Functional Safety and Control Reliability

Func­tion­al safe­ty describes the abil­i­ty of a con­trol sys­tem to per­form safe­ty func­tions cor­rect­ly over the life­time of a machine. These prin­ci­ples apply to elec­tri­cal, elec­tron­ic, pro­gram­ma­ble and flu­idic con­trol sys­tems. In the past, the term “con­trol reli­a­bil­i­ty” was used to mean the same thing but is no longer wide­ly used.

Engi­neer­ing con­trols are the sec­ond lev­el in the Risk Reduc­tion Hier­ar­chy, imme­di­ate­ly fol­low­ing Elim­i­na­tion or Sub­sti­tu­tion of the haz­ard. Some engi­neer­ing con­trols, such as bar­ri­er guards, fixed, and mov­able guards do not, on their own, need to have a reli­a­bil­i­ty analy­sis done – as long as the basic design require­ments have been met.

Mov­able guards are required to have inter­locks by all mod­ern machin­ery stan­dards. Safe­guard­ing devices includ­ing light cur­tains, safe­ty mats, area scan­ners, and sim­i­lar pres­ence-sens­ing equip­ment must also be con­nect­ed to the con­trol sys­tem of the machin­ery. Since these devices are all required to work auto­mat­i­cal­ly to pro­tect work­ers when they may not be aware of poten­tial dan­ger, these sys­tems must be reli­able. The ques­tion is…how reliable?

Func­tion­al safe­ty require­ments are also applied to Emer­gency Stop systems.

Emer­gency stop and safe­guard­ing sys­tems are not the same and may have dif­fer­ing lev­els of reli­a­bil­i­ty require­ments, with safe­guard­ing typ­i­cal­ly requir­ing high­er lev­els of reliability.