What is Permissives, Protections and Interlocks?

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.


Detailed explanation of permissives, protections, and interlocks with examples related to each term in the context of industrial control systems:


Consider a situation in a chemical plant where certain chemicals need to be mixed in a reactor vessel to create a product. However, the reaction is exothermic (produces heat) and must be controlled at a specific temperature range to prevent overheating which can cause safety issues.

A permissive in this context might be a control system rule that states “the reaction process can start only if the cooling system is on and functioning”. So, the operation of starting the reaction process is tied to the state of the cooling system. If the cooling system isn’t working, the control system will not allow the reaction to start, preventing potential hazards.


Continuing with the above example, a protection mechanism might be a safety relief valve installed on the reactor vessel. This valve is designed to open if the pressure inside the vessel exceeds a certain threshold (which might happen if the reaction gets out of control), thus allowing excess pressure to be safely vented and preventing a possible explosion.

Alarms and shutdown systems also serve as protection, raising alerts or stopping operations entirely when unsafe conditions are detected.


An example of an interlock could be found in an industrial oven used for curing products. The oven might have a safety interlock on the door that cuts off the heat source whenever the door is opened. This prevents workers from being exposed to extreme heat or accidental burns.

Similarly, in a nuclear reactor, control rods (which are used to control the rate of the fission reaction) might have interlocks to ensure they can only be raised when cooling water flow is confirmed, ensuring the reactor doesn’t overheat.

These examples illustrate how permissives, protections, and interlocks work together to create a safe and reliable industrial system. Permissives provide conditions for normal operations to proceed, protections provide safety mechanisms for when things go wrong, and interlocks enforce the safe sequence of operations.