Abstract: A guard grid is used as protection in a ventilating shaft and similar passageways in structures with a considerable wall thickness, and the guard grid is particularly useful in nuclear power stations. In the passageway direction, the guard grid is divided into a number of grid-like members one following the other. The grid-like member on the outside of the structure has crossbars extending between opposite sides of the passageway and spacer bars extending perpendicularly to the crossbars. The crossbars and spacer bars form a honeycomb-like structure with a dimension in the direction of the passageway equal at least to the dimension of the spacer bars extending transversely of the passageway direction.

Abstract: A system for detecting changes in load on terrain and actuating an alarm in response thereto including a fluid system, a pressure-sensitive device in the fluid system having a first pressure signal output responsive to a load on a terrain, a second pressure signal output generating device responsive to said first pressure signal output, an alarm and a pressure-sensitive switch connected to said alarm, said switch being responsive to the second pressure signal output to actuate the alarm.