Safety Instrumented Systems (SIS)

Safety in industrial plants is ensured by layers of protection. The BPCS (Basic Process Control System), the control system itself is the lowest layer of protection and is responsible for the operation of the plant in normal conditions. If it fails or is not capable of maintaining control, then, the second layer, the Operator Intervention (OI) Layer attempts to solve the problem. If the operator also cannot maintain control within the requested limits, then the SIS (Safety Instrumented System) Layer must attempt to bring the plant in a safe condition. If SIS also fails in restoring to the normal operation, the hazard occurs.



What is a SIS?

SIS stands for SAFETY INSTRUMENTED SYSTEM. A SIS is intended to prevent hazard situations and mitigate their effects, by taking the process into a safe state. It is a specialized very reliable interlock system for high risk industries with explosion risk. Whether it is called SIS, Emergency Shut Down, Safety Shut Down or Safety Interlock System, it provides a protective layer for personnel, plant and environment. The need for a SIS and its level of complexity and protection is determined by Hazard and Risk Analysis.

Basically, a SIS is composed of field mounted sensors, relays, valves and PLC for processing of input signals, alarm and activates outputs in accordance with a Cause and Effect diagram.


What is a SIF?

SIF stands for SAFETY INSTRUMENTED FUNCTION. A SIF is a safety function implemented by a SIS in order to maintain or to restore a safe state.


What is SIL?

SIL stands for SAFETY INTEGRITY LEVEL. SIL is the way to show the amount of risk reduction of a SIF. There are 4 SILs defined, each representing a risk reduction order of magnitude. The higher the SIL is, the lower the acceptable failure rate must be.

The probabilities of failure are related to one of four SILs (IEC/EN 61508):


Probability of failure

Mode of operation: on demand

Mode of operation: continuous

Average Probability of Failure on Demand

Frequency of Dangerous Failures Per Hour


≥ 10-5 to <10-4

≥ 10-9 to <10-8


≥ 10-4 to <10-3

≥ 10-8 to <10-7


≥ 10-3 to <10-2

≥ 10-7 to <10-6


≥ 10-2 to <10-1

≥ 10-6 to <10-5


 There are many methods for risk assessment and SIL determination:

  • Preliminary Hazard Analysis (PHA)
  • Layer of Protection Analysis (LOPA)
  • Target Safety Integrity Levels (SIL) assignment: risk graph and safety layer matrix methods
  • Hazard Analysis and Critical Control Points (HACCP)
  • Hazards and Operability (HAZOP)


Do I need a SIS?

In order to decide whether or not to purchase and implement or to replace a SIS, the following questions need to be answered:

  • Does the existing system comply with current regulations and standards?
  • Is the personnel and equipment protected in case of emergency?
  • How much would a possible shutdown cost?
  • Is it more cost-effective to run and maintain the current SIS than implementing a new increase performance one?


Our offer:

As a system integrator, SIS S.A. designs and develops complete fire and gas monitoring systems to meet your unique plant safety needs. Our specialists provide a full range of services:

  • Consulting;
  • Engineering;
  • Project management;
  • Software development;
  • Factory Acceptance Tests (FAT);
  • Site Acceptance Tests (SAT);
  • Equipment delivery;
  • Installation;
  • Approvals;
  • Maintenance.