The term reliability was developed during the 1950s, by American manufacturing and research organizations to test and improve the life of electronics components, particularly radio valves used in civilian and military aircraft application.

Reliability is something different from quality. It is the probability that a product or a part or a system or equipment will perform satisfactorily for a given time under normal conditions of use. A product of better quality may not be reliable according to the customers.

Quality is related to the initial performance of a product or a service, but reliability is related to the continuation of performance over a period of time. A product with better initial performance may fail to give the same performance afterwards. In such a case a product is not considered reliable. Therefore, the manufacturers should produce not only quality products but also reliable products.

Reliability is considered most important because failure of a single part may cause disaster. Imagine if the, tank of the battlefield or a bomber plane has to land in the enemy place due to the failure of a small part.

If the maintenance of the product is cheaper then reliability becomes less important. Electric bulb, when burns out, can easily be replaced, failure is not a problem. But failure of the important part in the power generating machine in a thermal power station, which is not available in the country and lead time of procurement is longer, is considered very seriously. This brings about closure of the power generating machine for a long time till the part is replaced. Quick and easy maintenance does less harm.

Reliability can be measured only if it is expressed in quantitative terms. For the measurement of reliability, two points are considered:

1. Probability.
2. Time need consideration.

Reliability must be maintained throughout the process of manufacture and the useful life of a product. If a product is not found reliable, while it is used, it becomes useless. A product is reliable or not, depends on the probability of its failure during a given period and the length of time for which it gives the rated performances. The information relating to the failures or defectives during a given period of time are collected, analyzed and corrective actions are taken to improve reliability. The measurement of reliability depends on the type of products. For example

(i) Product function only on one occasion
(ii) Products have long life, but if they fail they cannot be repaired
(iii) Products have a long life and if they fail they can be repaired and then again can be put to use

A motorcar, having useful life of say 100,000kilometers, cannot be used continuously for the journey of 100,000 km. If a bulb, having expected useful life of 1,000 hours, if used continuously, it may not last even for 800 hours.

We conclude that measuring the reliability of a product, one has to consider the conditions of use also. A product or a part or a component, if not used properly or if it is not well maintained its useful life may be shortened. Its reliability may decrease. Today reliability tests are also specified to ensure that the specifications are not met initially only, but also maintained over some specified period.

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