the busbar where they are exposed in the gaps the concave surface of the bottom face of the protective element may also slope a little so as to guide the bubbles better towards one longitudinal end of the protective element fig8 and 9 show another embodiment of a support device of the invention respectively before and after assembly of the protective element the support device 40 shown in these figures differs from those described above in that the protective element 44 is also provided with lateral protective fins 45 the spacing and the width of the fins 45 are selected to fill in the empty gaps left between the electrodes 42 and between the electrodes 43 the protective element 44 may be fastened to the busbar 41 by adhesive or by fastener members of the screw fastener type once the support device 40 has been assembled under the busbar 41 the fins 45 cover the flanks of the busbar where they are exposed between the electrodes thereby protecting them against the bubbles of corrosive species escaping between two electrodes the protective element 44 shown in fig8 and 9 has a bottom face 44 a with a concave profile serving to channel the bubbles given off by the electrodes towards the longitudinal ends of the protective element nevertheless the protective element 44 may also have.