The SIL Number is a Rating
SIL (Safety Integrity System) is a confidence rating system. In some ways, it's similar in fire alarm systems to the classification system for signal pathways through the building (like Class A, Class B, C, D, E, N, X). The SIL number is a measure of how well the fire alarm panel, the detection, the controls, and the horns/strobes/speakers will perform in case there's a failure.
As a comparison between the SIL number and the fire alarm's circuit or pathway:
- With a Class B circuit or pathway, a broken wire will show trouble on the panel, but some or all of the devices won't work. A wire-to-wire short will shut down all the devices on the pathway.
- With a Class A circuit or pathway, a broken wire will still show a trouble on the panel. However, because there's a second, or redundant, pathway around the break, all of the devices will still work. A wire-to-wire short will still shut down all the devices on the pathway.
- With a Class X circuit or pathway, a broken wire will still show a trouble, and there's second or redundant pathway, around the break, so all of the devices will still work. However, if there's a wire-to-wire short, the short will be isolated so the pathway works even when shorted.
The Safety Integrity System (SIL) number is often used with an industrial processing plant. With a fire alarm panel, it shows the confidence level that the fire alarm system will perform in an overall industrial setting. It shows the failure rate of the system, and it shows the ability to continue working in case there's a failure.
The numbers range from SIL 1 to SIL 4. The lower the number, the less confidence, the higher the number the more confidence that the system will continue working when something goes wrong.
More information on Safety Integrity Systems (SIL) can be found at: