Created on February 10, 2017

A Simple basic concept in protection layers are “Don’t put all your eggs in one basket”. If one basket falls, there is remaining eggs in other baskets.

The attached figure represents risk reduction in losing the eggs when the basket falls. There are 9 eggs in the basket. If those eggs are kept in one basket and fall to the floor, we loss all eggs. If the eggs are kept in separate basket, the risk of losing all eggs when one basket falls reduced significantly (reduced 67%).

Similar with keeping eggs into several baskets, process safety protection layers as shown in the attached diagram, are made into several layers, some of which are prevention layers and other are mitigation layer.

Risk is a function of the probability (or frequency, or likelihood) of an event and the severity (or consequence) of that event. Multiple safety layers in a facility are designed to reduce one or other. Prevention layers are implemented to reduce the probability of hazardous event from ever occurring, and mitigation layers are implemented to reduce the consequence the event when it has already happened.

Source: Gruhn, Paul, Safety Instrumented System (SIS): Design, Analysis, and Justification, International Society of Automation (ISA), 2006.

Kind Regards,
Ir. Dedi Irawan, SFS
Process Safety Engineer

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