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Enter each unit’s MTBF and MTTR, the current and proposed arrangements and the downtime cost to see system availability, downtime hours per year and the payback of installing a standby unit.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Hours per year with the system down, on a log scale, for each possible arrangement.
Downtime hours per year versus MTTR, for the current and proposed arrangements.
Cash flow of the improvement: investment in year 0 and annual savings escalated at the rate entered, discounted at the MARR.
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With an MTBF of 2,000 h and an MTTR of 24 h, one unit is unavailable about 1.2% of the time, more than 100 h per year. With one spare and 5% common cause, the system is down only a few hours per year.
It is a failure that hits all units together, such as loss of the power supply or a repeated maintenance error. It limits the benefit of redundancy, so reduce it with physical and electrical separation.
Hot standby runs alongside and takes over instantly, but wears. Cold standby sits idle and preserved, but can fail to start. Test the cold spare periodically and rotate units.
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