A safety instrumented system is one in which the system — or its equipment — is monitored and repaired based on the use of monitors and sensors. When a safety instrumented system's sensors detect something wrong with equipment, the equipment normally is fixed right then or put out of operation until it can be fixed. While this approach is very precise and usually safe, the monitoring equipment means it can be costly. People who are experienced with a particular machine tend to operate SIS sensors, because the sensors may not pick up everything.
When a safety instrumented system method is used, there must be many sensors and much equipment in place to check a machine. These sensors can determine many different details about the machine, and it largely depends on what the machine itself can do. For example, these sensors can determine how much power a machine is generating, its airflow and its software integrity, among other things. Each machine will have different acceptable safety limits and, when these limits are breached, it normally is best not to use the machine.
After safety instrumented system sensors pick up a problem that puts the machine and its operators at risk, the machine usually is fixed immediately. These problems normally are not immediate risks, and it may be possible to continue using the machine a bit longer, but this increases the safety risk and is considered irresponsible. If the machine cannot be fixed immediately, whether because of a lack of workers or a lack of parts, it normally will be put out of commission until the repairs can be made.
The safety instrumented system approach typically is safer than just trusting humans to check for errors, but one problem with SIS adoption is the use of expensive sensors. These sensors are essential to SIS, but they cost so much, as of 2011, that many small businesses cannot afford them. At the same time, they tend to lower costs after being installed, usually because fewer routine repairs are needed.
While it is possible to use anyone to read the SIS values, only experienced professionals that know how the machine works are normally given this task. This is because, while SIS sensors can determine almost any safety risk, they are not always 100 percent accurate, and there is a chance that there is a safety risk that the sensors cannot determine. For this reason, the human worker and the sensors function together to determine risk and when repairs are needed.