We focus in this paper on the contribution of a subsystem to the environmental impact of a system. In this way we propose to explore some limits related to Life Cycle Assessment (LCA), in particular the consideration of the use phase specificities (for example lifetime, technology or energy mix). Two cases studies concerning AREVA T&D's aluminium electrolysis conversion substations are proposed to illustrate these problems. The first one considers the environmental contribution of a transformer to the electrical substation, whereas the second one studies the contribution of the substation to primary aluminium production. We show that the context specificities of a product should be taken into account in order to assess its real environmental impacts. To ignore them can lead to false conclusions, what is essential to avoid when a company wants to define its eco-design strategy.