constructed from two aerodynamic shells an upper shell 62 and a lower shell 64 which are formed from a glass fibre cloth and resin composite the shells 62 64 are supported by a tubular structural spar 66 formed from glass fibre and carbon fibre the spar 66 forms the primary strengthening structure of the blade 60 at the rear of each shell 62 64 and towards the trailing edge 68 of the blade 60 the shells 62 64 are formed with a sandwich panel construction in which a foam core 70 is bonded between an inner sheet 72 and an outer sheet 74 of glass fibre these sheets 72 74 are also known as u2018 skins u2019 the foam core 70 is used to separate the glass fibre skins 72 74 and to keep the shell 62 64 stiff in this region the wind turbine blade 60 incorporates a lightning protection system in the form of a first series of lightning receptors 76 on a suction surface 78 of the blade 60 and a second series of lightning receptors 80 on a pressure surface 82 of the blade 60 the lightning receptors 76 80 in each series are located at five metre intervals along the length of the blade 60 two lightning receptors 76 80 are visible in the cross sectional view of fig2 a the lightning receptors 76 80 are attached via conducting straps 84 to a cable 86 which runs longitudinally inside the blade 60 and is earthed fig2 b is a close up view of one of.