How to use N+1 redundancy
The term 'N' is the number of components required for a system to function. A common strategy for increasing the MTBF of a system is to add (+1 or more) parallel paths. Each path has an exponential failure rate of λ per hour, and the MTBF is calculated as 1/λ hours.
It is easy to show that the MTBF of two parallel paths is 3/(2λ), thus increasing the MTBF by 50%. Initially, this does not seem like much, however if the failed path is quickly replaced the MTBF at system level can be increased dramatically.
For example, the power supply system pictured opposite has two elements each with a proven MTBF figure of 100,000 hours. From this value we calculate the Annual Failure Rate (AFR) of a single path using the number of hours in a year, 8760.
AFR1 = 8760 / 100,000
= 0.0876 or 8.76% ............91.24% availability
The value for the maintained N+1 system however, is AFR 2
AFR2 = 0.0077 or 0.77% .............99.23% availability
Using the value of AFR2 we can calculate the system MTBF.
MTBF = 8760 / 0.0077
= 1,137,662 hours !
Of course this is a very simple example and there will always be some non-redundant elements. Maintenance is expensive, but so is down-time, and critical systems often require multiple redundant elements. If your system has N+1 redundant features they can be incorporated within any of the reliability models listed.
If you would like further information on MTBF for systems with N+1 features, or would like a quotation for any of our listed reliability models please contact us on the numbers below
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