The failure distribution appropriate to the specific electronic, electrical, electromechanical, and

mechanical items should be used in computing the component reliability. In most cases, the failure

distribution will not be known and the exponential or the Weibull may be assumed. The α and β

parameters of the Weibull equation are normally empirically determined from controlled test data or field

failure data. This ETL presents a procedure for estimating these values. If the β value in the Weibull

function is unknown, a value of 1.0 should be assumed. The flat failure region of mechanical and

electrical components is often much longer than the other two regions, allowing this assumption to be

adequate. Once the component reliability values are determined, the RBD method is used to evaluate

their relationship within the system to determine the total system reliability. Appendices D and E contain

more information on determining component reliability. In Appendix F, the mechanical and electrical

subsystem reliability data from Appendixes D and E are applied to the overall system to determine an

overall lock and dam system mechanical and electrical reliability value.

The necessity for determining the reliability of a system requires that the reliability be considered from

two perspectives, basic reliability and mission reliability. Both are separate but companion products that

are essential to quantify the reliability of a system adequately. The incorporation of redundancies and

alternate modes of operation to improve mission reliability invariably decreases basic reliability. A

decrease in basic reliability increases the demand for maintenance and support. Basic reliability is

normally applied to evaluate competing design alternatives.

intended to measure overall system reliability. It is used to measure the maintenance and logistic support

burden required by the system. A basic reliability model is an all-series model. Accordingly, all elements

providing redundancy or parallel modes of operation are modeled in series. In a series system, the

components are connected in such a manner that if any one of the components fails, the entire system

fails. Care should be taken when developing this type of model since the final value of the basic

reliability of the system is inversely proportional to the number of components included in the evaluation;

i.e., the more components there are, the lower the reliability. Such a system can be schematically

represented by an RBD as shown in Figure 6.

For a system with *N *mutually *independent *components, the system reliability for time *t *is

(10)

It can also be shown that if *h*s(*t*) represents the hazard rate of the system, then

n

∑

(11)

i=1