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INTRODUCTION Before 1954, the concept of breakdown of machines had not considered by any author. In 1954 Johnson has considered the effect of break down of machines on the completion times of jobs in an optimal sequence. Later on many researchers such as Adiri et al , Akturk and Gorgulu , Schmidt , Chandramouli  have discussed the various concepts of break down of machines. The functioning of machines for processing the jobs on them is assumed to be smooth with having no disturbance on the completion times of jobs. But there are feasible sequencing situations in job shops where machines while processing the jobs get sudden break down due to failure of a component of machines for a certain interval of time or the machines are supposed to stop their working for a certain interval of time due to some external imposed policy such as stop of flow of electric current to the machines may be a government policy due to shortage of electricity production. In each case this may be well observed that working of machines is not continuous and is subject to break for a certain interval of time. This paper takes in to account the effect of break down time interval during which machines stop to work the processing of jobs in an optimal sequence.