Diagram of a w:Strong link/weak link system.
The nuclear weapon contains strong links (green) and weak links (purple). The strong links function to only permit valid signals to penetrate the energy barrier (blue) and enter the exclusion zone. Within the exclusion zone, weak link will fail before the strong links, meaning that by the time a strong link fails in a way that passes a signal into the exclusion zone, the weak links have already failed.
Weak links can be dedicated devices (top weak link), or be part of critical system components: for example the capacitors in the firing set, or the detonators themselves can be or contain weak links.

Also shown in this diagram is the concept of the Unique Signal (UQS) for the two shown inputs. Hardware devices, such as Environmental Sensing Devices or Permissive Action Links turn inputs such as human intent, altitude, pressure, heading, acceleration and so on into unique signals. The unique signals are designed to not be confused with natural signals, even in abnormal conditions. Furthermore, incorrect signal patters will "lock out" the strong links. This lock-out may, or may not be, resettable remotely.
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