UPS availability calculations


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I have a Server that gets 115V power from a redundant UPS system - two UPS's in parallel switched through a static switch(automatically switches to other 115VAC supply if primary fails)
The static switch is on the output side of the UPS's. If the MTBF is known for each UPS how is it possible to calculate the availabity of the 115V power to the server?? Can anyone help with this? I can provide more info if needed.



To calculate the availability (probability of failure) for the UPS you need to know what the failure rate or MTBF is for the transfer switch. Then probability of failure of you UPS be
[pf(UPS1)*pf(UPS2)]+ pf(static switch, where
pf mean probability of failure which is 1/MTBF.
"*" mean boolean AND and "+" mean boolean OR.
The above calcuation will give you the pf of the UPS. Subtract from one to the availability.

Personally, I don't think a calculation will be very meaningful for this (although I'm sure the vendors will be glad to provide you one at no
charge). The only things that really bring down redundant systems like this are common cause factors (e.g., the switch, batteries not being maintained, human errors, environmental factors, etc.) The switch is a single component, and depending upon it's reliability, may prove to be the weak link. Do you have failure rate data for the switch? Switches must be tested.

Paul Gruhn, P.E., C.F.S.E.
Siemens, Houston, TX
Thanks for your input
I don't have the switch MTBF. In fact I cant get an MTBF number from either MGE or Cyberex switches - they said the products are new and do not have those numbers yet. I don't think the numbers are meaningful either but I think marketing will love them.