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Question about availability and reliability in complex system

Question about availability and reliability in complex system

Question about availability and reliability in complex system

(OP)
Dear all,

This is my first topic in your (our :)) forum. The matter is how to calculate availabily (A) in complex system.

I know that for a single components A=mtbf/(mtbf+mttr).

In some articles i've readed that:

for series of two components (just a simple example) Aseries= A1*A2 and for a redundant schemes Ared= 1-(1-A1)*(1-A1).

Is it correct all of this? This relations are mathematically correct for reliabilities but i've some problems to accept them for the availabilities, because there are already mean values.

I think that i should calculate firts the mtbf of serial or parallel and mttr of serial or paralle and than the availability as the definition.

Thanks

RE: Question about availability and reliability in complex system

(OP)
Thanks IRstuff for your reply.

I prefer to use a model mathematically correct but I'm reading, for my work , some documents in which there are calculations very approximate. For example there is mtbf of a complex system calculated as an mtbf equivalent (let me say) calculated, that is:

mtbf_eq= avail/(1-avail) * MTTR (where has been considered deterministic value)

The problem is: I'm finding often an MTBF equivalent that is greater than the minimum mtbf of a series. Can be possible ?

If I reduce the system to a RBD series of subsystems, I think that the system MTBF must be equal/lower than the minimum MTBF. It's right?

Thanks







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