The instantaneous failure rate of the system is calculated. You can use the failure rate function to study the behavior of the system’s failure over time. Furthermore, the failure rate function, λ(t), is an important representation in the lifetime modeling of the system due to its intuitive interpretation as the amount of risk of failure associated with an item at time t.

The failure rate is calculated as:

Where:

R(t) = The reliability of the system over time t.

λ(t) = The failure rate at time t.

When Account for repair in reliability is selected, reliability is calculated using component repair information, and this function is then used to calculate failure rate.

Even if a system is composed of units with exponential failure distributions (constant failure rates), the overall failure rate of the entire system may not be constant with respect to time; this means system failure times may not be exponentially distributed.

Plotting a curve of the reliability function over time for a simple two-unit parallel system with constant failure rates, where one unit is required for operation, can prove this. If these same two units are put in series, when the reliability function is plotted over time, the reliability curve for this parallel system is not the smoothly decaying exponential curve that you get from the series system. Thus, even though a system is comprised of constant failure units, the failure rate of the system is not necessarily a constant failure rate.

Examples appear for two-unit series and parallel systems in the following topics: