These terms are commonly used while doing maintenance of power plants.Read the following to know about these.
Permissives are set of conditions required to be satisfied before start of an activity to safeguard the system/equipment/process.
Thus, permissives are implemented in series.
Once an activity starts based on system parameters, protections are set of conditions that initiate the closure/stoppage of that activity to safeguard the system/equipment/process.
As any condition in the set should stop the activity, they are implemented in parallel.
Interlocks, as the name itself suggests, are the set of conditions for an activity, where one activity initiates another activity to safeguard the system/equipment/process.