 The SH-55 is larger, and the circular main body is not as ribbed, it uses the same VS-N fuze as the other mines.The VS-N fuze is air pressure driven with pressure on the top plate of the mine forcing air into a diaphragm which exerts pressure on the striker until a retaining ball is forced into a recess.Shocks from overpressure are too short to inflate the diaphragm, this requires gradual pressure giving even greater resistance to shock.