The Right Way To Think About Capacity in any system

For a non-constraint, capacity can be: 

(1) Productive capacity 
Productive capacity is equal to the constraint’s capacity. The ability to produce the number of units that the constraint can

(2) Protective capacity 
Protective capacity is capacity needed to restore buffers after a disruption. Restore before another disruption occurs.

(3) Excess capacity
Excess capacity is capacity over and above productive and protective capacity.

Protective capacity is one of the most vital aspects of any stable business. If there is insufficient protective capacity, we will not be able to refill buffer. 
Since an hour lost at the constraint is an hour of lost output, downtime at the constraint is expensive. 

Protective capacity should be idle when the buffer is in an ideal state. 

The non-constraint uses only enough capacity to produce at the constraint’s pace.

 
Once a buffer leaves its ideal state, all non-constraints must use their protective capacity. They must restore the buffer to an ideal state before some other problem threatens to idle the drum.