second metal sheet 221 each have a peripheral fixing zone 224 225 and are fixed together by welding at their peripheral fixing zone 224 225 in operation the safety device 201 acts as follows when the electronic central unit detects a collision entailing the triggering of the said safety device 201 it delivers electrical current into the two electrical wires and this causes the initiation of the two ignition beads 209 210 these produce hot gases which come directly into contact with the flat sheet 206 constituting the pyrotechnic charge this flat sheet 206 then generates hot gases which pass through the orifices 217 of the side wall of the perforated piece 215 and allow the impervious inflatable metal bag 205 to be fully inflated starting from its centre this safety device 201 is even more compact than those described previously and has a maximum height h of the order of 1 cm furthermore depending on the desired location and use of the safety device 201 the first metal sheet 220 and the second metal sheet 221 may have a circular or rectangular or even triangular overall shape for example finally the pyrotechnic charge may also be produced using loose powder contained in a combustible sachet which is itself held in place by the perforated piece 215 referring to fig4 it can be seen that a safety device 301 according to another preferred alternative form of embodiment of the invention consists of a cylindrical body 302 in which an ignition device is fixed the said body 302 having a flat top surface 304 to which an impervious inflatable metal bag 305 containing a pyrotechnic charge is attached more precisely the body 302 has the shape of a plate with a central.