filler in that field can be used according to the invention even if chalk clay china clay talc and mica each on its own or in any desired mutual combination with each other form examples of particularly preferred fillers the most preferred filler is however chalk the barrier layer 12 consists as previously mentioned of a mixture of plastic and fine particle electrically conductive material mixed in the plastic the mixture of plastic consists partly of a plastic of the same type and partly a plastic of a different type from the plastic in the skeletal layer 11 the plastic of the same type as the plastic in the skeletal layer can thus be a propylene homopolymer with a melting index of under 1 according to astm 2 16 kg 230 u00b0 or an ethylene propylene copolymer with a melting index of between 0 5 and 5 according to astm 2 16 kg 230 u00b0 the plastic of a different type from the plastic in the skeletal layer 11 must be a plastic with good oxygen tight properties examples of plastic of this type are an ethylene vinyl alcohol copolymer evoh polyvinylidine chloride and polyamide among which the ethylene vinyl alcohol copolymer evoh is the most preferred the fine particle electrically conductive material in.