Reliability analysis for power to fire pump using Fault Tree and RBD

One of the most common questions in the early stages of designing a new facility is whether the normal utility supply to a fire pump is reliable enough to “tap ahead of the main” or whether the fire pump supply is so unreliable that it must have an emergency power source; typically an on-site generator. Apart from the obligation to meet life safety objectives, it is not uncommon that capital on the order of $100,000 to $1 million is at stake for a fire pump backup source. Until now, that decision has only been answered with intuition- using a combination of utility outage history and anecdotes about what has worked before. There are processes for making the decision about whether a facility needs a second source of power using quantitative analysis. Fault Tree Analysis (FTA) and Reliability Block Diagram (RBD) are two quantitative methods used in reliability engineering for assessing risk. This paper will use a simple one line for the power to a fire pump to show how each of these techniques can be used to calculate the reliability of electric power to a fire pump. The paper will also discuss the strengths and weakness of the two methods. The hope is that these methods will begin tracking in the NFPA documents that deal with fire pump power sources and can be used as another tool to inform design engineers and authorities having jurisdiction about public safety and property protection. These methods will enlighten decisions about the relative cost of risk control with quantitative information about the incremental cost of additional 9´s of operational availability.