Functional Safety

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

Engi­neer­ing con­trols are the sec­ond lev­el in the Risk Reduc­tion Hier­ar­chy, imme­di­ate­ly fol­low­ing the Elim­i­na­tion or Sub­sti­tu­tion of the haz­ard. Some engi­neer­ing con­trols, such as bar­ri­ers and fixed and mov­able guards, do not need a reli­a­bil­i­ty analysis—as long as the pri­ma­ry design require­ments have been met.

All mod­ern machin­ery stan­dards require mov­able guards to have inter­locks. Safe­guard­ing devices must also be con­nect­ed to the machin­ery’s con­trol sys­tem, includ­ing light cur­tains, safe­ty mats, area scan­ners, and sim­i­lar pres­ence-sens­ing equip­ment. 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 reli­a­bil­i­ty require­ments, with safe­guard­ing typ­i­cal­ly requir­ing high­er lev­els of reliability.

Standards

There are three pri­ma­ry stan­dards used for machin­ery func­tion­al safety:

• Safe­ty of machin­ery – Safe­ty-relat­ed parts of con­trol sys­tems – Part 1: Gen­er­al prin­ci­ples for design, ISO 13849–1
• Safe­ty of machin­ery – Safe­ty-relat­ed parts of con­trol sys­tems – Part 2: Val­i­da­tion, ISO 138349–2
• Safe­ty of machin­ery – Func­tion­al safe­ty of safe­ty-relat­ed con­trol sys­tems, IEC 62061

Both stan­dards can be used for elec­tri­cal, elec­tron­ic, mechan­i­cal, or flu­idic (hydraulic or pneu­mat­ic) con­trol sys­tems. Accord­ing to ANSI B11.0, CSA Z432, or ISO 12100, the ana­lyt­i­cal process starts with a risk assess­ment. The required per­for­mance or safe­ty integri­ty lev­el can be deter­mined based on the required risk reduction.

Fol­low­ing the design of the safe­ty-relat­ed con­trol sys­tem, either stan­dard can be used to ana­lyze its reli­a­bil­i­ty char­ac­ter­is­tics and deter­mine the val­i­da­tion plan require­ments. The ana­lyt­i­cal report, val­i­da­tion plan, and val­i­da­tion test report become part of the machine’s tech­ni­cal file.