What does NEC4 clause 60.1(19) mean?

Clause 60.1(19) is the prevention event, and it is the hardest paragraph in the list to satisfy. The event has four cumulative conditions. It stops the Contractor completing the whole of the works, or stops the Contractor completing them by the date shown on the Accepted Programme; neither Party could prevent it; an experienced contractor would have judged at the Contract Date that it had such a small chance of occurring that it would have been unreasonable to allow for it; and it is not one of the other compensation events. Each has to be shown. Stops means stops, so an event that only makes the work slower or dearer is outside the paragraph. Neither Party could prevent it excludes anything within either Party's control, including their suppliers. The experienced contractor test is the same test used for physical conditions in clause 60.1(12), applied at the Contract Date, so a known possibility at signing fails it. The Contractor notifies the event under clause 61.3 within eight weeks of becoming aware that it has happened, and should give an early warning under clause 15.1 the moment it is foreseeable, because clause 61.5 and clause 63 require an event to be assessed as if an early warning had been given where an experienced contractor could have given one. The change to the Prices is assessed under clause 63.1 as the effect on actual Defined Cost by the date of the notification and forecast Defined Cost afterwards, plus the Fee, generally standing resources, preservation of the works and remobilisation. What most often goes wrong is that the Contractor notifies a general disruption, such as a supply shortage that made materials dearer, when nothing stopped completion, or cannot show that the event was unforeseeable at the Contract Date. Any delay to the Completion Date is assessed separately under clause 63 by the programming expert.