By-pass systems provide facilities to redirect steam from entry to the turbine. There are several reasons why this may be needed and they differ, depending upon whether the plant is fossil-fired or nuclear.
The one common reason (and possibly the most important) is when there is a large loss of turbine load. For a more detailed discussion of the reasons for by-pass systems refer to Chapter 1.
Though nuclear and fossil-fired by-pass systems differ somewhat in design, their fundamental modes of operation are similar. They both redirect steam from the turbine inlet and pass it to the condenser via a series of isolating valves, pressure control valves and dump tubes.