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Old July 30th, 2015, 09:28 PM
N.Chaitanya Kumar Reddy N.Chaitanya Kumar Reddy is offline
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Default Bellcore Prediction

For a component is there any difference from Unit Failure rate to Total failure Rate, if we are using SR-332,Method I Case3. Any factors affecting between these two failure rates?:
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Old August 4th, 2015, 09:00 AM
David David is offline
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Default Re: Bellcore Prediction

The system failure rate is the sum of the unit failure rates. The system standard deviation is the square root of the sum of the squares of all the unit steady state failure rates.

If your system consists of only a single unit, then the system failure rate will be identical to the unit failure rate and the standard deviation will be identical to the unit standard deviation.


I hope this helps.
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Old August 4th, 2015, 09:48 PM
N.Chaitanya Kumar Reddy N.Chaitanya Kumar Reddy is offline
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Default Re: Bellcore Prediction

Hi david, thank you so much for your answer,

For example for a capacitor failure rates(FR) are
- 1 component- unit FR 0.15 and total FR 0.169
- 2 component- unit FR 0.15 and total FR 0.327
- 3 component- unit FR 0.15 and total FR 0.483

Can you please help me to find what is the factor affecting the deviation from unit FR to total FR.

Is there any calculation example by considering confidence level and standard deviation.

Thank you.

Last edited by N.Chaitanya Kumar Reddy; August 4th, 2015 at 10:37 PM.
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Old August 10th, 2015, 05:02 PM
David David is offline
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Default Re: Bellcore Prediction

Unfortunately, I am not sure what would cause the total failure rate for a component to be larger than the unit failure rate. It does not appear that you are trying to use redundancy in your system as that would make the total failure rate smaller than the unit failure rate instead of what you are describing. In other standards (MIL-217 for example), the total failure rate is expected to be higher than the unit failure rate. This is because the connection failure rate is considered separately from the unit failure rate. However, in Telcordia, the failure rate due to how the unit is connected to the system is built in to the unit failure rate so a separate calculation is not required, and therefore the unit and the system failure rate should match.
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