Which of the following is an example of a non-seated explosion?

A natural gas explosion is classified as a non-seated explosion because it results from the rapid expansion of gases due to combustion, typically occurring in an open environment. In such cases, the explosion is linked to the buildup of flammable gas that ignites, causing a violent release of energy. This type of explosion does not rely on a defined confinement or control over the combustion process, distinguishing it from seated explosions that involve a particular containment or structural integrity.

In contrast, flashover refers to the rapid spread of fire throughout a room, which is a thermal phenomenon and not an explosion in the traditional sense. Spontaneous combustion involves a fire starting without an external ignition source due to the self-heating of materials, which does not fit the criteria for an explosion. An electrical short circuit might lead to overheating and potential fires, but it typically does not create the rapid atmospheric pressure changes associated with explosions. Thus, among the options, a natural gas explosion stands out as a clear example of a non-seated explosion.