User manual

Publication 1756-RM093F-EN-P - January 2010 19

Safety Integrity Level (SIL) Concept Chapter 1

GuardLogix PFD and PFH

Specifications

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

demand mode, or in a high demand/continuous mode. IEC 61508

quantifies this classification by stating that the frequency of demands

for operation of the safety system is no greater than once per year in

the low demand mode, or greater than once per year in high

demand/continuous mode.

The Safety Integrity Level (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, 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 values are associated with each of the three primary

elements making up a safety-related system (the sensors, the logic

element, and the actuators). Within the logic element you also have

input, processor, and output elements.

For PFD and PFH values and functional verification (proof) test

intervals for CIP Safety I/O modules, see Appendix

E, Probability of

Failure on Demand (PFD) and Probability of Failure per Hour (PFH)

Data.

PFH Example

DeviceNet EtherNet

Logix5562S

Logix55LSP

Sensor

1791DS-IB12

GuardLogix Controller

1791DS-IB8XOB8

1791DS-IB4XOX4

Actuator

LOOP 1

LOOP 2