Functional Safety Categories

ISO 13849-1:1999 uses five basic types of designated system architectures based on the old EN 954-1:1996 standard (which became obsolete on Dec. 31, 2011):

Category B: The safety-related parts of control systems and their components must be selected and used in accordance with their respective standards and are designed to withstand expected environmental stresses (i.e. electrical load, vibration, EMC).
Risk – An error can lead to a loss of the safety function.
Safety principle is based on selection of components.

Category 1: Single channel – Not redundant
Requirements of Category B + the use of proven safety components and safety principles. (i.e. isolation of cable, wiring & mechanical protection, such as conduit or junction boxes).
Risk – An error can lead to a loss of the safety function, but the probability of occurrence is smaller than in category B.
Safety principle is based on selection of components.

Category 2: Single channel – Not redundant
Requirements of Category B + 1 + the safety function is tested or inspected at suitable intervals. This is not always the most cost-effective solution versus a Category 3 design because of the requirement for ongoing testing and inspection.
Risk – An error can lead to a loss of the safety function between checks, but the error will be detected through the check or inspection.
Safety principle is based on selection of components + control design.

Category 3: Dual channel – Redundant separate interrupts
Requirements of Category B +1 + single errors will not prevent the safety function (failsafe). Single errors are detected whenever practical (in accordance with the latest technology).
Risk – If an error occurs, the safety function is maintained, but an accumulation of undetected errors can lead to the loss of the safety function.
Safety principle is based on selection of components + control design.

Category 4: Dual channel – Redundant separate interrupts
Requirements of Category 3 + single errors are detected when or before performing the safety function. Accumulated faults will not prevent the safety function.
Safety principle is based on selection of components + control design.

EN 954-1 also includes a Risk Graph, shown below (left), which is based on the following risk factors:

S      Severity of injury
S1   Slight (normally reversible injury)
S2   Serious (normally irreversible injury or death)

F      Frequency and/or exposure to hazard
F1   Seldom to less often and/or exposure time is short
F2   Frequent to continuous and/or exposure time is long

P     Possibility of avoiding hazard or limiting harm
P1   Possible under specific conditions
P2   Scarcely possible

ISO 13849-1 was updated in 2006 and utilizes a similar risk graph, shown below (right), to determine the required performance level (PLR) of a particular safety function. It uses the same risk factors as the EN 954-1 standard (Severity, Frequency, and Possibility), but the S1 line now subdivides and the performance levels are now identified as a, b, c, d, and e (with “a” representing the lowest level of risk and “e” representing the highest level of risk). By comparing the old EN 954-1 categories with the new ISO 13849-1:2006 performance levels, one can correlate Category 4 to Performance Level e and Category 3 to Performance Level d.

ISO_13849-1_chart

ISO 13849-1:2006 also assesses performance levels using reliability data, diagnostic coverage and other factors which are beyond the scope of this basic overview. For more information on ISO 13849-1:2006, reference the publications listed below, or contact MPSA for help with the application of this important safety standard.

 

RESOURCES:

Future Safety Design, Revision of ISO 13849- and Performance Level, Omron/STI

Specific Background Information on EN ISO 13849-1:2006, Schmersal

Safety Standard ISO 13849-1, SMC

Machinery safety in accordance with EN ISO 13849, Pilz

Safety in control systems according to EN ISO 13849-1, Machine Safety – Jokab Safety products

The Buzz About ISO 13849-1: The Good, the Bad and the Ugly, EHS Today