An International society of automation (ISA) technical report provides guidance on evaluating the effectiveness of fire and gas systems (FGSs) in reducing risks in processes using an event tree risk model in preference to layers of protection analysis (LOPA) which was viewed as too limited for application to FGSs. This article shows that LOPA can be used instead of event trees to produce equivalent results and that LOPA is preferred for both technical and practical reasons. One of the examples from the ISA technical report is analyzed using LOPA to show how this is accomplished. (c) 2011 American Institute of Chemical Engineers Process Saf Prog, 2012