Page 557 - IRSEM_Main Book

4.7 Requirement of Safety Related System:

a) Safety function requirements i.e., what is function supposed to do

b) Safety integrity requirements i.e., what is the likelihood of the correct operation
of the safety function

4.8 Safety Integrity: The ability of a safety related system is to achieve its required safety
function under all stated conditions within stated operational environment and within
a stated period of time. If you introduce a protection system to reduce risk you are
now “Trusting” it, to do the job. Safety Integrity is a measure of that Trust.

4.9 Safety Functions in Railway Signalling:- Important elements of Signalling System:

(a) Interlocking System
(b) Train Detection System

(c) Point Operation

(d) Signal
(e) ATP and/or ATC Infrastructure

The functions performed by the above systems can be considered safety functions. SIL
is associated with these safety functions and not the equipment.

4.10 SIL is used for two purposes:

a) SIL is used to give an interval for the rate of safety critical failures. Applies to
random faults mostly due to intrinsic physical processes such as aging.

b) SIL defines measures to be applied in the design and during the manufacturing
process to keep systematic failures low.

4.11 SIL Integrity Level (SIL) Table:

Tolerable Hazard Rate Safety Integrity Level
(THR)
(SIL)

-8
-9
10 THR 10 SIL 4
-7
-8
10 THR 10 SIL 3
-7
-6
10 THR 10 SIL 2
-5
-6
10 THR 10 SIL 1

Annexure:G1: Reliability, Availability, Maintainability & Safety (RAMS) Page 500 of 535