Parallel availability and time to switch

[egabrum]

Hello all,

Say that you have to units (power supplies) working in parallel. The availability of each units is A1=A2=0.5 (50%). Therefore, the total system availability is A=1-W1*W2= 1-(1-0.5)*(1-0.5)=0.75

But, although both units are running in parallel (not stand-by), only one of them is connected to the load at a time. That is, there is a manual switch no connect the 2nd unit whenever the 1st unit fails. Therefore, there is a “time to switch”=X downtime and the real availability is less than 0.75

Do you guys know how can I calculate analytically the real availability of the system if I know the “time to switch” time? I al looking for a general solution, not the specific solution for the 1+1, 50%+50% case.

Thanks

[David]

From what I understand, what you described does represent a standby situation. Unfortunately, I do not know of an analytical equation to calculate system availability while accounting for the switch delay. However, BlockSim can account for the switch delay using simulation.

The reliability of a standby system can be calculated analytically, but it does not account for the switch delay. Only the reliability of the components and the switch are accounted for. This is discussed at http://www.weibull.com/SystemRelWeb/standby_components.htm under the Reliability of Standby Systems with a Switching Device section. See Eq. 28. I just wanted to provide this for your reference.

A free evaluation version of Blocksim can be downloaded at http://www.reliasoft.info/download/product.php?productid=10&cat=6&page=1.

I hope this helps.

[egabrum]

Thanks David.

I didn't realize that the case I was describing is a particular case of standby redundancy with reliability(active)=reliability(inactive).

I really wanted to learn to do it analytically. I have to calculate many different combination and, as far as I know, reliability packages allow little or no programming at all. The data-entry GUI is normally cumbersome to my taste too.

But I certainly appreciate your suggestion on how to solve the problem with BlockSim.

Thanks