With two sides of an ABS-R2 power system in parallel, the ship would have no power if both sides have failed. For such a system, we multiply the unreliability (probability of failure) of each side to obtain the unreliability of the total system
Parallel connection on ship system
Total parallel ship system unreliability = 0.05 × 0.05 = 0.0025.
That is, there is a 0.25 chance that the ship will have no power at all over a one-year period. Expressed positively for a management report, we say that this ship has a 99.75% reliable power system.
If two components were in series, each with the probability of failure 5%, such that any one failing will cause the entire chain system to fail, then we multiply the reliabilities (probability of working) of each component to obtain the reliability of the total system
Serial connection on ship system
Total serial ship system unreliability 0.95 × 0.95 = 0.9025 or 90.25%.
This is quite a reduction from 99.75% with parallel components. That is why modular systems with identical components in parallel to share the total load are always preferred and implemented in practice.
