Accidental Nuclear Holocaust over North Carolina?

A newly declassified report from Sandia National Laboratory confirms that in 1961  B-52 broke apart over North Carolina dropping two 4 megaton bombs. One fell to the ground unarmed. But the second assumed it was being deliberately released over an enemy target - and went through all its six arming mechanisms save one. OK - that's certainly scary. But how scary exactly? The journalist currently publicizing this states  "only the failure of a single low-voltage switch prevented disaster".  In contrast, the declassified report states that a short would have been required i.e. the sixth switch appeared to function as it should have.  How close were we actually? In nearly 70 years we haven't had a single nuclear weapon accidentally detonated, but we need to be careful that it never happens in the future.