Series, parallel, and where redundancy actually helps

Every reliability block diagram is built from two primitives. Get the intuition for these and the rest is just bookkeeping.

Series: the weakest link

Components in series all have to work for the system to work. Reliabilities multiply:

R_system = R1 × R2 × ... × Rn

Because each R is below 1, a long series chain erodes fast — ten components at 99% each give a system around 90%. In series, your system is never more reliable than its worst block.

Parallel: share the load

Components in parallel give redundancy — the system survives as long as one path works. It's easiest to reason about the failure probabilities, which multiply instead:

F_system = F1 × F2 × ... × Fn
R_system = 1 − F_system

Two units at 90% in parallel reach 99%; three reach 99.9%. Redundancy buys the most when each unit is already fairly reliable and the units fail independently.

The catch

Parallel maths assumes independent failures. Shared power, a common controller, the same corrosive environment — any of these can take out both "redundant" paths at once, and the real reliability falls short of the formula. Model the common element explicitly as its own series block rather than trusting the redundancy to cover it.

In Reliafy you can wire all of this on the canvas — series, parallel, k-of-n, and standby — and read off system reliability, MTTF, and which block matters most.