1. What is a SIS?

A SIS is a Safety Instrumented System. It is designed to prevent or mitigate hazardous

events by taking the process to a safe state when predetermined conditions are violated.

A SIS is composed of a combination of logic solver(s), sensor(s), and final element(s).

Other common terms for SISs are safety interlock systems, emergency shutdown

systems (ESD), and safety shutdown systems (SSD). A SIS can be one or more Safety

Instrumented Functions (SIF).

2. What is a SIF?

SIF stands for Safety Instrumented Function. A SIF is designed to prevent or mitigate a

hazardous event by taking a process to a tolerable risk level. A SIF is composed of a

combination of logic solver(s), sensor(s), and final element(s). A SIF has an assigned

SIL level depending on the amount of risk that needs to be reduced. One or more SIFs

comprise a SIS.

3. What is SIL?

SIL stands for Safety Integrity Level. A SIL is a measure of safety system performance,

or probability of failure on demand (PFD) for a SIF or SIS. There are four discrete

integrity levels associated with SIL. The higher the SIL level, the lower the probability of

failure on demand for the safety system and the better the system performance. It is

important to also note that as the SIL level increases, typically the cost and complexity

of the system also increase.

A SIL level applies to an entire system. Individual products or components do not have

SIL ratings. SIL levels are used when implementing a SIF that must reduce an existing

intolerable process risk level to a tolerable risk range.

4. What does functional safety mean?

Functional safety is a term used to describe the safety system that is dependent on the

correct functioning of the logic solver, sensors, and final elements to achieve the desired

risk reduction level. Functional safety is achieved when every SIF is successfully carried

out and the process risk is reduced to the desired level.

5. Why were the ANSI/ISA 84, IEC 61508, and IEC 61511 standards

developed?

The standards were a natural evolution for the need to reduce process risk and improve

safety through a more formalized and quantifiable methodology. Additionally, and

specifically for IEC 61508, as the application and usage of software has evolved and

proliferated, there was an increased need to develop a standard to guide system /

product designers and developers in what they needed to do to ensure and “claim” that

their systems / products were acceptably safe for their intended uses.

Click here for additional information on Standards.

6. When do I need a SIF or a SIS?

The philosophy of the standards suggests that a SIS or SIF should be implemented only

if there is no other non-instrumented way of adequately eliminating or mitigating

process risk. Specifically, the ANSI/ISA-84.00.01-2004 (IEC 61511 Mod) recommends a

multi-disciplined team approach that follows the Safety Lifecycle, conducts a process

hazard analysis, designs a variety of layers of protection (i.e., LOPA), and finally

implements a SIS when a hazardous event cannot be prevented or mitigated with

something other than instrumentation.

7. What is a proof-test interval?

Proof testing is a requirement of safety instrumented systems to ensure that everything

is working and performing as expected. Testing must include the verification of the

entire system, logic solver, sensors, and final elements. The interval is the period of

time that the testing occurs. The testing frequency varies for each SIS and is dependent

on the technology, system architecture, and target SIL level. The proof-test interval is

an important component of the probability of failure on demand calculation for the

system.

8. What is a Process Hazard Analysis (PHA) and who conducts this?

A PHA is an OSHA directive that identifies safety problems and risks within a process,

develops corrective actions to respond to safety issues, and preplans alternative

emergency actions if safety systems fail. The PHA must be conducted by a diverse team

that has specific expertise in the process being analyzed. There are many consulting and

engineering firms that also provide PHA services. PHA methodologies can include a

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What-If Analysis, Hazard and Operability Study (HAZOP), Failure Mode and Effects

Analysis (FEMA), and a Fault Tree Analysis.

9. What voting configurations are required for each SIL level?

Obtaining a desired SIL level is dependent on a multitude of factors. The type of

technology employed, the number of system components, the probability of failure on

demand (PFD) numbers for each component, the system architecture (e.g., redundancy,

voting), and the proof testing intervals all play a significant role in the determination of

a SIL level. There is not a standard answer for what voting configurations are required

for each SIL level. The voting architecture must be analyzed in the context of all the

factors noted above.

10. Will a SIL rated system require increased maintenance?

SIL solutions are certainly not always the most cost-effective solutions for decreasing

process risk. Many times, implementing a SIL solution will require increased equipment,

which inevitably will require increased maintenance. Additionally, it is likely that the

higher the SIL level, the more frequent the proof testing interval will be, which may

ultimately increase the amount of system maintenance that is required. This is why the

standards recommend a SIL based solution only when process risk cannot be reduced by

other methods, as determined by LOPA.

11. Can a F&G system be a SIF or SIS?

A Fire and Gas (F&G) system that automatically initiates process actions to prevent or

mitigate a hazardous event and subsequently takes the process to a safe state can be

considered a Safety Instrumented Function / Safety Instrumented System.

However, it is absolutely critical in a F&G system to ensure optimal sensor placement. If

there is incorrect placement of the gas / flame detectors and hazardous gases and

flames are not adequately detected, then the SIF / SIS will not be effective.

Correct sensor placement is more important than deciding whether a F&G SIF / SIS

should be SIL 2 or SIL 3.

12. What is SIL 4?

SIL 4 is the highest level of risk reduction that can be obtained through a Safety

Instrumented System. However, in the process industry this is not a realistic level and

currently there are few, if any, products / systems that support this safety integrity

level.

SIL 4 systems are typically so complex and costly that they are not economically

beneficial to implement. Additionally, if a process includes so much risk that a SIL 4

system is required to bring it to a safe state, then fundamentally there is a problem in

the process design which needs to be addressed by a process change or other non-

instrumented method.

13. Can an individual product be SIL rated?

No. Individual products are only suitable for use in a SIL environment. A SIL level

applies to a Safety Instrumented Function / Safety Instrumented System.

14. What type of communication buses or protocols are applicable for SIL 2 or

SIL 3 systems?

The type of communication protocol that is suitable for a SIL 2 or SIL 3 system is really

dependent on the type of platform that is being used. Options include, but are not

limited to: 4-20 mA output signal, ControlNet (Allen Bradley), DeviceNet Safety (Allen

Bradley), SafetyNet (MTL), and PROFIsafe. Currently, the ISA SP84 committee is

working on developing guidelines for a safety bus, to make sure that the foundations

comply with IEC 61508, and IEC 61511 standards. The first devices with a safety bus

should be available by 2008. The Fieldbus Foundation is actively involved in the

committee and working on establishing Foundation Fieldbus Safety Instrumented

Systems (FFSIS) project to work with vendors and end users to develop safety bus

specifications.

15. For General Monitors, how can I access the PFD and MTBF data for the

products?

The General Monitors SIL certificates have the PFD, SFF, and SIL numbers that

correspond to each product. MTBF data can be provided by request. 16. Can a manufacturer state their products are “SIL X certified” rather than

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“suitable for use in a SIL X system”?

Individual products are only suitable for use in a SIL environment. A SIL level applies to

a Safety Instrumented Function / Safety Instrumented System.

Product certificates are issued either by the manufacturer (self-certification), or other

independent agency to show that the appropriate calculations have been performed and

analysis has been completed on the individual products to indicate that they are

compatible for use within a system of a given SIL level.

However full IEC 61508 certification can apply to a manufacturer’s processes. Full

certification implies that a manufacturer’s product development process meets the

standards set forth in the appropriate parts of sections 2 – 3 of IEC 61508 (including

hardware / system and software). However, this does not mean that the individual

products are more reliable or more safe. It just means that the engineering process has

been reviewed.

There are very few nationally accredited bodies that can issue nationally accredited

certifications. Other consulting firms issue certificates that indicate that the product

and / or process has been reviewed by an independent third party. 17. Can a manufacturer state their products meet all parts of the requirements

of IEC 61508 parts 1 to 7?

IEC 61508 consists of the following parts, under the general title Functional Safety of

electrical/electronic/programmable electronic safety-related systems:

Part 1: General requirements

Part 2: Requirements for electrical/electronic/programmable electronic safety-related

systems

Part 3: Software requirements

Part 4: Definitions and abbreviations

Part 5: Examples of methods for the determination of safety integrity levels

Part 6: Guidelines on the application of parts 2 and 3

Part 7: Overview of techniques and measures

The following section have provisions to which it may be necessary to conform to in

order to be able to claim compliance with the standard: part 1 (excluding annexes); part

2 (including annexes); part 3 (including annexes A and B, excluding annex C); and part

4 (excluding the annex).

Sections 5, 6 and 7 are informative only, and assist in understanding the standard, but

are not necessary to conform to in order to be able to claim compliance.

Manufacturers of products generally meet Section 2 requirements to determine through

a FMEDA analysis that their products are suitable for use within a given SIL level

Companies choosing to certify their engineering processes and receive full IEC 61508

certification will also comply with Section 3 as it relates to software development.

18. What does SIL X suitable mean, is this a valid statement as per the

standard IEC 61508 or can any other wording be used?

SIL stands for Safety Integrity Level. A SIL is a measure of safety system performance,

or probability of failure on demand (PFD) for a SIF or SIS. There are four discrete

integrity levels associated with SIL. The higher the SIL level, the lower the probability of

failure on demand for the safety system and the better the system performance. It is

important to also note that as the SIL level increases, typically the cost and complexity

of the system also increase.

A SIL level applies to an entire system if it reduces the risk in the amount corresponding

to an appropriate SIL level. Individual products or components do not have SIL ratings.

SIL levels are used when implementing a SIF that must reduce an existing intolerable

process risk level to a tolerable risk range.

Only the end user can ensure that the safety system is implemented to be compliant

with the standards. It is up to the user to ensure that procedures have been followed

properly, the proof testing is conducted correctly, and suitable documentation of the

design, process, and procedures exists. The equipment or system must be used in the

manner in which it was intended in order to successfully obtain the desired risk

reduction level. Just buying SIL 2 or SIL 3 suitable components does not ensure a SIL 2

or SIL 3 system.

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