TABLE OF CONTENTS Availability is a measure of the time an equipment is able to perform to specified standards, in relation to the time it is in service. The item will be unable to perform when it is down for planned or unplanned maintenance, or when it has tripped. Note that it is only required that the equipment is able to operate, and not that it is actually running. If the operator chooses not to operate it, this does not reduce its availability.
Some items are only required to operate when another item fails, or a specific event takes place. If the first item itself is in a failed state, the operator will not be aware of its condition since it is not required to work till another event takes place. Such failures are called hidden failures. Items subject to hidden failures can be in a failed state any time after installation, but we will not be aware of this situation. The only way to know if the item is working is to place a demand on it. For example, if we want to know whether a fire pump will start, it must be actually started - this can be by a test or if there is a real fire. At any point in its life, we will not know whether it is in working condition or has failed. If it has failed, it will not start. The survival probability gives us the expected value of its up-state, and hence its...
