A Review of Standby Redundant Systems

This article critically reviews the existing literature on standby redundant systems. Concepts related to standby systems have been defined. Parameters in which system designers and engineers may be interested have been discussed. Some of these parameters are mean-time-to-system failure, s-expected number of visits to a state, steady-state availability, and s-expected profit rate of the system. The types of systems discussed in literature and various methods employed by different workers in the analysis have been reviewed. Existing literature has been classified with respect to above listed features of standby systems. An up-to-date bibliography is presented systematically.     

A Computer Algorithm for Optimal Maintenance of Standby Redundant Systems

A class of standby redundant systems with exponential failure-time and repair-time distributions is considered. There are several alternatives available in each state (up or down) to maintain the system. A computer algorithm is developed to obtain the maintenance policy that maximises s-expected profit of a standby redundant system.     

Stochastic Behavior of 2-Unit Standby Redundant Systems with Imperfect Switchover

We discuss two models of 2-unit standby redundant systems with imperfect switchover. In Model I the switch assumes up and down states repeatedly, independently of the behavior of the main unit. We obtain the stochastic behavior of the system, i.e., the first-passage time distributions, the expected numbers of visits to a certain state, and the transition probabilities. In Model II the failure of the switch can be detected only when it is used.     

Random Point Processes Applied to Availability Analysis of Redundant Systems with Repair

An approach to calculate the stationary state probabilities and in particular the stationary point availability of redundant systems with repair is presented. The approach is based on the theory of random marked point processes. The tractibility of the results is demonstrated by an example.     

Effect of Human Error on The Availability of Periodically Inspected Redundant Systems

The engineered safeguards of nuclear power plants contain many redundant systems which are not operating under normal conditions but they are called upon to act when emergency conditions develop. To ensure their operability, the systems are periodically tested (usually once a month). In this paper, unavailability formulae for several common logic configurations are developed, taking into account the probability of failure of components due to human error in the simultaneous testing scheme. The probability of the component being down due to human error is assumed to depend on the number of components which have gone through the tests consecutively prior to the inspection of the component under consideration. The system unavailability is the sum of unavailability due to hardware failure alone and the unavailability contribution due to human error.     

A Standby Redundant Model with Noninstantaneous Switchover

This paper considers the case of a standby redundant system where the switchover is not instantaneous. The duration of time between the instant at which action is initiated on the standby subsystem in order to bring it to the active state and the instant at which it becomes operating standby is called switchover time, and is considered to be a random variable. A policy specifying the instant at which action is initiated on the standby subsystem in order to bring it to the operating standby state is proposed and under this proposed policy, the Laplace transform of the probability density function of time to system failure and the expression for expected time to system failure are derived.     

高可用度的冗余UPS

简述通信行业标准《通信局(站)电源系统总技术要求》(YD/T1051)对UPS可用度的要求,介绍适用于电信系统的高可用度冗余UPS。     

Reliability of a 2-Unit Standby Redundant System with Repair, Maintenance and Standby Failure

A repairable 2-unit warm-standby system with repair and preventive maintenance is discussed. Two models are presented. In each of the models the mean time to system failure and the steady state availability are calculated. Some numerical calculations illustrate the results.     

Reliability of Systems with Standby Components

A method is introduced for approximating the reliability of a large complex system with standby components. It is done by substituting an approximate simple model for the more complex system. Each part of the model has a simple formula for its reliability.     

Reliability of a 2-Unit Standby Redundant System with Constrained Repair Time

This paper considers a 2-unit warm-standby redundant system with repair. The repair of a failed unit is constrained as follows: Associated with each failure of a unit is a random variable termed the Maximum-Repair-Time (MRT) of the failure. If the repair of a failed unit is not completed within the MRT, the unit is rejected for further use. Two types of failure stituations for the system are considered: 1) No allowed down time, and 2) Some allowed down time. Expressions for the Cdf of the Time to System Failure (TSF) and the mean TSF are derived by using Markov renewal processes.     

Reliability of Some Modularly Redundant Systems

A mathematical model is established for the reliability of modularly redundant systems with unequal failure rates for the operating and standby units. The failure modes include failures of the active and standby units, three types of switch failures, and failures on system recovery. System reliability is considered for cases of both similar and dissimilar units, and for various restrictions on the failure parameters. It is shown that the most important failure parameters are those related to catastrophic failures, and that putting more reliable units as basic units, which operate initially, is important when switches and recovery are imperfect.     

Optimal Availability of Maintainable Systems

Operating a system for optimum availability is a key aspect in many a complex system. Failure of the system can have serious consequences both in monetary terms as well as in loss of production or disruption of service. Optimal control theory provides a convenient tool to obtain the optimal maintenance policies of maintainable systems. These optimal policies are obtained by applying the generalized maximum principle of Gamkrelidze. The prescribed objective function is represented in terms of the cost of repair as well as the downtime cost. The models considered are Markov in that their present state provides sufficient information about the system. Constant as well as time-varying optimal maintenance policies during the planning or operating period can be considered by the proposed method. The optimal maintenance policy very much depends on the failure rates of the individual system components.     

Time Dependent Solution of a Complex Standby Redundant System under Preemptive Repeat Repair Discipline

The system has two classes of components. Each component in one of the classes is essential to operation. Components in the other class are helpful but not essential. The Laplace transforms of various state probabilities have been obtained; a particular case has been derived.     

Redundant Systems with Appreciable Exchange Time

In dealing with redundant systems, the time required to exchange the failed unit for a spare one is often assumed to be negligible and only the time required for repair is considered. If exchange time is indeed appreciable, numerical methods are applied to analyze the system because it is usually thought to be too complicated to be solved analytically. This paper proposes an analytic solution, in which states of the system are grouped into rows; states having the same number of up-units are in the same row. Steady state probability of each state is expressed in terms of the probabilities of the states in the same row and in the row immediately above. Through these recursive relationships and simple substitutions, the steady state probabilities are solved.     

Analysis of 7 Models for the 2-Dissimilar-Unit, Warm Standby, Redundant System

The paper is in 2 parts. In all models the failure rates are constant, but repair rates need not be constant. The method of supplementary variables is used for solving the models. Part I considers the effect of priorities on reliability and availability for 4 basic models; 1) priority in both repair and operation; 2) priority in repair; 3) priority in operation; 4) no priority. Models 1 and 2 treat 2 repair disciplines: a) preemptive-repeat, b) preemptive-resume. We obtain 1) Laplace transforms of availability and reliability and 2) explicit expressions for steady state availability and for mean time to system failure. The effect of priority assignment to maximize steady state availability is discussed. Part II considers the effect of having different repair rates, depending on whether the failure was from standby or from operation. We obtain 1) Laplace transforms of availability and reliability and 2) explicit expressions for mean time to system failure.     

Reliability of Redundant Systems with Unreliable Switches

Two models of redundant systems with unreliable switches are formulated. These models are analyzed under conditions which are more general than those known in literature. Compact expressions for the first two moments are obtained.     

Repair Priority Effect on Availability of a 2-unit System

This paper is concerned with a 2-unit parallel system with priority repair. The priority rule is a mixture of several disciplines. The optimal priority rule minimizes the s-expected total discounted time in which the system is failed. The optimality of Late Preemption Rule is shown and the effect of the rule on the availability of the system is investigated.     

Reliability of Some Redundant Systems with Repair

A mathematical model is established for the reliability of modularly redundant systems with repair. The model allows different hazard rates for active units and for standby units. The hazard rates are assumed to be constant. The cases of constant repair rate and constant repair time for a two unit system are evaluated using the reliability and mean time between failure. The approach is then extended to systems with more than two units. A system parameter, relating to certain types of sensing, switching, and/or recovery has a very significant impact on system reliability for modularly redundant systems with repair.