88 / 1145
InstMC FS2016 (Rev 3.0)
Page
2
of
10
Nicol Instrument Engineering Limited
In some cases, the definitions still differ from the definitions of the same terms used in IEC 61508-
4:2010 because of the terminology used in the process sector, however there is no difference in the
technical meaning between IEC 61511-1 edition 2.0 definitions and the definitions of the same terms
in IEC 61508-4:2010.
Notable deletions are; Electrical/electronic/programmable electronic system, External risk reduction
facilities, Proven-in-use, and Safe failure fraction.
Notable additions or clarifications is on:
Definition item
Comments
“Bypass”
New definition of an action or facility that prevent all or parts of the SIS
functionality from being executed
Common cause / mode
failures
Clarification on the definition.
Conservative approach
New definition on cautious way of doing analysis and calculations.
Harmful event,
hazardous event,
hazardous situation
New definition added.
MRT, MTTR, MPRT
New definitions for repair or restoration.
Mode of operation
Clarified of a low demand mode being where the SIF is only performed
on demand and where the frequency of demands is no greater than
one per year, a high demand mode is where the SIF is only performed
on demand and where the frequency of demands is greater than one
per year, and continuous mode is where the SIF retains the process in
a safe state as part of normal operation.
Operating environment
New definition added on the conditions inherent to the installation of
a device that potentially affects its functionality and safety integrity,
etc. etc.
Process operating mode
New definition added for any planned state of process operation,
including modes such as start-up after emergency shutdown, normal
start-up, operation, and shutdown, temporary operations, and
emergency operation and shutdown.
Performance
New definition added for achieving a given action or task measured
against the specification.
Prior use
This definition replaces proven in use, and outlines a documented
assessment for a device is required to show suitable for use in a SIS,
and should be based on previous operating experience in similar
operating environments.
Process safety time
New definition outlining the time period between a failure occurring in
the process or the basic process control system
and the occurrence of the hazardous event if the SIF is not performed.
Protection layer
New definition for an independent mechanism that reduces risk by
control, prevention or mitigation.
Application program
life-cycle
New definition for activities occurring during a period of time that
starts when the application program is conceived and ends when the
application program is permanently disused.
Systematic capability
New definition for measure of the confidence that the systematic
safety integrity of a device meets the requirements of the specified SIL,
in respect of the specified safety function, when the device is applied
in accordance with the instructions specified in the device safety
manual.