Automation Canada Machine Safety Issue - 6

A CLOSER LOOK AT SAFETY VALVES AND THE
VALUE OF PERFORMANCE LEVEL E CNT'D.

The next requirement is to determine how well the safety function
needs to perform which will in turn identify the required Performance
Level (PLr). For this it is necessary to determine severity or harm when
someone would be exposed to a hazard, the potential frequency of that
exposure and finally the possibility of avoiding the hazard. There are
charts and scales available that allow you to consider these risks and
determine the SIL or PLr to meet the safety needs for that machine.
It is prudent at this point to consider safety system structures. This is
designated architecture defined as categories from B-4 with each safety
function is designated with an input - logic - output device. Examples
could be light curtain as an input, a safety PLC as a logic and a safety
exhaust valve as an output. This is important when evaluating if the
safety measures of the system meet the required Performance level.
* Category B & Category 1 = Have a single channel architecture with
no resistance to faults, meaning if it fails the system is no longer safe
* Category 2 = Requires some frequent testing, used primarily for
electrical systems
C A N A D I A N A U TO M AT I O N

* Category 3&4 = Have dual channel architecture, or redundant
systems and differ only by the amount of diagnostics, or cross
monitoring of the system in place
* Category 4 = Has more diagnostics to monitor every channel of the
system with a higher frequency
By assessing in this way, you can verify if the performance level of the
safety functions put in place are equal or greater than the required
performance level.
There are defined parameters that need to be determine for the safety
components used in the safety system. You would need to know the
diagnostics coverage, common cause failure values, design
architecture and the mean time to dangerous failure calculations for
each input, output, and logic component of the safety system. And as
noted performance levels must always meet or exceed the analysis
levels determined by the safety analysis.

VOLUME 2, ISSUE 6

6
Automation Canada Machine Safety Issue

Table of Contents for the Digital Edition of Automation Canada Machine Safety Issue
