Probability Logic Table
Voting | Formula | Comment |
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1oo1 | PA | Single element system |
1oo2 | PAPB | Both devices fail |
2oo2 | PA + PB – PAPB | Either device fails |
1oo3 | PAPBPC | All devices must fail |
2oo3 | PAPB + PAPC + PBPC – 2PAPBPC | Any two devices must fail |
3oo3 | PA + PB + PC – PAPB – PAPC – PBPC + PAPBPC | Any device fails |
PFD Equations
PFD equations (not simplified) assuming all devices in the voting scheme have the same lambda:
Voting | PFD | PFD Avg |
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1oo1 | 1−e−λt | 1+λTe−λT−1 |
1oo2 | 1−2e−λt+e−2λt | 1+λT2e−λT−2+2λT1−e−2λT |
2oo2 | 1−e−2λt | 1+2λTe−2λT−1 |
1oo3 | 1−3e−λt+3e−2λt−e−3λt | 1−6λT11+λT3e−λT−2λT3e−2λT+3λTe−3λT |
2oo3 | 1−3e−2λt+2e−3λt | 1−6λT5+2λT3e−2λT−3λT2e−3λT |
3oo3 | 1−e−3λt | 1−3λT1+3λTe−3λT |