Strong link weak link
A Strong link/weak link and Exclusion zone nuclear detonation mechanism is a type of safety mechanism employed in the arming and firing mechanisms of modern nuclear weapons. It is a form of automatic safety interlock. 
The safety mechanism starts by enclosing the electronics and mechanical components used to arm and fire the nuclear weapon with a mechanical and electrical isolation barrier, forming the Exclusion zone. This is insulated from mechanical, thermal, and electrical disruptions (such as static electricity, lightning, fire).
Between the exclusion zone and the actual detonators, a normally disconnected link mechanism is used, such as a switch which has a built in motor to activate it. The arming system has to activate the switch in order to connect the firing circuits to the detonators in the weapon. This disconnection, which requires the arming mechanism to operate, is called the strong link.
It is possible for an accident (rocket explosion, airplane crash, accident while weapon is being moved) to disrupt the weapon and break the integrity of the exclusion zone. As a safety mechanism, a weak link is also built into the system. This is a set of components designed to fail at lower stresses (thermal, mechanical, and electrical) than will cause failure of the strong link mechanisms. The weak link acts to break the connection to the detonators before the strong link could be disrupted and fail by the stress of an accident.
These mechanisms do not prevent misuse of the weapon, which is restricted by Permissive Action Link code systems, or an accident from physically causing initiation of the explosives or detonators directly from extremely high temperatures, impact forces, or electrical disturbance such as lightning. The risk of accidental direct detonation is significantly reduced by using insensitive high explosives such as TATB, which is extremely unlikely to detonate due to fire or impact or electricity. While TATB may decompose or burn in a fire, it is extremely unlikely to detonate as a result of that action.