Page 458 - Offshore Electrical Engineering Manual

CHAPTER

Notes on Safety Integrity 3

Level Assessment for

Electrical Engineers

INTRODUCTION
As electrical engineers, we may be asked to attend Hazard and Operability (HAZOP)
study and Electrical Safety and Operability (ESAFOP) study meetings, as well as
safety integrity level (SIL) assessment meetings. All such meetings are run in a simi-
lar manner, and usually involve the breaking down of the system or installation into
a number of areas and control loops and following a set script of questions which are
applied to each item being studied. Usually software is used to present and organise
the questions and pictorially highlight the location on the plan or the loop being
studied. The intention of this chapter is to concentrate on the subject of SIL assess-
ment and provide some practical guidance on the process and also the pitfalls to be
avoided. For those who wish to take the subject further, I have included a number of
web addresses at the end of the chapter. Some HAZOP and ESAFOP references are
also provided.

SAFETY FUNCTION
There is a difference between good design of a device and the provision of functional
safety. For example, an electrical motor may have an encapsulated rotor designed not
to fly apart at high speeds, but it may also be fitted with an overspeed sensor which
trips the motor at a maximum safe speed setting. The overspeed device is a provision
of functional safety. A safety function is a function to be implemented by a safety
instrumented function (SIF), other safety technology or external risk reduction facil-
ity, which is intended to achieve or maintain a safe state for the process in question,
with respect to the specific hazard in question.

SAFETY INSTRUMENTED FUNCTION
An SIF is a safety function with a specified SIL, necessary to achieve functional
safety. This can be achieved by either implementing a safety instrumented protec-
tion function or a safety instrumented control function (see IEC61511 Definition
3.2.71).

Offshore Electrical Engineering Manual. https://doi.org/10.1016/B978-0-12-385499-5.00050-9 445
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