The specific seismic class shall be considered for important electro technical equipment, particularly for nuclear power stations, where the acceleration to which the equipment is subjected will be specified in the response spectrum. As you know, the vibratory stresses to which the functional devices will be subjected must be evaluated, and there must be assurance that they would function correctly when put under these stresses. Therefore the nature of the transfer of ground seismic solicitation to functional cells (e.g. protective relays), must be taken into account at the design stage. The fragility threshold of functional devices (level of acceleration at which the equipment no longer performs its function) has to be determined be forehand. When a structure (generator protection panels) undergoes floor acceleration “a (t)” (i.e. an earthquake) everything happens, supposing that the floor is fixed, as if each element of mass m of the structure was subject to an inertia load Ft = -m . a (t) The equation of motion for damped forced vibrations is Ft =mx”+cx’+kx In the first mode of vibration, the acceleration of mass increased by its displacement. Therefore, the acceleration of mass in top of protection panel is more than its bottom portion. As a result, when generator protective relay installed in top of panel, it is possible to exceed acceleration of relay mass from its vibration withstand.