On page 15, the guide points out a hidden drawback in over-engineering power systems:
"For modern critical facilities, the benchmark availability is in the range of 99.999% (“five nines”) to 99.9999% (“six nines”). To achieve six nines availability, the engineered systems will have to incorporate designs that include system+system [2(N+1)] redundancy. It is worth noting that engineered systems in a critical facility are often over-designed to include too much redundancy. That is, systems become more complex than they need to be, which leads to decreased reliability."