Concept Chapter 1

Performance Level and Safety Integrity Level (SIL) CL3

For safety-related control systems, Performance Level (PL), according to ISO

13849-1, and SIL levels, according to EN 61508 and EN 62061, include a rating

of the system’s ability to perform its safety functions. All of the safety-related

components of the control system must be included in both a risk assessment and

the determination of the achieved levels.

Refer to the ISO 13849-1, EN 61508, and EN 62061 standards for complete

information on requirements for PL and SIL determination.

PFD and PFH Definitions

Safety-related systems can be classified as operating in either a Low Demand

mode, or in a High Demand/Continuous mode:

• Low Demand mode: where the frequency of demands for operation made

on a safety-related system is no greater than one per year or no greater than

twice the proof-test frequency.

• High Demand/Continuous mode: where the frequency of demands for

operation made on a safety-related system is greater than once per year or

greater than twice the proof test interval.

The SIL value for a low demand safety-related system is directly related to order-

of-magnitude ranges of its average probability of failure to satisfactorily perform

its safety function on demand or, simply, average probability of failure on demand

(PFD). The SIL value for a High Demand/Continuous mode safety-related

system is directly related to the probability of a dangerous failure occurring per

hour (PFH).

PFD and PFH Data

These PFD and PFH calculations are based on the equations from Part 6 of

EN 61508 and show worst-case values.

This table provides data for a 20-year proof test interval and demonstrates the

worst-case effect of various configuration changes on the data.

Table 2 - PFD and PFH for 20-year Proof Test Interval

Attribute Value

PFH [1e-9] 4.09

PFD [1e-4] 3.90

SFF % 99.5

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 41 42

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Centro Barbecue 6500 Safe use Specifications Page 11